Reference Edition
This chapter is part of the Air Force Dental Laboratory Manual (2005) – Digitally Restored Edition.
This edition preserves the original publication while correcting OCR errors, restoring formatting, reconstructing damaged tables where necessary, and improving digital readability.
The technical content has not been rewritten, modernized, expanded, or altered.
It is provided as a professional reference. Modern instructional material is published separately throughout DentalTechnology.org.
The high increase in the value of gold, beginning in the late 1960s, has been the main force behind the development of gold substitute alloys for dental casting uses. The greatest development has been toward base metal alloys, commonly called nonprecious alloys, which contain no noble metals. The most popular of these have been the nickel-chromium alloy systems. These alloys are not new. Like many other materials used in dentistry, they were initially developed for purposes outside the dental profession. Nickel-chromium (NiChrome™) has been used in industry for many years as heater wire or conductive rod material in the manufacture of heating elements. Nickel-chromium alloys were selected for fixed restoration casting primarily because of their high heat characteristics.
The most popular current use of the nonprecious nickel-chromium alloys in fixed prosthodontics is the casting of substructures or copings for metal-ceramic restorations. An advantage of the base metal alloy in this application is its high sag or distortion resistance at the temperatures needed for the firing of the veneer porcelain. This is attributed to the higher fusion and melting temperatures of the base metal alloy over gold-based alloys designed for comparable use. Because the base metal alloys equal or exceed the mechanical properties of gold in many respects, base metal alloys have been used in all cast metal fixed restorations, with and without porcelain.
Practically all of the current nonprecious alloys for fixed prosthodontic uses are nickel-chromium systems. Composition ranges for the many brands available are approximately 67 to 81 percent nickel and 2 to 20 percent chromium. Other elements such as molybdenum, manganese, aluminum, silicon, and beryllium are added in small quantities ranging from approximately 0.1 to 5.2 percent. The trace amounts of these elements control cast metal grain size, fusion and melting temperatures, hardness, and tensile strength; and they impart other needed characteristics to the alloy depending on its intended use. As a general rule, those additions to the alloy that provide lower melting temperatures usually produce higher hardness and, therefore, restorations that are more difficult to finish. Higher melting alloys are generally less hard and less difficult to finish.
3.4.1. The element nickel may be one of the most common causes of allergic dermatitis. It may be responsible for more allergic reactions than all other metals combined. Laboratory technicians who have been shown to be nickel sensitive by medically valid evidence should be advised of some potential risk with long-term exposure.
3.4.2. Some base metal alloys contain the element beryllium to control hardness of the cast alloy and reduce the fusion temperature. Inhalation of beryllium-containing dust particles is known to be a potential health hazard. Industrial safety precautions must be observed and adequate ventilation provided when grinding and finishing beryllium-containing alloys.
3.5.1. The well-established laboratory techniques developed over many years of premium gold alloy use cannot be directly applied to the base metal alloys. This is because base metal alloys, due to their chromium content, have high melting temperatures which usually range between 2000 and 2600 °F. Such a heat range is beyond the capability of a conventional gas-air torch. Instead, a gas-oxygen torch with multiorifice tip is required to melt base metal alloys. The use of acetylene as a fuel should be avoided because the flame produced from such a source can become too hot. Also, an oxyacetylene flame is rather dirty. When using a gas-oxygen torch, the gas pressure should be between 6 and 8 pounds while the oxygen pressure should be adjusted to 20 pounds.
3.5.2. Base metal alloys are susceptible to oxidation of their component metals and to carbon inclusion in the molten state. For this reason induction casting is preferred for base metal alloy casting and produces the most consistent results.
3.5.3. Due to their high melting temperatures, base metal alloys undergo greater shrinkage or contraction than gold alloys when cooling from the liquid (molten) to the solid state. Consequently, more expansion of the mold is necessary to produce a casting of sufficient size to fit the die without considerable grinding on the internal surfaces of the casting. To compensate for the unusually large amount of shrinkage, burnout temperatures of 1500 to 1600 °F are recommended. Because gypsum-bonded casting investments cannot be used for such temperature levels, either phosphate or silicate bonded investments must be used. Even these high heat investments show considerable surface breakdown when contacted by overheated metal, a common occurrence with torch-melted alloy.
