Introduction to Mold Making and Casting of Small Parts

What is mold making?

Mold making is the art of making a mold which will be used to make a casting or finished part. The mold could be made several different ways, however the simplest and most cost effective way is to make an RTV rubber mold from an original pattern.

Why use RTV rubber to make the mold?

RTV rubber allows an almost exact replication of an original pattern without the tremendous cost of having an injection mold made in a machine shop. It is an excellent way for a model maker to make repeated copies of an original at a very reasonable cost.

When should you make an RTV rubber mold?

Any time you need repeated copies of a part or accessory that is not commercially available.

How do you make a mold?

You start with your original pattern. Glue your pattern down to a smooth flat surface. Next build a mold box around it leaving at least 1/2 in. All the way around your pattern, and making sure that when you pour the RTV rubber in the mold box that it will also cover the highest part of your pattern with at least 1/2 inch of rubber.

Mix the RTV rubber according to the instructions included with the rubber. If the rubber you are using requires de-airing, put the rubber into a vacuum chamber and de-air at 29 inches of mercury for at least two minutes. Slowly pour the rubber in the mold box allowing it to flow over and around your pattern. Allow the mold to cure for 24 hours and then carefully remove the pattern from the mold.

After your pattern has been removed from the mold, you are ready to begin making castings.

What are castings?

Castings are objects which are made by pouring some sort of casting material in a mold. The mold can be used repeatedly to make castings which are identical to the original pattern.

Why use Alumilite to make the castings?

Alumilite is a two part polyurethane plastic material which hardens in as short a time as three minutes. It is almost odorless, non toxic and non carcinogenic. Alumilite will allow an almost exact copy of an original pattern without the tremendous cost of having an injection mold made in a machine shop. After it hardens, it can be drilled, sanded, machined, and painted.

When should you make an Alumilite casting?

Any time you need repeated copies of a part or accessory that is not commercially available.

How do you make a casting?

First prepare the mold by heating it in an oven or microwave. This heat will help to speed up the curing process. Next, coat the inside of the mold with a parting agent such as a commercially prepared release agent, or you can paint the inside of the mold with a lacquer base paint. The paint will add color to your casting and also act as a release agent. The paint will chemically bond to the plastic as it cures. If you do not use a release agent you can use baby powder on the inside of the mold.

Mix the Alumilite according to the instructions included in the package. Make sure to shake the "b" side of the mixture before combining it with the "a" side. Measure equal amounts of part "a" & "b" and pour them into a separate mixing container. Stir until thoroughly mixed and pour into your mold. If you want a flat surface on the back of the mold, you can over pour the mold and put a piece of glass on the mold. This will give you a nice flat surface on the back of your casting.

The casting will begin to harden within three minutes, and will be ready to remove from the mold in around six minutes.

Reference materials

1) Alumilite instruction manual (included with the 28 oz. kit)

2) Making and casting RTV rubber molds (m.s. Productions)

Materials, tools and other equipment

1) stirring sticks and mixing cups

2) scale

3) Styrene plastic or other material to make a mold box

4) super glue or other glue to glue the mold box together

5) clean clay or a sulfur-free modeling clay

6) spray lacquer

7) baby powder

8) oven or microwave

9) vacuum pump and chamber

10) latex rubber gloves

11) Dow Corning RTV (room temperature vulcanizing rubber)

12) Alumilite

13) cleaning solvents such as tolyulene or xylene, mineral spirits, acetone, mek or denatured alcohol.