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Silicone Mold Making Guide

Mold Making - Part 1

I've decided to put together a basic "how-to" guide that covers making liquid silicone molds and plastic resin molding and casting . I learned quite a bit over the last 5 years about silicone rubber while building my B9 Robot and more than 35 Twilight Zone Mystic Seers replicas. Hopefully you will pick up a few tips here that will help you avoid some of the pitfalls. These silicone rubber mold making tips can be used by the hobbyist who want to create molds for making chocolate, or to create soap molds and candle molds as well. When making food molds be sure to only use food safe silicone materials. If you wanted to know what is silicone or how to use it for molding, you have come to the right place. This is the same type of mold making used by all prototype plastic molders as well as short run plastic molders. I've added links to most of the necesary supplies where available at Amazon.

If you're going to get serious in molding, you'll want to purchase a vacuum pump. I got mine on ebay for about $100. It's an industrial pump and it is very rugged. You may want to buy one of the smaller hobby vacuum pumps available, but I went for the commercial one as it should last me longer, and it actually cost about the same. You want one that can pull 29 "inches of mercury" for a complete vacuum. There are several on Amazon that should do the trick.


First, a few things you'll need. Get yourself a good 400+ piece basic set of LEGOS. Yes, I said LEGOS! If you're lucky you have a son that went through the LEGOS phase. If you're like me, you may still have yours from when you were young. Either way, they are a must if you want to quickly make molds of various sizes. You want a set with just the basic pieces and none of the fancy special pieces. Your best bet might be hitting up a few garage sales. The smallest set I could find on Amazon was 650 pieces which is probably more than you'll need for smaller molds.


If you want to save yourself some money and get something almost as good purchase one of the immitation Lego sets on Amazon.

While they are not as good, quality wise, as genuine Legos, the 1000 piece "Building Block set includes many more of the larger pieces you will need when making your molds and they are less than half the price.

Bonus Fact: The name "Lego" is derived from the Danish phrase "leg godt", which means "play well".


You may want to pick up one of the "starter" molding liquid silicone rubber kits and plastic resin kits. I have tried kits from both Smooth-On and Por-a-Cast. I prefer Smooth-On as they offer so many different products. Smooth-On has what they call a "Mold Making and Casting Pourable Starter Kit" that contain enough rubber for several small molds and dozens of small resin parts.


Read more: Mold Making - Part 1

Mold Making - Part 2

Prepare you original; - If you are reproducing a part or existing object you have, clean it well. The item can not be porous or it will get stuck in the mold. You may have to use a sealer on the surface.

Choose your parting line carefully. You need to decide where your parting line will be. That is where the two halves of the mold will separate. Look for undercuts in your part that will prevent you mold from coming apart. Knowing that you may have a seam visible on your finished part will also help you decide where it should be. Try to line it up with a straight or flat side of you part if you have one. Also make note of the original part's seam, if there is one. Try to match your seam to the original one, if you can.

Build your molding box -; I use my son's LEGOS to make the molding box. It's quick and cheap. You can adjust the size to fit you project. Now if you are molding something of size, say bigger than a softball, you may want to build a wood or metal mold box. For smaller hobbyist type of molding, the LEGOS are sufficient.


Add a layer of molding clay about 1/2 inch thick.


Carefully press your original into the clay. I then use the rounded end of a pen to make registration "holes ". These holes will help keep the mold together and align the top and bottom half of the molds. Also notice the "cone " on the top of my part. That will form the funnel opening where the resin will be poured into. You can form this out of clay. In this case I used a piece of plastic and topped it off with a little clay. This opening need to be on the top of the mold (where you will be pouring the resin in). Try to put it on a spot on your part that will not be noticeable.


Read more: Mold Making - Part 2

Mold Making - Part 3

After the mold is filled, you will see some small bubbles come to the surface. Some pros use a vibration table to force the bubbles to the surface. I happen to have a table sander mounted to the corner of my work bench. I turn this on for 3-4 minutes and it vibrates the table pretty good. Any stubborn bubbles that raise to the surface and don't pop, I poke with the end of an Exacto knife. Allow the rubber to cure at least 16 hours. I usually allow 20 hours.


Carefully flip over the mold and take out one of the Lego pieces. This will allow you to slowly peal the clay off of the part you are molding. Be gentle.


You don't want to disturb the part as it is half buried in the bottom half of the silicone mold. Peal it off gently leaving the part in place.


Read more: Mold Making - Part 3

Mold Making - Part 4

Fill it as high as you can. A thick wall is better and will make your mold last longer Vibrate it if you can and pop any bubbles that raise to the surface. Allow the rubber to cure at least 16 hours. I usually go 20 hours.


Start taking apart the Legos. You will find that the bricks will easily separate from the rubber.


If you did it right and the spray worked, you should be able to find the parting line. Don't worry if it does not fall right apart at the line. The rubber may take a bit of work to separate but once you get it started it will separate.


Read more: Mold Making - Part 4