Burnout Process of heating an invested mold to eliminate the embedded wax or plastic pattern.
Casting (1) The process by which a wax pattern is converted to a metallic replica of a prepared tooth restoration. (2) A dental restoration formed by the solidification of a molten metal in a mold.
Hygroscopic expansion Amount of setting expansion that occurs when a gypsum-bonded casting investment is immersed in 38 °C water during setting. (See Chapter 13 , Auxiliary Materials, for more information on this process.)
Refractory Capable of sustaining exposure to a high temperature without significant degradation.
Sprue The mold channel through which molten metal or ceramic flows into a mold cavity.
Sprued wax pattern A wax form consisting of the prosthesis pattern and the attached ingate or sprue network.
The lost wax method has been used to fabricate prostheses made of metal, ceramic, and polymers. Although the principle of the process is the same for all three classes of materials, there are distinct differences unique to each material. Fabrication of ceramic- and polymer-based prostheses was presented in Chapters 10 (Fabrication of Metal-Ceramic Prostheses) and 11 (Heat-Activated Denture Base Resins) , respectively. This chapter will focus on the metals only.
The process requires a die that duplicates the oral tissue needing a replacement or prosthesis. A wax pattern ( Figure 14-1 ) of the prosthesis is built on the laboratory indirect die by carving wax to the exact dimension. The wax pattern is then removed from the die and placed in a casting ring to be filled with investment materials. After the investment sets, the casting ring is heated to melt and burn out the wax. The investment material in the casting ring is now a negative of the final prosthesis. The negative mold is then filled with molten metal. After the metal cools, the investment is fragmented and removed, leaving the prosthesis. This is a very brief description of the process. In practice, there are many fine details. The focus of this chapter is first to prepare a die suitable for fabricating the wax pattern. The wax pattern is then fixed in a casting ring with cylindric segments known as an ingate or sprue for the flow of molten metal to fill the mold cavity, made by investing in a refractory material ( Figure 14-2 ). The molten metal is introduced to the empty space with pressure. Finally, there is a discussion of casting defects.
What methods can be used to increase the abrasion resistance of master dies?
Preparation of the Master Die
The most commonly used die materials are type IV and type V improved gypsum dental stones ( Chapter 13, Types of Gypsum Products ). Type IV stones have a setting expansion of 0.1% or less, whereas type V stones may expand as much as 0.3%. This greater expansion is useful in compensating for the relatively large solidification shrinkage of base-metal alloys.
The chief disadvantage of the type IV gypsum die is its susceptibility to abrasion during carving of the wax pattern. To overcome deficiencies of gypsum dies, several modifications or different materials have been used:
Silver or copper plating, coating the surface with cyanoacrylate adhesive, or adding a die hardener to the gypsum can be used to improve abrasion resistance of gypsum master dies.
Several coats of resin-based spacer, such as nail polish, can be painted on a stone die away from the margins of the restoration to produce relief space for cement luting agent and ensure complete seating of an otherwise precisely fitting casting or coping.
To eliminate the possibility of distortion of the wax pattern on removal from the die or during the setting of the investment, one technique is to pour up the die with an investment material with comparable properties to die materials.
Nongypsum die materials, such as acrylic, polyester, and epoxy resins, are used because of their superior abrasion resistance. Compatibility between die material and the impression material is specific to the brand rather than to chemical types of impression materials. Even though these dies are generally undersized in comparison with the prepared tooth, they are used successfully, presumably because dimensional compensation has occurred during the investing and casting procedure.
Several gypsum die stones have been compounded with resins to provide the advantages of both materials. These modified die stones maintain the low expansion of conventional die stone, but they also have the increased toughness and resistance to carving imparted by the resins.
Metal dies produced from electroplated impression material have moderately high strength, adequate hardness, and excellent abrasion resistance. The first step is treating the impression to make it conductive to electrical current. The metallized impression is the cathode in the electroplating bath, along with a silver plate as the anode. A direct current is applied to transfer silver from the plate to the impression, a reverse process of corrosion discussed in Chapter 3, Electrochemical Properties . The impression is then filled with dental gypsum stone. When the stone sets, a die with an electroformed metal shell is obtained.
