MAKING AN RTV RUBBER MOLD

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MAKING A 2-PART RTV RUBBER MOLD - CLAY METHOD
This page is a pictorial about making a 2-part room temperature vulcanizing (RTV) silicone rubber mold using modeling/non-drying clay. The mold kit I am using is MicroMark's 1:1 4-Hour Cure kit (you can order this kit along with the other materials described here directly from MicroMark: http://www.micromark.com/). I have also used other brands of RTV silicone rubber, including Smooth-On's Oomoo 30, which is similar to MicroMark's kit but is a little less costly, and Alumilite's 10:1 4-hour cure kit, a little less easy to use, and a bit more expensive.

PREPARATION: The first step is to gather all the materials necessary for this project. That includes the rubber mold mix, a good mold release, a mold box (here made out of plastic building bricks), non-drying (modeling) clay, rubber gloves, mixing cups, stirring sticks, rags/paper towels, and of course, the original or "pattern" I will be replicating. In this case, it is a pair of plastic resin/Super Sculpey headsculpts I will be replicating for one of my projects.

STEP ONE: I determined the best way to divide the mold (for a 2-part mold) to minimize the cast lines and undercuts that would trap air bubbles.

STEP TWO: Next, I had to build a mold box that can collapse and be set up again later. Many people recommend regular plastic styrene sheets cut to the right size that you can keep together with tape. But if you have them handy, LEGO® bricks work well, too, since they're made to be taken apart and reassembled and make very tight joints. Here I am using clone bricks so the joints are not so tight but still sufficient given the viscosity of the mold mix. I made the box large enough to accommodate the patterns with at least a quarter -inch clearance on all sides. Then I covered one half of the patterns with modeling clay, building it up to the brick walls, being careful that no gaps are between the pattern and the clay.

STEP THREE: After I got the patterns into the clay and positioned loosely in the box, I made peg holes in the clay that will let me align the 2 parts of the mold and lock them to each other when I'm ready to cast.

STEP FOUR: I completed building the mold box by building the walls up at least a quarter in above the highest point of the angled pattern, in both cases, the chin.

STEP FIVE: The next step is to mix the RTV compound. WARNING! Wear gloves for this step, because uncured rubber is toxic to bare skin and can give you a chemical burn. First, I made sure that the 2 parts, A and B, were thoroughly mixed before I poured equal parts of each into its own measuring cup. Then, I poured both into a larger container and mixed those together very thoroughly. I used my Dremel moto-tool with its Flex Shaft and a mixing paddle made from an old coat hanger to mix all the compounds at the lowest possible speed through this step. The compound stays active and workable for about 30 minutes, so there's no rush to pour. Once mixed, I placed my mold box on a level surface, and started pouring the RTV rubber into one corner of the box in a thin stream. This is to minimize air bubbles. I let the compound slowly spread from this corner, until the patterns were covered and there was at least a quarter -inch clearance of compound over them. I set aside the mold box for 4 hours.

STEP SIX: After the first half of rubber cured, I dismantled the mold box and removed the clay from the new mold half. I cleaned the mold and the patterns with rubbing alcohol to make sure I removed all the clay. DON'T REMOVE THE PATTERN FROM THE RUBBER. I turned the mold over and reassembled the box around it, pattern side up. I brushed the rubber with a mold-release agent (even "Pam" cooking spray will work) paying special attention to the alignment pegs. It is very important to get every exposed rubber surface since if you don't, when you get to step 8, you will have to pull apart the 2 mold halves forcibly, possibly tearing and distorting the rubber. So be sure to take extra special care applying the mold release - it doesn't hurt to apply 2 coats, in fact.

STEP SEVEN: Repeat step 5 (mix and pour the compound). Set aside for 4 hours on a level surface.

STEP EIGHT: After the rubber has completely cured, I dismantled the mold box again and separated the 2 halves of my mold. If you did a good job with the mold-release in step 6, you shouldn't have a problem taking your new mold apart. I was able to remove the patterns by flexing the mold and removing the embedded pieces.

STEP NINE: You will need to cut two chutes or channels into your mold, one for pouring the casting material, the other as a vent to let air escape, which is especially important for undercut areas where air bubbles would otherwise be trapped. After I made these cuts using a hollow metal brush handle, I reassembled the 2 halves of my mold and for added protection, secured them together with rubber bands. Now I'm ready to cast with my new RTV silicone rubber mold!

NOTE: the first resin piece made with the new mold is rarely useable. This is normal for newly cured rubber molds. In fact, it might take 2 or 3 casts before the mold will produce a useable piece.