What is pressure casting? Will it completely eliminate bubbles from my plastic casting?

Pressure casting is the process of using 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. 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.

How Pressure Casting Works

  1. A liquid plastic (Smooth-Cast® Plastics, Crystal Clear® Plastics or TASK® plastics) is mixed and poured into a rubber mold.
  2. The entire mold structure is placed into a pressure chamber.
  3. The door or lid to the pressure chamber is closed and tightly secured.
  4. A compressor hose is then connected to an air port on the pressure chamber.
  5. The compressor is then turned on and the mold and resin are subjected to 60 psi pressure until the resin gels.
  6. The bubbles are collapsed into solution and the result is a bubble free casting.

Making the Mold Under Pressure

If you are going to use a rubber mold on a regular basis for pressure casting, it is suggested to make the rubber mold under pressure as well. 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".

Important Notices

  • Note: 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.

Related FAQ: How long should I keep a material under pressure when pressure casting?

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Disclaimer
This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.