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Plastic Resin Casting Guide

Plastic Resin Casting Guide - Part 2

Here is Part Two of basic "how-to" guide that covers silicone rubber mold making as well as plastic resin molding and casting. Many of my projects, over the last 10 years (B9 Robot, Twilight Zone Mystic Seer and Back to the Future Flux Capacitor) have forced me to learn how to make molds in silicone and to cast plastic resin parts. Be sure to start with Part 1.

Stirring the liquid plastic resin

I use popsicle sticks to help mix it up. They are also useful when pouring the resin into the mold. You can pour down the stick into the opening. It allows you to aim the stream better.

Plastic Resin Solenoid in moldHere's the mold filled with resin. 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 raise 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.

Allow the resin to set the required time. With the resin I use, sets in about 10 minutes, but I wait 20-30 minutes before I take the part out of the mold.

Here's the finished part. A chip off the old block.

Pressure Vessel

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.