casting procedures and defects

INTRODUCTION:

History of cast restorations

Lost wax technique

Dimensional changes in the Lost wax Technique:

Spruing the wax pattern

Theory of compensation

Casting ring

Ring liners

Preparing the wax pattern for investment

Casting procedure

Wax Elimination & Heating or Burnout

 Casting Machines

Casting Technique

Recovery of the Casting

 

INTRODUCTION:

Casting is the process by which a wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is sued make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.  Because castings must meet stringent dimensional requirements, the casting process is extremely demanding.  In dentistry virtually all casting is done using same form or adaptation of the lost wax technique. The lost wax technique has been used for centuries but its use in dentistry was not common until 1907 when W.H. Taggart introduced his technique with the casting machine. The process consists of surrounding the wax pattern with a mold made of heat resistant investment material, eliminating the wax by heating and then introducing molten metal into the mold through a channel called sprue. In dentistry the resulting casting must be an accurate reproduction of the wax pattern in both surface details and overall dimension. Small variation in investing or casting can significantly effect the quality of the final restoration. Successful castings depend on attention to detail and consistency of technique. An understanding of the exact influence of each variable in the technique is important so rational decisions can be made to modify the technique as needed for a given procedure.

 

History of cast restorations:

Copper was cast inMesopotamiain about 3000 B.C.  The art of casting was latter introduced intoEgyptwhere the lost wax moulding process was first developed.  The most ancient dental prosthesis fabricated from gold wire was found inEgyptand dated as far back as 2500 BC.  In approximately 500 BC, the Etruscans produced bridges made of soldered gold bands.  The renaissance craftsman and sculptor Benvenuto Cellini (1500-1571) in his autobiography described his method of casting in both gold and bronze by coating his finely detailed wax model with a reinforced refractory shell. This was heated to eliminate the wax and molten metal was then poured into the mould cavity. The oldest dental castings were gold inlays found in teeth from the natives of Ecnador dating about first Century AD.  After the introduction of dental cement (Oxychloride of zinc) in 1860 various materials were used for construction of dental inlays.  These included pieces of porcelain ground to fit the cavity in an attempt to overcome the aesthetic problems of the popular gold foil fillings.  Another method was to burnish a platinum or pure gold matrix to the cavity and fill this with gold solder or a combination of gold foil and solder. In 1897 Philbrook read a paper entitled ‘Cast Filings’.  He described a method of casting watts metal into a mould formed from a wax pattern for the restoration of posterior teeth. However, this technique was soon abandoned because of the inadequacy of the cementing medium.  In 1903 Lentz cast occlusal surfaces to banded gold crowns by the lost wax method of mould formation but apparently did not apply his technique to the fabrication of cast inlays.

 

It was W.H. Taggart who in 1907 finally described a casting process which launched the cast restoration as it is known today. His process was to take direct wax pattern of the lost tooth structure, sprue and invest it, heat the investment to remove the wax and cast gold into space formed using air pressure. Taggarts investment was composed of Plaster of Paris 37.5%, silica 57.5% and graphite 5%.  Its properties were a setting expansion of 0.45%, hygroscopic expansion of about 1% and a thermal expansion of 0.77%.  Taggart was unable to overcome the problem casting shrinkage which resulted in restorations which were under size. He mistakenly believed that applying air pressure would prevent alloy shrinkage.  However, his investment did not provide sufficient mould expansion and therefore compensation at the temperature she used, probably because no liner was used in the casting ring to allow effective investment expansion.  Lane believed that all castings made by Taggart method were undersize and attributed this to the shrinkage of gold.  As a result Lane conceived the idea of casting into an enlarged mould which he achieved by using an investment containing high percentage of silica (75% approx) plus plaster of Paris, heated to about 650ºC.  He was thus the first to introduce mould expansion as a compensation technique in the dental casting process.

 

In 1910 Van Horn introduced a different method of compensation recommending that the wax pattern be invested at a temperature equal to mouth temperature.  He stated that the result and wax expansion and available investment expansion would, either singly or in combination compensate for alloy shrinkage.  He used a high silica content investment mixed with warm water (46ºC) and after the pattern was invested placed the sealed inlay ring in a water bath held at 43ºC until the investment had set.  Access of water to the selling investment was prevented, the water bath was used only as a convenient way of maintaining the required temperature of the wax pattern and the investment. Van Horn did not attempt to determine the amount of expansion needed, but he contributed greatly to understanding some of the factors related to the accuracy of dental casting by the “expanded pattern” Technique. In the following year Prince using a vitreous silica mould  determined a value of 1.64% linear contraction for pure gold as it solidified at a constant temperature of 1063ºC and a 2.2% linear contraction from this freezing point to room temperature. He concluded that all the contraction of the gold except that which occurred after the metal had solidified was compensated for by the addition of liquid gold from the crucible. However it should be noted that his apparatus did not stimulate dental casting conditions. Many years elapsed before any further fundamental contributions were made to the art of dental casting.  During this time operators argued over whether casting should be done as soon as the wax was eliminated by the rapid boil technique or the high temperature furnace burnout method, as against cooling to room temperature after elimination of the wax before casting. These operators did not appreciate that the investment contracted upon cooling.

 

The low heat casting technique was finally abandoned in 1928 wen Coleman published his research paper No.32 for the United States National Bureau of Standards in which he demonstrated the great shrinkage which occurred when investment is cooled after heating.  This paper also gave a number of formula for dental investment available at the time in which the proportion of plaster of Paris varied from 19 to 56% the balance being quartz, some graphite, colouring material and in some cases boric acid. Weinstein in 1929 found that adding boric acid to customary investment mixtures prevented the shrinkage which occurred when the investments were heated to about 370ºC and with the addition of this chemical quartz based investments expanded approximately 0.9% when heated to 700ºC.  Moore(1993) discovered by the addition of chlorides to investments he could obtain a thermal expansion of as much as 1.1% from quartz based investment.

 

Two important developments occurred in the next few years; the use of cristobalite in investment and hygroscopic setting expansion technique.  Sweeney, Paftenbarger  (1930-33) studied use of cristobalite as a refractory in dental casting investment and found that a cristobalite based investment (75% cristobalite & 25% plaster) heated to temperatures between 400ºC and 800ºC produced dimensionally accurate castings.

 

Phillips (1935) further refined the above technique by the use of a “Control powder” mixed with a cristobalite investment in varying proportions designed to give different thermal expansion to the investment.  In 1932 Scheu presented a end technique which used the setting expansion of investment to compensate totally for the gold alloy shrinkage.  He found that if the invested pattern was placed in water after its initial set (10 min) there would be sufficient expansion to compensate for gold shrinkage and the greater expansion was achieved with a thicker mix. He recommended that the investment remain in water bath for between 20 and 30 minutes. Although Scheu was the first to use this technique deliberately for compensation, this form of expansion was in fact being used unintentionally after the introduction of the wet asbestos liner.  Dr. Wilmer souder recognized that the thermal expansion of the investment was greatly inhibited by the rigid metal casting ring and advocated lining the ring with soft asbestos to act as a heat resisting cushion which would permit the investment to undergo its full thermal expansion.   It has since been established that whether used wet or dry the asbestos liner definitely produces hygroscopic expansion of the investment but of an undetermined amount.

 

Lost wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burnt out (‘lost’) to creates space into which molten metal is placed or cast.  A wax pattern is first formed on a die of the tooth to be restored or occasionally, directly on the tooth.  All aspects of final restoration are incorporated into the wax pattern including the occlusion, proximal contact and marginal fit.  Once the wax pattern is completed, a sprue is attached which serves as a channel for the molten metal to pass from the  crucible into the restoration.  Next the pattern and sprue are invested in a ceramic material and the invested pattern is heated until all remnants of wax are burned away. After burnout, molten metal is cast into the void created by the wax pattern and sprue.  Once the investment is broken away the rough casting is pickled to remove oxides.  Finally the sprue is removed and the casting is polished and delivered to the patient.

 

Dimensional changes in the Lost wax Technique:

The management of these dimensional changes is complex, but can be summarized by the equation

Wax shrinkage + metal shrinkage = wax expansion + setting expansion + Hygroscopic expansion + Thermal expansion  

 

Wax shrinkage:  The amount of contraction which inlay waxes undergo from mouth temperature to room temperature is important when the direct technique is used to form wax pattern.

 

Goldberg (1937) recommended the use of the  wax with 0.1% Contraction for the direct technique and the wax with 0.38% contraction for the a  patterns made on a die at room temperature in the indirect technique.

 

Coefficient of linear thermal expansion (300-600 x 10^-6/K).  Thus  temperature changes occurring during manipulation  of inlay wax are a potential source of dimensional in accuracy in wax patterns.  If the thermal contraction is prevented by physical restraint, as when the pattern cools on a tooth from the wax softening temperature to mouth temperature or cools on a die from the wax softening temperature to ambient temperature residual stresses will be set in the wax and these can lead to subsequent distortion by stress relief (Hollenback 1959). Fusayama (1959) also stated that the shrinkage of the inlay wax could not be overcome by applied pressure during solidification on a room temperature die.

 

Casting shrinkage:

The shrinkage occurs in 3 stages:

1. The thermal contraction of the liquid metal between the temperature to which it I heated and the liquidus temperature.
2. The contraction of the metal inherent in its change from the liquid to the solid state.
3. The thermal contraction of the solid metal that occurs on further cooling to room temperature.

 

The first mentioned contraction is probably of no consequence, because as the liquid metal contracts in the mold with a properly designed sprue system more molten metal can flow into the mold to compensate for such a shrinkage.  The casting technique allows for such flow of molten metal.

Coleman used the expression ‘casting shrinkage’ to include the contraction which occurred during all the three stages already described and employed the term ‘net casting shrinkage’ to describe the casting contraction measured in his experiments.  Earnshaw (1957) suggested the following nomenclature to give a more precise meaning for the term “casting shrinkage”.

 

a)      Inherent casting shrinkage:  The maximum contraction that could occur during the casting of a given alloy, that is the  unrestricted thermal contraction of the alloy when cooling from its solidus temperature to ambient temperature.

b)      Actual casting shrinkage:  the contraction of the alloy which occurs when a casting is  made.  It can also be described as the ‘observed’ or measured casting shrinkage.

c)      Net casting shrinkage:  The difference between the compensation provided by the available investment expansion and the actual casting shrinkage of the alloy, thus describing the amount by which the final casting differs in size from the original wax pattern. Since it is possible that castings may be either oversize or undersize, this value can be positive or negative.

 

Effect of alloy composition on casting shrinkage:  varies with alloy composition, the variation amongst gold alloys normally used for ordinary cast inlays, crowns and bridges is not large enough to warrant changes in investment expansion (Morey 1991).

 

Effect of shape of the casting on casting shrinkage:  Interference could have a greater effect in castings of more complex shapes, especially if those shapes allow interlocking of the solidified alloy and the mould size and shape of the casting effect the linear casting shrinkage (Paffenbarger). An interrelated and equally important factor is the compressive strength of the investment. An investment with high compressive strength at the casting  temperature will be more effective than a weaker one in restricting casting shrinkage of a complex casting and the effect will be greatest when interlocking of casting and mould in maximal (Earnshaw, R) 1969.

 

When small dental castings (inlays and crown) are considered it is obvious the effect of these two interrelated factors would be least in simple class I and class V castings where interlocking between the alloy and mould would be minimal.  ON the other hand, in full crowns the effect would be greatest as the investment cores opposes alloy contraction especially in the directions important to the fit of the casting.

 

MOD inlay casting: restraint imposed by the walls of the mould is more complex and varies in different directions.  It is greatest in the mesiodistal direction where the investment cores against opposes alloy contraction and least in buccolingual and occluso-gingival directions where there is little interlocking of the casting and mould (Earnshaw) 1969.

 

Effect of alloy shrinkage on the fit of MOD inlay casting:  Alloy contraction could therefore be expected to be greatest in directions in which interlocking of castings mould was minimal (that is in buccolingual and occlusogingival directions) and this differential contraction could cause a distortion of the casting. Therefore if an MOD inlay casting seats fully on its preparation, discrepancies could be expected in both occlusal and proximal sections which would be masked by the marginal bevel.

 

All these observations support the contention that while for a given alloy the inherent casting shrinkage is determined onlay by its coefficient of thermal expansion and its solidus temperature, the actual or observed casting shrinkage will be lower by a varying amount depending on the size and shape of the casting and the compressive strengths of the investment mould at its burnout temperature (Moorey 1991).

 

 

Linear solidification shrinkage of casting alloys Type I (Au-based) 1.56%, Type iI (Au-Based) 1.37%, Type III (Au-based 1.42%, Type IV (Ni-Cr based) 2.30%, Type IV (Co-Cr) 2.30%.

 

The values for casting shrinkage differ for the various alloy, presumably because of differences in their composition. For ex platinum, palladium and copper are effective in reducing casting shrinkage of an alloy.  IN general casting shrinkage values are les than their linear thermal contraction values, even though the casting shrinkage as obtained includes both the solidification shrinkage and the thermal contraction from the solidification temperature to room temperature. The reasons are:

When the mold becomes filled with molten metal the metal starts to solidify at the walls of the mold, because the temperature of the mold is less than that of the bulk molten metal  2) During initial cooling, the first layer of metal to solidify against the walls of the mold is weak and it tends to adhere to the mold until it gains sufficient strength as it cools to pull away.  When the metal is sufficiently strong enough to  contract independently of the mold, it shrinks thermally until it reaches room temperature. 3) There may be constraints by the mold on the metal contraction during cooling because of the typical geometry of dental castings.

