Can I do this?

This is a very simple mold, the masters are glued on the tupperware with rubber cement.  The masters are a section of a bridge for a battleship model designed by Classic Warships.   (Images are courtesy of Steve Wiper, Classic Warships)

Actually it is more difficult to bake a cake that it is to start making molds and casting those items. The design of the mold is the most critical part of the process. For that reason we will take some time to explain how to make simple, yet very effective molds.

Step 1:

Find something to hold the RTV when it is poured over the master. We have used many different things to contain the RTV material, things like PVC pipe, Crystal boxes (like those in the Complete Kit) and leggos. Of all of the choices, we prefer to construct containers using cardboard or foam core board and hot glue.  However anything will work for a mold container, Tupperware, empty butter or yogurt containers work just fine and a simple dam of clay will do just fine for a small master.

Step 2:

Set up the master to be molded. This is the most important part of the whole process because the success of pulling out perfect copies depends on the design of mold. (A simple hint on how to set-up the master is to look at how other resin companies have set-up similar masters.) For example, many times the best way to mold a wheel is to set it up with the tire on the wheel straight up and down and add some sheet plastic on a square trough. (see example below)  The trough has two functions; 1st, the resin has a place to collect and allow air bubbles to work their way up and away from the part to be cast.  The second function is make removing the cast part from the mold easier.  The important thing is to think of how air bubbles are going to work their way out of the casting.  So, you need to think from the inside out and the bottom up. It is not as hard as it sounds...  (We usually try to mold as many things as we can as single piece mold.)

Example 1	Example 2
Fill any holes in the wheel with paint or scotch tape	These masters are set-up similar to the wheel.

Step 3:

Now you have the master set-up in the molding box, its time to pour the RTV into the mold.  Mix the RTV 10 parts of silicone to 1 part activator.  Look for a nice pale light blue color.  Don't worry about being too precise, we usually pour a little more activator than exactly 1 part to 10.  Stir the RTV thoroughly so that there are no streaks of white.  Mixing the RTV thoroughly introduces air into the mixture.  There are a few ways to combat any air ending up on your master when the mold is cured.  The simple way is to use a vacuum chamber to draw out any air bubbles from the mixed RTV.  However most of us do not have a vacuum chamber, so we need to attack this problem a different way. Start by pouring just a small amount on the master, use a old brush or an airbrush to force out any air bubbles you see on the master.  This process is called skinning the master.  Now pour the remaining RTV into the molding container from one corner and allow the RTV to completely fill  from that one corner.  Some folks have rigged up a fish tank air pump to work as vibrator to work air bubbles.  Now allow the RTV to set-up.  The RTV material likes a warm moist place as it sets, we usually leave molds in the bathroom.

Step 4:

Its time to remove the master from the mold; and now you get to see how well you have designed your mold.  Carefully work the mold away from the master as you pull the master out from the mold.  Many times it helps to use baby powder to separate  the mold from the master.  (Blow out the baby powder after you get the master out.  The mold does not need to be perfect clean, however there should not be any powder loosely sitting at the bottom of the mold.

Step 5:

Mix the resin 1 part to 1 part.  Pour the "A" side first and then the "B".  (Tip: pour just a little more of the  "B" side, in cooking they call it a pinch!) Stir the resin thoroughly, so the mixture is clear without any "strings" in the mix.  Now pour the resin into the mold, filling the trough.  Use a popsicle stick or toothpick to work any air bubbles.  Allow to resin to cure. 

Step 6:

Pull the cast from the mold after about 20 to 30 minutes.  Check the temperature.  If its cool to the touch its ready to pull.  Now enjoy your exact replica of your original.  Check the part for any incomplete casting or air bubble.  We call this "Learning the mold".  The next time you pour the mold you will better know where to better work out any air bubbles.

Step 7:

Refrain from celebrating too much.