How Rubber Molds Are Made

In order to give you some idea of the process involved in making a rubber or latex mold, here is a quick rundown.

Part 1 - The easy part, decide what it is you want to copy or make your mold of and get one.

Part 2 - This is the long part. It could take 5 or 6 days if you are only able to paint a couple of times a day. You cover your original - lets say it is a frog - with release agent. You may not always have to do this. Some plastics and resins do not need it. Now paint the frog with the rubber paint. Let it sit untill it is almost dry - you can put your finger on it and no paint sticks to your finger. Paint it again and again. Do this until you have built up a thickness of about 3/16" or so. You can judge this by painting a piece of wood or something every time you apply a coat to the frog. Look at the wood to see how thick your test paint is.

Part 3 - After the paint has dried for 2 or 3 days you can start to make the support for the rubber mold. If you can picture the mold as a rubber glove, when you fill the glove with water it will stretch. You have to stop the stretch. Most of the time this is done with a fiberglass case or backing that you will make to go over the mold. You may have to make this case in 2-3-4-5 or more pieces that will bolt together. This is so you can take the case off easily. If there are a lot of undercuts in your frog then chances are your case will be several pieces. You can avoid having a lot of pieces by filling undercuts on the outside of your frog before you start to make the case. This means more time painting on the rubber paint. This is a good thing because the fewer pieces in the case the easier it is to use. When you make your case it has to have a good big base, it will be holding the rubber mold upright when you fill it with concrete. You don't want it to fall over just when you get to the top!

Part 4 - Take the rubber off the original. Be careful! It will be inside-out. Now you can sprinkle a little talc on it and turn it inside-in. Put it into the fiberglass case you made, bolt or clamp the case closed, invert the case so that it is sitting on that big base we where talking about. Mix your concrete and fill the mold. Let it sit to cure for a couple of days. This is the test run for this mold and case so you want the concrete extra hard when you do your first de-mold.

Part 5 - Unbolt the case and take out your casting. Carefully take the rubber mold off the casting. If you are a skilled mold maker it should be right the first time!! If not you need to look at any problem areas and decide what you can do to correct them.

Now you can see why the rubber or latex molds cost so much and why they take a couple of weeks or more to make.

The building time can be shortened by using fast drying silicone based rubber, it is a little more expensive but.... Plaster can be used instead of fiberglass if the mold is not going to be used a lot. This is cheaper but not nearly as good as fiberglass.

My thought is that for a person to start into the casting business the best way is to start with a few plastic molds - no fiberglass backing needed - do a few pours and see if you want to keep at it. If you decide you do, then as your skills increase, start to think about investing in a simple latex mold. You may be able to find a used one in good shape. Try it and see if you want to do that type of casting.

The results are great once you have mastered the process but it is not a cheap or easy thing to do as a novice.