3.5.4. In addition to casting shrinkage compensation, the high casting ring temperature is required to maintain the molten or liquid state of the nickel-chromium alloy for as long as possible. Because base metal alloys have only half the density (approximately 9 grams per cubic centimeter [g/cc]) of their gold counterparts, more time is required for the molten metal to fill the casting investment mold cavity by centrifugal force of the casting machine. The speed of the centrifugal arm of the casting machine must be significantly increased to provide the centrifugal force required for base metal casting. The lower density of these alloys also requires special spruing and venting techniques for wax patterns to consistently produce complete castings.
3.5.5. Basic research in laboratory techniques for fixed restorative construction with base metal alloys has shown that manufacturer’s instructions for handling of investments are not adequate to produce castings large enough to fit the die. Therefore, modification of the manufacturer’s instructions are often necessary to establish routine laboratory procedures for the production of clinically acceptable restorations. Once established, such procedures must be closely followed, otherwise the technique sensitivity of the base metal alloys will result in a product less than satisfactory for clinical use. Remakes of unacceptable restorations quickly offset any potential savings in alloy cost.
3.5.6. Soldering of base metals is also very technique sensitive. The thick oxides that form when soldering can affect or weaken the chemical bond of the porcelain. Therefore, ill-fitting FPDs are usually remade rather than soldered.
Due to the low density of base metal alloys, wax pattern spruing requires the following special attention:
3.6.1. Direct Spruing of Individual and Multiple Unit Patterns. Use 8-gauge round wax or plastic for pattern sprue formers. Preformed sprue formers with spherical reservoirs are preferred. Place sprue former attachments at the area of maximum bulk of the pattern. If additional bulk is necessary, add to a noncritical portion of the pattern for sprue attachment. Make sure the point of attachment blends well. It is not unusual for castings to exhibit “cold tear” or “shrink spot” porosity in these attachment areas, but this may be removed when excess metal is cut away. If preformed sprue formers with reservoirs are used to directly sprue multiple unit patterns, attach the sprue formers to the pattern so there is contact between the reservoirs. NOTE: Connector areas are sometimes chosen as sprue attachment sites.
3.6.2. Indirect Spruing of Individual and Multiple Unit Patterns. Use 8-gauge round wax for pattern sprue formers. Use 6- or 8-gauge round wax for the runner and main sprue former leads. The length of the pattern sprues as well as the main sprues should be adjusted to position the runner bar within the thermal zone and the pattern above it. See paragraph 1.52.4.3 for more details on indirect spruing.
The higher melting temperature of base metal alloys requires maximum compensation for casting shrinkage. To provide enough expansion of the investment mold cavity, modifications and combinations of traditional investing and burnout techniques may be required. Under ideal laboratory conditions, a procedure including vacuum mixing of investment, hygroscopic set, and high temperature burnout provides consistently high quality castings with acceptable fit on the dies without grinding and force fit.
3.7.1. Line the casting ring with 0.040-inch thick KAOLINER®, which is a trade name for a mat of finely spun fibers of kaolin used as an investment ring liner. It allows maximum expansion of the investment mix while setting and during burnout. Asbestos is generally not resilient enough to allow the expansion needed for base metal alloys. One 0.040-inch thick strip of KAOLINER is equivalent to a double layer of asbestos and does not pose the respiratory health problem of asbestos.
3.7.2. The high heat investment of choice for casting base metal alloys is the phosphate-bonded type. It produces smoother surface castings than the silicate-bonded type. Phosphate-bonded investment is supplied as a powder-liquid system. The liquid portion of the system is a colloidal silicate that can be diluted with water to provide various amounts of investment expansion. Undiluted liquid provides maximum expansion and should be used that way for base metal castings of fixed prosthodontic restorations. Vacuum mix the investment for base metal alloys according to the manufacturer’s instructions, followed by 15 seconds of vibration under vacuum to remove escaping gas bubbles.
3.7.3. Immediately place the filled ring in a 100 °F water bath for a minimum of 45 minutes followed by overnight bench set. Rings may then be placed in a cold oven and brought to 1500 °F in 1 hour and heat soaked for an additional hour. On completion of burnout, the investment should appear white. Dark areas of investment indicate not all the carbon residue has been eliminated.
