This Instructable shows how to modify and use a pressure paint tank for bubble-free resin casts. The first part shows the modification of a paint tank and the following section depicts the casting process. I assume you know how to make a mold and cast material (here is a great Instructable on making a mold and video on making a simple block mold like I'm using below).
A professional 5-gallon pressure chamber costs upwards of $700, not including the compressor. A modified paint tank should cost under $120, and of course you will still need an air compressor.
You will need:
- Pressure Paint tank
- Air compressor with Air hose
- 1/4" Compressor Coupler
- 1/4" Female Compressor Plug
- 1/4" npt cap
- 3/8" npsm (fine thread) cap
- Thread Seal Tape
in addition to a mold, casting material, and a release agent.
The main image shows two casts of the same material (Smooth On's popular 325 urethane plastic), one that was pressurized and another that was not. I used the exact same mold and process for each cast except for pressurizing the first cast. Hopefully this image is enough to convince you that pressurizing is ESSENTIAL if you are serious about casting resin/urethane plastic.
Note 1: After conducting more research and talking to a number of techs, it it imperative that the mold used with the pressure chamber have its silicone vacuum-degassed. If this is not the case you will get "measles," as tiny air bubbles will become exposed when pressurized (essentially ruining the mold). Again - the ultimate way to remove air bubbles is to first vacuum-degass the silicone used for the mold, let that fully cure, and then vacuum-degass the resin (casting material) after mixing, pour vacuumed material into a mold, and then put mold into pressure chamber (as a few people have also pointed out in the comments below). If you're wondering why you can't put the mold in a vacuum chamber, its because the material expands before contracting so it would spill out of mold. Also if you're wondering why you need the pressure chamber, its because you'll likely induce additional air bubbles when pouring. Again, the ultimate way to remove air bubbles is to combine both processes. This Instructable covers the second part of that process.
Note: Some good mixing techniques include keeping the stir stick on bottom of container, and pouring "high" for silicones (also know as. "ribbon technique") or "low" for resins (discussed later) to minimize air bubbles in both the mold and the cast. These are certainly worth practicing, but will not achieve results that come close to a pressure cast.