It is important to note that some of the methods described can alter the die dimensions slightly, thus reducing accuracy ( Table 14.1 ).
|DIMENSIONAL CHANGE (%)|
|Type IV die stone||0.06||0.00|
|Type IV stone plus hardener A||0.16||0.08|
|Type IV stone plus hardener B||0.10||0.10|
|Silica-filled epoxy resin||−0.15||−0.26|
|Aluminum-filled epoxy resin||−0.14||−0.19|
What casting deficiencies may result when: (1) the sprue former is too small in diameter, (2) the sprue former is attached without flaring to thinner areas, (3) the sprues are oriented toward thin areas of a wax pattern, or (4) the sprues are of inadequate length to position the wax pattern less than 6 mm from the end of the invested ring?
Wax Pattern and Sprue Design
To provide a pathway or ingate to the mold cavity for molten metal, the wax or resin pattern must have one or more cylindrical wax segments attached at the desired point(s) of metal entry; this arrangement is termed a sprued wax pattern. A sprue is the channel in a refractory investment mold through which molten metal flows. After the wax pattern has been made, a sprue-former base is attached to the sprued wax pattern, an investment ring is pressed into the sprue-former base, and an investment slurry is vibrated into the ring to embed the wax pattern in the investment, usually under vacuum. Examples of sprued wax patterns on a sprue-former base are shown in Figure 14-1 .
The diameter, the length of the sprue former (also referred to simply as the sprue ), and the position of attachment depend on the type and size of the pattern, the type of casting machine to be used, and the dimensions of the ring in which the casting will be made .
The sprue former should be attached to the wax pattern with the pattern on the master die, provided that the pattern can be removed directly in line with its path of withdrawal from the die. Any motion that might distort the wax pattern should be avoided during removal. The gauge selection and design for the sprue former are often empirical, but optimal performance during the casting process is based on the five general principles discussed in the following subsections.
Select a sprue former with a diameter that is approximately the same size as the thickest bulk 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’s diameter is too small, it may solidify before the casting itself, and localized shrinkage porosity (“suck-back” porosity) may develop. As shown in Figure 14-3 , reservoir sprues are used to help overcome this problem.
Sprue Position and Attachment
The sprue-former connection to the wax pattern is generally flared for higher-density gold alloys but often restricted for lower-density alloys. Flaring of the sprue former may act in much the same way as a reservoir, smoothing the entry of the fluid alloy into the pattern area with less turbulence. The position of the sprue-former attachment is often a matter of individual judgment and intuition, based on the shape and form of the wax pattern. As indicated earlier, the ideal area for the sprue former is the point of greatest bulk in the pattern to avoid distorting thin areas of wax during attachment to the pattern and permit complete flow of the alloy into the mold cavity.
Then, it is best for the molten alloy to flow from a thick section to surrounding thin areas (e.g., the margins). The sprue former should not be attached to a broad, flat surface at a right angle. Such an orientation leads to turbulence within the mold cavity and porosity in this region ( Figure 14-4, A ). When this same pattern is sprued at a 45-degree angle to the proximal area, a satisfactory casting is obtained ( Figure 14-4, B ).
The sprue former should be long enough to position the pattern properly in the casting ring within 6 mm of the trailing end ( Figure 14-2 ) and yet short enough so that the molten alloy does not solidify before it fills the mold.
A reservoir should be added to a sprue network to prevent localized shrinkage porosity ( Figure 14-3 ). When the molten alloy fills the heated casting ring, the pattern area should solidify first, and the reservoir last. The molten metal in the reservoir remains molten to furnish additional liquid alloy into the mold as it solidifies.