3.8.1. Manual Casting. Manual casting of base metal alloys, using a gas-oxygen torch and broken arm casting machine, is similar to casting other metal-ceramic alloys. The major difference between these alloys is the appearance of the molten metal when it is ready to cast. Safety precautions are required for eye protection. (Use welder’s goggles or glasses approved by Occupational Safety and Health Administration [OSHA] standards.) The following guidelines are for casting with a torch:
3.8.1.1. Adjust the oxygen to 20 psi. Adjust the torch to produce a stable flame with 1/4 inch blue inner cones. A distinct hissing sound should be evident.
3.8.1.2. Place the alloy into a preheated crucible. Lay the round ingots on edge to take advantage of the ability to roll them under pressure of the torch. Place multiple ingots in contact with each other.
3.8.1.3. Heat the alloy using the tips of blue cones 1 1/2 to 2 inches from the ingots. Guide the torch tip in a circular motion to heat all metal evenly.
3.8.1.4. Load the ring when the ingots begin to slump; oxides will not allow the alloy to pool as precious metals do. Molten alloy under the oxide layer will begin to roll and move under the pressure of the flame. Shake the crucible carriage with tongs as you slide it forward. When ingots collapse, cast them immediately.
3.8.2. Induction Casting. Induction casting of base metal fixed restorations can be done by using casting machines such as the Ticonium Modular 3. Operating instructions for this machine are as follows:
3.8.2.1. Preparing the Machine.
3.8.2.1.1. First, choose and place the crucible for the alloy being used. Carbon crucibles should not be used for base metal alloys because the carbon contaminates alloys containing nickel or palladium. Ceramic crucibles are predominantly used for base alloys, but they can be used for all alloys. Select a cradle and balance the casting arm for the ring size being used. Turn the circuit breaker on and rotate the power switch from 0 to 1 to turn machine on. The white pilot light should now be on.
3.8.2.1.2. Raise the coil around crucible by rotating the casting arm until crucible is over the coil. With your left hand, push the crucible carrier back to the center crucible over the coil. Place the fingers of your right hand under the black handle and lift while pushing the silver lever to the left with your thumb. Ensure the reference pin aligns with the hole beneath it and release the silver lever to lock the coil in the up position.
3.8.2.1.3. Remove any slag from previous castings and adjust the crucible so the spout aligns with the sprue hole in mold.
3.8.2.2. Premelting the Alloy (for Multiple Ingots)
Load the alloy into the crucible by gently placing ingots into place using tweezers. (Dropping them may break the crucible.) Close the cover and turn the reset switch onto the proper number to begin melt. (A higher number will produce a faster melt.) Turn the reset switch clockwise for a ceramic crucible and counterclockwise for a carbon crucible. To set the electronic eye during premelting:
3.8.2.2.1. Using appropriate eye protection, observe the alloy as it melts. Use visual indicators to determine correct casting setting. Experienced technicians often describe observing “shadows” in the heated alloy to judge this. At the instant a correct melt occurs, observe the numerical readout.
3.8.2.2.2. Raise the lid to stop the melt, push the set point toggle switch down, and rotate the set point knob to obtain numerical reading observed earlier. Record this number for future reference if the machine is used for different alloys.
3.8.3. Casting. Close the cover to resume heating. Just prior to reaching casting temperature, open the cover and place the mold into the cradle. When the amber light glows, immediately push the silver lever to the left with your left hand and push the black handle down firmly with your right hand. When the black handle reaches bottom, the arm will begin to spin automatically. Allow the arm to spin for 10 to 15 seconds before pushing the red stop button. The lid will not open until the arm stops completely. NOTE: Ticonium also makes a flask support that adapts the Ticomatic for casting FPD castings. The only other adjustment that must be made is to set the relay range knob for the metal being cast.
Due to their superior physical and mechanical properties, base metal castings are more difficult to grind than their softer, gold-based counterparts. There is a tendency to overwax the patterns for gold castings because it is easier to remove the excess bulk in the finished casting.
3.9.1. Make sure the patterns for casting base metal alloys are highly refined and as close to the final form as possible. Keep overwaxing of margins to a minimum. Remove bulk such as sprue cutoff on high-speed equipment (such as a high-speed lathe), using conventional abrasive disks. Abrasives like TiCor® and TiHi®, used to polish RPD frameworks, are good polishing agents for base metal fixed restorations also.
3.9.2. To prevent contamination of the area to be covered with porcelain veneer, use an air-abrasive device such as a microblaster to deliver an aluminum oxide abrasive. This technique offers a clean, conditioned surface with the best opportunity for mechanical and chemical bonding of porcelain.
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