 

The thermal shrinkage of the first weak solidified layer is initially prevented by its mechanical adhesion to the walls of the mold.   During this period it is actually stretched because of its interlocking with the investment material, which has a lower thermal contraction coefficient. Thus any contraction occurring during solidification can be eliminated by a well designated sprue system that feeds new liquid metal to the sites undergoing solidification.  Also, part of the total thermal contraction can be eliminated with the result that the observed casting shrinkage is les than might be expected on the basis of possible stages of shrinkage.  Because thermal contraction as the alloy cools to room temperature dominates the casting shrinkage, the higher melting alloys tend to exhibit greater shrinkage.

 

If the mold is not made correspondingly larger than the original wax pattern, resultant casting will be that much smaller for inlays and cast dowel cores, which are intracoronal and intraradicular, respectively a slight net shrinkage is acceptable.  However, if the restoration is a crown which is extracoronal net shrinkage may prevent it from seating completely on the tooth preparation.

 

Theory of compensation:  Compensation for the shrinkage of wax and gold by investment expansion was studied scientifically in the early 1920s when Weinstein and Coleman at the United States National Bureau of Standards evaluated the three materials used in the production of a dental casting.  Many researchers have since attempted to prove that theoretically, if the shrinkages of the wax and gold alloy are known, the mould can be expanded an equal amount, thus solving the problem of compensation (Phillips).

 

Contraction occurs both during cooling of the liquid alloy and during solidification.  However under correctly controlled casting conditions, feeding of additional molten alloys from the sprue, reservoir or button has been shown to compensate fully for the shrinkage of the alloy (Price, Coleman, Sonder).  Thus observed liner casting shrinkage of an alloy is simply the thermal contraction which occurs when the solidified casting  cools from its solidus temperature to ambient temperatures.

 

 

Setting expansion:  Setting expansion of the investment occurs as a result of normal crystal growth.  The expansion probably is enhanced by silica particles in the investment interfering with the forming  crystalline structure of the gypsum, causing it to expand outward (Phillips).  This expansion in air normally is about 0.4% (Max setting expansion in air for Type I  investment ANSI/ADA specification No.2 allows 0.6%). The pattern metal casting ring and compressibility of the ring liner all influence this expansion.  The effectiveness of the setting expansion in enlarging the mold containing the wax pattern may be related to the thermal expansion of the pattern caused by the heat of reaction that occurs coincidentally with the setting of the investment.  It follows from such a theory that the setting expansion is effective only to the extent that the exothermic heat is transmitted to the pattern. The amount of expansion of an investment depends on the gypsum content of the investment, therefore, the setting expansion of an investment with a comparatively high gypsum content is more effective in enlarging the mold than is a product with a lower gypsum content.  Likewise, manipulative conditions that increase the exothermic heat increase the effective  setting expansion ex: lower the water/powder ratio or drier the mix for the investment greater  is the effective setting expansion. The use of less water increases the setting expansion and results in a slightly larger casting.  Use of an additional ring liner increases the setting expansion, as well a slight increase in mixing time. If a smaller casting is desired, more water can be used or the liner can be eliminated, both of which curtail the amount of expansion. As the investment sets, it eventually gains sufficient strength to produce a dimensional change in the wax pattern and mold cavity as setting expansion occurs. The inner core of the investment adjacent to mesial-occlusal distal (MOD) wax pattern can actually force the proximal walls outward to a certain extent. If the pattern has a thin wall, the effective setting expansion is some what greater than for a pattern with thicker walls, because the investment can move the thinner wall  more readily.  Also, softer the wax, greater the effective setting expansion, because the softer wax is more readily moved by the expanding investment. If a wax softer than a Type II inlay wax is used, the setting expansion may cause a serious distortion of the pattern.

 

Hygroscopic expansion:   Changed setting expansion which occurs in the presence of excess water.  The phenomenon was extensively studied by Scheu who named it hygroscopic expansion.  Contact with water may be achieved by immersing the filled casting ring in a water bath, by the addition of a known quantity of water to the exposed surface of unset investment in the casting ring or by lining the casting ring with wet asbestos prior to investing the wax pattern. The magnitude of hygroscopic setting expansion is directly related to the quantity of water added during the setting period until a maximum expansion has occurred.  No more expansion is evident regardless of any further addition of water.  The ANSI/ADA specification Nos for Type II investments requires a minimum setting expansion in water of 1.2%, the maximum expansion permitted is 2.2%.

 

Effect of Added water:

The presently accepted mechanism for hygroscopic setting expansion also relates to the normal setting expansion that occurs when the investment mix sets in contact with air the basis for this mechanism lies in the role played by surface tension of the mix water. After the investment is mixed, water surrounds the components of the setting investment. As the reaction of the calcium sulphate binder progresses, the surrounding water is reduced and growing gypsum crystals impinging on the surface of the remaining water whose surface tension inhibits outward crystal growth. When the water needed for the reaction is used up and the reaction is virtually completed, the growth of gypsum crystals stops in its inhibited form. If the investment is poured into a casting ring having a waterfilled liner, the gypsum crystals can grow further but only until the new water surface provided by the additional water in the liner is reached; surface tension then inhibits further growth. If water is supplied to the mixed investment mass by immersing the invested ring in water bath, no new surface is close enough to provide inhibition of crystal growth.  However, when the investment is setting in an confined ring, hygroscopic expansion is limited by the confinement of the ring.   For hygroscopic expansion the additional water provided must be presented to the investment during setting. This is significantly different than adding more water to the premixture components (high W/P ratio).  Anther requirement for hygroscopic expansion is that the additional water be presented before the observed  “loss of gloss” which is when the setting reaction is not complete and the mix water can still be observed on the surface of the setting investment.  This allows the additional water to join the remaining mix water and extend the water surface. So that the action of surface tension is either delayed or inactive.

 

The term hygroscopic in a strict sense  is a  misnomer. Although the water may be drawn into the setting material by capillary action, the effect is not related to hygroscopy.  Furthermore on the basis of the theory hygroscopic setting expansion is as normal a phenomenon as that which occurs during normal setting expansion.

 

Effect of confinement:  Hygroscopic expansion dos not occur in an unrestricted trough or an expandable investment ring. However in a lined rigid, metal ring the increase in the effective setting expansion during immersion of investment in a 37.7ºC (100ºF) water bath is apparently not only the result of hygroscopic expansion. Rather it may be caused mainly by heating and expanding the wax pattern and softening the pattern at the water temperature permitting an increase in effective setting expansion. The latter results from a combination of thermal expansion of the wax pattern plus the softened condition of the wax, reducing its confining effect on the expansion of the setting investment.

 

The following factors tend to increase hygroscopic expansion:

Increased silica content of the investment & hemihydrate as the binder.

Increased setting expansion of the binder

Thicker mix of investment

Increasing mixing time (spatulation)

Immersion of the investment at or before its initial set length of time it is immersed.

Temperature of water bath

Lining the ring with asbestos liner

Using a split ring or one made of flexible rubber

Storage at 100% humidity

Hygroscopic technique with the pattern in the upper apart of the ring.

 

Wax pattern expansion:  Expansion of the wax pattern while the investment is still fluid occurs when the wax is warmed above the temperature at which it was formed.  The heat may come from the chemical reaction of the investment or from a warm water bath in which the ring is immersed. Thermal expansion of the wax pattern can be obtained by maintaining the temperature of the water investment mixture at approximately 40ºC to 42ºC. However, such a treatment will produce stress relief of the wax and consequent distortion and for this reason this technique is not recommended.

 

The low temperature burnout technique employs a combination of wax pattern expansion and thermal expansion of mold. After the investment, filled ring is removed from a 100ºF (38ºC) water bath the ring is heated to only 900ºF (482ºC) before casting to produce the additional expansion needed.

 

Thermal and inversion change expansions: Suitably compounded quartz investments show good setting and hygroscopic expansion. This together with the small thermal (450ºC) expansion occurring before the temperature of the inversion change of quartz can compensate fully.  Other quartz materials with a laser hygroscopic expansion require the addition of the inversion change expansion and are heated to 700ºC.  Cristobalite shows a high inversion change expansion at 450ºC.  To obtain good inversion change expansion, an investment mould containing quartz should therefore be cast above 650ºC and one of cristobalite above 350ºC.  This mans that a quarter mould must be heated to between 700º to 750ºC to allow for any cooling which may take place before casting is completed. When hygroscopic expansion is the main method of compensation, an investment containing quartz is commonly used.  The mould is heated to 450-470ºC so that only a slight thermal expansion occurs.

 

Type II investments ANSI/ADA specification Nos.T.E. is 0.6% at 500ºC (932ºF) for Type I which rely principally on thermal expansion for compensation, the thermal expansion must be not less than 1% n or greater than 1.6%.

 

The high temperature burnout technique relies primarily on the thermal expansion of mould.

 

The silica refractory material is principally responsible for this because of solid state phase transformations. Crislobalite changes from α to the β (high temp) form between 200ºC (392ºF). This transition involves a change in crystal form, an accompanying change in bond angles and axis dimension and a decreased density producing a volume increase in the refractory components (a displacive transformation) Greener 1972.

 

Expansion requirements for castings:  Thin ¾ crown – 1.80%, Class I, and II, small MOD – 1.85%, Large MOD, ¾ crown 1.40 Overlay, pin pontic – 1.95%, Bulky ¾ crown – 2.00, small class V, Full crown – 2.10%, Large class V crown 2.40.

 

Gypsum bonded investment:  Gypsum is used as a binder, along with cristobalite or quarts as the refractory material.  The cristobalite and quartz are responsible for the thermal expansion of the mould during wax elimination.  Because gypsum is not chemically stable at temperatures exceeding 650ºC. These investments are typically restricted to castings of conventional Type II, III & IV Gold alloys.

 

Phosphate bonded investments:  Because most of metal ceramic alloys fuse at around 1200ºC (2300ºF) (as opposed to conventional gold alloys at 925ºC (1700ºF) additional shrinkage occurs when the casting cools to room temperature.  To compensate for this, a larger mold is necessary. The added expansion is obtained with phosphate bonded investment.

 

The principal difference between gypsum bonded and phosphate bonded investments is the composition of the binder and the relatively high concentration of silica refractory material in the later. The binder consists of MgO and an ammonium phosphate compound contrary to gypsum bonded investments, this material is stable at burnout temperatures above 650ºC (1200ºF) which allows for additional thermal expansion.  Most phosphate bonded investments are mixed with specially prepared suspensions of colloidal silica in water.  Some phosphate bonded investments contain carbon and therefore are gray in color. Carbon containing materials should not be used for casting base metal alloys because the carbon residue affects the final alloy composition.  They may be used for casting high gold or Palladium alloys.

Expansion: Compared to gypsum bonded investments, phosphate bonded investments after greater flexibility in controlling the amount of expansion. The liquid-powder needs only slight modification to effect a significant change in selling expansion.  Increasing the proportion of special liquid (colloidal silica also increases expansion). 

 

Working time:  Phosphate bonded investments have a relatively short working time compared to gypsum materials. Their exothermic setting reaction accelerates on the temperature of the mix rises during manipulation. A longer mixing time significantly accelerates the setting reaction and temperature and thus reduces the working time even further.  The addition of water to the colloidal silica suspension increases the working time, with some loss of setting expansion.  Gas is formed during the reaction and must be removed for a sufficiently long period to minimize nodules on the casting.  Maintaining a vacuum for about 60 seconds appear to be adequate for this.

 

Hygroscopic expansion:

Hygroscopic (submersion in water bath)

Water bath immersion technique was developed by Scheu. The investment filled ring was submerged in a water bath held at 38ºC while the investment set – 1932.

 

Hollen back method:  Hollen back combined Scheus method when he lined casting rings with wet asbestos and suggested the immediate immersion of the filled ring in a warm water bath (1943).

 

Modified Hygroscopic technique: Markley  1958 observed distortion caused by resistance of wax pattern to hygroscopic and setting expansion is excessive with hygroscopic expansion technique. Hygroscopic expansion can be minimized by keeping water level ¼ inch below the top of the ring (So W/P ratio not affected).

 

Hygroscopic Technique with controlled water added technique: Asgar Mahler & Peyton 1955.  Instead of submerging in water bath they added specified amount of water during setting of investment so that desired amount of hygroscopic expansion is achieved.

 

Padal Technique: Marginal integrity is utmost importance.  Margins of wax pattern are cut away 1 or 2 mm and dead (Dual wax) soft wax is added into these areas for accurate margin reproduction and used pretempered distilled water (81.5º) for mixing investment.

 

Spruing the wax pattern:

                

                    

               

Sprue oringate is the channel in a refractory investment mold through which molten metal flows.

 

Sprue former (Sprue pin) is a small diameter pin or tube made of wax, plastic, (resin) or metal.

