Plastic Casting - Part 2
I've decided to put together a basic "how-to" guide that covers rubber mold making and resin plastic casting and molding. I learned quite a bit over the last 5 years while building my B9 Robot and a couple dozen Mystic Seers'. This is will not cover plastic injection molding but rather the type of molding you can do at home. Hopefully you will pick up a few tips here that will help you avoid some of the pitfalls. If you haven't already, you should first check out my Silicone Rubber Mold Making - How-to Guide.
Here's the mold filled with resin. I use a rubber band to hold the mold together. You don't want the rubber band too tight or it will distort the mold. The registration "holes" you created when you made the mold should help hold the mold together. Unlike custom injection molding, this mold is not under any pressure from the resin. This is also a good time to tap the mold a few times to get any bubbles to rise to the surface. If you have any undercuts in your mold you may have to tip it back and forth a few times as well. You will learn real fast where your mold can trap air. For a few of my molds I pour with the mold setting at an angle. Then I set it upright. This prevents the air from being trapped while pouring.
Now here are a few tips is you find you are getting small bubbles in your cast parts. The most expensive solution (and best) is to get yourself a paint pressure tank for about $85. These are used by professional painters but are available at Amazon and Harbor Freight. Molding under pressure will compress any air bubbles to a much smaller size. You'll need an air compressor as well. You want to place your mold into the tank as soon as you have poured it. Tighten the top and apply at least 25 PSI.
A much cheaper solution, that works pretty well is to dust your mold with talc-based baby powder. You don't want much, just a small bit sprinkled into the mold and the gently blow out the extra. You'll want to wait until your mold release has completely dried or even try it in place of your mold release.