CASTING: casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.
CASTING PROCEDURE: It involves both the clinical and the laboratory steps:
- Hand mixing and the use of the vibrator to remove air bubbles.
- Vacuum mixing- This is the better method because it removes air bubbles as well as gases that are produced and thus produces a smoother mix.
- Hand investing
- Vacuum investing
- If hygroscopic expansion of the investment is to be achieved then immediately immerse the filled ring in water at the temperature of 37C.
Step 6: CASTING THE METAL
- The force exerted by the machine is adjusted by turning 3-4 turns of the arm to wound the spring and kept in that wounded position with the help of a stop rod.
- Balancing the machine should have been done before the ring is heated by placing the ring on the casting machine so that the arm is balanced to compensate for the weight of the ring and the investment.
- Preheating the alloy to its melting point is done by using the reducing zone of the torch flame in ceramic crucible attached to the broken arm of the casting machine. Use of reducing zone only is necessary to avoid carburization of the metal and because it is the hottest part of the flame. Reducing zone is blue in color.
- Then the ring is immediately taken out of the heating furnace and place firmly against the back plate of the machine. Then the crucible is moved up against the sprue hole end of the ring. The crucible also has a hole in it. Thus both the holes are up against each other.
- The alloy is reheated again until it spins, and looks bright red hot (1100C) with shiny mirror like surface. This indicates its proper fusion.
- At this stage torch flame is removed and arm of the machine is released by dropping the stop rod simultaneously.
- mandrels, abrasive disks.
- Rubber cup polishers, bristle brushes.
- Wool mop
Casting procedure for chrome cobalt removable partial denture:
- Investment must withstand the high temperature of melting chrome-cobalt alloy that is above1250C.
- investment must have sufficient expansion to compensate for the high casting shrinkage of the metal.
Shrink spot or localized shrinkage porosity:
- Kennth J Anusavice, Philips science of dental materials 11th edition W B Saunders publication 2003
- Rossenstiel, Land, Fujimoto : Contemporary Fixed prosthodontics 3rd edition Missouri Mosby 2001
- Shilingburg, Herdert : Fundamentals of fixed prosthodontics ;3rd edition Chicago