Сraig. Dental Materials

ventional melting facilities are inadequate for most available alloys because the melting range of the alloys is too high. Generally, phosphatebonded investments and gas-oxygen torch are used in casting this type of alloy. Phosphate investments are stronger and denser than are gypsum-bonded investments (see Chapter 13). Although the technique for using these special alloys and investments is not complicated or elaborate, the technique recommended by the manufacturers should be followed faithfully to achieve satisfactory surface character and fit of the casting.

Because the phosphate-bonded investment used for porcelain-fused-to-metal castings is dense, special attention should be given to the manner of spruing of the wax pattern. Proper spruing should facilitate the escape of gases from the mold cavity before the molten metal solidifies completely. The bar type of spruing used for casting these alloys is shown in Fig. 17-4. With this type of spruing, the sprue bar is placed inside the mold as a reservoir to help keep the alloy in the molten stage for a longer time so a more complete casting can be attained. For single units, the bar is shortened or a ball is used. Note that with a normal spruing method, the pattern is sprued in such a manner as to reduce turbulence in the flow of molten alloy. However, with the bar-spruing method, an attempt is made to increase the turbulence so the investment close to the sprue maintains a higher temperature, which keeps the alloy molten for a longer time. The main disadvantage of the bar type of sprue is the increased amount of alloy required to make castings; for this reason the bar type is not used to make castings of alloys with low melting ranges and gypsum-bonded investments.

If a bar type of sprue is used with a centrifugal casting machine, the casting ring should be placed in the casting machine so the bar sprue is vertical. The leader from the sprue button to the bar should be attached 1to 2 mm below the top of the bar. In this manner, when the alloy at the tip of the bar sprue freezes, the alloy 1 to 2 mm below the tip remains molten, and feeding from the sprue button to the bar is still possible. The leader from the bar to the wax pattern should be

attached to the highest part of the wax pattern. Centrifugal casting machines tend to feed molten alloy straight (as a result of the centrifugal force) and down (from gravity). If the leaders from the sprue button to the bar or from the bar to a one-piece cast bridge are placed in the middle of the bar or bridge, there is a high probability of a miscast. In such a sprue arrangement, the molten alloy must fill the lower portion of the bar and the mold cavity before being forced upward against gravity to fill the upper portion of the bar and the bridge. Proper attachment of the leaders to the bar and to three copings are shown in Fig. 17-4.

Other steps in the investing and casting procedure of porcelain-fused-to-metal alloys normally include vacuum investing, careful wax elimination, a gas-oxygen torch or other hightemperature melting facility, and centrifugal casting with adequate casting pressure. Special solders may be required with these alloys for certain techniques; with some alloys and solders, the operation requires skilled management only obtained with extensive practical experience.

Casting Co-Cr and Ni-Cr Alloys and Partial Denture Frameworks Posterior crowns are often cast in base-metal alloys but, because of their high melting temperatures, phosphate investments are almost always used. Also, as a result of their high freezing temperatures, more shrinkage of the alloy must be compensated for than in gold-based alloys to obtain accurately fitting castings. The extra compensation can be obtained by (1) painting a die spacer (varnish) on the die, but short of the margins, before preparing the wax pattern; and/or

(2) using two layers of ceramic paper liner in the investing ring to make the setting expansion of the investment more effective.

The methods of casting relatively large partialdenture frameworks in base metals differ from the casting of simple restorations such as crowns, although the two operations are similar in principle. In cast partial-denture construction, a suitable cast of refractory material serves as the structure on which the wax pattern is formed. This is done because the wax pattern is too large

to be freestanding and must be supported during investing and casting. This refractory cast is prepared by duplicating the master stone model, usually by using the agar duplicating materials described in Chapter 12. Fig. 17-15, A, shows a duplicating flask containing the master model before pouring the duplicating material. Fig. 17- 15,B, shows the duplicate impression in agar and the separated refractory cast that was prepared in the agar mold.

Once the refractory cast is prepared, the wax pattern for the partial framework is prepared on the cast and invested, in place, on this cast. Fig. 17-16, A, shows a wax pattern on a refractory cast. Note that the pattern is sprued through the base of the cast. After the wax pattern is formed, the refractory cast with the wax pattern is invested in a casting ring as shown in Fig. 17-16, B, where the sprue button-former has been removed. A gypsum-bonded investment is used when a low-fusing point (<1300•‹ C liquidus) alloy is used, but a phosphateor silica-bonded investment is used when high-fusing point alloys (>1300•‹C solidus) are to be cast. The invested pattern is burned out and the investment is heated to the casting temperature, causing thermal expansion. As for any casting, the sum of the setting and thermal expansions compensates for the casting shrinkage of the base metals. A cast restoration is shown in Fig. 17-17.

