A: Pressure casting is the only way to guarantee bubble free castings. Pressure casting most commonly refers to a process incorporating a pressure chamber and compressor to eliminate bubbles from a casting resin or mold rubber. It is the best method for ensuring that castings are bubble free.
A simplified explanation of how pressure casting works; A liquid plastic (Smooth-Cast® Plastics, Crystal Clear® Plastics or TASK® plastics) is mixed and poured into a rubber mold. The entire mold structure is placed into a pressure chamber. The door or lid to the pressure chamber is closed and tightly secured. A compressor hose is then connected to an air port on the pressure chamber. The compressor is then turned on and the mold and resin are subjected to 60 psi pressure until the resin gels. The bubbles are collapsed into solution and the result is a bubble free casting.
The pressure casting method is commonly used by professional prototype model makers who cannot tolerate a single bubble. A single bubble on the surface of a casting will result in the casting being discarded.
Suggested by professional model makers: if you are going to use a rubber mold on a regular basis for pressure casting, make the rubber mold under pressure. This means mixing and pouring mold rubber over your original model, placing the structure in your pressure chamber and applying 60 psi until the mold cures.
Reason: If you pressure cast resin in a rubber mold that has not also been pressurized, any bubbles present in the rubber mold may implode under the pressure and the collapsed bubbles may be reflected in the finished casting as bubble positives or "dimples".
Limitation: when making a mold of a hollow model under pressure it may collapse under pressure
Warning: use commercial pressure chambers only. More than one resin caster has learned the hard way that "home-made" pressure chambers can explode violently and cause serious injury.
Important: ensure that the air running from your compressor (used to pressurize the pressure chamber) is dry. Moisture in the air line will react with urethane resins and rubbers causing bubbles or foaming.
Air line driers are available from industrial supply stores.