The Lost Wax Process
Part 3: investment and burnout
Once wax duplicates are made from the RTV mold, they are ready to be sprued onto the rubber flask base. The rubber base has a raised cone in the center. This will form a funnel in the prepared mold and also creates a reservoir of extra metal (called the button) to fill the mold.
The wax models can be attached directly to the base, or attached to a sprue, which is melted onto the rubber base, forming a “tree”.
Next a stainless steel tube is placed around the rubber base and the mold is ready to be “invested”.
The investment is a plaster-like powder that is carefully mixed using precise ratios of water to powder. It is vacuumed until it boils,and one minute thereafter, then carefully poured into the prepared flask. Then it is vacuumed again to make sure no air bubbles remain clinging to the wax model. It is left to dry for at least 2 hours.
Once the investment has set, it is time to begin the “Burnout”. The flask is placed into a special oven, nozzle down, and the temperature is gradually increased over the course of hours to cure and temper the investment in preparation for being injected with molten metal.
The oven is preheated to 300 degrees and the flasks are placed in for a few hours, then the temperature is raised to 700, then 1350 degrees, finally the temperature is brought down to the proper casting temperature for the metal to be cast.