Сraig. Dental Materials

Fig. 17-5Diagram showing the position and area of attachment of a single sprue to a small inlay restoration pattern. The location (A), diameter, length, and angle of the sprue (B) are critical to preserving the proper freezing order during casting. Furthermore, the attachment of the sprue to the restoration is critical. In

x, the sprue is attached with rounded corners to avoid turbulence and the risk of investment fracture. In y, the sharp corners of attachment are not desirable.

The location of the sprue attachment to the wax pattern is paramount for proper casting. In general, the sprue should be attached to the bulkiest portion of the wax pattern, as shown in Fig. 17-5. Placing the sprue away from the fine margins minimizes distortion of this delicate area of the pattern. When attaching the sprue, the point of contact should be flared, as shown in insert x of Fig. 17-5, B. This allows more-even flow of the metal into the mold and less porosity

in the casting at the point of contact. In addition, not flaring the sprue connections, as shown in insert y, results in the formation of sharp projections of investment that can break off and incorporate into the metal during casting. The direction of the sprue is another factor to consider. In general, the sprue should be directed toward the margins such that it minimizes the turbulence of the flow of the molten metal and favors the fine margins of the wax pattern. Finally, the point of attachment of the sprue should be selected, keeping in mind that the contour of the restoration will be altered by the sprue's presence. Although the contour can be restored once the casting is complete, such recontouring is often much more difficult in metal.

The diameter of the sprue, in conjunction with the pressure of the casting machine and density of the molten metal, controls the rate of flow of the molten metal into the mold cavity. The larger the diameter of the sprue and the higher the density of the molten metal, the faster the molten metal should enter the mold cavity. However, the mold cavity is filled with various gases before the entry of the molten metal, and the mold cavity cannot be filled completely unless all gases are driven out through the pores of the investment. As mentioned in Chapter 13, a requirement of dental investments is that they be sufficiently porous to allow gases to escape. Thus not only do the sprue diameter and pressure of the casting machine have an effect on the rate of filling the mold cavity, but the rate of elimination of gases from the mold cavity also has an effect. Incomplete castings may result from using a sprue with a diameter that is too small. In this case, the molten metal may solidify in the sprue area before completely filling the mold cavity. The proper sprue diameter for dental castings, depending on the size of the wax pattern, may vary between 6 and 12 gauge (4.1 and 2.0 mm, respectively). Theoretically, the diameter of the sprue should be larger than the thickest part of the wax pattern; however, practically it could be slightly smaller.

The length of the sprue is also important to ensure a good casting. The wax pattern should be positioned approximately 6 mm from the end

of the casting ring (Fig. 17-6). 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, the metal in the sprue remains liquid and flows to the casting until the casting is completely solidified.

Once the sprue is attached to the restoration, the other end of the sprue is attached to a sprue base usually made of hard rubber (see Fig. 17-61. As with the attachment of the sprue to the restoration, the base-sprue attachment should avoid sharp corners to ensure smooth, nonturbulent flow of the metal into the pattern during casting. Sharp corners may also encourage fracture of pieces of the investment during casting, sending these pieces of investment into the mold and possibly the restoration. Once the pattern has been completely sprued on the base, it is ready for investing.

INVESTING THE WAX PATTERN

Investing is the process by which the sprued wax pattern is embedded in a material called an investment.The investment must be able to withstand the heat and forces of casting, yet must conform to the pattern in a way such that the size and surface detail are exactly reproduced. In dentistry, gypsumand phosphate-bonded investment materials are the two types of materials used for this purpose; details about the composition, setting reaction, and expansion properties of these materials are covered in Chapter 13. After spruing, the pattern appears as shown in Fig. 17-6 (left).A casting ring is added to contain the investment while the investment material is poured carefully around the pattern.

For the setting and hygroscopic expansion of an investment to take place more uniformly, some allowance must be made for the lateral expansion of the investment. Solid rings do not permit the investment to expand laterally during the setting and hygroscopic expansions of the mold. To overcome this lateral restriction, a ce-

A drawing of the sprue base, wax patter, inlay ring, and liner in position is shown in Fig. 17-7. The ceramic paper liner is cut to fit the inside of the metal ring and is held in place with the finger. The ring containing the liner is then dipped into water until the liner is completely wet and water is dripping from it. The ring is shaken gently to remove the excess water. After the liner has been soaked, it should not be touched or adapted further with a finger because this reduces its cushioning effect, which is needed for the lateral expansion of the investment. A liner that is about 3 mm short at each end of the ring is preferred. When the liner is equally short at each end of the ring, the investment is locked into the ring, and uniform expansion of the cavity form occurs.

