Learning the Art of Molding and Casting

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As you might have seen on my Facebook page, or over at the Minimate Multiverse, I am working on molding and casting a series of Minimate-sized dioramas. The first one is pictured above, and I will have at least 4 more dioramas ready to order in the next two weeks. This is a project I have had in the works for a few months, and most of that time was spent ironing out all of the kinks in the molding and casting process. I wanted to share part of that process with you today.

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Each diorama started with sculpting the parts I was going to mold and cast. The piece I am focusing on today is a claw from the Training Room Diorama #2 that will debut next week. This set will have two mechanical claws reaching out to trap your heroes as they learn to work as a team. I sculpted this piece using Super Sculpey, and gave it a quick paint job to make I didn’t miss any large flaws.

Once you have the item you want to mold, you will need liquid silicone rubber, a mold release agent, and a box to pour the mold into.

I learned along the way that you need to think about how you are going to mold and cast the piece before you even start sculpting it. Think of it as pouring batter into a cupcake pan. If the pan is shaped like a 3D torus knot, there is no way you are getting it out of the pan. I had several pieces fail because I made them too complex. These shapes might work in an industrial factory, but not a small one-man operation.

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The next step was to build the box that would contain the liquid rubber for the mold. I used legos because once the mold has cured, I can simply remove the legos brick by brick and then reuse them again in another mold. I used mailing tape around the edges to keep the liquid rubber from spilling out underneath it.

I tried using a brush-on rubber mold solution, but it didn’t work well for irregular shaped pieces that had to stay level when I made casts of them. After doing some research online, I settled on the pour-on liquid rubber offered by Smooth-On. I picked up a small quantity of this at my hobby shop to start with.

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A one-part mold simply has you pour the liquid rubber on top the piece, and then when the piece is removed, you have the large opening to pour the liquid plastic in to. A two-part mold is kind of like a waffle maker, with the liquid plastic being contained on all sides. This claw item is a one-part mold.

One of the first things I learned was that you need to spend at least 2 full minutes mixing the liquid rubber, and you need to pour it between two different containers, and use two different stirring sticks, or else it won’t set correctly. I also learned that some of the pieces I was trying to mold had a tendency to float in the solution, and generally at an odd angle, which was not going to work. So I starting gluing the pieces to the base, and then scraping the glue off my original sculpt later after I had removed it from the mold.

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The rubber takes 18 hours to set, and then the piece can be removed, leaving you with a mold that can be used over and over again, as long as you take good care of it. To cast pieces in this mold, you need liquid resin or epoxy, a mold release agent, and optionally some colored pigment.

I made  a simple mistake early on, and it cost me a lot of money in supplies, and a lot of time in ruined molds. You need to treat your mold with a release agent in order to be able to remove your cast from it. Apparently there is a world of difference between “Rubber Mold Release” and “Universal Rubber Mold Release”, and the nice man at my hobby shop didn’t know there was a difference. I had many molds rip and become unusable after making only two or three casts in them, because I was using the wrong release agent. I got very frustrated, and finally called the manufacturer, who set me straight.

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Now that I was using the correct release agent, I could pour eight casts at once, demold them, and prepare the molds for a new cast, all within 25 minutes. Pictured above is the first cast of the claw. I didn’t use quite enough black pigment in the mix, so it didn’t get as gray as I wanted. But the shape is a perfect replica of my original sculpt.

Taking the cast out of the mold does require some patience and a steady hand, as you can still rip the mold even when you are using the correct release agent. But this is also related to how you sculpt it. The Broken Wall Diorama, with all of its cracks and holes, is a lot harder to remove from the mold than the smooth claw pictured above. After several tries I have started to learn what will work and what will be a disaster.

I am looking forward to sharing pictures of the entire first series of Mini-Figure Dioramas next week, and I am starting to plan out pieces will be in the second series, which will be released in March of next year. After the new year I will also start working on a molding and casting tutorial, so you can make your own parts for customizing!

I hope you enjoyed this peek into my molding and casting projects!

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3 Responses

  1. Jcastick says:

    That was a great article, a lot of good tips in there. I tried my first mold with liquid latex not too long ago and it was a major failure.
    I want to try this stuff from Smooth-On and see what happens.
    Thanks!

  2. Bondjunkie says:

    where are those zombie decals? Have you given up?

  1. November 28, 2009

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