 

Sprue design varies depending on the type of restoration being cast, the alloy used and the casting machine. There are three basic requirements:  1) The sprue must allow the molten wax to escape from the mold  2) The sprue must enable the molten metal to flow into the mold with as little turbulence as possible  3) The metal within it must remain molten lightly longer than the alloy that has filled the mold. This provides a reservoir to compensate for the shrinkage that occurs during solidification of the casting.

 

Types and Desired characteristics in a sprue former:

There are three types of sprue formers each made of different material  1) Resin  2) Wax 3) Metal  4) Plastic.

Wax sprue formers:  Wax sprue formers are preferred for most castings because they melt at the same rate as the wax pattern and thus allow easy escape of the molten wax. Round wax is a most commonly used sprue material for many restorations of all sizes. Wax has the advantages of being inexpensive, easy to manipulate, easy to burn out, and available in a variety of diameters.  Wax sprues can also be easily designed for complex castings that requires multiple sprues and vents.  Have low thermal conductivity and thus transmit a minimal amount of heat to the pattern with minimal possible distortion before evaporation.  Disadvantage: lack rigidity.

Plastic: Sprue formers has the rigidity of metal an advantage but can still be burned out although longer burnout times may be required than with wax and may block the escape of wax resulting in increased casting roughness.

 

Advantages: Easily  burnable so there is no need to remove them like metal ones. The fusion temperature of the resin or wax sprue formers should be lower or at most the same as that of the pattern wax in order to allow for the evaporation of the pattern wax has low thermal conductivity.

 

Metal sprue formers:  Can be solid or hollow. Hollow sprue formers are preferred since they hold less heat than a solid sprue and so will cause less heat transfer to wax pattern resulting in less distortion. Also they are more retentive.  To further improve retention and reduce thermal conductivity, the sprue former can be filled with sticky wax. The sprue former must be mechanically removed prior to burnout.  This could cause investment to loosen from the walls.  To avoid this metal sprue formers are uniformly coated with wax before investing so that at the time of burn out the sprue former comes out on its own because of melting of wax.  This way the loosening or breaking of investment can be avoided.

 

Sprue former surfaces should be very smooth to prevent an irregular inlet walls. Irregularities in thee walls can separate from the investment and can become incorporated in the stream of the melt; leading to obliteration of some details or creation of internal voids.  This risk is very real with metal sprue formers, less with resin ones and minimal with wax types.

 

The sprue former material should not rust or react with any ingredient of the environment, thereby avoiding the formation of product materials that could modify the properties of the restorative material or act as dislodged pierces or foreign bodies.

 

The sprue formers should be perfectly cylindrical in shape so as to create a perfectly rounded inlet for the mould which is the easiest inlet for the melt ingress.

 

Diameter of sprue:  The diameter of the sprue is the most important factor in dictating the speed with which the melt enters and fills the mold. Melt velocity is directly proportional to sprue diameter.

 

The velocity of the melt into the mold is also governed by:

1)      Density of the cast metal:  More density faster it will be driven into mold.

2)      Melt velocity: which is directly proportional to the energy supplied by the casting machine.

3)      The velocity with which the air and gases can be evacuated: greater the velocity faster the melt will be sucked and driven into the mold.

4)      The viscosity of the melt – lower the viscosity of the melt is, the faster it can be driven in

5)      Length of the sprue former – the  shorter the sprue is, the faster the melt will ingress into the mold

6)      Flaring (Funnelling) – the more flared the sprue is as it comes close to the mold, the greater the speed of melt ingress will be.

7)      The size of the pattern – larger the formed wax pattern is, the slower the velocity of the ingresing melt will be

8)      The dimensions of the walls surrounding the mold, the greater the thickness of those walls, the slower the rate of heating will be, the slower the escape of gases will be and less the vacuum and slower the speed of filling the mold with the melt.

Speed of filling the mold should not be faster than the speed of evacuation of the gases and air from the mold since this can create a mixture of cat materials and gases in the mold with a subsequent porous or incomplete casting.

 

Select a sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern.

• If the pattern is small, the sprue former must also be small because attaching a large sprue former to a thin, delicate pattern could cause distortion. On the other hand, if the sprue former diameter is too small this area will solidify before the casting itself and localized shrinkage porosity (suck back porosity) may develop. Preferably the diameter of the sprue former should not be more than ¼ of the total area of the wax pattern.
• If gold is to be melted immediately above the entrance (air pressure technique) the sprue former should be small in diameter so that the molten gold will not e able to flow into the sprue hole before the pressure of the casting is applied.

 

American brown and Sharpe wire gauge  numbers and wire diameter

6-18 gauge No. (4.115 mm – 1.024).

• 10 gauge – full crown, large ¾ crown, large MOD.

If more than one sprue is used for the above use 12 gauge (2 mm) 12 gauge – for medium sized patterns.

 

Point of Attachment or Location:  In general sprue should be attached to the bulkiest part of the pattern, away from margins and occlusal contacts.  Placing the sprue away from the margins minimizes distortion of this  delicate area of the pattern.

 

Bulkiest portion because: 1) minimize the effect of released residual stresses by the heat of attaching the sprue.

It will ensure that the thinner cross section of the mold will be completely filled  3) melt will always be fluid enough and available until all lesser dimensions are completely filled.

Normally the large non centric cusp is used. The point of attachment should permit a stream of metal to be  directed to all parts of the mold without having to flow opposite  the direction of the casting force.  The sprue must also allow for proper positioning in the ring. This can be critical because the expansion within the mold is not uniform. Ex spring on cup tip can give good results but spruing on the proximal contact may produce a casting that is too wide mesiodistally and too short occlusocervically.

 

Sprue formers should be attached to the least anatomical area in the wax pattern ex: are of no grooves, cuspal anatomy, fossae or ridges.

 

Direction or Angulation of the sprue former:

Should be directed away from thin or delicate parts of the pattern, because the molten metal may abrade or fracture investment in this area and resulting in casting failure.  The melt should not but this areas of the mold at 90º to avoid  fracture failure, missing details or excess. It should be directed away from or at 45º to these details. Sprue is never directed at right angle to a flat portion  of a wax pattern as this will create a reverse flow (with turbulence of melt) causing Hot pot (suck back porosity).  Can create a concavity in this wall opposite to the sprue. The mold concavity will be reproduced as a convexity in the restoration preventing its seating and making the restoration rock.  Angulation should be planned to assure the easiest and most efficient way of flowing the melt.

 

Length of the sprue:  Major fact or governing the sprue length is the length of the ring. The wax pattern should be positioned (1/4) 6 mm from the end of ring (GBI) and 3.25 mm (1/8”) for BBI. This position provides sufficient thickness of investment to contain the molten metal and reduces the amount of investment through which gases must escape.  Positioning the pattern close to the surface  also ensures that the casting cools more rapidly than the more centrally located sprue. When this  is done metal in sprue remains liquid and flows to the casting until the casting is completely solidified.

 

 

Number of sprues:  Patterns may be sprued directly or indirectly for direct spruing the sprue former provides a direct connection between the pattern area and the sprue base or crucible former area. With indirect spruing a connector or reservoir bar is positioned between the pattern and the crucible former. It is common to use indirect spruing for multiple single units or fixed partial dentures although several single unit can be sprued with multiple direct sprue formers should the wax pattern have a thin area between the sprue and periphery of the pattern, the melt will solidify at the reduced cross sectional area preventing complete filling of the mold (MOD) Y shaped sprue. Here two sprues can be sued.  If multiple sprues are used, they should join together at the crucible former level in a reservoir larger in diameter than all the sprues combined.

 

 

 

Sprue former attachment – (Sprue-wax pattern joint) should be smooth and uninterrupted. If high velocity ingress of the melt into the mold is required (due to the rapid solidification of the alloy, a large surface area of the mold etc) the sprue wax pattern joint should be flared with smooth uninterrupted surfaces.  Flaring will make his joint the area of maximum dimension in the sprue. Wax pattern complex thereby ensuring a fluid melt and non obstructed flow (SAME AS RESERVOIR).Attached at largest cross sectional areas. This design minimizes the risk of turbulence.  The attachment should not be restricted because necking increases casting porosity and reduces mold filling. Not flaring sprue connections results in formation of sharp projections of investment that can break off and incorporate into metal during casting.  If velocity ingress is required (due to high viscosity of melt, slow escape of gases from the mold, piston injection used to drive the melt into the mold).  A constricted function should be made where the sprue meets wax pattern. This configuration will introduce shear stressers in the melt passing at the constriction, thus decreasing the viscosity of the melt and making it more wetting.

 

To minimize the stress release while joining a sprue former to the wax pattern a drop of sticky wax should be applied to the wax pattern.  Then the sprue also with the bead of sticky wax is brought in contact with the sticky wax on the wax pattern and held immobile until the sticky wax is solidified. The flare shape of the function with the wax pattern also ensures the retention of the sprue in the wax pattern.

 

Reservoir: Occasionally, wax is added around the sprue former 1-2 mm from the pattern in order to create an area in the mold (a reservoir) with dimensions far exceeding  that of the thickest portion of the pattern. It is always indicated when the sprue is long or thin or for any other reason that might cause the flow of the melt to be interrupted before all the mold details re filled.  A reservoir is added to sprue to prevent localized shrinkage porosity. Because of its large mass of alloy and position in the heat center of the ring, the reservoir remains molten to furnish liquid alloy into the mold as it solidifies. When molten metal strikes against the mold wall at 90º angle and if the mold metal temperature differential is more it creates hot spot, the molten metal in this area will solidify last leading to localized shrinkage porosity.  The reservoir provides the molten metal to compensate for this localized shrinkage porosity at the casting sprue function. If we use a sprue, the diameter of which is greater than the diameter of the cross sectional area of the pattern, there is no need for providing reservoir. The sprue itself will provide the molten metal.

 

Venting:  Small auxillary sprues or vents have recommended to improve casting of thin patterns. Their action may help gases escape during casting (Strickland 1959), or ensure that solidification begins in critical areas by acting as heat sink (Rawson RD) 1972.  In some situations there is some doubt about the speed with which the mold gases will escape relative to the speed the melt is entering. This could be due to considerable thickness of investment surrounding the investment, the use of pattern material whose residue may clog the investment pores or wax pattern with a lot of minute details or thin cross sectional areas that are difficult for the melt to wet.  These cases require attaching a wax rod to the furthest or close to the furthest part of the pattern, which will stop short of the ring (investment) surface. In most cases they are curved towards the sprue. Gases that will not escape fast enough ahead of ingressing melt will be compressed and trapped in thee vents for infection moulding of cast ceramics venting tubes are built into the vesting flasks that connect the mold with the flask surface.

 

 

 Crucible former (Sprue base, sprue former base):  It is cone shaped. It is also important factor. If a pressure casting machine is used, the crucible should be rounded at sprue hole. If a centrifugal machine is used, the crucible former should be in the nature of a broad cone. By being conical in shape it allow the molten gold to enter the mold chamber rapidly and as easily as possible. Sprue formers that have been altered to provide a large flat area to attach a greater number of patterns. Can be the cause of centrifugal casting problems. When the molten gold strikes this flattened surface it becomes turbulent. A rolling action results as the molten gold attempts to enter the sprue openings.  The slowing down of the molten alloy allows it to drop in temperature fluidity and velocity and can result in void areas in the castings. Crucible former is tall to allow use of a short sprue and allow pattern position.

 

Casting ring

 Holds the investment in place during setting and restricts the expansion of mold  with the use of solid metal rings, provision must be made to permit investment expansion.  The mold may become actually smaller rather than larger because of the reverse pressure resulting from the confinement of the setting expansion. To overcome this split ring (ringless casting system) or flexible rubber ring that permits the setting expansion of the investment or a ceramic paper liner is placed inside the ring against which the investment can expand to enlarge the mold.  If there is no room for outward expansion, the expansion forces will be exerted inward towards the mold resulting in distortion of the casting.

 

The metal used in the construction of a ring should be non corrodible, hard with a thermal expansion similar to investment used.  Stainless steel has been found to produce the most acceptable rings.  The thermal expansion of SS is 1.20% at 700ºC  which is compatible with the expansion of investments provided a liner is used (Ray 1933).  The position of the pattern in the casting ring affects expansion, so far consistent results a single crown should be placed within the ring equidistant from its walls. The dimensions of the ring may vary according to the desire of the operator but the average dimensions are approximately 29 mm (1 1/8”) in diameter and 38 mm (1 ½”) in height.

 

Ringless casting system:    With use of higher strength phosphate bonded investments, the ringless system has become quite popular. This method uses paper or plastic casting ring and is designed to allow unrestricted expansion (Engelman 1989). Also called as power cast ringless system consisting of three sizes of rings and formers, preformed wax sprues and shapes, investment powder, and a special investment liquid. The crucible former and plastic ring are removed before wax elimination, leaving the invested wax pattern.

 

Ring liners:

Need for liner:  A resilient liner is placed inside the ring to provide a buffer of pliable material against which the investment can expand to enlarge the mould. If there is no liner present, the investment is in direct contact with the walls of the mold and will not e able to expand outward and so will expand in the direction providing less restriction  i.e. towards the center of the mold thus resulting in distortion of the casting. The liner if wetted allows for semi hygroscopic expansion of the investment. It  becomes easier to remove the investment and casting from the ring, if a liner is used.