The length of the sprue former depends on the length of the casting ring. If the sprue is too short, the wax pattern may be so far removed from the end of the casting ring that gases cannot be adequately vented to permit the molten alloy to fill the ring completely. When these gases are not eliminated, the casting will be incomplete. The sprue length should be adjusted so that the top of the wax pattern is within 6 mm of the open end of the ring for gypsum-bonded investments ( Figure 14-2 ) and within 3 to 4 mm of the top of the phosphate-based investment.
The material for making the mold must be refractory and thermally stable so that it can withstand exposure to the high temperatures of molten metal as the metal solidifies and cools to room temperature. In addition, the mold or investment material must not interact chemically with the metal surface, and it must be easy to remove from the metal casting.
Generally, two types of investments—gypsum-bonded and phosphate-bonded—are employed, depending on the melting range of the alloy to be cast. A third type is the ethyl silicate–bonded investment, which is used principally for the casting of removable partial dentures made from base metals (cobalt-based and nickel-based alloys). Because the thermal contraction coefficients of the alloys are higher than that of the investment mold, the dimension of the prosthesis when cooled to room temperature will be smaller in dimension than the wax pattern’s intended dimension. The difference of the dimension is called casting shrinkage. To obtain a prosthesis of the intended dimension, the investment mold needs to expand during setting and expand more during burnout to compensate for the casting shrinkage.
The gypsum-based materials represent the type traditionally used for casting gold alloys. There are two classifications by International Organization for Standardization (ISO) 7490 (American National Standards Institute [ANSI]/American Dental Association [ADA] 126): type 1 is used for casting inlays, onlays, crowns, or other fixed prostheses, and type 2 is used for removable partial denture frameworks.
The ingredients of gypsum-based investments are α-hemihydrate of gypsum as binder, polymorphs of silica as fillers, and other ingredients as modifiers.
The binder holds the filler together and provides the investment strength. When the binder is heated at temperatures sufficiently high to completely dehydrate the investment, it shrinks considerably, and occasionally fractures. As Figure 14-5 shows, when the temperature is raised to 200 to 400 °C, it dehydrates and shrinks. A slight expansion occurs between 400 and 700 °C, and a large contraction then takes place at higher temperatures, caused by decomposition of the calcium sulfate. Thus it is imperative that gypsum investments not be heated above 700 °C.
To compensate for the inherent contraction associated with gypsum and casting shrinkage, polymorphs of silica are incorporated in the investment. Silica exists in at least four polymorphs: quartz, tridymite, cristobalite, and fused quartz. When quartz, tridymite, or cristobalite is heated, a change in crystalline form occurs at transition temperatures characteristic of each polymorph of silica ( Figure 14-6 ). This crystalline transition is called an inversion. Each crystalline transition from lower temperature (α phase) to higher temperature (β phase) is accompanied by a linear expansion. For example, the inversion completes at 573 °C for quartz, at 200 and 270 °C for cristobalite, and at 117 and 163 °C for tridymite. In powdered form, the inversions occur over a temperature range rather than instantaneously at a specific temperature. By the quantity of thermal expansion shown in Figure 14-6 , quartz and cristobalite are of particular dental interest.
In addition to silica, certain modifying agents, coloring matter, and reducing agents, such as carbon and powdered copper, are present. The reducing agents are used to provide a nonoxidizing atmosphere in the mold when a gold alloy is cast. Some of the modifiers not only regulate the setting expansion and the setting time but also prevent most of the shrinkage of gypsum when it is heated above 300 °C. In some instances, the modifiers are needed to regulate the setting time and setting expansion, as described for the dental stones.
Typically, the setting expansion of these investments is controlled by retarders and accelerators for the gypsum. The exothermic heat of gypsum setting can result in expansion of the wax pattern before the investment sets, influencing the effective setting expansion. The expansion of the investment may cause distortion of the wax pattern. Consider a mesio-occulso-distal (MOD) restoration as an example; the investment inside of MOD can force the proximal walls of the wax pattern outward as it sets. If the pattern has a thin wall, the effective setting expansion is somewhat greater than that for a pattern with thicker walls because the investment can move the thinner wall more readily. Also, the softer the wax, the greater the effective setting expansion because the softer wax is more readily deformed by the expanding investment. If a softer wax is used, the setting expansion may cause excessive distortion of the pattern.
