Casting machines for dental technology
Casting machines are electric units for casting molten materials in dental technology. This mostly involves metals but can also include glass-ceramics and plastics (injection moulding). The casting procedure comprises two processes: melting and casting. The solid material is warmed in a crucible (mostly ceramic or carbon), often temperature-controlled, to a liquid smelt.
The various types of casting machine generate heat using different methods: Arc melting (electric/gas discharge on tungsten electrode) is usually used when casting titanium.
Other metallic materials such as precious and non-precious alloys can be fused with a precisely adjusted open flame (propane/butane-oxygen, "open flame casting") or induction melting (water-cooled, alternating current induction coils induce heat in the metal ingots). The temperatures reached by resistance heating are usually only high enough for melting preciousmetals.
The casting mould is normally produced by waxing up the pattern with wax or resin and investing it in a casting ring using soft investment material (controllable expansion for compensating for the setting shrinkage) which is then allowed to harden. The casting ring is then heated in a preheating furnace to fully burn out the wax or resin. This results in a refractory, single-use, mould for the lost-wax casting technique, which is later destroyed during devesting. It is heated until immediately prior to casting before being secured in the holder of the casting machine. The metal melted in the casting machine must then be forced to flow into the mould.
There are various options for applying the necessary force. Depending on the system in use, gravitational force (swivelling mechanisms), compressed air ("pressure casting"), vacuum or centrifugal force may act alone or in combination. As the metal shrinks, i.e. loses volume, during cooling (preferably "directional", i.e. the thin marginal areas furthest from the crucible cool first), the casting machine must also be designed to feed the excess molten metal into the mould.
Centrifugal casting machine
To prevent the molten metal oxidising, casting machines include various measures such as vacuum for evacuating undesirable gases from the investment material, casting in shrouding gas (e.g. argon) or the use of special carbon crucibles which release CO2 (carbon dioxide) shrouding gas and CO (carbon monoxide) reducing agent from their surfaces when heated. When casting with an open flame, CO is also released in the "reducing zone" and addition of flux powder also prevents oxidation.