 

Material of liner:  Most commonly used technique to provide investment expansion is to line the walls of the ring with a ring liner.  Traditionally asbestos was the material of choice but it can no longer be used because its carcinogenic potential makes it a biohazard. It has been proved that asbestos fibers can cause asbestosis, bronchogenic lung cancer, mesothelioma. The currently accepted threshold limit value for asbestos fibers range from 2 x 10⁵ – 20×10⁵ fibres/m³.  The limit is considerably exceeded when dental castings are recovered from asbestos lined ring.

 

Asbestos – white – less toxic – used in dentistry

          – brown

          – blue

Alternatives to asbestos liners have been introduced, these include absorbent cellulose (paper) and non absorbent ceramic materials. The latter are manufactured by standard paper making technique from fibers of alumino silicate glass derived from  Kaolin.  Cellulose fibers readily absorb water when immersed and therefore like asbestos wetted before use.  Ceramic liners are virtually nonabsorbent when immersed in water under atmospheric pressure and therefore can be used dry. It has been recently suggested that fliers from ceramic liners can be of similar dimension to those of asbestos liner and so may also be potential health risk. But other studies claim that these fibers  dissolve quickly in lung tissues and so have little harmful effect.  Most studies carried out to determine suitability of these alternative liners have concluded that both cellulose and ceramic liners could produce satisfactory castings provided some alterations were made to the investing technique, which is routinely used with asbestos. When using ceramic liners one should not use vacuum investing technique. A dry ceramic liner produces very inconsistent castings, which are clinically unacceptable. So one can saturate the liner with a dilute wetting agent prior to investing.  An investment with an increased potential expansion should be used. The relative incompressibility ad high water sorption of cellulose liners must be considered and also the fact that they burn out of the ring at temperatures over 600ºC.  Ceramic liner could be compressed 50% of its original thickness when dry or wet.

 

How to use liner: A liner can be used dry or wet. A dry liner will allow greater normal setting expansion in the investment.  Theoretically, a wet liner will allow greater. Normal setting expansion and semi hygroscopic expansion, but it also reduces the powder-water ratio, which in turn will reduce the thermal expansion of the investment. As a result, net expansion with a dry liner will be greater than with a wet liner.  However because the effect of dry liner depends on its volume relative to that of the investment, which varies with the diameter of the ring, a damp liner is preferred for sake of consistency.

 

The maximum thickness of liner is 1 mm. A thicker liner or two layers can also be used. The liner is cut to fit the inside diameter of the ring with no overlap. The length of liner is a controversy.  Few authors view that when a liner is placed 1/8-14” short at each end of the ring, the investment cannot expand laterally at the ends of the ring. In the central portion of the ring it does expand laterally to a limited extent.  Thus the mold is distorted. Many others feel that expansion of the investment is always greater in the unrestricted direction (longitudinal) rather than its lateral direction (towards ring itself). Thus it is important to reduce expansion in the longitudinal direction.  Liner should be placed 1/8” short at the ends to get less distortion of the mold. Others believe that placing the liner flush with the open end of the ring give max expansion. When cellulose liners are used, they burn out before the casting is made.  This deprives the investment of support by the ring and may result in cracking the investment.  Thus 3 mm of the ring is left unlined at each end to support the investment.

 

After the liner has been placed in the ring the ring I dipped in water for 10 sec. (incase of asbestos or cellulose liners) and gently shaken to remove excess. Squeezing the liner would resulting removal of excess and variable amounts of water and non uniform expansion. Although ceramic liner my not absorb water like a cellulose liner, it network of fibers can retain water on the surface.

 

Preparing the wax pattern for investment:

The wax pattern should be cleaned of any debris,  grease or oils. A commercial wax pattern cleaner or diluted synthetic detergent is used.  Any excess liquid is shaken off, and the pattern is left to air dry while the investment is being prepared.  The thin films of cleanser left on the pattern reduces the  surface tension of the wax and permits better.  Wetting of the investment to ensure complete coverage of the intricate portions of the pattern. The surface of a wax pattern that is not completely wetted with investment results in surface irregularities in the casting that destroy its accuracy. The distortion of the wax pattern after its removal from the die is a unction of the temperature and time interval before investing.  The nearer the room temperature approaches the softening point of the wax the more readily internal stresses are released. Also longer a pattern is allowed to stand before investment, the greater the deformation that may occur, even at room temperature.  A pattern should therefore be invested as soon as possible after its removal from the die and it should not be subjected to a  warm environment during this interval.  In any case a pattern should not stand for more than 20 to 30 minutes before being invested.

 

Investment Techniques:

While the wax pattern cleaner is air drying, the appropriate amount of distilled water (gypsum) investments or colloidal silica special  liquid (phosphate investment) is dispensed. The liquid is added to a clean dry mixing bowl and powder is gradually added to the liquid  to minimize air entrapment. Mixing is performed gently until all powder has been wet, the unmixed powder may inadvertently be pushed out of bowl. Although hand mixing is an option. It is for more common to mix all casting investments mechanically under vacuum.

 

Hand investing, the cover of the bowel containing the investment mix is placed over the bowl. The cover contains mechanical mixer and the mixing is done by hand usually for 100 turns of the spatulator. The setting rate of an investment depends on the number of spatulation turns, which also affects hygroscopic expansion. Mechanical mixing under vacuum removes air bubbles created during mixing and evacuates any potentially harmful gases produced by the chemical reaction of the high heat investment . Once mixing is completed the pattern may be hand invested or vacuum invested.

 

Hand investment:  The sprue base which holds the sp5rue and wax pattern is held in one hand and the investment is painted over the wax pattern with a camel hair brush.  In painting the pattern the investment should be teased ahead of the brush to prevent incorporation of air adjacent to the pattern.  Furthermore  to avoid distorting the pattern, the brush itself should not touch the pattern. After the pointing is completed the pattern is vibrated very gently with the sprue base held firmly with the fingers and the underside of hand resisting on the vibrator. This method relieves any minor air bubbles that might have been trapped around the wax pattern. After the pattern has been painted with the investment, the mixed investment is poured into the ring which is held at a slight angle (tilting) so  the investment flows slowly down its side to fill from the bottom to the top. IN this manner the possibility of trapping air in the ring or around the pattern is reduced to a minimum. It is often helpful to hold the assembled ring and sprue base in one hand which rests gently on a vibrator, while investment is being poured into the ring. When the ring is completely filed it is  leveled with the top by the edge of a plaster spatula. Excessive vibration should be a voided because it can cause solids in the investment to settle and may lead to free water accumulation adjacent to the wax pattern  resulting in surface roughness.  Excessive vibration may also dislodge small patterns from the sprue former resulting in mix cast.  When a phosphate bonded investment is used the ring is slightly overfilled, the top of the ring is not leveled off and the investment is allowed to set. After the investment has set the excess  investment is ground off using a model trimmer. This procedure is necessary because a non porous glassy surface results which must be ground off to  improve the permeability of the investment and allow for gases to readily escape from the mold during casting.

 

Double investment technique:  Used to ensure that the investment is well a adapted to the pattern. In this method, a thin layer of fine investment material is painted on the pattern by means of a camel hair brush. Before this layer has set, investment powder is dusted on until a more or less spherical and porous mass lf investment surrounds the pattern. By this means,  any excess water in the first mix is soaked by the dry powder and the resultant thick consistency of investment is said to ensure high setting expansion. This lump of investment  with the pattern in its centre is embedded in a further mix of investment of normal consistency. Some hygroscopic expansion of the initial layer of investment will take place when it is wetted by the new investment material. Uneven expansion may occur and may lead to distortion of the wax pattern because of the uneven thickness of the investment round the pattern. Also the two different mixes of investment may not have the same thermal expansion, the cracking of the inner layer may occur.  It is preferable to use one measured mix of investment and to point the pattern with a portion of this to ensure the elimination of air bubbles. Then complete the investment procedure using the same mix of material.

Vacuum investing: special equipment is used  to facilitate the investing operation.  Vac-U-spat investor whip mix combination unit and Girrbach vacuum at with this equipment the powder and water (or special liquid) are mixed under vacuum and the mixed investment is permitted to flow into the ring and around the wax pattern with the vacuum present. First hand spatulate the mix, with the crucible former and pattern in place attach the ring to the mixing bowl.  Attach the vacuum hose and mix according to the manufacturers recommendations. Inner the bowl and fill the ring under vacuum. Remove the vacuum hose before shutting off the mixer. Remove the filled ring and crucible former from the bowl. Although the vacuum investing does not remove all the air from the investment and the ring  the amount of air is usually reduced enough to obtain a smooth adaptation of the investment to the pattern. Vacuum investment often yields castings with improved surfaces when compared with castings produced from hand invested patterns. The degree of difference between the two procedures depend largely on the care used in hand investing. If a high temperature (1200ºF, 650ºC) burn out technique will be used, place the casting ring and crucible former in a humidor (a covered plastic container or sealed plastic bag with wet paper towels in the bottom) and let set at room temperature. If a low temperature (900ºF, 480ºC) burnout technique is to be used immediately immerse the ring in a 100ºF (38ºC) water bath to promote expansion of the wax pattern.

 

Casting procedure:

Once the investment has set for approximately 1 hour for most gypsum and phosphate bonded investments it is ready for burnout.  The crucible former and any metal sprue former are carefully removed. Any debris from the ingate area (funneled opening at the end of the ring) is cleaned with a brush.  If the burnout procedure does not immediately follow the investing procedure, the invested ring is placed in a humidor at 100% humidity.  If possible the investment should not be permitted to dry out. Dehydration of set investment that has been stored for an extended period may not replenish all of the lost water.

 

Wax Elimination & Heating or Burnout:  The invested rings are placed in a room temperature furnace and heated to the prescribed maximum temperature.  For gypsum bonded investment, this temperature can either be 500ºC for the hygroscopic technique or 700ºC for the thermal expansion technique.  With phosphate bonded investment the maximum temperature setting may range from 700ºC to 1030ºC, depending on the type of alloy used.  The temperature setting is more critical with gypsum bonded investment than with the phosphate type because the gypsum bonded investments are more prone to investment decomposition.  During burnout some of the melted wax is absorbed by the investment and residual carbon produced by ignition of the liquid wax becomes trapped in the porous investment.  It is also advisable to begin the burnout procedure while the mold is still wet.  Water trapped in the pores of the investment   reduces the absorption of wax and as the water vaporizes, it flushes wax from the mold.  This process is facilitated by placing the ring with the sprue hole down over a slot in a ceramic tray in the  burnout furnace. When the high heat technique is used the mold temperature generates enough heat to convert carbon to either carbon monoxide or carbon dioxide.  These gases can then escape through the pores in the heated investment.

 

During wax elimination the investment expands thermally which is necessary to compensate for casting shrinkage. Although wax melts at a comparatively low temperature its complete elimination requires much higher temperature. Usually if elimination is not complete small bits of the wax residue are retained in the fine margins of the mould and prevent the formation of a complete casting. Because waxes are organic materials they are composed of carbon, hydrogen, oxygen and nitrogen.  When heated to higher temperatures any organic material decomposes and forms carbon dioxide Co₂, water (H₂O), or nitrogen oxide No₂ all of which are gases and can be easily eliminated.  However formation of these gas depends on the presence of a sufficient supply of oxygen, the relative high temperature of the oven and adequate heating time of ring.  If insufficient oxygen is available to the wax in the mold cavity the temperature of the oven is not high, or the wax pattern is heated for only a short time, incomplete reaction between the wax and oxygen may result.  A satisfactory way to eliminate the wax pattern is to set the mold in furnace in the sprue hole placed downwards at first, so most of the wax drains out and is eliminated as liquid. The ring is then invested with the sprue hole placed upwards.  In this position the oxygen in the oven atmosphere can circulate out more readily into the cavity, react with the wax and form gases rather than the fine carbon that interferes with the venting of the mold cavity. The lower the mold temperature and larger the wax pattern, the longer the mold should be left in the oven. For a 500ºC oven temperature and larger wax pattern, the mold should remain in the oven for approximately 1 hour with the sprue hole spaced downwards during half of this time and upward during the other half. If more than one ring is placed in an oven, a longer period is required for wax elimination. The general rule is to add 5 minutes to the burnout time for energy extra ring placed in the oven at 500ºC.  With the oven temp of 600ºC to 700ºC  shorter time may be sufficient to completely eliminate the wax.

 

Investment materials are poor conductors of hat which results in a temperature difference between the inner core and the outside portion of the mold. Although this difference is relatively great at the beginning of the burnout, it diminishes as time progresses and is zero at the time of casting. The difference in temperature between the inner and outer portions of the mold are even greater if the mould is placed in an hot oven or if the heating rate of the oven is too rapid.  The outside of the mold, being exposed to a higher temperature expands somewhat more than does the inner part,. This expansion may cause the mold to crack during heating.  Because of the high thermal expansion containing cristobalite, these temperatures difference may be especially important. For best results this type of investment should always be placed in a room temperature oven and the oven temperature increased slowly. These temperatures differences do not affect quartz containing investments, probably because their rate of thermal expansion  is lower than that of cristobalite investment. When the wax pattern is completely eliminated and the mold has reached the casting temperature the gold alloy is melted by an appropriate method and cast into the mold cavity immediately upon removal of the mould from the burnout oven. However, the investment should never be allowed to cool before casting because the expansion of the investment during burnout is essentially irreversible.  If the mold cools before it is cast, the only recourse is to discard and start over by making a new wax pattern.

