MFC Mobile Banner
MFC main Banner (Small)

Plastic Resin Casting Guide

Plastic Resin Casting Guide - Part 1

Here is my basic "how-to" guide that covers 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. Hopefully you will pick up a few tips here that will help you avoid some of the problems I encountered. These resin casting tips can be used by the hobbyist to make plastic resin copies of anything from missing toy parts to turn signal lenses for that vintage car.


When molding parts, I use Smooth-On's "Smooth Cast 300" series (300, 305, 321 & 326) with great success. You measure it out by volume so it is pretty easy to use. It's also pretty forgiving if your ratio is off a little.

I have also used a 2 part resin called Por-A Kast "Mark-2".


If you want your resin part to have a specific color you will have to experiment with some coloring. I use So-Strong colorants. There are very strong and very reliable once you figure out how much to use. To get the desired pink-flesh tone I have to mix a drop of orange with a drop of red. That is enough to tint about 4 ounces of resin. I mix up a small batch of the Part A of the resin and keep it in an air tight container.


Silicone Mold Release Spray - If you want to extend the life of your molds you'll want to have to spray on mold release. Lightly spray both halves of your mold and let it dry a few minutes.


Measuring the liquid plastic resinMeasuring the liquid plastic resin - Measure out an appropriate amount of Part A and Part B of the plastic resin. I use three 2 oz plastic "portion" cups for smaller jobs. One for Part A and one for Part B. The third one is for the final combined resin.


Mixing the liquid plastic resinMixing the liquid plastic resin - Combine the two parts. I pour them back and forth a few times to make sure I get it all mixed. Then pour it into a third clean cup. Now use a new Popsicle stick so that you don't have any unmixed resin on it. This in key to getting a fully mixed solution.


Continued in Part 2

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.