Because the minor alloying elements of carbon, nitrogen, and oxygen influence the properties of a base metal, it is generally recognized that a pronounced variation in properties can result from the use of variable casting conditions (see Chapter 16). Variables such as mold temperature, temperature of the molten alloy, and the sprue size and arrangement affect the properties of the finished casting as much as does the composition. Therefore, Co-Cr and Ni-Cr alloys are generally considered technique-sensitive. Another reason for this sensitivity is that almost all elements in these alloys, such as chromium, silicon, molybdenum, cobalt, and nickel, react with carbon to form carbides, even though only a relatively small amount of carbon is present in the alloys. Depending on the mold and alloy casting temperature, cooling rate, and other tech-

nical variables, carbides of any one of these elements may form, which changes the properties of the alloy. Careful control of manipulative variables in the casting operations is therefore essential.

The alloy melting temperature is an important factor in the selection and control of the melting and casting equipment and in the choice of technique and mold equipment used for the casting of base-metal alloys. Only a base-metal alloy that melts below 1300" C can be cast into a calcium sulfate-bonded investment. It is also possible to cast a low-melting point nickelchromium alloy against wrought platinum-gold- palladium wire when a flexible partial-denture clasp is required. Because of their higher melting points, other cast base-metal alloys cannot be melted with the conventional blowtorch used for gold-based alloys. It has therefore been necessary to develop special electric or induction melting facilities or, less commonly, to melt the alloy with an oxygen-acetylene torch. Either method is acceptable in the hands of a skillful operator.

Regardless of the method employed to melt the alloy, it is possible to cause severe damage to the properties of a base-metal casting if proper melting practices are not observed. Two sprue buttons from base-metal alloy casting, one sound and free from defects and the other with some porosity and surface irregularities, are shown in Fig. 17-18. More-severe damage from excessive overheating and resulting porosities and surface reaction with the mold materials is not uncommon. Castings with poor surface appearance usually also possess inferior physical properties. It is probable that the proper control of the factors related to the casting operation is more important in controlling the properties of the finished structure than are the variations in composition or the choice of different products.

When casting any of the base metals into molds designed to accommodate the higher melting temperatures of these alloys, certain problems may be encountered that are less common when casting alloys of lower melting temperatures. One problem is that of trapping gases in the mold during the casting process. To have

shock, some investments for cast base-metal alloys lack sufficient porosity for the rapid escape of gases from the mold cavity when the hot metal enters. Gases may therefore be trapped in the mold cavity and produce voids and casting defects. The effect of such trapped gases on one casting of a cobalt-chromiurn alloy is clearly shown in Fig. 17-19. The general view of the

casting in A shows the location of the defect in a critical area of the appliance. The magnified view of the defective area in B reveals that a large gas bubble became trapped in the molten metal at the time the mold was filled. Before it could be dissipated, the metal solidified. A higher temperature of the casting alloy would have assisted in overcoming this difficulty. Numerous other

methods have been proposed to overcome such defects, such as venting to the surface of the mold to permit rapid elimination of gases. Such a method is used in the preparation of cast test bars for specification testing purposes. The skillful spruing and venting of the mold, combined with complete elimination of the wax residue and adequate heating of the metal, tend to reduce this type of defective casting.

When properly designed and cast, the cast base-metal alloys give acceptable removable partial-denture restorations. A typical appliance of this type, with an acrylic denture base material and artificial teeth attached in the proper relationship is shown in Fig. 17-20. Much clinical study has been given to the choice of clasp material and the proper design of the appliance to give stability and support to the appliance and to the remaining teeth. The mechanics of the design of such restorations are an important aspect of clinical procedures, and rely on the appropriate physical properties of properly cast alloys.

Casting Titanium Titanium has many desirable properties for use in dentistry, but it is

difficult to cast in comparison with the common dental casting alloys because it requires relatively complex and expensive equipment. Two problems in casting titanium are its high melting point and the tendency for the molten metal to become contaminated. The melting point of commercially pure titanium is 1671' C, whereas other dental casting alloys have liquidus temperatures below 1500" C. Titanium readily absorbs several gases when in the molten state. If hydrogen, oxygen, and nitrogen are absorbed, the mechanical properties are adversely effected. To prevent absorption of gases, titanium is cast under the protective atmosphere of argon or in a vacuum. To achieve the high melting temperatures, arc melting in either graphite or water-cooled copper crucibles is used. The casting systems force the metal into the mold using either pressure or centrifugal casting techniques.

The casting design for titanium casting is similar to that of other more common dental alloys. A wax pattern is prepared and sprued, as before, but here only the more temperature resistant investments can be used. Both phosphatebonded and silica and magnesia investments produce good castings and give casting dimensions

that are within the accepted range for base-metal partial-denture and crown castings.

CASTING PROBLEMS

Unless every 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 steps in achieving an acceptable cast restoration. Some common casting problems are described in the following paragraphs and Fig. 17-21. The complete description of casting problems is beyond the scope of this chapter; detailed information is available in books on casting and metallurgy.