 

Hygroscopic Low heat technique:  This technique obtains its compensation expansion from three sources:  1) The 37ºC water bath expands the wax pattern  2) The warm water entering the investment mold from the top adds some hygroscopic expansion and  3) The thermal expansion at 500ºC provides the needed thermal expansion.

 

The warming influence of water bath causes a wax pattern expansion and also nerves to soften the wax thereby reducing restriction to investment expansion.

 

When the setting has occurred and some hygroscopic expansion has occurred, the crucible former is removed and the ring and invested pattern are placed in an electrical furnace and the temperature of furnace is not allowed to exceed 500ºC (932ºF).  Complete wax elimination requires 60-90 min.

 

Compensatory exp = T.S.E (N.S.E + H.S.E) 1.5% + T.E 0.55% = 2.05%whereas for high heat Technique = T.S.E (0.7) + T.E (1.25) = 1.95%.  The other important aspect of heating is that it raises the temperature of the investment to a level closer to the temperature of the molten metal that will be cast into it. If the thermal shock produced by a  very hot liquid metal setting a very cold ceramic mold is too great the mold may crack and cause a defective casting.  Heating the investment to such a high temperature reduces the likelihood that this will happen.

 

Advantages:  1) Less investment degradation  2) a cooler surface for smoother castings  3) Convenience of placing the molds directly into 500ºC furnace.

 

Disadvantages:

Care must be taken to allow sufficient burnout time, because the wax is more slowly oxidized (eliminated) at low temperature. The mold should remain in the furnace for at least 60 min. and they may be held up to 5 hr longer with little damage.

 

Fine carbon may be retained to reduce venting of the mold thus back pressure porosity is a greater hazard in the low heat technique than in the high heat technique, since the investments generally employed with the low heat technique my be more dense.

 

The standardized hygroscopic technique was developed for alloys with a high gold content, the newer noble alloys may require slightly more expansion.  This added expansion may be obtained by making one or more of the following changes;  1) Increasing the water bath temperature to 40ºC  2) Use two layer of liner  3) Increasing the burnout temperature to a range of 600ºC to 650ºC.

 

High heat thermal expansion Technique:  This approach depends almost entirely on using a high heat burnout to obtain the required expansion while at the same time eliminating the wax pattern.  Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern and the water entering the investment from the wet liner adds a  small amount of hygroscopic expansion to the normal setting expansion.

 

Gypsum Investments:  These castings are relatively fragile and require the use of a metal ring for protection during heating.  The molds are usually placed in a furnace at room temperature, slowly heated to 650ºC to 700ºC in 60 min and held for 15-30 minutes at the upper temperature.

 

The rate of heating has some influences on the smoothness and n some instances on the overall dimension of the investment.  Initially, the rapid heating can generate steam which can cause flaking or spalling of the mold wall. Too many patterns in the same plane within the investment often cause separation of a whole section of investment, because the expanding wax crates excessive pressure over a large area.

 

Too rapid a heating rate may also cause cracking the investment. In such a case the outside of the layer of investment expands more than the center sections.  Consequently the outside layer starts to expand thermally resulting in compressive stresses in the outside layer which counteracts tensile stresses in the middle regions of the mold. Such a stress distribution causes a brittle investment to crack from the interior outwardly in the form of radial cracks.  These cracks in turn produce a casting with fins or spines. This condition is especially likely to be present in cristobalite investments.  The comparatively low inversion temperature of cristobalite and the rapid rate of expansion during the inversion makes it especially important to heat the investment slowly.

 

Breakdown of the dental investment and the resulting contamination and brittleness of the gold alloy casting probably occur more frequently than in generally realized.  The mechanism of this investment decomposition and alloy contamination is related to a chemical reaction between the residual carbon and calcium sulphur binder.  Calcium sulphate per se does not decompose unless it is heated above 1000ºC.  However, the reaction of calcium sulphate by  carbon takes place rapidly above 700ºC.

 

CaSO₄ + 4C à Cas + 4CO, 3 CaSO₄ + Cas à 4CaO + 4SO₂

The sulphur dioxide as a product of this reaction contaminates gold casting and makes them extremely brittle (avoid burn out temperatures above 700ºC particularly if investment contains carbon).

 

After the casting temperature has been reached the casting should be made immediately. Maintaining a high temperature for any length fo time may result in sulphur contamination of the casting and also in a rough surface on the casting because of disintegration of investment.  To avoid this use furnaces with heating elements on all four sides, reducing burnout time. Notwithstanding all of these precautions and reasons for using a slow burnout technique the desire for rapid results had led to improved investment formations.  A few gypsum products same with a considerable amount of cristobalite are now offered for use with a much more rapid burn out procedure. Some suggest placing the model in a furnace at 315ºC for 30 min and following with very rapid heating to the final burnout temperature.  Few investments may be placed directly into a furnace at the final burnout temperature held for 30 min and cast (Accelerated casting method by Marzonk and Derby in 1989 JPD).  Design of the furnace the proximity of mold to the heating elements and availability of air in the muffle may affect size and smoothness.

 

Phosphate investments:  After certain advantages over gypsum bonded investments.  They are more stable at high temperatures and thus are material of choice for casting metal ceramic alloys. They expand rapidly at the temperatures used for casting alloy and their size can be conveniently controlled.  They obtain expansion from following sources: 1) Expansion of wax pattern this is considerable because the setting reaction raise the mold temperature substantially  and softens wax.  2) Setting expansion – this is usually higher than in gypsum products especially because special liquids are used. 3) Thermal expansion: This is greater when taken to temperatures higher than those used for gypsum  bonded investments.

 

A total expansion of 2% or more is required for alloys to produce metal ceramic prosthesis, since these gold based, palladium based and base metal alloys have higher melting temperature and solidification temperatures.  Although phosphate investments are usually much harder and stronger than gypsum investments they are nevertheless guite brittle and subject to same unequal expansion of adjacent sections as phase changes occur during heating.

 

The usual burnout temperatures for PBI range from 750ºC to 1030ºC.  The highest temperatures are required for base metal alloys. The heating rate is usually slow to 315ºC and is quite rapid thereafter reacting completion after a hold at the upper temperature for 30 min. Most burnout furnaces are now capable of being programmed for heating rates and holding times.

 

To save time:

Availability of some investments that can be subjected to two stage heating more rapidly, placed directly in the furnace at the top temperature held for 20 to 30 min and then cast.  Eliminated use of a metal ring and liner, the metal ring being replaced by with a plastic ring that is tapered so that once the investment sets it can be pushed out of the ring, held for specified time to ensure complete setting, and then placed directly into hot furnace (expansion is different when lined ring is sued which should be adjusted by varying liquid concentration).  More silicasol and less water = more expansion less silicasol & more water = less expansion.

 

However castings made from PBI are rougher than those made with GBI and are more difficult to remove from the investment. Because PBI have lower porosity, complete mold filling becomes difficult.  Castings also are more likely to have surface nodules which must be removed (vacuum mixing and a careful investing technique help reduce but do not eliminate the occurrence of nodules).

 

Carbon free PBI are available for use with base metal alloys that are brittle in presence of carbon.  Burnout ovens are available with manual, semi automatic or fully programmable controls.

 

Investing with phosphate Bonded investments – used for investing and casting alloys with higher temperatures ex: Ag-Pd, AuPd and Ni-Cr.  To obtain sufficient expansion for crown of these alloys, the model must be heated to 1400ºF (760ºC) or higher temperature and that would cause decomposition of calcium sulphate binder with the resultant release of contaminating sulphur in the mold  (O’brien 1959). IN generally an alloy with a casting temperature in excess of 2100ºF (1150ºC) as differentiated from fusion temperature which is 100ºF to 150ºF lower should be cast into an investment with a binder other than gypsum.  (Because dowel cores do not require as much expansion of the mold as do the crown, they can be cast with a Ag Pd alloy into gypsum bonded mold heated to only 1200ºF (650ºC)

 

PBI investment has poor wetting characteristics – trapped air bubbles are more during investing.  Either open or in vacuum investment can be sued and allowing the investment to set in a pressure pot will reduce the size and number of bubbles.

 

For lower fusing gold alloy casting, sprue formers run directly from crucible former to wax pattern to provide rapid. Turbulence free aces of the metal to the mold during casting.  Patterns for metal ceramic fixed partial denture must be sprued by an indirect pattern because these alloys used fuse and solidify at much higher temperatures because the ambient  air is much colder than the molten metal the exposed button is likely to solidify while the metal at the center of the ring is still liquid. This means tat the button cannot serve as a reservoir to prevent shrink spot porosity.

 

Technical considerations for PBI – As with any investment that has a high thermal expansion, especially when marked changes in expansion or contraction occur, it is necessary to use slow heating rate during burnout to prevent possible crackling or spalling. Some furnaces provide slow rates of heating. For those that do not it is advisable to use a two stage burnout, holding at 200ºC to 300ºC for atleast 30 min before completing the burnout wax softens and then expands much more than does the investment.  When investing it is desirable to leave3 to 6 mins of investment around each pattern and to stagger the patterns if several are placed in the same ring. A number  of patterns placed along a plane can exert tremendous pressure and fracture almost any investment but particularly the PBI. The rapid expansion of cristobalite investment at approximately 300ºC requires slow heating to prevent fracture. After the temperature reaches 400ºC the rate of heating can be safely increased. After the burnout usually at a final temperature of 700ºC to 1030ºC depending on the alloy melting range, the casting is made.  Permeability of the PBI is low compared with that of GBI, therefore the required casting pressure should be greater than GBI.   Recovery and cleaning of the castings  are more difficult when a PBI is used because such materials do not contain the soft gypsum products.  Also the particles usually include large grains of quartz. In some instances, such as with gold containing alloys the investment adheres tenaciously usually requiring cleansing in an ultrasonic scaler. Neither the phosphate binder nor the silica refractory is soluble in Hydrochloric or sulphuric acid.  Cold Hydrofluoric acid dissolves the silica refractory quite well without damage to a gold based or a Pd-Ag alloy but this must be used carefully with other alloys.

 

HF-tissue injury – Alternative solutions such as No-San can be with greater safety.

Time available for casting:  The investment contracts thermally as it cools when thermal expansion or high heat technique is used the investment lose heat after the heated ring is removed from the furnace and the mold contracts. Because of the liner and low thermal conductivity of the investment a short period can elapse before the temperature of the mold is appreciable affected. Under average conditions of casting approximately 1 min can pass without a noticeable loss in dimension.

 

In the low heat casting technique, the temperature gradient between the investment mold and the room is not as great as that employed with the high heat technique.  Also thermal expansion of the investment is not as important to the shrinkage compensation.  However, the burnout temperature lies on a fairly steep portion of the thermal expansion serve rather than on a plateau portion, as in the high heat technique.  Therefore, in the low heat casting technique, the alloy should also be cast soon after removal of the ring from the oven; otherwise a significant variation from the desired casting dimension ma occur.

 

Casting Machines:

The alloys are melted in one of the four following way, depending on the available type of casting machine.

1)      The alloy is melted in a separate crucible by a torch flame and cast into mold by centrifugal force.

2)      The alloy is melted electrically by resistance heating or induction furnace then cast into the mold by motor or spring actin.

3)      The alloy is vaccum arc melted and cast by pressure in an argon atmosphere.

4)      The alloy is melted by induction heating then cast into the mold centrifugally by motor or spring action.

 

In addition to these three melting methods, the molten metal may be cast by air pressure or vacuum or both.

 

Casting Machines:  All casting machines accelerate the molten metal into the mold either by centrifugal force or air pressure.

 

The selection of the casting and melting techniques is heavily influenced by the type of alloy and restoration to be cast.

 

 

Centrifugal machines: A variety of centrifugal casting machines are available.  Some spin the mold in the plane parallel to the table top which the machine is mounted, whereas others rotate in a plane vertical to the table top. Some are spring driven and others re operated by electric power. An electric heating unit is attached to some machines to melt the alloy before spinning the mold to throw in the metal.  Others have a refractory crucible in which the alloy is melted by torch before the casting operation is competed. Each of these machines depends on the centrifugal force applied to the molten metal to  cause it to completely fill the mold with properly melted metal. 

 

Advantages

Simplicity of design and operation, the opportunity to cast both large and small castings on the same machine.

 

Casting machine is spring wound 2 to 5 turns (depending on particular machine and the speed of casting rotation desired. The alloy melted is melted by a torch flame in a glazed ceramic crucible attached to the broken arm of the casting machine.  The broken arm features accelerates the initial rotational speed of the crucible and casting ring thus increasing the linear sped of the liquid casting alloy as it moves into and through the mold. Once the metal has reached the casting temperature and the heated ring is in position, the machine is released and spring triggers the rotational motion. As the metal fills the mold, a hydrostatic pressure gradient develops along the length of the casting. The pressure gradient from top of the casting to the bottom surface is quite sharp and parabolic  inform reaching zero at the button surface.  Ordinarily pressure gradient at the moment before solidification reaches about 0.21-0.28 MPa (30-40 Psi) at the tip of casting. Because of this pressure gradient there is also a gradient in the heat transfer rate such that the greatest rate of heat transfer to the mold is at the high pressure end of the gradient (tip of casting). Because  this end  also is frequently the harp edge of the margin of a crown, there is further assurance that the solidification progresses from the thin margin edge to the button surface.