Hygroscopic Setting Expansion
As illustrated in Figure 14-7 , the hygroscopic setting expansion may be six or more times greater than the normal setting expansion of a dental investment. If greater expansion is needed, there are many factors in the control of hygroscopic expansion.
The magnitude of hygroscopic setting expansion is generally proportional to the silica content of the investment, which becomes greater with a finer size of silica. Filler plays no role in hygroscopic expansion; it is the interface between filler and gypsum that allows added water to diffuse through the setting material and increased expansion.
As discussed for gypsum products, the higher the water/powder (W/P) ratio of the original investment water mixture, the less the hygroscopic setting expansion. As the mixing time is reduced, the hygroscopic expansion is decreased. The older the investment, the lower is its hygroscopic expansion. The greatest amount of hygroscopic setting expansion is observed if the immersion takes place before the initial set. The longer the immersion of the investment in the water bath is delayed beyond the time of the initial set of the investment, the lower the hygroscopic expansion. The effects of the W/P ratio, mixing, and shelf life on the maximal hygroscopic setting expansion are illustrated in Figure 14-8 relative to the amount of water added during setting.
The effect of cristobalite compared with that of quartz is demonstrated in Figure 14-9 . Because of the much greater expansion that occurs during the inversion of cristobalite, the normal contraction of gypsum during heating is readily eliminated. Furthermore, the expansion occurs at a lower temperature because of the lower inversion temperature of the cristobalite in comparison with that of quartz. A reasonably good fit of the casting is obtained when a gold alloy is cast into the mold at temperatures of 500 °C and higher. The thermal expansion curves of an investment provide some idea of the polymorph of the silica that is present. As can be seen from Figure 14-9 , the investments containing cristobalite expand earlier and to a greater extent than those containing quartz. Most current investments are likely to contain both quartz and cristobalite.
The magnitude of thermal expansion is related to the amount of solids present. This effect is demonstrated by the curves shown in Figure 14-10 . The same figure also shows that it is imperative to measure the water and powder accurately for proper compensation.
The addition of small amounts of sodium, potassium, or lithium chlorides to the investment eliminates the contraction caused by gypsum and increases the expansion without the need for an excessive amount of silica.
The strength of an investment is usually measured under compressive stress and is increased according to the amount and type of gypsum binder present. The use of chemical modifiers increases the strength because more of the binder can be used without a marked reduction in thermal expansion. The strength of the investment is affected by the W/P ratio in the same manner as any other gypsum product; the more water employed during mixing, the lower the compressive strength. After the investment has cooled to room temperature, its strength decreases considerably, presumably because of fine cracks that form during cooling.
Although a certain minimal strength is necessary to prevent fracture of the investment mold during casting, the compressive strength should not be unduly high. When the alloy is still hot and weak, the investment can resist alloy shrinkage by virtue of its strength and constant dimensions. This can cause distortion and even fracture of the casting if the hot strength of the alloy is low.
Porosity of Set Investment
As the molten metal enters the mold under pressure during casting, the trapped air must be forced out ahead of the inflowing metal. If the air is not completely expelled, a back pressure builds up to prevent the molten alloy from completely filling the mold. The simplest method for venting the mold is through the pores of the investment. Thus it is important that the end of a wax pattern that is nearest to the end of the investment ring not be covered by more than 6 mm of investment to allow sufficient interconnectivity of the porous network for the escape of gas from the mold cavity during filling of the mold with molten metal.
Gypsum-based investments should be stored in airtight and moisture-proof containers. During use, the containers should be opened for as short a time as possible. All investments are composed of several ingredients, each of which possesses a different density. There is a tendency for the components of the investment to separate as they settle, according to their specific gravity. It is advisable to purchase prepackaged investments in relatively small quantities if investments are needed on an infrequent basis.
