Moldmaking

Step Three — the production mold
Your waste casting is all refined and sanded and epoxied and beautiful. The waste mold served its purpose, but you don’t want to do another complicated, nasty blanket mold! Who can blame you? I don’t. Here’s the solution, which is equally complicated to make, but works much better for producing multiple castings.

What is a block mold?

This is the type of mold we used for making hordes of Heathers.

Rather than brushing liquid rubber onto the model, which results in a thin blanket of rubber, the model is placed in a container and rubber is poured onto it. This is usually done in two separate “pours.” Some models require three or more pours due to their complexity, but for our purposes I’ll just give a demo of the two-part.

This method simplifies the mold, which reduces stress when extracting the model. The rubber is less apt to tear or get stretched out of shape, and the model is less apt to bend or snap legs. A block type mold has more mass (like a solid brick; hence the name “block”), and tends to hold its shape well. A mother mold is still used, but it plays somewhat less of a vital part than with the thinner, more flexible blanket mold.
I’ll use this photo again. This is a block-type production mold. It is much thicker than the blanket mold. One half is on the table, and Dad is holding the other. You can see how easily the two halves can come apart, and go together. Our mother mold is simple and encompassing. If it looks like we have used two different silicones, you’re right. The pale, colorless rubber is softer than the blue rubber. We thought it would release the model more easily, but found that it wasn’t necessary. Our later molds were done with one kind of rubber, the harder blue silicone.
Claying up

This is the reason you can’t make a two-piece production mold from your soft, squishy original. (Unless you’re really good.)

First, it is important to visualize seam lines on your model. This is difficult! My suggestion is to study a resin in the same general pose as your model and examine it for seam lines. Where do they go? Why do they go where they go? Heather, because of her trotting pose, is a particularly good candidate for a two-part mold. She is relatively linear and flat, and all four legs are exposed independently of one another. I know that sounds extremely weird, but try to picture it. This is just one of those things that requires experience to understand. You may need to experiment a bit to get a feel for it.

Okay, let’s try to dissect what’s going on here. Examine the below photo.

Seam lines have been planned and drawn onto the cleaned-up Heather waste casting, and now we are starting to apply a clay table all around her, following those seam lines very carefully. The clay, when possible, is kept at a ninety-degree angle to the model.

The marble and wires are part of a venting system for her feet. When this is all flipped into negative space, the wires will be hollow, and will allow resin to flow through to the marble. This will keep structural bubbles out of her extremities. The idea behind using the marble is to keep the vents inside the mold, and eliminate that annoying resin leakage to the outside. This keeps the whole production simpler and cleaner.

This procedure must be done precisely, and kept very clean and smooth (no little nobules) to ensure good seams and mold locking. Although there will be two pours of rubber, the claying-up only needs to be done once.
The start of the claying-up stage.
When the claying-up is fairly advanced, the next step is to make a container for the liquid rubber. We made a cardboard sleeve for Heather. A larger model would require something heftier, such as a wooden box.

The clay is built up and extended to meet the cardboard, and a good seal is made.
The working side of the clay. The rubber will be poured onto this surface, and contained by the cardboard. The bottom side. Try to keep this as a clean mass as well, for structural integrity. Be sure to support the model and clay from below! Don’t trust your little clay seal to hold up the entire lake of silicone that will soon be poured onto it.
The first pour

Your model is clayed-up and ready to go! Now comes the silicone.

Again, apply a release agent to the model, clay and container. Just like the blanket mold, it’s important to have a detail coat first. Mix up a small batch of rubber and paint it thinly onto the model and clay, trying to eliminate all bubbles.
Dad mixes silicone for Heather. We use a small Ohaus scale that reads in grams. This particular type of Smooth-On silicone has an unequal one-to-ten mix, with the A being the body of the rubber, and the B a very fluid colorant and catalyst.

In the background is our vacuum chamber, which is a good investment if you plan to do this a lot.

Also shown are our professional mixing cups (we save our drink cups when we eat out), professional stirring implements (a popsicle stick and a chopstick), paper towels, and a stiff-bristled homemade brush for the detail coat.
A batch of silicone in the vacuum chamber. Weird! It bubbles and foams like a living sponge alien as the air is sucked from it. This process eliminates most or all of the bubbles within the silicone itself. It will boil over out of the cup unless the cup is at least triple the size of your batch.

The chamber is simply an aluminum cooking pot (with no dents!), a thick Plexiglas lid with a homemade rubber seal, and a tube leading to the vacuum pump.
The third installment of the “first pour.” You really only need two (the detail and the body). Let the first pour (the detail pour) set, then pour more rubber till the mold is full to the top. You don’t need to let the rubber set between these final pours.
The second pour

Once your silicone has set (it’s good to wait 24 hours), carefully turn the whole mold, horse, clay and container over and thoroughly and gently remove the clay. You should have something like this shown below.
A close-up. You can see that Dad didn’t follow the drawn-on seam lines exactly. It’s hard to pre-visualize, and we had to change our plans slightly as we went along. It is very important to keep the clay sealed to the model. Handle your in-progress mold very gently!
This is kind of what you will end up with. This mold half does not have a container because this was a disaster (due to not supporting the model well enough when we poured). Picture spraying this with release, and pouring again. Simple, huh?

Apply a release agent again, to prevent the silicone from bonding to the rubber that is already there. I don’t usually trust a mere release agent (it would be a bad thing if they bonded to each other!) so I mix up a batch of Vaseline and mineral spirits to spray on as well. The mineral spirits will evaporate and leave the vaseline as a very thin physical barrier between mold halves.

Wait another 24 hours for this side to set, apply a mother mold (see photo at the top of this page), and voila! You now have a production mold. Time to take it for a test drive.
Onward to Production Casting!

Or wimp out and go home