1:72 Scale Modelling Techniques
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. 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 one-piece mold. Any object with one flat side, i.e. with no detail on that side, will work. 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 decide to use GE 3110 RTV resin, 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.
Tools and Materials
- Alumilite resin
- CASTALL RTV
- GE 3110 RTV
- 4″ × 4″ Flat Glass
- 1 mm Sheet Styrene
- Modelling Clay
- Hot Glue
- Mineral Spirits
- Disposable Brush
- Baby Powder
The techniques described in this article may be used in conjunction with various resin casting materials available on the market today. The author recommends CASTALL RTV, GE 3110 RTV and Alumilite resin, but any similar products should produce good results as well. If you are new to resin casting, do yourself a favor and contact manufacturers directly to obtain their literature on the subject. Alumilite Corp has a small and very informative booklet on the subject, which will point beginners in the right direction.Alumilite resin
315 E. North St.
Kalamazoo, MI 49007
1-800-447-9344 Toll Free
Company representatives were very friendly and helpful to the author.
CASTALL RTV is an industrial product which you probably will not be able to purchase. 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 the product in about 1-2 hours. I am also told that this RTV does not need to be de-aired. Though I have not experimented with it yet, this is what I will try when my CASTALL runs out.
Preparing the Mold Box
I used a 4″ × 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 in the center of the glass with its flat side down. Now build a «mold box» around the master. I use pieces of .040″ (1mm) thick styrene to build a box around the master. Allow at least ½″ of space all around the master, including above it. You may use any type of glue on the box that you would like, but be sure that the mold does not leak, the RTV must be held 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 the box in place around the master. Reader suggestion: Dave Wachter recommends using LEGO blocks to build mold boxes of any shape or size. The LEGO blocks seal perfectly, and they can be used again and again.
Pouring the Mold
Mix the 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.
Using 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 which resin you choose, it is very important to read, understand and follow the manufacturer’s 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 a 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 this 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.