Moldmaking

Step Two — the waste casting
Pouring the waste casting

Congratulations! If you’ve gotten this far, you have a waste mold. But where’s the horse? He still needs to be created.

Gently clean your silicone blanket mold with mild soap, warm water and a soft brush to get rid of the oil and clay bits that have broken away from your original clay model. After it has dried thoroughly, give the inside a light spray of mold release, a commercial product available where you buy your silicone rubber. Piece the blanket mold back together with lots of wiggling and jiggling to align the seams properly, and click the mother mold back onto it. I use many rubber bands to hold the parts of the mother mold together tightly.

You need two-part resin to make a hard copy of your horse. Many artists in the industry use Smooth-On’s Smooth-Cast 300, or Smooth-Cast 305. Both are bright white, dense, fairly UV stable polyurethane resins that capture extremely fine detail, stand up to sanding and epoxy work well, and don’t shrink or expand over time.

Paper cups work well for mixing the resin (we save ours from the coffee shop). The two parts should be mixed well according to instructions. You’ll have to estimate the amount of resin to use the first time.

Since your original clay horse was mounted on a pipe, this can be used as the sprue. The sprue is the hole leading to the mold cavity. It’s kind of an umbilical cord attaching your nonexistent horse to the life-giving force of existence. Aren’t you glad you have a sprue?
In this cut-away diagram the negative space is white, silicone is blue, mother mold is pink, liquid resin is red, and the funnel (an important part of pouring resin!) is gray.
Planning ahead is always a good idea, and especially so when molding and casting. How are you going to prevent bubbles? Picture the above mold staying stationary, with resin pouring in from the sprue hole. Chances are, air will be trapped in the upper extremities because there is no way for it to escape.

There are two ways to eliminate major bubbles. One is rotational casting. It sounds scary, but this can be as simple as plugging the sprue hole and rotating the mold by hand for 20 minutes as the resin gels and sets inside. Rotocasting is great for making a hollow model that saves on material costs, and eliminates most structural bubbles by allowing resin to flow into all the voids.
Tom rotates Heather’s waste mold by hand for twenty long minutes. Again, wear gloves and a mask!! By the way, it’s great to have a brilliant dad who figures out how to do most of the nasty technical stuff!
Another method of avoiding bubbles is to vent your mold. This involves making long cuts (like with sharpened metal model airplane fuel line, or some such) from the high points where bubbles are likely to form up to the outside of the mold.
The venting system. Each model is so different it’s hard to generalize, but try to picture the highest points when the mold is in pouring position, and cut from there. You will have resin leakage to the outside, which can be a bear to block.
As most resins set, the chemical process that bonds A with B to make a hard substance generates a lot of heat. A good way to tell if your model is ready to be pulled out of the mold without stretching and weirding is to peel back a seam and feel the temperature of the casting. When it’s cool it’s safe to take out.

Whew, good grief. That was complicated and nasty! But if everything worked with everything else just perfectly (which is usually a total miracle) you should have something like this.
From the depths of the purple monster’s grasp emerges the first Highland Heather casting!

You can see the ends of the vents on her ears. Though we rotated the mold, we needed to vent her ears simply because they’re too small to allow proper resin flow.
This is truly an exciting moment. Even if your first casting doesn’t come out perfectly, it’s still just so cool to see your own work in hard white resin! Take a moment to revel.
Making the resin master

If you’re not satisfied with the detail or perhaps other points of your model, you may want to clean up or even change the casting with sanding, scraping, epoxying, etcetera. It’s a good idea to make more than one casting from the waste mold so you’ll have some options.
Sometimes truly horrrible things can happen to the waste casting. Here Ravenhill tries out life as a dachshund.
And here he gives his back a much-needed stretch after being trapped in that darned mold all summer!
This waste casting has been cleaned up with hours of epoxy work and sanding. Now she is ready for the production mold.

Oh no, another mold?
Yes, another mold is in her future! Go on to the Production Mold if you dare!

Or, just go home...