Introduced by Jameson in 1907.  The machine basically has a strong spring encased in the base of the casting machine which can be wound into tension by rotating the arms with the weights at one end and casting ring at the other end.  In front of ring is a separate crucible in which the gold alloy is melted. When the spring is released the two arms rotate rapidly and the molten metal in forced into the mold by centrifugal force. The “broken arm” principle is incorporated into the machine.  It further accelerates the effective initial rotational speed  of the crucible and casting ring, thus increasing the linear speed of the liquid casting alloy into the mold. Time required to cast gold alloy by centrifugal force depends on the cross sectional area of the sprue and number of winds of the machine.  When the size of the sprue is increased the casting time is reduced. When the winds is increased, therefore force is increased, time is reduced.  Cross sectional area has greater influence than no. of winds

Centrifugal force

     -     square of the sped of the machine in rev/sec

-          radius of the circular path (length of the arm)

-          weight of the metal directly over the sprue

-          rapidly with which rotating arm comes into actin.

 

Disadvantages:

Centrifugal force is not a completely scientific procedure because the forces generated are impossible to evaluate and control

2) Metal instead of being thrown gently into the mold is thrown into it with a high impact

 

 

 

 

Air pressure Type Machine (Taggart 1907)  either compressed room air or some other gas such as carbon dioxide or nitrogen can be used to force the molten metal into the mold. The air pressure is applied to the molten metal through a suitable value mechanism. This type of machine is satisfactory only for making small castings.  For titanium and Titanium alloys, vacuum arch heated argon pressure casting machines are used.  The alloy is melted in the hollow left by crucible former and then air pressure is applied through a  piston, which is pushed downward into contact with the top of the ring enclosing the molten gold alloy.  A pressure of 10-15 pi is usually applied. The pressure gradient along the sprue axis and casting is nil, the thinner the section, the faster it solidifies.  The button is the thickest portion so it solidifies last.

 

Disadvantages:  suitable for small castings

The diameter of the main sprue is limited to 3 mm.

 

Vacuum casting: An important point concerning both the centrifugal and pressure casting in the question of what happens to air inside the mould. The flow of the alloy could easily press the air into porosities. Frank (1907) was one of the first to use an evacuated mold. He positioned the casting ring on a disc model with a hole connected to a syringe acting a a vacuum jump. If the ring and the disc were sufficiently airtight the mould could be evacuated through a porous investment and the outside atmospheric pressure would assist in driving the alloy into the mould.

 

Vacuum and Pressure method:  Surface tension of liquid alloy plays a role in casting. In attempts to cast only in a vacuum, thin parts  of the mould would not be filled completely. By admitting compressed air quickly into casting chamber at about 2.8 x 10⁸ N/m² the melt is forced home into any still empty cavities.

 

Piston plunger forces: The pre made crucible will be filled with a softened raw ceramic. The plunger piston which snugly fits the crucible is driven into the filled crucible driving the softened ceramic into the mold.  Casting machines both centrifugal and gas pressure are available with an attached vacuum system designed to assist the molten metal filling the mold.

 

Electrical melting units of various designs are used to melt the alloys during casting. The advantage of these units is that slightly less skill is required than is necessary for controlling torch.  However man of these electric heating units have no limiting controls and the operator must therefore exercise judgement regarding the proper condition of the alloy to be cast. The Electric units are heated either by induction or resistance heating systems.

Resistance Heated:  Current is passed through a resistance heating conductor and automatic melting of alloy occurs in graphite or ceramic crucible. This is an advantage especially for alloys such as those used for metal ceramic prostheses which are alloyed with base metals in force amount that tend to oxidize on overheating.  Another advantage is that the crucible in the furnace is located flush against the casting ring.  Therefore the alloy button remains molten slightly longer again ensuring solidification progresses completely from the tip of the casting to the button surface. A carbon crucible should not be used for melting high Pd alloys and Pd-Ag Alloys , Ni-Cr alloys or Co-Cr base metal alloys. They take longer time to complete the  heating and casting operation than do torch heated units.

Induction Melting Machine:

Alloy is melted by an induction field that develops within a crucible surrounded by water cooled metal tubing. The electric induction furnace is a transformer in which an alternating current flow through  the primary  winding coil and generates magnetic field in the location of the alloy to be melted in a crucible.  Once the alloy  reaches the casting temperature in air or in vacuum, it is forced into the mold by centrifugal force, by an pressure or by vacuum. More commonly used for melting base metal alloys. Melt alloy much faster than those heated by torch, if the procedure is not watched closely alloy can easily be overheated. An electric monitor is useful for indicating temperature.

Direct Current Arc melting machine: The direct arc is produced between two electrode, is the alloy and the water cooled tungsten electrode. The temperature within the arc exceeds 4000ºC and the alloy melts very quickly. This method has a high risk of overheating the alloy and damage may result after only a few seconds of prolonged heating.

 

Casting crucibles:  Generally four types of casting crucibles are available clay, carbon, quartz and zirconia alumena. Clay crucibles are appropriate for many of the crown and bridge alloys such as high noble and noble types. Carbon crucibles can be used not only for high noble crown and bridge alloys but also for the higher fusing, gold based metal-ceramic alloys. Crucibles made from alumina quartz, or silica are recommended for high fusing alloys of any type. They are especially suited for alloys that have a high melting temperature or those that are sensitive to carbon contamination.  Crown and bridge alloy with a high palladium content such as Pd-Ag alloys for  metal ceramic coping, and any of the nickel based or cobalt based alloys are included in this category.

 

 

Torch Melting: Most common method of heating dental alloys for full car metal restorations is by using a gas-air torch. A properly adjusted torch develops and adequate temperature for melting dental alloy whose melting ranges are between 870ºC to 1000ºC. Completing the melting operation promptly also depends on the proper adjustment of the torch flame. Poorly adjusted flames can waste time during melting and considerably damage the alloy through excessive oxidation or gas inclusion. Small and irregularly shaped flames should not be used to melt moderate or large quantities of alloy for casting purposes.  A well defined torch flame is the hottest and most effective for such melting operations. The properly adjusted flame contains well defined components.  Parts described as inner and outer cones or portions having different color intensities.

 

Gas-air Torch for conventional alloys

Gas oxygen Torch for metal ceramic alloys

Multi orifice gas oxygen or oxy acetylene torch for Base metal alloys

 

The conical flame First cone à the mixing zone is  a cool, colorless one. Around this area is a greenish blue combustion zone in which partial combustion takes place this is an oxidizing zone.  Next is a dim blue tip the reducing zone. This is the hottest area in the flame and is the only part of the flame used to heat the casting alloy.  Beyond this is another oxidizing zone which is final combustion between the gas and surrounding air occurs.  Neither of these oxidizing zones should be used for heating. They are not as hot as the reducing zone and if the alloy comes in contact with them copper and other non noble metals will be oxidized, changing properties of alloy.  This can result in reduced strength and altered solidification shrinkage. The oxides may also become incorporated in the casting as impurities.

 

Although an improper flame is the most common cause of oxidation of base metals in the alloy, even a properly adjusted flame causes  oxidation if it is held too close to the metal being heated, too far from it, or to  on side or another, or if it is moved over the surface or away from the alloy.  Modified torches may be used that combine natural gas and oxygen or certain “tank” gases such as acetylene and oxygen.

 

Melting of Gold alloy:  (Noble metal alloys)

When the reducing zone is in contact, the surface of the gold alloy is bright and mirror like. The alloy first appears to be spongy and then small globules of fuse alloy appear. The molten alloy soon assumes a spheroidal shape. At proper casting temperature the molten alloy is light orange and tends to spin or follow the flame when the latter is moved slightly. At this point, the alloy should be approximately 38º to 66ºC above its liquids temperature.  The casting should be made immediately when the proper temperature is reached.

 

• Nicr and Cobalt alloy are ready to cast when the harp edges of the ingot round over.

Regardless of the method of melting the alloy or the type of casting machine used the following objectives should be kept in mind when casting

1)      The alloy should be heated quickly as possible to completely molten condition (above the liquidus temp)

2)      Oxidation of the alloy should be prevented by heating the metal with a well adjusted torch (or other method) and small amount of flux on the metal surface.

It is desirable to use flux for gold crown and bridge alloys to id in minimizing porosity.  Flux increases the fluidity of the alloy and prevents oxidation.  Reducing fluxes containing powdered charcoal are often used but small bits of carbon may be carried into the mould and cause a deficiency at critical margin. Better flux is equal parts of fused borax powder ground with boric acid powder. The boric acid aids in retaining the borax on the surface of the alloy.

3)      Adequate force should be applied to force the well melted metal into the mold.

4)      After the molten metal is forced into the mold cavity the casting machine should be allowed t continue to provide pressure on the metal while the metal is solidifying encouraging complete casting of the margins.

 

Casting Technique:

1)      Preheating crucible (particularly in area in contact with the alloy, voides excessive slag formation during casting crucible which in cool can freeze the alloy (incomplete casting)

2)      Making the melt

3)      When alloy is molten, the casting ring is removed from the furnace and placed in cradle

4)      Tongs are used to slide the crucible platform into contact with casting ring

5)      Orifice of the crucible aligns with the sprue

6)      Heating continues for a few seconds so the melting is complete and casting can proceed

7)      Casting arm is pulled forward until the pin drops

8)      Centrifugal force carries the melt into the mold cavity

 

The machine is allowed to spin until it has slowed  enough that it can be stopped by hand and the ring is removed with casting Tongs.

 

Recovery of the Casting:  After the red glow has disappeared from the button, the casting ring is plugged under running cold water into a large rubber mixing bowl quenching

1)  noble metal alloy is left in an annealed condition for burnishing, polishing and  similar procedures.

2) When water contacts  the hot investment, a violent reaction ensues, resulting in a soft granular investment that is easily removed.

Often the surfaces of the casting appears dark with oxides and tarnish. Such a surface can be removed by a process known as pickling, which consists of heating the discolored casting in an acid. One  of the best pickling solutions for GB is a 50% Hcl solution which aids in removal of any residual investment as well as oxide coating. Disadvantages with Hcl:  1) fumes from acid corrode laboratory metal furnishings  2) fumes are health hazard and should be vented via fumehood. However pickling process can be performed ultrasonically while the prosthesis is sealed in Teflon container.  Ultrasonic devices are useful for cleaning the casting as are commercial pickling solution made of acid salts. A solution of sulphuric acid maybe advantageous in this aspect.

 

Abrasive blasting devices are also useful for cleaning the surface of castings.  The best method for pickling is to place the casting in a test tube or dish and to pour the acid.  It may be necessary to heat the acid, but boiling avoided because fumes re produced.  After pickling acid is poured oft and the casting is removed picking solutions should be renewed frequently because it is likely to become contaminated after reusing the solution several times.  In no case should the casting be held with steel tongs so that both the casting and tongs come in contact with the pickling solutions because this may contaminate casting.  The pickling solution contains in dissolved from previous casting. When steel tongs contact this electrolyte a small galvanic cell is created and copper is deposited on the casting at the point where tongs grip it. This copper deposition extends into the alloy and is a future source of discoloration in the area.

 

It is common to practice to heat the casting and drop into the pickling solution. 

Disadvantage: delicate margin may be melted in the flame or casting may be distorted by the sudden thermal shock when plunged into the acid.

 

Gold based 8 Pd-based metal ceramic alloys and bas metal alloys are bench cooled to room temperature before the casting is removed from the investment. Castings from this alloys are generally not pickled and when pickling is recommended for certain metal ceramic alloys, it is only to selectively remove specific surface oxides.

 

Base metal alloys require sandblasting with fine Alumina Acids should not used for cleaning base metal alloys. The selection of the appropriate PBI must be made on the bases of the composition of alloy to be used. Carbon containing investments are well suited for Gold based crown bridge casting alloys and metal ceramic alloy. However, if alloy is carbon sensitive (such as Ag-Pd, high Pd, Pd-Ag, NiCr Be, NiCr and Co-Cr) a non carbon investment should be used.

 

REFERENCES

Operative Dentistry Modern theory and practice-Maonk

Dental materials – Philiphs – Anusavice

Full mouth rehabilitation – Kornfield

Restorative Dental materials – Craig

Fixed prosthodontics – Rosenstiel

Fundamentals of fixed prosthodontics – Schillinburg.

Science & Technique of Cast restorations – Hollenback

Dental Materials – Ferra cane

Dental Materials –Anderson

Australian Dental Journal 1991 36 (5): 391-6

1992 37(1) 93-54

1991 36(4): 302-9

1992 37 (2): 91-7

Notes on Dental Materials – E.C. Coombe

British Dental Journal 1972: 428-435.

 

Introduction

Classification of defect in casting/common causes of casting defects.

