Chapter 4:

Advanced Casting Techniques

Alternative Casting Materials

Plasters, gypsum cements & dental stones

Plastics: polyurethanes, polyesters & epoxies

Composite materials & special mixes

Specialized Techniques

Hollow casting

Centrifugal casting

Lay-up and reinforcement

Casting Case Studies

Casting a one-sided dental plaque in plaster

A mold was presented which consisted of a solid piece of Silastic® E RTV. The impression was of the lower dental arcade of a modern human and the mold was about 10 cm square. It was fashioned such that Silastic® E walls rose about 2 cm above the level of the specimen impression, and were continuous around the edge of the mold, forming a reservoir for the casting material. The cast to be produced was of the dental arcade mounted on a 9 cm square plaque. It was decided to pour the cast in Coecal® dental stone with the syringe-stylus method.

The mold was filled with diluted Permaflex® mold dressing to a depth of about 1 mm, and 200 g of Coecal® were weighed out. As the mold was one sided, and therefore did not need to be 'closed', the plaster could be mixed much thinner than normal. The common problem of discolored and soft efflorescences would not be a problem because when the plaster stratified from over-dilution, the dense plaster would settle into the specimen impression, and the lighter, more watery plaster would rise to the back of the plaque where it could easily be sanded off without affecting the information on the cast. Therefore, [38%] by weight, or [76] g of Stalite® hardener were weighed out, about [5%] more than usual.

The plaster slurry was mixed in the 500 ml Vac-U-Mix® bucket and held on the Power Mixer at 29 inches of mercury for a full 45 seconds, approximately twice as long as usual, to ensure the mixture was very fine and homogenous. The extra mixing enables the slurry to pass through a finer syringe tip than usualwithout clogging the tip. The plaster slurry was then placed in a plaster bowl and the mixing bucket was rinsed. The last 1/5 of the tip of a 12 cc Monoject® disposable syringe with a tapered, curved tip was removed with an X-Acto® blade.

The plunger was removed from the syringe and a finger was placed over the opening in the cut tip of the syringe to prevent spillage. The empty syringe was held against the vibrator, which was set on high. The plaster was then poured into the syringe, avoiding bubble entrapment by decanting slowly at an angle down one of the inside walls of the syringe. The syringe was 75% filled and left on the vibrator for an additional 5 to 10 seconds to give bubbles a chance to rise out of the mixture.

The mold was drained of the bulk of the wetting agent and placed on the vibrator. The syringe, with the plunger still removed, was inserted into the anterior teeth impressions first. As with a fountain pen or stylus, the plaster used from the tip of the syringe is continually replenished at a slow, even rate. The deepest portions of the teeth with their complex crenulations and fine [apices] were filled first, and then the plunger was replaced so that work could proceed at a quicker pace. The teeth and the rest of the specimen impression were completely filled with plaster from the syringe, and the remainder of the plaster was poured into the mold to a depth of 8 mm to form the backing. The edges and corners of the mold were probed with a blunt dental tool to remove any entrapped air bubbles.

The mold was set aside on a countertop to harden, the vibrator was turned off and the syringe and plaster bowl were rinsed. After 15 minutes, the cast was pulled and found to be free of defects on all surfaces except the back, which had moderate efflorescences. These were lightly sanded off, and the cast was set aside to completely dry.