Silicone Molding and Resin Casting

This is a little tutorial for resin casting. There are two parts for resin casting: making the mold and making the casts. The first step is to make the molds. I like to use legos to make the mold containers because they’re plastic and I can basically build a box of any shape and size. The master piece is measured and a box is built around the master. The master is then placed inside the box.

The material I’m using is RTV silicone rubber and comes in two parts, the silicone base and the activator. The base and activator is to be mixed by weight, at a ratio of 10 parts base to 1 part activator. Your standard digital kitchen scale is very handy here when measuring. Using some disposable cups, the RTV is mixed.

Once thoroughly mixed, the silicone is poured into the box. I have small bits of cut up old silicone molds that I use as filler, this saves on the amount of raw silicone I have to use. So as I pour, I fill with the bits of cured rubber. The rubber bonds very well with itself so it works as the perfect filler. Once the silicone is poured, you will notice little bubbles slowly surfacing. This is the escaping air that is trapped when the silicone is poured. Now if all these bubbles don’t escape, your mold will have small bubbles. To counter this effect, I have a pressure pot that is powered by my airbrush compressor. The mold is placed into the pressure pot chamber and sealed. The compressor is turned on and a constant pressure of 40 psi is filled. This is enough to force all the air out of the mold creating the most bubble free molds.

A pressure pot is not absolutely necessary, as you can get fairly decent molds without one. But for the cleanest and most bubble free molds, the pressure pot facilitates this. The mold is left in the pot over night to cure. The normal cure times for this particular RTV is about 6-8 hours. And this will vary depending on relative humidity and temperature.

A few days later, the mold is ready to be removed. However, after checking on the mold and removing it from the lego casing, the rubber didn’t feel completely cured. To lower the risk of ruining the mold, the mold is left to sit for a few days so that it can fully set up and cure.

After about a week, the mold has completely set up. The mold may not have completely set up due to a problem when mixing the silicone. If not enough activator is added, the mold will cure too slowly or not cure at all, so it is very important to measure out the correct mixture ratios. The mold is solid and we can now un-mold the clay master. Using a hobby knife, the mold is cut carefully down. Once the clay master is exposed, carefully cut around the master while slowly releasing the master from the mold. I don’t cut all the way through the mold creating a clam shell mold. Once the master is removed, the mold is cleaned up with an old toothbrush to remove any clay that may have been left from the demolding process.

The mold is then sprayed with some mold release that will help preserve the life of the mold as well as help in resin removal from the mold. The mold release is heat activated. So a hair dryer is used to heat up the mold, activating the mold release. With the molds ready the resin casting can begin.

The resin is two part urethane resin. They are mixed at a 1 to 1 ratio by weight, so using the kitchen scale and a disposable cup, the two parts are mixed. The two resin components react exothermically, and learning from an experience when casting in the middle of winter – it is a good idea to preheat the resin components in a hot water bath before mixing. I had once ended up with a mixture that never fully cured because the base parts were too cold when mixed. The reaction is fairly quick, so you need to work quickly after mixing the two ingredients and pour it into the silicone molds.

Once poured into the molds, the molds are quickly placed into the pressure pot. Again, the pressure pot is an optional piece of equipment, but to reduce the amount of bubbles that may form, the pressure pot is employed. The resin will cure very quickly, within minutes. A full cure will occur after about 15-20 minutes. I usually allow the pot to sit for about an hour before de-molding.

After about an hour, the part is de-molded. Since I have extra resin, I poured some into another mold, and removing that mold, the resin didn’t fully fill the mold, this happens. I can just mix more resin and pour another mold and continue until I get the perfect replicated part. The casted resin head came out nice. In hindsight, I could have marked off where the original master fit and cut the mold so that the mold lines are easily removed. But these castings are pretty clean. I can cast an unlimited number of these parts now.

Products and where to buy

I have picked up the silicone RTV rubber from various locations. I have seen aluminite at local arts and crafts stores as well as brookhurst; however, since these products do expire, I usually like to hit up my local plastic retailer: Plastic Depot located in Torrance. They also sell various resins, but the resin product I have been using is Silwhite from; and this company is located in Pomona where folks local to Southern California can pick up the products. Baring that, I would search your local plastics dealer and inquire about silicone RTV and urethane resin products.

Pressure Pot

THe pressure pot I picked up at my local harbour freight needed some modifications before being used as a pressure pot for resin casting. The first picture is the complete setup; there are a few things that need to be attached to the out of the box pressure pot. Several fittings need to be added to plug up the various ends of the pot and an optional modification will help make the pot easier to use for casting larger items.

Starting with the optional modification. The pot has a metal tube that runs down the pot for extracting paint. I cut this tube out with a pipe cutting tool. Since the tube runs down the pot, removing it allows for casting of bigger items. The pot can be used with the tube in place, but in my uses, I have found that the tube just gets in the way.

Next I plugged up the exterior end of where the metal tube is attached. This is the end where a hose attachment that leads to a paint gun is attached for spraying paint. So this end needs to be closed up. The connection end is a 3/8″ pneumatic male connection. Since I wanted to use a drain valve that is a 1/4″ fitting, I needed to get an adapter fitting from 3/8″ to 1/4″. From there I have a coupler for which the drain valve is attached.

Next up, is the regulator/gauge setup. I plugged both ends with quick connect female ends. These are both 1/4″ fittings. I can then connect from the compressor to one of these ends, and with everything plugged up tightly, I can start pressure casting.