Dimensional Inaccuracies or Dimensional errors in casting

Distortion

 Surface roughness

Surface irregularities

    Nodules

    Fins

    Ridges/Veins on casting surface

 Discolored castings

Pits (inclusion porosity)

Incomplete casting

Porosity:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Causes of Defective Castings:

An unsuccessful castings result in considerable trouble and loss of time. IN almost all instances defects in castings can be avoided by strict observance of procedures governed by certain fundamental rules and Principles. Unless energy step in the casting operation is handled properly the cast restoration may not fit the prepared tooth with the desired accuracy. Naturally a proper cavity design, an accurate impression of the prepared cavity, a good and accurate die and proper waxing and investing are all important step in achieving an acceptable cast restoration. Seldom is a defect in a casting attributable to factors other than carelessness or ignorance of the operator. With the present technique casting failures should be the exception, not the rule.

 

Classification of defect in casting/common causes of casting defects.

According to Rosenstiel – rough casting, large nodule Multiple nodules Nodules in occlusal surface, Fins, Incomplete castings, Incomplete castings with shiny rounded defects, suck back porosity, Inclusion porosity, marginal discrepancy inadequate or excessive expansion.

According to Obrien: A) General problems – problem with accuracy, problems with distortion, problems with bubbles, problems with fins, problems with short rounded margins, problems with miscastings, problems with pits  B) problems with internal porosity, problems with localize shrinkage porosity, problems with subsurface porosity, problems with microporosity  C problems with external porosity: problems with back pressure porosity.

According to Anusavice 1) Distortion  2) surface roughness and irregularities  3) Porosity  4) Incomplete  or missing detail

Porosities:  I. Solidification defects – localized shrinkage porosity, Microporosity  II. Trapped gases – Pinhole porosity, Gas inclusions, subsurface porosity  III. Residual air

Dimensional Inaccuracies or Dimensional errors in casting – The final fit of a casting depends on a balancing out of contraction and expansion which occurs during its construction.  The major dimensional changes involved re the casting shrinkage of the alloy which should be compensated for by the selling expansion, hygroscopic expansion, thermal expansion and inversion of the investment.  The castings can be either too small or too large.

• Casting too large is due to excessive mould expansion eliminated by  1) use of correct temperature  2) correct type of investment.
• Casting too small – is due to too little mild expansion eliminated by Heating the mold sufficiently.

 

A casting should be as accurate s possible although a tolerance of + 0.05% for an inlay casting is acceptable. The tolerance limit of dental castings are approximately one tenth the thickness of human hair. To obtain casting with small tolerance limits rigid requirements should be placed not only on the investment materials but also on the impression materials, waxes and die materials. Technical procedures and the proper handling of these material are equally important. The values of selling, hygroscopic, and thermal expansion of the investments materials my also vary from one product to another and slightly different techniques may be used with different investments.

 

Control of Dimensional accuracy: By reference to the values for the various expansions given by the manufacturer of the investment material, a technique can be evolved. The precise degree of casting shrinkage varies with different alloys. When once accurate castings have been produced, the techniques should not be varied.  Use mixing water of correct temperature (usually mouth temperature), use constant water powder ratio, For hygroscopic expansion immerse the investment before it reaches initial set, and control time and temperature, cast into the heated mould while it is still at the correct temperature.

 

Ideally the size of the casting should be exactly the same as that of wax pattern.  It has been suggested however, for inlays the expansion may be varied slightly according to the type of inlay being cast. When an inlay fit inside the tooth structure and is completely surrounded by it, over expansion will produce an inlay which will not go into the cavity.  With a slight under expansion the inlay will be a somewhat loose fit.  Of the two errors the second is perhaps the one to be preferred. Conversely for an inlay which encloses tooth structure, any under expansion will produce a non fitting inlay and over expansion a lose one such methods of varying the expansion only try to overcome errors of manipulation and are not advised. A correct technique should be established and will produce a perfect castings on each occasion, provided one starts with a perfect pattern.

 

Distortion:  Any marked distortion of the casting is probably related to wax pattern distortion This type of distortion can be minimized or prevented by proper manipulation of the wax and handling of the pattern.

Unquestionably some distortion of the wax pattern occurs as the investment hardens around it. The setting and hygroscopic expansion of the investment may produce a non uniform expansion of the walls of the pattern. This type of distortion occurs in parts from the non uniform outward movement of the proximal walls. The gingival margins are forced apart by the mold expansion whereas the solid occlusal  bar of wax resists expansion during the early stages of setting (avoided by using  dual wax technique). The configuration of the pattern, the type of wax and thickness influence the distortion that occurs. Distortion increases as the thickness of wax pattern decrease less the setting expansion of investment, the  less the distortion, generally this is not a serious problem, except that it accounts for some of the unexplained inaccuracies that may occur in small castings.  Nevertheless, not a great deal can be done to control this phenomenon.

 

The distortion of the wax pattern after its removal from the die is a function of the temperature and time interval before investing.  The nearer the room temperature approaches the softening point of the wax the more readily internal stresses are released. Also longer the pattern is allowed to stand before investment the greater the deformation that may occur even t room temperature.  Pattern should therefore be invested as son as possible after it is removed from the die and it should not be subjected to  warm environment during this interval. In any case a pattern should not stand for more than 20 to 30 minutes before being invested.  Once it is properly invested and the investment has set, there is no danger of further pattern distortion even if it remains for some hours before the final stages of wax elimination (burnout) and casting (Delayed investment)

 

Distortion can occur during spruing the pattern because of the heat transferred to the pattern. Placing the sprue away from the margins minimizes distortion of this delicate area of the pattern.

 

Controlling distortion: Proper manipulation of wax and handling the pattern.  The mix is too thick it cannot be applied to the wax pattern without distortion.

 

Pattern should be removed carefully from the die, and should be invested immediately.  Dieslone investment combination (Divestment technique) wax pattern constructed on the die made of disinvestment material. Then the entire assembly (die and pattern) is invested in a mixture of divestment and water thereby eliminating the possibility of distortion of the pattern on removal from the die or during setting of the investment.

 

Surface roughness:  The surface of a dental casting should be an accurate reproduction of the surface of the wax pattern from which it is made excessive roughness or irregularities on the outer surface of the casting necessitates additional finishing and polishing whereas irregularities on the cavity surface prevent a proper sealing of an otherwise accurate casting.

 

Definition:  Relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern.

 

Even under optimal conditions, the surface roughness of the dental casting is invariably somewhat greater than that of wax pattern from which it is made. The difference is probably related to the article size of the investment and its ability to reproduce the wax pattern in microscopic detail. With proper manipulative techniques, the normal increased roughness in the casting should not be a major factor in dimensional accuracy.

 

Improper finishing of wax pattern – Proper finishing of wax pattern. Improper water powder ratio – increases surface roughness (water films)

Excess surfactant – will combine with the investment which touches the pattern and will inhibit its setting reaction. This weak investment surface crumbles on heating and will be crushed by the force of the metal entering the mold.

Direct wax pattern – saliva, mucous and blood contamination.

Raid heating rates – because of the flaking of the investment when the water or steam pours into the mould. Furthermore such a surge of steam or water may carry some of the salts used  modifiers into the mould and these salts are left as deposits on the walls after the water evaporates.

Underheating – causes incomplete elimination of wax residues which plays a role in low temperature investment technique.

Prolonged heating (high heat casting technique) cause disintegration of the  GBI and walls of the mould are roughened products of decomposition are sulphur products that my contaminate the alloy to the extent that the surface texture is affected.  Such contamination may be the reason that the surface of the casting sometimes does not respond to pickling.

If the alloy is heated to too high a temperature before casting, the surface of the investment is likely to be attacked.

Too high a pressure during casting can produce rough surface.

Composition of investment – ratio of binder to the quartz influences the surface texture on the castings.  Coarse silica causes a surface roughness. If investment meet ANSI/APA specificatinNo.2 composition is not a probable factor.

When Foreign bodies: get into mold rough crucible former with investment clinging to it may roughen the investment on its removal so that bit of investment are carried into the mold with the molten alloy.  Carelessness in removal of sprue former.

Sulphur contamination from investment breakdown or high sulphur content in the torch flame.

Impact of molten alloy – shrinking at 90º to the investment surface.

Pattern position – cracking and breakdown if the space between patterns (several patterns invested) is less than 3 mm because of the greater expansion of wax than that of investment.

Carbon inclusions from crucible, improperly adjusted torch or carbon containing investments.

 

Control of surface roughness: proper finishing of wax patterns measuring amount of water and powder accurately.

• Wetting agent be applied in thin layer. It is best to air dry the wetting agent because any excess liquid dilutes the investment causing roughness.
• Before starting to invest a direct wax pattern taken from the mouth it should be washed in water at mouth temperature to remove the debris such as mucus and blood.
• Mold should be heated gradually at least 60 min should elapse during the heating of the investment filled ring from room temperature to 700ºC.  The greater the bulk of investment, the more slowly it should be heated.
• When the thermal expansion technique is employed the mould should be heated to the casting temperature never higher and the casting should be made immediately.
• A gauge pressure of 0.10 to 0.14 Ma in air pressure casting machine or 3 to 4 turns of the spring in an average type of centrifugal casting machine is sufficient for small casting.
• Spring properly so as to prevent direct impact of the metal on to weak portions of the mould surface.
• PBI cause more roughness.
• More patterns are invested in same ring, they should not be in same plane, minimum of 3 mm spacing between each pattern.

Surface irregularities: are isolate imperfections such as nodules that are not characteristic of the entire surface area.

 

Nodules:  Bubbles of gases trapped between the wax pattern and the investment produce nodules on the casting surface.

 

Large nodule- Air trapped during investing

Multiple nodules – Inadequate vacuum during mixing

        Improper brush technique

                              Lack of surfactant.

 Nodules on occlusal surface – Prolonged vibration after pouring

 

Even minute nodules can limit seating of the casting  to a considerable degree. When they are large or situated on a margin they usually necessitate remaking the restoration. When small they often can be removed with a No. ¼ or ½ round bur. A binocular microscope is extremely helpful to detect and remove nodules. Remove a slight excess of metal to ensure the nodule does not interfere with complete seating.

 

Avoiding nodules is careful investment technique a surfactant, vacuum spatulation and careful coating of wax pattern with investment. Castings made with phosphate bonded investment are especially prone to such imperfections and experience and care are required to produce castings that are routinely free of nodules.

 

Fins caused by cracks in the investment that have been filled with molten metal.

 

Cause:

Weak mix of investment (high W/P ratio) – crack excessive casting force.

Steam generated during too-rapid heating

Reheating an invested pattern, an improperly situated pattern (too close to the periphery of the casting ring), or even premature or rough handling of the ring after  investing, cracking of investment, dropped ring, rapid heating of wet or unhardened mold, liner in flush with end of ring, excessive casting force.

Double investment technique: do not have the same thermal expansion and cracking the inner layer may occur.

 

Problems: More time required for finishing and polishing, remade if they are present in critical areas.  If the crack in the mold lead outside, some alloy may be lost and may result in incomplete casting.

 

Avoid finning:  Avoid prolonged and rapid heating of the mold, use correct water powder ratio, adequate casting pressure, proper spruing so as to prevent direct impact of the molten metal at an angle of 90º.  If several patterns are invested 3 mm between  each pattern and should not lie in same plane, pattern should be within 6 mm of the trailing end of the ring, careful handling of the mold.

 

Ridges/Veins on casting surface:  Wax is a repellent to water and if the investment becomes separated from the wax pattern in some manner a water film may form irregularly over the surfaces. 

Causes: Pattern slightly moved, fared or vibrated after investing, or if the painting procedure doe not result in intimate contact of the investment with the pattern. Too high L/P ratio.

Wetting agent is of aid in prevention of such irregularities.

 

Discolored castings:  With calcium sulphate bonded investments when the color of the casting in black after removal from the investment, the cause is probably one of the following:

1. wax not completely eliminated
2. Mold remains in the oven too long
3. Oxidizing flame was used in melting the alloy
4. The investment did not contain deoxidizing agents.

 

Wax pattern is not completely eliminated, very fine particles of carbon cover the investment pores through which the gases in the mold cavity is supposed to escape. Depending on the amount of carbon remaining on the walls of the mold cavity when the molten metal enters the casting may be complete but black in color or it may be incomplete the black color in this instance cannot be cleaned by routine pickling action, because most pickling solutions are acidic and most acids are not effective in removing carbon from the surface of noble and high noble alloys.

 

However, if the black colour of the gold castings is removed by the normal pickling procedure,  it has caused mainly b copper oxides formed during casting.  If the investment is left too long in the oven, all the deoxidizers will be decomposed and eliminated.  Thus when the molten metal alloy enters the mold cavity the oxidation of copper is not prevented and casting will be black. Castings that are black because of oxidant of some f the elements in the alloy can easily be cleaned by normal pickling procedures.

 

Sulphur contamination – breakdown of investment if overheated.

Carbon inclusions from crucible, improperly adjusted torch or carbon containing investments formation f carbides or even crate visible carbon inclusions.

Failure to use flux.

 

Pits (inclusion porosity) Particles of investment dislodged during casting, Debris in the mold, Dirty wax, Loose debris in crucible, Mold temperature too hot.

