The following is a report on my first attempts at resin casting. I appologize for the length, but, I'm trying to make it as easy as possible for those modellers who think it is too difficult.
225 Parsons St.
Kalamazoo, MI 49007
I suggest you do yourself a favor and contact this company for their literature if you are attempting casting for the first time. They will include a small booklet that is very informative and will get you going in the right direction. They are also very friendly and helpful and will answer any questions you may have.
CASTALL RTV - This is an industrial product that you probably will not be able to get. Based on conversations and research may I suggest that beginners substitute GE 3110. This is available from many sources including Alumilite, who also have a quick cure catalyst that will cure this in about 1-2 hours. I am also told that this RTV does not need to be de-aired. Though I have not tried it yet this is what I will try when my CASTALL runs out.
If you have not tried resin casting yet by all means do. It is not beyond the ability of any experienced modeller and will open up a world of possibilities to you. You may pick any subject you choose for a first attempt, but, you will be more successful if you choose something that will work in a 1 piece mold. This should be something with 1 flat side with no detail on that side. By starting this way you will not have to be concerned with "gates" and "runners", the channels that carry the resin into the actual mold cavity and allow air to exit. Get some experience before you tackle gates and runners. Also, if you choose to use GE 3110 RTV you should pick a master that does not have very deep undercuts as this material is not flexible enough to allow extraction of your master if it has more than moderate undercuts.
All that said let's get to it. I used a 4" X 4" piece of ordinary glass to build my mold on because it is cheap, easy to obtain, easy to clean, and will not adversly affect the cure of RTV as wood might. Using just a tiny drop of white glue, glue your master, flat side down in the center of the glass. Now build a "mold box" around the master. I used pieces of .040" (1mm) thick styrene to build a 4 sided box around the master. Allow at least 1/2" all around the master, including above it. You may use any type of glue on the box that you would like, but be sure it does not leak as this is what will hold the RTV in place around the master as it cures. Hot glue would work well for this and it is very quick. Use modelling clay or hot glue to seal this box in place around the master.
You are now ready to pour your mold. Mix your RTV following the directions that came with it. I can't emphasize this strongly enough. I am told by the experts that most problems stem from not thoroughly understanding the directions for the products used. When you pour the RTV begin pouring directly over your master and pour very slowly. You are trying to avoid air bubbles on the surface of the master as these will render your mold useless. You may want to use a disposable acid brush to paint a coat of the RTV on the master to ensure that there are no bubbles on the surface. I did not find this to be necessary, but, keep it in mind as a solution to the problem of bubbles.
I have found that no mold release agents are needed on the master. You may want to use it on the glass and the inside of the mold box to make removing the mold easier. I used a mold release made from petroleum jelly and mineral spirits. Mix a 1/8" diameter ball of Vaseline to a teaspoon of the paint thinner. Use a Q-Tip to apply this sparingly to the glass and inside of the mold box. Allow the RTV to cure for the time that the manufacturer specifies. When cured remove the mold box and remove the mold from the master. If you have trouble removing the mold from the master, soak it in warm water to break the white glue joint. Once the master is free from the glass bend and flex the mold as needed to free your master from the mold.
It is now time to pour a casting. Dust your mold cavity with Johnson's baby powder and blow off any excess. This will help prevent any air bubbles at the surface of the cast part. If you use Alumilite resin you may want to refridgerate it for about 20 minutes to slow the cure and give you a little more working time as this stuff "kicks off" fast. No matter what resin you choose READ, UNDERSTAND, AND FOLLOW THE DIRECTIONS. Using a disposable plastic mixing cup (available at your local hobbyshop) mix a batch of resin. Be sure you mix enough to fill your mold cavity. As soon as it is mixed pour it into the mold. Pour slowly and start pouring into the deepest part of the cavity. Fill the mold slightly above the surface and then slide your piece of glass over the surface of the mold. This will ensure a flat bottom to your cast part. It will cause some flash, but it will be minor and is easily removed. The resin will cure in a matter of minutes. When cured remove the cast part from the mold in the same way you removed your master. VIOLA, your first resin cast part.
HAVE FUN! - Bill Rittner