During investing, the water-based gypsum material must flow around the pattern and capture every surface detail. However, the wax sur-

3 mm clearance

- Ceramic paper m liner

- Metal ring

- 3 mm clearance

Sprue base

Fig. 17-7 Cross-sectional diagram of the assembled sprue-based and metal casting ring shown in Fig. 17-6. A single sprue connects the sprue base to the inlay pattern. After investing but before burnout, this metallic sprue pin will be removed. If the sprue is made

of wax, its removal is not necessary. The ceramic liner is 3 mm short of the top and bottom of the ring to lock in the investment during burnout and casting.

faces generally are not easily wetted by water. The surface of a wax pattern that is not completely wetted with investment results in surface irregularities in the casting that destroy its accuracy. These irregularities can be minimized by applying a surface-active wetting agent on the wax. The function of the wetting agent is to reduce the contact angle of a liquid with the wax surface. Wetting agents also remove any oily film that is left on the wax pattern from the separating medium. The spreading of water is illustrated by the comparison in Fig. 17-8,which show silhouettes of advancing water droplets on a polished inlay wax surface, A, and on the same surface treated with the surface-active agent before the water droplet was applied, B. In B, the agent was applied and blotted dry with lens paper. The contact angles are 98' for the plain wax surface and 61" for the treated wax surface. The lower contact angle indicates that the treated wax surface has an affinity for water, which results in the investment being able to spread more easily over the wax. Because the surface-active agents are quite soluble, rinsing the wax pattern with water after the application defeats the purpose of their

Fig. 17-8 Diagram illustrating the contact angle of a liquid like water on the wax pattern. In A, the wa-

ter has a high contact angle and does not wet the wax well. In B, a wetting agent was applied, which decreases the contact angle and improves the wetting. In C, the contact angle is increased again because the wetting agent was rinsed off. For successful invest- ing, the water-based investment must wet the wax pattern well.

use, as shown in Fig. 17-8, C. The same wax as was used in Fig. 17-8, B, was rinsed with tap water and blotted dry. The contact angle in this case was 91•‹, closely approaching that of the original untreated wax surface.

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, the longer a pattern is allowed to stand before investing, the greater the deformation that may occur, even at room temperature. A pattern should therefore be invested as soon as possible after it is removed 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. 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.

Investment Techniques During investing of the pattern, the correct watedpowder ratio of the investment mix, a required number of spatulation turns, and a proper investing tech-

nique are essential to obtain acceptable casting results. There are two methods of investing the wax pattern: hand investing and vacuum investing. In both cases, the proper amount of investment powder and water should be used, following the manufacturer's instructions exactly. The water is added first, followed by the slow addition of the powder to encourage the removal of air from the powder. The powder and liquid are mixed briefly with a plaster spatula until all the powder is wetted.

In hand investing, the cover of the bowl containing the investment mix is placed over the bowl (Fig. 17-9). The cover contains a 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 the hygroscopic expansion. The investment, after being spatulated, is placed on the vibrator to eliminate some of the air bubbles from the mix and to collect all of the mix from the sides of the rubber bowl into the center. The sprue base, which holds the sprue and wax pattern, is held with 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 ad-

jacent to the pattern. Furthermore, to avoid distorting the pattern, the brush itself should not touch the pattern. After the painting is completed, the pattern is vibrated very gently with the sprue base held firmly with the fingers and the underside of the hand resting 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 investment, the mixed investment is poured into the sing, which is held at a slight angle 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 minimuin. It is often helpful to hold the assembled ring and the sprue base in one hand. which rests gently on a vibrator, while the investment is being poured into the ring. When the ring is completely filled, it is leveled with the top by the edge of a plaster spatula. The filled ring is then set aside for the investment to set completely, which usually requires 45 to 60 minutes. 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 nonporous, glassy surface results, which must be ground off to iinprove the permeability of the investment and allow for gases to readily escape from the mold during casting.

In vacuum investing, special equipment is used to facilitate the investing operation. A popular vacuum machine used in dentistry is shown in Fig. 17-10. 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. Although 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 investing often yields castings with improved surfaces when compared with castings produced from hand-invested patterns. The degree of dif-

Fig. 17-10 Picture of a powered vacuum unit for investing. The investment powder and liquid are added to the mixing bowl and mixed with a spatula to ensure wetting of the powder. A vacuum line is then attached to the mixing bowl and the top of the power unit. The casting ring is inverted and attached to the mixing container. The power is switched on, and the mixing bowl is attached to the bottom of the mixer, causing the mixing blades to rotate in the

bowl. After mixing 10 to 15 seconds, the mixing bowl is inverted such that the investment slowly enters the casting ring. Vibration is applied by the lever protruding from the right side of the mixer. Alternatively, the mixer may be removed from the mixing bowl after mixing and the investment poured by hand into

the ring.

ference between the two procedures depends largely on the care used in hand investing. Whether handor vacuum-investing procedures are used in filling the casting ring, the investment should be allowed to harden in air before burnout of the wax.

