Pressure Casting with Epoxy or Acrylic
Once a satisfactory mold has been produced using degassed RTV, the second stage of producing replacement knobs takes place. As mentioned earlier, if the void produced within the RTV by the "master" object is filled with a suitable medium, once that medium becomes stable it can be removed in the same form of the master.
Air bubbles are as troublesome a factor in this process, as they are when producing the RTV mold. When air is present in the casting medium, it can prevent contact between the mold and the medium, resulting in dimples or actual holes being present on the finished casting once it has cured.
Since most casting liquids are two part solutions which require stirring during their preparation, the possibility of air being introduced into the solution is high. Moreover, casting liquids have relatively fast curing times so the use of a vacuum to degass the solution may not be practical.
By placing the uncured casting within a pressure chamber and introducing compressed air via a holding tank/compresser arrangement, the casting can be pressurized while it is still in a liquid state.
When subjected to moderately high air pressure during the curing process, any air trapped within the casting solution will itself compressed by the external pressure. Bubbles that begin the size of a match head in the solution will be compressed to the size of a grain of sand by the pressure.
Although the finished casting may not be truly bubble free, it will be devoid of any visible air induced flaws...
It should be stated that working with pressurized air is a potential killer. If you don't know what you are doing.....DON'T DO IT!!!!!!