Preparing a new plaster mold (Part 1)

Having just finished a new plaster production mold, I thought it might be helpful to see how a new mold requires adjustments to work well. Production molds are made from the rubber master, but like anything handmade, each one is a little bit different. If I’ve learned anything in the last few years, it’s that there are no perfect molds and each comes with its own quirks.

Unfortunately, you don’t really know what you are getting until the mold has dried, which can take weeks. In the picture above, I’ve opened the new mold (for the drafter Finn) for the first time and am dusting it with a soft brush to remove any dust or dirt.

At this point I check the mold for obvious flaws like these bubbles in the mane. Textured areas like manes, tails and feathering are prime locations for these kinds of problems.

I use a small hook tool to remove and smooth casting bubbles. This particular mold only has the two bubbles. I’ve found that I rarely get bubbles with new rubber masters, but after a number of production molds the masters tend to pit. This pitting can result in bubbles in the production mold. Fortunately the problem is easy enough to fix.

Once this is done I am ready to pour the first test casting. This casting is not usable, since there is usually a bit of mold release still in the design cavity. It’s sole purpose is to check the mold.

Here I have poured slip into the closed mold. Those straps holding the mold together are heavy-duty rubber bands. I need to keep it constantly full (like the top picture) because the slip in the pour hole creates the pressure the pushes the liquid up into the horse’s legs. (Remember that the pour hole enters the belly, so the horse is being cast legs-up.) One of the hardest lessons to learn about casting is that this is not the time to multitask. Do not go do anything else! You will lose track of time and find the pour hole drained and (later) a horse without toes. Not fun.

The slip sits in the mold for a while, forming a skin. When I started casting, I wanted hard rules about how long the slip should sit. What was frustrating is that there were no rules. It depends on the mold, and the thickness of the slip, and the local humidity, and other mysterious factors I just haven’t figured out yet. The only way to know it’s time to drain the mold is by spilling a bit of slip out, and testing the thickness of the skin with a plastic mold trimming knife.

When the skin is the right thickness, the mold is turned upside down to drain. At this point you should have a hollow pony inside the mold – sort of like the model horse version of a chocolate Easter Bunny. (I have always wanted to put those yellow candy eyes on one…)

Here the mold is sitting upside-down on two plaster pedestals to finish draining. (The pedestals were made by pouring excess plaster into the bottoms of some plastic cups.)

Now there is more unpredictable waiting. The time it takes to get the casting to the right ‘leatherhard’ consistency is dependent on many of the same factors as the draining. Unlike the wait for refilling the mold, this is actually a really good time to multitask. Pick loooong chores. That’s because no matter how soon you think the casting will be ready to demold, it is probably not ready. I hate to admit how many castings I have lost to my own impatience.

Molds are almost always designed to have a specific piece lift first. For Finn, the piece that I am holding is the first. I’ve taken off the rubber bands and removed the pour spout with the mold trimming knife (the blade is there just to the right), and am testing to see if it is ready to pull. When the horse inside is dry enough, the mold piece will pull, though there is often a soft “pop” as the suction is broken. If the side does not separate (at least a crack), the casting is still too soft.

It won’t always pull completely free, though, at least not at this point. That’s part of what this first casting is checking. There are almost always small areas along the seams where the plaster wants to grab the clay, and that will often pin the mold piece in place. The trick is to wait until the mold will crack open easily, but won’t pull further. By carefully forcing the piece off, the clay will give and stick to the areas that need adjusting.

Here is that first piece with the tell-tale clay on the outer edge of the tail. Left as it is now, the mold will always want to hang there. When flaws like this happen in the wrong place, the demolding process can tear the casting apart. That’s not likely in the case of the tail end, but leaving it would mean that part of the tail will have to be reconstructed each time since it would get scraped off with each demold.

It’s better to just trim the excess flash from the mold so the tail pulls freely. Imperfections like this one are easy enough to fix.

But not all problems can be fixed. There is a chip in the piece along the shoulder (marked in the previous picture) that happened when the shoulder insert was broken from from the side piece. Fractures like that often happen where the sides butt up against a hand-formed piece. There isn’t really a way to fix it, so each casting will have a larger-than-usual seam along that shoulder that will need to be trimmed. Fortunately trimming seams is pretty easy with greenware; far easier than with resin castings.

So now the casting is visible and I am ready to start removing the rest of the mold pieces. Like the first side piece, these are usually designed to come off in a specific order. I’ll pick up there with my next post, starting with that inset shoulder piece that caused such problems when it separated.

, ,

No comments yet.

Leave a Reply