22: INVESTING AND CASTING

22 INVESTING AND CASTINGimage

KEY TERMS

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.

An understanding of the exact influence of each variable in the technique is important for making rational decisions to modify the technique as needed for a given procedure.

PREREQUISITES

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

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).

Attachment

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.

Crucible Former

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.

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Fig. 22-6 Rubber crucible formers.

(Courtesy of Whip Mix Corporation, Louisville, Kentucky.)

Casting Ring and Liner

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

Spruing Technique

Step-by-step procedure for a single casting

A 2.5-mm (10-gauge) sprue form is recom-mended for molar crowns or metal-ceramic castings, and a 2-mm (12-gauge) sprue for premolar and partial-coverage restorations. The procedure is as follows:

6. Place the ring over the pattern to ensure that it is long enough to cover the pattern with about 6 mm of investment (Fig. 22-10F). If necessary, the sprue may be shortened, or a longer ring may be chosen as an alternative.
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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.

MATERIALS SCIENCE

M. H. Reisbick

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-Bonded Investments

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.

Expansion

Three types of expansion can be manipulated to obtain the desired size of casting: setting, hygroscopic, and thermal.

Thermal expansion

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.

Phosphate-Bonded Investments

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.

SELECTION OF MATERIALS

Selecting a Casting Alloy

The choice of casting alloy largely determines the selection of investment and casting techniques and therefore is discussed first.

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.

Factors to be considered

Composition

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.

Cost

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.

Laboratory performance

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

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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.)

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on 22: INVESTING AND CASTING
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