Resin Casting for Dummies! by Matt Bishop

Resin casting is not an art, although a few guys take it to such levels. With trial and error any hobbyist, with a little patience, can cast small parts and even bodies at home. Casting complete bodies was just an experiment for me. I wanted to see if I could do it! I did do it and couldn't believe how good the results were. I will attempt to explain in the following directions, the steps I go thru in casting a simple body and trim parts.

The body used in the example is the Strombecker 1/24 Dirt Track Racer. It's one of my favorite cars to build. The first step is to take the body you will use to make your mold and fill the inside cavity with modeling clay. Also fill all holes in the body so the mold rubber does not run into the underside of the body. A small dimple may be left, so that the holes can be opened up in the proper location after you pull your resin body from the mold. I like to leave enough clay under the body to raise it about 1/4". I use regular modeling clay and have had no problems, but Clean clay for this purpose is sold thru suppliers and they recommend it. Another important step in preparing the master is to use a screwdriver blade or other such flat surface to scrape a 1/8" relief in the clay along all the lower body surfaces. This leaves a definitive lower body edge and wheel well openings in your finished mold. Always keep in mind that this body will be covered with silicone and the silicone will flow to all open areas, such as the relief around the lower edge.

You should have a body filled with clay at this point and enough on the bottom to raise the body about 1/4" . You need to make a suitable mold box next. This can be a convenient plastic box, cardboard box or just cardboard taped together. Allow 1/2"-1" around the edges and make the box at least 1/2" higher than the body. Be sure the inside is smooth, with no place for the silicone to get trapped. Also be sure there are no holes in the box.. You will put your upright body in the mold box and fasten it down. This doesn't take much. I usually run a screw thru the bottom of a cardboard box and into the clay. This holds the body down sufficiently. At this point the body is in the box in an upright position, about 1/4" from the bottom. You should now be ready to pour the silicone. Mix according to the directions, and don't scrimp. this is not the time to try to save $4 worth of rubber. Be sure you have enough silicone to completely cover your master. Slowly pour around the edges, allowing it to flow slowly under the relief you cut in the clay along the lower body edges. Keep pouring until you have the body covered by at least 1/2". Note that most molding silicone de-airs as it cures and covering your part with silicone, as opposed to pushing your part in silicone, leaves the air bubbles an easy escape path. When the silicone is cured, 2-12 hours or more, depending on what kind you use, you can take the screw out of the bottom and cut away your mold box.

You will be left with a big piece of silicone rubber. When you turn it over, you should see the clay that filled the bottom of the body. gently push the rubber away from the sides of the body. After you loosen it up all the way around, you should be able to slowly work your master out of the mold. You will be left with the silicone mold for making new bodies. It doesn't hurt to oil the inside of the mold very lightly. Spray cooking oil is good for this purpose. If you use oil, spread it around with your finger, don't spray a lot in the body. I don't use oil on body molds. This may effect the life of the mold, but I haven't had any trouble. For wheel inserts and other pieces with holes in them, the oil does make the mold last longer and parts are removed easier.

You've got the finished mold, it's time to make a body. Mix resin as per directions. No more than a teaspoon of the two parts. Mix thoroughly, this is important. Pour it into the body mold and slowly tilt the mold so the resin flows into all areas and covers the whole inside of the mold. You have a nice border around you lower body edges, so there should be no problem with the resin staying in the mold when you tilt it. After you coat the whole inside, you can turn the mold up and let excess resin run out. If you do this, you will need to repeat the above procedure at least 4-5 times. I opt to cover the whole inside of the mold and then lay the mold on one body side. This lets the excess resin run into that side and form thick sidewalls to the body. I repeat this step after the resin dries and put the body on its' opposite side. If the front or back of the body look too thin, I pour more resin in and after coating the whole inside, I stand the mold on end so the end of the body has additional thickness. You get the picture, pour resin several times. Allow to dry between pourings and lay the mold on it's different sides to make all sides of the body thick. Experiment, trial and error. After the resin completely cures, read the directions for de-molding time, you can slowly pull the mold away from the side of the body. until the body is loose in the mold. Pull the body out of the mold by the angle of least resistance. You should have a usable body, now.

For a lot of trim and small pieces, a simple push mold works very good. Although not technically a push mold, this method is about the same thing, it just allows the air to not be trapped in the mold. Take a small box and figure out which side of your small parts can be unfinished or flat. Glue the original parts in the bottom of the mold box. Glue lightly, with the flat or unfinished side down. Pour silicone over these small part and let it cure. After you let it cure use an Xacto or other small tool to loosen the edges of the silicone in the mold box. You should be able to work it loose and gently and pull the rubber mold out of the box. The mold you pull out should look like the one shown on the right. This is my mold for a Monogram driver, exhaust, fuel injector, steering wheel and other parts. This type of mold is very good for a lot of small parts and trim.

If your part has no flat or back side, the original part can always be cut in half and copies made in halves that glue together when done. Experiment with ways to cast parts that look impossible to do. A third type of mold that works fine for a lot of parts is a split mold. For certain parts, a push mold as described above just won't work. Sometimes, you can take the part you wish to copy and completely submerge it in silicone. After curing, take an Exacto knife and slice one side of the mold open, to allow the original piece to be pulled from the silicone (see picture on the left). This is another driver mold. You now have a mold that you can flex open, but it will stay closed when you release it. You can always put a rubber band around it to hold it together, if it doesn't want to close all the way. If there tends to be air pockets in a mold like this, you can flex it open, pour the resin in and then close it to force the air out of the resin.

The picture on the right shows one of my driver molds and also a mold for fuel injector pipes. I had lots of trouble with air in the injector pipes. It is recommended that a straight pin be used to get the air out of such places. I found it much easier to cut the mold in half. leaving it attached only at the bottom. I just spread it open, pour in the resin and force it closed. All the air is expelled at that time. Obviously, you may need to do a little more finishing work on your resin parts, but you can do this at home and it's not rocket science. The pros use vacuum tanks to pull out all the air in their resin. The experienced guys also make nice inner bucks, so their bodies have nice uniform thickness. While this is great, and produces outstanding work, you can make parts yourself without all the hardware, training and extra equipment.

There are a lot of good rubbers and resins on the market. Alumilite is used by many and easily available. Micro Mark sells rubber and resin that is particularly easy to use. Both these companies sell on-line and over the phone. Needless to say, whatever you use, follow directions on mixing ratios and mix completely. For mixing silicone and resin, nothing beats paper bathroom cups and free plastic spoons from the fast food restaurants we all frequent!

I am indebted to Chris Etzel for advice. While his company and techniques go way beyond this, he gave me advice on the simple techniques that I used here. I freely admit that I am just a hobbyist and only do simple resin casting. I'm sure you can easily get results better than me if you just try!