BURNOUT OF THE WAX PATTERN

After the wax pattern has been invested, it should be set aside until the investment has hardened, usually 45 to 60 minutes. The metallic ring is

Fig. 17-11 Picture of a casting ring that has been filled with investment, but before burnout. The wax sprue is visible as a small dark dot in the middle of the investment. The casting ring is inverted and the sprue base has been removed. This ring is ready for burnout.

placed directly into the burnout oven to begin eliminating wax from the mold. As discussed previously, investment must withstand the impact force exerted by the molten metal when it enters the hot mold during casting. These forces can be considerable. The metallic ring will support the investment as the molten metal enters the mold (Fig. 17-11). In spite of the support given by the ring, it is imperative that the investment be mixed properly and allowed to set completely so it will obtain a sufficient strength.

During burnout, the mold is placed in an oven to completely eliminate the wax, thereby forming a cavity into which the molten metal is cast. During wax elimination, the investment expands thermally, which is necessary to compensate for the casting shrinkage. Although wax melts at a comparatively low temperature, its complete elimination requires much higher temperatures. Usually, if elimination is not complete, small bits of the wax residue are retained in the fine mar-

gins of the mold and prevent the formation of a complete casting. When the molten metal enters the spme hole, the resulting force causes the air in the mold cavity to be driven out through the pores of the investment, thus the mold cavity is filled completely. This process takes less than a second. The presence of any foreign material in the mold cavity slows or possibly prevents the air or other gases from being driven out of the mold before the molten metal solidifies. As a result, the castings may be incomplete or the margins irregular, in which case the casting should be repeated, starting with a new wax pattern.

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 (CO,), water (H,O), or nitrogen oxide (NO,), all of which are gases and can be easily eliminated. However, formation of these gases depends on the presence of a sufficient supply of oxygen, the relatively high temperature of the oven, and adequate heating time of the ring. If insufficient oxygen is available to the wax in the mold cavity, the temperature of the oven is not high enough, or the wax pattern is heated for only a short time, incomplete reaction between the wax and oxygen may result. The internal walls of mold cavities placed in a roomtemperature oven, set at 500' C, and left for different times are shown in Fig. 17-12. The longer the ring remained in the oven, the better the elimination of carbon residue from the mold cavity. Castings made under these four conditions showed that for conditions seen in Fig. 17-12, A and B, the castings were not complete and part of the pattern did not cast. For the condition seen in Fig. 17-12, C, depending on the total amount of wax used, the number of investment rings in the oven at the same time, and other such variables, the castings were either incomplete (as shown in A and Bl or appeared complete but had short and rounded margins. These results indicated that elimination of gases from mold cavities was rather slow because of the presence of carbon deposits, and the molten alloy solidified before all the gases around the marginal area could escape. For all three condi-

ment molds on the presence of carbon residue. Each mold was placed in a room-temperature furnace that was then set at 500' C, heated for the times indicated (A to D), and sectioned to examine the internal appearance. Notice the dark gray area of carbon residue around the mold cavity in C even after 45 minutes of heating.

tions, the surfaces of the castings were black and could not be cleaned by normal pickling (deoxidization) action. This black color indicated that the surfaces were covered with fine particles of carbon residue, because the acid in pickling solutions is not effective in dissolving or removing carbon particles. Castings made under the condition shown in Fig. 17-12, D, were complete, with sharp margins, and on pickling the casting surfaces had a typical yellow color. These experiments demonstrate the importance of adequate burnout time.

A satisfactory way of eliminating the wax pattern is to set the mold in the furnace with the sprue hole placed downward at first, so most of the wax drains out and is eliminated as a liquid. The ring is then inverted with the sprue hole placed upward. In this position the oxygen in the oven atmosphere can circulate 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 500" C oven temperature and larger wax patterns, the mold should remain in the oven for approximately 1hour, with the sprue hole placed downward 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 every extra ring placed in the oven at 500" C. With an oven temperature of 600" to 700" C , a shorter time may be sufficient to completely eliminate the wax.

Investment materials are poor heat conductors, 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 mold is placed in a hot oven or if the heating rate of the oven is too rapid. The outside of the mold, being exposed to a higher ternperature, expands somewhat more than does the inner part. This expansion may cause the mold to crack during heating. Because of the high thermal expansion of investments containing cristobalite, these temperature differences may be especially important. For best results, this type of investment should always be placed into a roomtemperature oven and the oven temperature increased slowly. These temperature difference~ do not affect quartz-containing investments, probably because their rate of thermal expansion is slower than that of cristobalite investments.