The investment supplied in bulk packages should be weighed, and the water should be measured according to the proportion of the investment mix. In this manner, one can control the setting or the thermal expansion in relation to the compensation needed for the casting shrinkage and other important properties. One need only measure the gauging water.
One should be aware of slight variations in the weight of premeasured packets of powder. The quality control of investment products is related not only to the homogeneity of particulate components but also to variations in the weight of powder in the packets.
The use of alloys with higher melting temperature ranges, such as those for metal-ceramic restorations, usually leads to greater thermal contraction after solidification. This necessitates an investment material that is more heat resistant with greater expansion. Phosphate-based investments are designed primarily for alloys used to produce copings or frameworks for metal-ceramic prostheses and some base-metal alloys ( Chapter 9, Manipulation of Base Metal Alloys ).
Phosphate-based investments contain refractory fillers and a binder. The silica filler is typically 80% by weight. The particle size varies from submicron to that of a fine sand. The binder consists of magnesium oxide (basic) and monoammonium phosphate (acidic). A colloidal silica suspension in place of water is used for mixing phosphate investments.
The chemical reaction for the phosphate-based binder system is as follows:
The reaction product is predominantly colloidal multimolecular (NH 4 MgPO 4 .6H 2 O) n aggregate around excess MgO and fillers. On heating, the binder of the set investment undergoes thermal reactions that appear to be the decomposition of (NH 4 MgPO 4 .6H 2 O) n . It begins to lose water around 50 °C and dehydrates to (NH 4 MgPO 4 .H 2 O) n at 160 °C. Further heating to 300 to 650 °C expels the remaining water and ammonia from the phosphate compound and results in a noncrystalline polymeric phase of (Mg 2 P 2 O 7 ) n . As the temperature increases, the noncrystalline phase begins to crystalize. The resulting reaction products of phosphate-bonded investments are crystalline Mg 2 P 2 O 7 , excess MgO, and essentially unchanged silicas. In addition, Mg 3 (P 2 O 4 ) 2 may be formed if the investment is grossly overheated above 1040 °C or if the molten metal contacts the mold-cavity surfaces.
Setting and Thermal Expansion
The setting reaction yields a slight expansion, which can be increased considerably by using a colloidal silica solution (special liquid) instead of water. Figure 14-11 shows the effect of the concentration of colloidal silica in aqueous suspension on the setting and thermal expansion. Figure 14-12 shows the thermal expansion of a typical phosphate investment mixed with water compared with the same investment mixed with its accompanying special liquid. The early thermal shrinkage of phosphate investments is associated with decomposition of the binder, magnesium ammonium phosphate, and is accompanied by the evolution of ammonia, which is readily apparent by its odor.
Working and Setting Time
The working and setting time of phosphate investments are affected by temperature. The warmer the mix, the faster it sets. Increased mixing time and mixing efficiency result in a faster set and a greater rise in temperature. The ideal technique is to mix as long as possible yet have just enough time for investing. Mechanical mixing under vacuum is preferred. The liquid/powder (L/P) ratio also affects the working and setting time; an increase in the L/P ratio increases the working time.
Surface Quality of Cast Metals
The detail reproduction and surface smoothness of a metal-ceramic gold cast in a phosphate-bonded investment are considered inferior to those of a conventional gold alloy cast in a gypsum-bonded investment. Increasing the ratio of “special liquid” to water used for the mix enhances casting surface smoothness but can lead to oversized castings. Improvements in the technique and the composition of phosphate-bonded investments now make it possible to fabricate castings having few surface imperfections on low-fusing gold alloy, high-fusing gold alloy, or base-metal alloy.
Ethyl Silicate–Bonded Investment
The binder of ethyl silicate–bonded investments is a silica gel that reverts to silica (cristobalite) on heating, and the fillers are silicas and magnesium oxide. A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol, and water. The reaction, in its simplest form, can be expressed as follows: