22 INVESTING AND CASTING
The lost-wax casting technique has been used since ancient times to convert wax patterns to cast metal. It was first described1,2 at the end of the 19th century as a means of making dental castings.
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 the sprue. In dentistry, the resulting casting must be a highly accurate reproduction of the wax pattern in both surface details and overall dimension. Small variations in investing or casting can significantly affect the quality of the final restoration. Successful castings depend on attention to detail and consistency of technique.
When the wax pattern has been completed and its margin has been reflowed (see p. 578), it is carefully evaluated for smoothness, finish, and contour (see Chapter 18). The pattern is inspected under magnification, and any residual flash (wax that extends beyond the preparation margin) is removed. A sprue is attached to the pattern, then removed from the die, and attached to a crucible former (Fig. 22-1). The wax pattern must be invested immediately because any delay leads to distortion of the pattern as a result of stress relief of the wax.3
Sprue design (Fig. 22-2) varies depending on the type of restoration being cast, the alloy used, and the casting machine. There are three basic requirements, as follows:
The shape of the channel in the refractory mold is determined by the sprue that connects the wax pattern to the crucible former. The sprue can be made from wax, plastic, or metal. Wax sprues are preferred for most castings because they melt at the same rate as the pattern and thus allow easy escape of the molten wax. Solid plastic sprues soften at a higher temperature than the wax pattern and may block the escape of wax, which results in increased casting roughness. However, plastic sprues can be useful when casting fixed dental prostheses in one piece because their added rigidity minimizes distortion. Also, hollow plastic sprues that allow the escape of wax are available.
If a metal sprue is used, it should be made of noncorroding metal to avoid possible contamination of the casting. Metal sprues are often hollow to increase contact surface area and strengthen the attachment between the sprue and pattern. They are usually separated from the investment at the same time the crucible former is. Special care must then be taken to examine the orifice for small particles of investment that may break off when such a sprue is removed, because these can cause an incomplete casting if undetected (see p. 701).
In some casting techniques other than the commonly used centrifugal technique, a narrow sprue, or a sprue design that narrows at the point of attachment to the wax pattern, is essential. For instance, with air-pressure machines, the melt is made directly in the depression created by the crucible former and then forced into the mold by the sudden change in air pressure. With this technique, a narrow sprue prevents the molten metal from flowing into the mold prematurely.
The sprue should be attached to the bulkiest noncritical part of the pattern, away from margins and occlusal contacts. Normally, the largest nonfunctional cusp is used (Fig. 22-3). The point of attachment should allow a stream of metal to be directed to all parts of the mold without having to flow in an opposite direction to the casting force (Fig. 22-4).
Fig. 22-4 Incorrect sprue placement in the central fossa obliterates occlusal anatomy and may result in poor mold filling because the molten metal is not pushed into the cusp tips by centrifugal force.
The sprue must also allow for proper positioning of the pattern in the ring. This can be crucial because expansion within the mold is not uniform.6,7 For example, spruing on the cusp tip can give good results, but spruing on the proximal contact may produce a casting that is too wide mesiodistally and too short occlusocervically.
The sprue’s point of attachment to the pattern should be carefully smoothed to minimize turbulence. For the centrifugal casting technique, the attachment area should not be restricted because necking increases casting porosity and reduces mold filling.8 Similarly, excessively widening the attachment can cause this part of the cooling melt to solidify last, causing a void on the internal aspect of the casting, known as shrink-spot porosity.
Small auxiliary sprues or vents have been recommended to improve casting of thin patterns. Their action may help gases escape during casting9 or ensure that solidification begins in critical areas by acting as a heat sink10 (Fig. 22-5).
The sprue is attached to a crucible former* (Fig. 22-6), usually made of rubber, which serves as a base for the casting ring during investing. The exact shape of the crucible former depends on the type of ring and casting machine used. With most modern machines, the crucible former is tall, to allow use of a short sprue and also to enable the pattern to be positioned near the end of the casting ring.
The casting ring serves as a container for the investment while it sets and restricts the setting expansion of the mold. Normally a liner is placed inside the ring to allow for more expansion, because the liner is somewhat compressible. Use of two liners allows for additional compression and enables increased setting expansion of the investment material. At one time, asbestos was used as the liner, but this has been replaced by other materials to avoid the health risks associated with asbestos fibers. Like many other factors that come into play in achieving consistent casting with the proper quality of fit, changes in the liner are important. Wetting the liner increases the hygroscopic expansion of the mold and should be carefully controlled. An absorbent dry liner removes water from the investment and makes a thicker mix, which leads to increase in the total expansion.11,12 To prevent expansion restriction, care must be taken not to squeeze the liner against the ring. Increased expansion can be obtained by placing the mold in a water bath. This is because of hygroscopic expansion (Fig. 22-7). The position of the pattern in the casting ring also affects expansion. For consistent results, a single crown should be centered in the ring, equidistant from its walls. When fixed prostheses are cast as one piece, greater accuracy is achieved if the pattern is placed near the center of a large or special oval ring, rather than if a portion of a multiunit wax pattern is only partially centered and partially near the edge of a smaller ring.6
With the use of higher strength, phosphate-bonded investments, the ringless technique has become quite popular (Fig. 22-8).13 The method entails the use of a paper or plastic casting ring and is designed to allow unrestricted expansion.14 This can be useful with higher melting alloys that shrink more because of a larger cooling trajectory.
Fig. 22-8 Ringless investment technique. Crucible formers and cone-shaped plastic rings for a ringless casting system. The crucible former and plastic ring are removed before wax elimination, leaving the invested wax pattern. The systems are designed to achieve expansion that is unrestricted by a metal ring.
(Courtesy of Whip Mix Corporation, Louisville, Kentucky.)
Fig. 22-10 Spruing technique for a single casting. A, Attaching the sprue to the pattern. B, Removing the pattern from the die (see p. 566 for a description of this technique). C, Positioning the pattern on the crucible former. D, Application of surfactant. E, A ring liner increases the setting expansion. F, The pattern must be positioned sufficiently away from the end of the ring.
When more than two units are being cast together, each is joined to a runner bar (Fig. 22-11). A single sprue is used to feed the runner bar. Two units may be cast with a runner bar, or each unit may be fed from a separate sprue.
Several investment materials are available for fabricating a dental casting mold. These typically consist of a refractory material (usually silica) and a binder material, which provides strength. Additives are used by the manufacturer to improve handling characteristics.
When investments are classified by binder, three groups are recognized: gypsum-bonded, phosphate-bonded, and silica-bonded investments. Each has specific applications. The gypsum-bonded investments are used for castings made from American Dental Association (ADA) type II, type III, and type IV gold alloys. The phosphate-bonded materials are recommended for metal-ceramic frameworks. The silica-bonded investments are for high-melting base metal alloys used in casting partial removable dental prostheses. However, because of their limited application in fixed prosthodontics, silica-bonded investments are not included in the following discussion.
Gypsum is used as a binder, along with cristobalite or quartz as the refractory material, to form the mold. The cristobalite and quartz are responsible for the thermal expansion of the mold during wax elimination. Because gypsum is not chemically stable at temperatures exceeding 650°C (1200°F), these investments are typically restricted to castings of conventional types II, III, and IV gold alloys.
The water/powder ratio can be altered to reduce or increase the amount of 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 does 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. When attempting to alter setting expansion, the clinician should not deviate more than minimally from the manufacturer’s recommendations, to ensure that there are no changes in the essential properties of the investment.
Hygroscopic expansion occurs when water is added to the setting gypsum investment immediately after the ring has been filled. Usually this is accomplished by submerging the ring in a water bath at 37°C (100°F) for up to 1 hour immediately after investing. A significant amount of additional setting expansion results, enabling the use of a slightly lower wax elimination temperature. A wet ring liner also contributes hygroscopic expansion to the portion of the mold with which it is in contact (see Fig. 22-7).
As the mold is heated to eliminate the wax, thermal expansion occurs (Fig. 22-12). The silica refractory material is principally responsible for this because of solid-state phase transformations. Cristobalite changes from the α to the β (high-temperature) form between 200°C (392°F) and 270°C (518°F); quartz transforms at 575°C (1067°F). These transitions involve 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.
Because most metal-ceramic alloys fuse at approximately 1400°C (2550°F) (as opposed to conventional gold alloys at 925°C [1700°F]), additional shrinkage occurs when the casting cools to room temperature. To compensate for this, a larger mold is necessary. The added expansion can be obtained with phosphate-bonded investments.
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 latter. The binder consists of magnesium oxide and an ammonium phosphate compound. In contrast to gypsum-bonded products, this material is stable at burnout temperatures above 650°C (1200°F) (Fig. 22-13), which allows for additional thermal expansion. Most phosphate-bonded investments are mixed with a specially prepared suspension of colloidal silica in water. (Some, however, can be mixed with water alone.)
Some phosphate-bonded investments contain carbon and therefore are gray in color. Carbon-containing materials should not be used for casting base metals because the carbon residue affects the final alloy composition. They may be used for casting high-gold or palladium content alloys.
Compared to gypsum-bonded investments, phosphate-bonded investments offer greater flexibility in controlling the amount of expansion. The liquid/powder ratio needs only slight modification to effect a significant change in setting expansion. Increasing the proportion of special liquid (colloidal silica) also increases expansion.
Phosphate-bonded investments have a relatively short working time in comparison with gypsum materials. Their exothermic setting reaction accelerates as the temperature of the mix rises during manipulation. The filled ring feels warm to the touch even shortly after it has been filled. 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. Many technicians therefore vary the quantity of special liquid and water between batches and make trial mixes for each new shipment. This has been a reliable means of adjusting expansion.15
Gas is formed during the reaction and must be removed for a sufficiently long period to minimize nodules on the casting.16 Maintaining a vacuum for about 60 seconds appears to be adequate.
The number and variety of alloys suitable for casting have expanded dramatically, largely because of changes in the price of gold. Many alloys are available, especially for metal-ceramic restorations (see Chapter 19). The dentist must be able to make a rational choice on the basis of current information.
In 1965, the ADA adopted the specifications of the Fédération Dentaire Internationale (FDI), which classified casting alloys according to their physical properties (specifically their hardness), as follows:
Porcelain-type alloys with a high noble metal content were found to have hardness similar to that of type III alloys, and base metal alloys were found to be harder than type IV alloys (see Chapter 19).
Manufacturers place considerable emphasis on the color of their alloys, and color preference is often given to gold over silver. The patient’s views on the subject should be sought if the metal will be visible in the mouth; otherwise, the color of the dental alloy is irrelevant.
To be accepted by the ADA as an alloy suitable for dental restorations,17 the manufacturer must list the percentage composition by weight of the three main ingredients and any noble metal percentage. The functional characteristics of corrosion resistance and tarnish resistance were traditionally predicted on the basis of gold content. In general, if at least half the atoms in the alloy are gold (which would be 75% by weight), good resistance to corrosion and tarnish can be predicted. Nevertheless, clinical evaluations have failed to show statistically significant differences in the tarnish resistance of high-gold (77%) and low-gold (59.5% to 27.6%) alloys.18 However, a poorly formulated alloy, even of high gold content, can rapidly tarnish intraorally.
Treatment plans are often modified to suit the financial capabilities of the patient or a third party. Base metal alloys have found favor principally because of their low cost. Similarly, alloys containing approximately 50% gold have been found to offer some economic advantage (although the savings are not proportional to the reduced gold content of the alloy). Alloys containing primarily palladium and only a small percentage of gold are an alternative for use in the metal-ceramic technique, although soldering procedures may be less predictable.
When the intrinsic metal cost of a restoration is calculated, the volume of the casting, rather than its weight, should be determined. Dental casting alloys can vary considerably in density from below 8 g/mL to over 18 g/mL (see Table 19-1). An “average” restoration has a volume of 0.08 mL; an all-metal pontic may have a volume reaching 0.25 mL.19 Therefore, it is conceivable that the cost of a large pontic cast in a low-density alloy would be equal to or less than the cost of a complete cast crown fabricated from a high-density alloy. When noble metal prices are high, more sophisticated techniques of scrap recovery become economically attractive. These can range from installing conventional metal catchers in all areas where castings are finished to equipping all work stations with filtered suction machines.
In most respects, clinical performance (biologic and mechanical) is more important than cost. Biologic properties that can be evaluated include gingival irritation, recurrent caries, plaque retention, and allergies. Mechanical properties include wear resistance and strength, marginal fit, ceramic bond failure, connector failure, and tarnish and corrosion.
A risk in choosing a new alloy is that defective clinical performance may fail to appear in laboratory testing or in short-term animal and clinical trials. For example, manufacturers introduced copper-based casting alloys with very poor corrosion resistance20 when the price of gold was rapidly rising.* Although the clinically established alloys all have disadvantages, their performance is likely to have been well documented, and the prognosis of restorative treatment can be more accurately predicted.
Sound laboratory data are essential in the selection of a casting alloy. Important areas of consideration are casting accuracy, surface roughness, strength, sag resistance, and metal-ceramic bond strength. Currently available data suggest that nickel-chromium alloys have lower casting accuracy21 and greater surface roughness22 than do gold alloys (Fig. 22-14) but higher strength and sag resistance because of their higher melting ranges.23
Fig. 22-14 A, Comparison of casting accuracies with different alloys. Au-Pt-Pd, gold-platinum-palladium; Ni-Cr, nickel-chromium. B, Influence of metal casting temperature and alloy selection on casting roughness.
(A, From Duncan JD: The casting accuracy of nickel—chromium alloys for fixed prostheses. J Prosthet Dent 47:63, 1982; B, from Ogura H, et al: Inner surface roughness of complete cast crowns made by centrifugal casting machines. J Prosthet Dent 45:529, 1981.)