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 mold 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 (see Chapter 13). If the mold cools before it is cast, the only recourse is to discard the mold and start over by making a new wax pattern.

CASTING

Casting Machines Several types and designs of casting machines are used to make dental castings. All casting machines accelerate the molten metal into the mold either by centrifugal force or air pressure. Numerous modifications and variations of these methods are used in different machines. The selection of the casting and melting techniques is heavily influenced by the type of alloy and restoration to be cast.

A variety of centrifugal machines are available. Some spin the mold in a plane parallel to the table top on which the machine is mounted, whereas others rotate in a plane vertical to the table top. Some are spring-driven, and others are operated by electric power. An electric heating unit is attached to some machines to melt the alloys before spinning the mold to throw in the

metal. Others have a refractory crucible in which the alloy is melted by a torch before the casting operation is completed. 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 (Fig. 17-13). Concerning the quality of the casting, a preference for either machine is not known. The main advantage of the centrifugal machines is the simplicity of design and operation, with the opportunity to cast both large and small castings on the same machine. Typical examples of centrifugal machines are shown in Fig. 17-14.

With the air pressure type of machine, 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

valve mechanism. This type of machine is satisfactory only for making small castings.

Casting machines, both the centrifugal and gas pressure, are available with an attached vacuum system designed to assist the molten metal in filling the mold. In some castings, the added vacuum may be advantageous, but in general, this addition shows little evidence of producing superior quality castings.

Melting the Alloy To cast a metal restoration, a suitable torch or other heating equip-

ment to melt the alloy is necessary. Various devices are available for each of these operations. The most common method of heating dental alloys for full cast-metal restorations is by using a gas-air torch. A properly adjusted torch develops an adequate temperature for melting dental alloys whose melting ranges are between 870" and 1000•‹ C. 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. Awell-defined torch flame is the hottest and most effective for such melting operations.

The literature contains many descriptions of the proper flame for heating metals and alloys. One practical method of checking and interpreting the flame condition is to apply the flame to a small piece of copper, approximately the size of a coin, placed on a soldering block. The torch is adjusted as it would be for making a casting and is then directed on the copper. If the copper turns bright and clean as it is heated, the flame and the torch manipulation are correct. If the copper turns to a dark, dull red color, oxidation is occurring and the heating is ineffective. The operator should make the necessary adjustments to eliminate these conditions.

The properly adjusted flame contains welldefined component parts described as inner and out cones or portions having differing color intensities. For example, the inner, well-defined, light blue central cone is usually accepted as the most effective for heating because it is the least oxidizing portion of the flame. Although an improper flame is the most common cause of oxidation of the base metals in alloys, even a properly adjusted flame causes oxidation if it is held too close to the metal being heated, too far from it, to one side or another, or if it is moved over the surface or away from the alloy. The dramatic oxidation and ineffective heating that can be seen on the copper are not as evident with noble and high-noble alloys, yet may be observed by the experienced operator who is aware of such conditions.

Modified torches may be used that combine natural gas and oxygen or certain "tank" gases, such as acetylene and oxygen. The natural gas and oxygen combination is mainly used for melting alloys designed for construction of porcelainmetal restorations. The combination of acetylene and oxygen has been used mostly to melt cobaltchromium and nickel-chromium alloys, which have higher fusion temperatures and are used for removable partial dentures.

Electric melting units of various designs are

used in some laboratories to melt the alloys during casting. The advantage of using these units is that slightly less skill is required than is necessary for controlling the torch. However, many of these electric heating units have no limiting controls, and the operator must therefore exercise judgment regarding the proper condition of the alloy to be cast. The electric units are heated either by induction or by resistance heating systems. Those units heated by induction melt alloy much faster than do those heated by torch; if the procedure is not watched closely, the alloy can easily be overheated. An electronic monitor is useful for indicating proper temperature. Melting units with resistance heating take longer to complete the heating and casting operation than do torch-heated units. The slightly longer heating time does not appreciably increase the temperature of the molten alloy, nor does it cause any significant problems.

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 as quickly as possible to a completely molten condition (above the liquidus temperature); ( 2 ) oxidation of the alloy should be prevented by heating the metal with a well-adjusted torch (or other method) and a small amount of flux on the metal surface; and (3) adequate force should be applied to force the well-melted metal into the mold. Finally, after the molten metal is forced into the mold cavity, the casting machine should be allowed to continue to provide pressure on the metal while the metal is solidifying. This pressure encourages complete casting of the margins. Although each of these operations demands care and attention, they are not difficult to master.

SPECIAL CASTING SITUATIONS

Casting Porcelain-Fused-To-MetalAlloys

Because of the characteristically high melting temperature of the alloys used in porcelain- fused-to-metal restorations, special investments and casting facilities are necessary. Calcium sulfate-bonded investment materials and con-