 

Join the sprue former, crucible former and pattern with continuous smooth surface with no faged areas of investment, which can break of and enter mold. Use clean, new wax for patterns and sprues, use clean crucible with each casting, avoid overheating, casting mold should be handled with sprue downwards so as to prevent the falling of broken pieces of investment or particles of dust down the sprue.

 

Incomplete casting:  Occasionally onlay a partial complete casting or perhaps no casting at all is found. The obvious cause  is molten metal alloy has been prevented in some manner, from completely filling the mold.

 

Insufficient venting of the mold

High viscosity of the fused metal.

Insufficient venting is directly related to back pressure exerted by an in the mold. If the air cannot be vented quickly the molten alloy does not fill the mold before it solidifies. In such cases insufficient casting pressure might have been used the back pressure cannot be overcome (rounded in complete margins).

 

Incomplete elimination of wax residues from the mold. If too many products of combustion remain in the mold, the pores in the investment may become filled so that the air cannot be vented completely. If moisture or particles of wax remain the contact of molten metal alloy with these foreign substances produces an explosion that may produce sufficient back pressure to prevent the model from being filled (rounded margins but shiny appearance). The shiny condition of the metal is caused by the strong reducing atmosphere created by carbon monoxide left by the residual wax.

Low W/P ratio less porosity

Too great a viscosity (insufficient heating) surface tension and viscosity of molten alloy are decreased with an increase in temperature.

• Inadequate spruing (sprue former too mall)
• The alloy is not able to enter thin portions of the mold
• Mould is too cold causing premature solidification of the alloy
• Insufficient alloy is used
• Sprues are blocked by foreign water  such as particles of flux or unburnt wax.
• Insufficient casting force
• Back pressure of gases in the mold
• Metal either not completely molten or allowed to cool before applying the casting force.
• Wax pattern thin.

 

Remedy: The temperature of the alloy should be raised higher than its liquidus temperature so that its viscosity and surface tension are lowered and so that it does not solidify prematurely as it enters the mold.

• Adequate venting the mold
• Sufficiently high casting pressure. Pressure should be applied for 4 seconds. The mold is filled and the metal solidifies in one second.  But it is very soft during early stages. Hence should be maintained for few seconds beyond this point.
• Accurate W/P ratio
• Use large sprue former place sprue former in such a way that all area of the mold are fed by molten alloy.
• Mold should soak heat approximately 1 hour at burnout temperature.  Mold should be removed from burn out oven and cast immediately.
• Ensure that no debris blocks the ingate ring should be held with sprue hole down when removing crucible former and sprue former (metal)
• Cast enough metal to allow for a good button in the crucible of the ring.

 

Lane (1904) “resistance” offered by the metal to the enrushing gold is the very high surface tension of gold alloys. This surface tension increases with decreasing thickness of metal to be cast as main reason for incomplete casting as it prevents the melt  from flowing into capillary cavities of crown and inlay moulds.

 

Porosity:

Porosity may occur both within the interior region of a casting and on the external surface. The later is a factor in surface roughness but also it is generally a manifestation of internal porosity.  Not only does the internal porosity weaken the casting but if it also extends to the surface, it may be cause for discoloration. If  severe it can cause plaque accumulation at the tooth restoration interface and secondary caries may result. Although porosity in a casting cannot be prevented entirely it can be minimized by use of proper techniques.

 

Classification:

I. Solidification defects  A) Localized shrinkage porosity  B) Microporosity

II. Trapped gases  A) Pinhole porosity  B) Gas incusons  C) Subsurface porosity  III) Residual air.

 

Localized shrinkage porosity:  is generally caused by premature termination of the flow of the molten metal during solidification. The linear contraction of noble metal alloys in changing from a liquid to solid in at least 1.25%.  Therefore continual feeding of the molten metal through the sprue must occur to make up for the shrinkage of the metal volume during solidification. If the sprue freezes in it cross section before this feeding completed to the casting proper a localized shrinkage void will occur in the last portion of the casting that solidifies.

 

Occurs near sprue casting junction  where the lat part of the casting to solidify was in the low melting metal that remains as the dendrite branches develop. Improper sprue design causes suck back porosity. The entering metal impinges onto the mold surface and creates a higher localized mold temperature in this region known as Hot pot. Hot spot may retain a localized pool of molten metal after area have solidified causing shrinkage void or suck back porosity.

 

 

Eliminated by of suck back porosity:

• Flaring the point of sprue attachment and reducing the mold-melt temperature differential that is lowering the casting temperature by 30ºC.
• Attaching on or more small gauge sprues (18 gauge) to the surface mot distant from the main sprue attachment and extending the sprues laterally within 5 mm of the edge of the ring. These small chill set sprues ensure that solidification begins within the sprues and act as cooling pins to carry heat away.
• Place a sprue in such away that hot pot formation is avoided.
• Do not use excessively long sprue
• Using a reservoir
• Y-shaped sprue can be used instead of a single sprue.  Only half of the molten alloy enters the mold through each leg of the Y sprue so the temperature of the investment under the sprue does not rise as high.
• Increasing the temperature of the molten alloy or using an extra turn on a centrifugal casting machine.  In an attempt to drive the alloy more completely into the mold does not help eliminate suck back porosity.
• In fact this strategy may increase suck back porosity by increasing the temperature of the investment in the local area as the alloy is driven across the investment at a higher ate. The higher forces of alloy entry also increases the chances of cracking the investment either from thermal shock or from mechanical failure.

 

Micro porosity:  Also occurs from solidification shrinkage but is generally presenting fine grain alloy castings when the solidification is too rapid for the microvoids to segregate to the liquid pool. This premature solidification cause porosity in the form of small irregular voids.  Such phenomena can occur from rapid solidification if the mold or casting temperature is too low. This type of defect is not detectable unless the casting is sectioned,. It is generally not a serious defect.

 

Increase in sprue thickness and increase in sprue length has no effect on microporosity.  Increase in melt temperature and mold temperature reduces microporosity.

 

Pinhole and gas inclusion Porosities are related to entrapment of gas during solidification. Both are characterized by spherical contour but they are decidedly different in size.  The gas inclusion porosities are usually much larger than pinhole porosity. Many metals dissolve or occlude gases while they are molten. Cu and Ag dissolve oxygen in large amounts in the liquid state.  Molten Platinum and Palladium have a st4rng affinity for hydrogen as well as oxygen. On solidification the absorbed gases are expelled and pin hole porosity results. The larger voids may also result from the same cause, but is more logical to assume that such void are caused by gas that is mechanically trapped by the molten metal in the mold or by gas that is incorporated during the casting procedure.  However the porosity should be kept to a minimum because it may adversely affect the physical properties of the casting. Castings that are severely contaminated with gases are usually black when they are removed from the investment and do not clean easily on pickling. Larger spherical porosities can be caused by gas occluded from a poorly adjusted torch flame or by using mixing or oxidizing zones of the flame rather than the reducing zone. This type of porosity can be minimized by premelting the gold alloy on  graphite crucible or graphite block if the alloy has been used before and by correctly adjusting and positioning the torch flame during melting.

 

Subsurface porosity – caused by simultaneous nucleation of solid grains and gas bubbles at the first movement that the alloy freezer at the mold walls. This type of porosity can be diminished by controlling the rate at which the molten metal enters the mold.

Back pressure porosity

Entrapped air on the liner surface of the castings sometimes referred as back pressure porosity which produces large concave depression.

 

Causes: Inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces their pocket towards the end of the investment  via the molten sprue and button.

 

Occasionally it is found outside surface of the casting when the casting temperature or mold temperature is so low that solidification occurs before the entrapped air can escape.

-          Entrapped air is present more when dense investments are used, by an increase in mold density produced by vacuum investing and by the tendency for the mold to clog with residual carbon when the low heat technique is used which slow the venting of gases from the mold during casting.

-          Insufficient casting pressure or underheating the alloy.

-          Turbulence and compression of these gases ensue in the investment core because the gases cannot find rapid escape. If the compressed gas exerts  a back pressure that exceed the casting pressure then the trapped gases will force its way into the molten metal thus causing porosity.

 

Recommendation for the correction of Back pressure porosity

1)       Since back pressure porosity occurs more generally in full crown type castings, insertion of a wax rod into the core of the investment when investing the pattern will provide a good means of venting the hot gases quickly. Attaching wax rods to the outside surface does not reduce back pressure porosity, proper sprue diameter, length, direction and flaring are essential.

2)       Use of reservoir also helps to eliminate this type of porosity.

3)       The instance from the top of the pattern to the outside of the investment is kept to ¼ inch.

4)       Furnace temperatures above 1100ºF help eliminate this back pressure porosity condition. A higher mold temperature causes a slow rate of cooling and consequently allows more gold alloy to draw from the reservoir.

5)       Increasing the number  of turns on a centrifugal machine and increasing the casting pressure on pressure casting machine (minimum 20 ponds).

6)       Performing the casting operation in a vacuum.

7)       Using more metal when casting so that a good size button is left us a important precaution.

8)       Hygroscopic low burn out technique produces more of this porosity than high heat techniques.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Castable Ceramics: Cast and cerammed crowns such as the obsolete product  Dicor are made using the lost wax technique. The molten glass is cast into a mild heat transferred to form a glass ceramic and  colored with shading porcelain and surface stains.

 

Dicor is a castable glass that is formed into an inlay facial veneer or full crown restoration by a lost wax casting process similar to that employed for metals. After the glass casting core or coping is recovered, the glass  is sandblasted to remove residual casting investments and sprues are gently cut away. The glass is then covered by protective “embedment” material and subjected to heat treatment that causes microscopic plate like crystals of crystalline material to grow within the glass matrix. This crystal nucleation and crystal growth process is called ceramming. Once the glass has been cerammed it is fit on the prepared dies ground as necessary and then coated with veneering porcelain to match the shape and appearance of adjacent teeth. Dicor glass ceramic is capable of producing surprisingly good aesthetics perhaps because f the ““hameleon” effect where part of the color of the restoration is picked up from adjacent teeth as well as from the tinted cements used for luting the restorations. Dicor glass contains about 55% of tetrasilisic fluormica crystals.  The ceramming process results in increased strength, and toughness increased resistance to abrasion, thermal shock resistance, chemical durability and decreased translucency.

Ease of fabrication, minimal processing shrinkage, good marginal fit.

 

Pressable glass ceramic:

Pressure molding is sued to make small, intricate objects.  This method uses a piston to force a heated ceramic ingot through a heated tube into the mold when the object has solidified the refractory mold (investment) is broken apart and the ceramic piece is removed. It is then debrided and  either stained and glazed or veneered with one or more layers of a thermally compatible ceramic.

 

IPS Empress is a glass ceramic provided as core ingots that are heated and pressed until the ingot flows into the mold. It contains a higher concentration of leucite crystals that increase the resistance to crack propagation. The hot pressing process occurs over a 45 min period at a high temperature to produce the ceramic substructure.

 

IPS Empress – leucite reinforced, IPS Empress2  in lithia disilicate reinforced glass ceramic.

 

After hot pressing, divesting and separation of ceramic units the sprue segments, they are veneered with porcelain containing leucite crystals in a glass matrix.

 

Accurate fit, Marginal adaptation better.

OPC, OPC 3G are pressable ceramics that are similar in nature to IPS Empress and IPS Empress 2 respectively. OPC is a leucite containing ceramic and OPC 34 contains lithia disilicate crystals OPC is (Optec pressable ceramics).

 

The restorations are waxed, pressed in a manner similar to gold castings.

 

Leucite based – they are pressed at high temperatures from 900ºC to 1165ºC (1650ºC to 2130ºF) into a refractor mold made by lost wax technique.

Ex: IPS Empress, optimal pressable ceramic, cerpress, Finesse

Lithium silicate based IPS Empress 2 pressed at 920ºC (1690ºF).

 

Fabrication:  Wax the restoration to final contour, sprue and invest as with conventional gold castings.  If veneering porcelain is used only the body porcelain shape is waxed.

 

Heat the investment to 800ºC to burn out the wax pattern. Insert a ceramic Ingot of the appropriate shade and alumina plunger into the sprue and place the refractory in the special pressing furnace.

 

After heating to 1150ºC the softened ceramic is slowly pressed into the mold under vacuum. After pressing recover the restoration from the investment by  air borne particle abrasion, remove the sprue and refit it to the die. Esthetics can be enhanced by applying an enamel layer of matching porcelain or by adding surface characterization.

 

Investments for All Ceramic Restorations – used for cast glass technique.  Lane thermal expansion compatible with ceramic contains phosphate bonded refractories contain fine grain refractor fillers to allow accurate reproduction of detail.

 

REFERENCES

Operative Dentistry Modern theory and practice-Maonk

Dental materials – Philiphs – Anusavice

Full mouth rehabilitation – Kornfield

Restorative Dental materials – Craig

Fixed prosthodontics – Rosenstiel

Fundamentals of fixed prosthodontics – Schillinburg.

Science & Technique of Cast restorations – Hollenback

Dental Materials – Ferra cane

Dental Materials –Anderson

Australian Dental Journal 1991 36 (5): 391-6

1992 37(1) 93-54

1991 36(4): 302-9

1992 37 (2): 91-7

Notes on Dental Materials – E.C. Coombe

British Dental Journal 1972: 428-435.

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