Improper solidification of metal causes many casting problems. As discussed earlier in this chapter, the shrinkage of wax and alloy is compensated for by various types of expansion of the investment. However, the shrinkage of alloys takes place in two stages as a result of (1) the transformation of the alloy from liquid to solid and (2) the coefficient of expansion of the solid alloy. While the molten alloy is cooling, the temperature eventually reaches the solidification range, causing the alloy to change from a liquid to a solid. This change of state is accompanied by a large shrinkage, which is compensated for only by adhering to a proper casting technique because the expansion of the investment cannot offset such a large shrinkage. The result of not having this expansion is shown in Fig. 17-22.Ide- ally, molten alloy located farthest from the sprue button should freeze first and molten alloy in the sprue and the sprue button should feed the rest of the pattern, thus compensating for the shrinkage as a result of the change of state.

As long as the remaining alloy is in the liquid state and the casting machine is rotating, molten alloy will feed the solidified portion of the casting, thereby compensating for the shrinkage. The next layer then solidifies, and this process continues until all the shrinkage resulting from the change of state is compensated for by the available molten alloy in the casting, sprue, and sprue

button. If solidification does not occur in this systematic manner and a portion of the alloy in the sprue freezes before the alloy in the casting, what is known as suckback porosity occurs, as shown in 17-21, A, and Fig. 17-22, B. Expansion of the investment material cannot compensate for this type of porosity.

Improper sprue design can also cause suckback porosity. As seen in Fig. 17-22,molten alloy may enter the mold cavity through a single sprue. Investment at the pulpal floor of a full crown pattern therefore heats up because of the higher temperature of the molten alloy, and it keeps the alloy in this area molten somewhat longer than other areas. Thus if alloy in the sprue solidifies before the alloy in the pulpal floor area, the molten alloy in this area would feed the solidifying alloy in the sprue area. When molten alloy in the pulpal floor area solidifies and shrinks because of the change of state, it cannot be fed. As a result, a large suckback porosity under the sprue occurs. Because the cause of the suckback porosity is the improper sequence in the solidification of the alloy, the precautions in the following paragraphs may help to prevent it.

A Y-shaped sprue can be used instead of a single sprue. In this instance, only half of the molten alloy enters the mold cavity through each leg of the Y-sprue, so the temperature of the investment under the sprue does not rise as high. However, the arms of the Y must be widely separated to prevent the investment between the arms from overheating, which could also cause suckback porosity. If a Y-sprue has already been employed, the diameter of each leg of the Y can be increased or an extra 1to 2 g of gold alloy can be used, which will keep the alloy in the sprue and the sprue button in the molten state somewhat longer and will better feed the solidifying alloy.

Another possible solution to prevent suckback is to increase the mold temperature from 500" C, which is used with many investments and techniques, to 650" or 700" C. With a higher mold temperature, the difference in temperature between the investment located around the sprue and the investment in the area of the pulpal floor of the full crown is decreased. This decrease

A

Last to freeze

Fig. 17-22A, Correct sequence of solidification of an alloy in the investment mold of a full cast crown. The margins of the casting should freeze first and the button should freeze last. This order allows the molten metal to compensate for the shrinkage realized when each increment of metal goes from the liquid to

solid state. B, Incorrect solidification sequence that results in a suckback porosity. The sprue area froze before the cusp area of the crown and the metal in the crown had to feed the shrinking alloy, causing a

void in the crown.

helps the molten alloy at the pulpal floor to solidify before the alloy at the sprue. Studies have shown that when a gold alloy with a solidification temperature of 940" C was heated to 1040"C and cast into a mold with a temperature of 500" C, the temperature of the investment 1 mm away from the sprue reached only 585" C, whereas the temperature of the investment 1 mm away from the area of the pulpal floor was as

high as 900" C. Thus the difference between the solidification temperature of the alloy and the temperature of the investment at the pulpal floor was only 40" C, whereas the difference near the sprue was 355" C. Therefore the molten alloy decreased in temperature faster around the sprue than at the pulpal floor, and the sprue solidified sooner. When the temperature of the mold was increased from 500" to 700 " C, the temperature of the investment 1 mm away from the pulpal floor increased from 900" C to 906" to 910" C, whereas the temperature of the investment around the sprue area increase from 585" to 800" C.Thus the difference between the solidification temperature of the alloy and the temperature of the investment in the area of the pulpal floor was 30" to 35" C,whereas around the sprue it was 140" C. In other words, by increasing the mold temperature before casting, the variation of the mold temperature in different areas after casting is reduced.

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 suckback porosity. In fact, this strategy may increase suckback porosity by increasing the temperature of the investment in local areas as the alloy is driven across the investment at a higher rate. The higher forces of alloy entry also increase the chances of cracking the investment, either from thermal shock or from mechanical failure. Miscasts can be caused by a host of other factors, including incomplete wax elimination, overheating of alloys, insufficient casting pressure, insufficient escape of mold gases, and incorrect spruing. These topics have been discussed in other parts of this chapter.

Other Casting Problems With calcium sulfate-bonded investments, when the color of a casting is black after removal from the investment, the cause is probably one of the following:

(1)the wax was not completely eliminated,

(2)the mold remained in the oven too long,

(3)the oxidizing flame was used in melting the alloy, or (4) the investment did not contain any deoxidizing agents. As mentioned earlier, when