The high frequency induction casting machine-
Induction systems are capable of melting alloys by passing a high-density electric current through a copper coil. This electric current produces a magnetic field around the coil. The polarity (direction) of the current is then altered rapidly from positive to negative.
Placing an alloy capable of having its polarity changed (usually magnetic alloys) within the magnetic field causes the molecules of the alloy to oscillate and the molecular bonds to break down. The intensity of molecular activity produces heat and the alloy becomes molten, in effect, the heat is generated from within the alloy itself, not the coil. In typical dental technology induction systems, the coil surrounds a tapered crucible into which an alloy is placed.
Gold alloys are not capable of polarising, therefore, a carbon crucible must be used so it is the crucible becomes hot and then transfers the heat to the alloy.
To prevent heat from the metal and crucible damaging the induction coil, water is circulated through the copper coil to keep it cool during the melting process.
Carbon arc melting is used to melt high fusing alloys. A high electric input (30 A) is used to create an electric arc at the end of two electrodes. The electrode arms are adjusted to 3-4 mm apart with the arc between them and this placed about 12 mm from the alloy to heat it. The manufacturer’s instructions must be closely followed because the arc can damage skin and eyes. The apparatus should be used in a separate room with the means to prevent access to unprotected personnel. Normally, the operator is required to wear a black apron to prevent glare and a specially designed facial visor. A foot operated control switch is generally used to allow both hands to be free during its operation.
In carbon resistance melting an electric muffle (similar to a small furnace) is used to melt alloys up to 1000°C. The alloy is placed in a carbon crucible, both are then placed in a muffle (usually a part of the casting machine). The carbon crucible heats up and melts the metal, usually gold.
The lost wax process-
The lost wax process is a relatively simple and inexpensive way of producing acrylic dentures and cast restorations. A wax pattern is embedded in a mould material (such as plaster or investment) and the wax is then eliminated either by melting it away with hot, running water or heating it to such high temperatures in a furnace that it is incinerated.
We can then force molten metal or acrylic down a channel or directly into the cavity, filling the void left by our original wax pattern. Thus a metal substructure or acrylic denture is produced.
The mould materials for casting alloy structures must be capable of withstanding very high temperatures without disintegrating. In addition, upon heating the mould must undergo sufficient expansion to counteract the contraction of the alloy on cooling to normal room temperatures in order to produce an accurate fitting metal substructure. This mould cannot be reused as it's destroyed when removing the pattern.
The wax pattern for a dental alloy casting may be created in two different ways:
- The duplicate refractory cast technique. Here, the master model is surveyed, the path of insertion selected and undesirable undercuts waxed out. A duplicate model is produced in refractory investment material and the wax pattern produced on this duplicate model. The wax pattern is invested on the duplicate model. This technique is commonly used in the production of partial or full metal denture bases.
- The "lift-off" technique. The wax pattern is formed on the master cast or die, lifted off the model and then invested. This technique is only used in simple designs (like crowns and inlays) because complex patterns are likely to distort.
