Molding and Casting: Part 1

Want to make copies of your sculpted creations? Molding and casting might be the answer! Basically you make a rubber mold of your part, and then make as many resin copies as you want. Here in part 1 of this series we will get through the steps and materials required to make the rubber mold. Part 2 will cover the process of casting your part in resin, including how to color the resin.

I tried a lot of different products, and the only ones that ever worked well for me are Mold Star 15 for making the mold, Smooth Cast 325 for the resin, and Mann Ease Release for spraying the molds each time. They are all made and distributed by smooth-on.com. These materials are not cheap – a small size of all of the materials above together will run you about $80. If you base it on the size of the object in this post, that $80 will get you about 6-8 rubber molds and 20-25 casts. If you really get into molding and casting there are larger sizes available, which makes it cheaper per piece.

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The object that I molded for this post is the Webbed-up Minimate Base that was formerly a bonus item with our Deadly Foes of Spider-Man Minimates. The first step is to build an enclosure around the part, in order to hold the rubber while it is in a liquid state. The easiest way to build an enclosure is with Legos, but I have seen people make them out of wood and other materials as well. The important thing is that thee enclosure can’t leak. I will be using Legos for this project. Start by selecting the proper size base. Remember that you will need room all around your part for Lego bricks, as well as room for the rubber to flow around the sides of your part.

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Next, build up the sides with bricks. Overlap the bricks to ensure that it will be strong and leak proof. Make the enclosure at least 1/4″ taller than the top of your part.

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Now it is time to mix the rubber. You will need two plastic cups, two popsicle sticks (or another disposable stir stick that is fairly strong), and the Mann Ease Release spray mentioned earlier. The cups should be large enough to hold the entire volume of the inside of the enclosure. Mold Star 15 mixes by volume, so if you want to measure exactly how much you will need, first pour water into your enclosure up to the top. Pour half of this water into each cup. You will need this much, plus extra, since the rubber is thick and will stick to the sides of the cups. Dry the enclosure and the cups with a towel, since you don’t want any water in the mixture. Put your part back into the enclosure, and give everything inside a quick spray with the Mann Ease Release.

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Now pour the correct amount of Part A and Part B of Mold Star 15 into each cup. Make sure the amounts are exactly the same. It might help to mark inside the cup with a sharpie before you pour it in. Each part has to be agitated before mixing, so put a popsicle stick into each mixture and stir each one separately for 20 seconds or so, making sure to scrape the sides and bottom.

Next it will be time to combine the mixtures. This mixture cures in 30 minutes, and you will only have about 6 minutes to work with it before it starts to thicken. So before you start this process make sure your part is ready, and that there are no distractions around you for the next 10 minutes or so. Use the popsicle stick to scrape the entire contents of one plastic cup into the other. Now begin stirring, once again making sure to scrape the sides and bottom very well. Stir for at least 2 minutes, until the color of the mixture is completely even.

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Begin by pouring the mixture around the base of your part, in order to hold it in place. If you part starts to drift, center it again with your stick. Next, pour the mixture over the top of your part. This mixture will automatically expel any air bubbles, as long as your part isn’t too complex. Continue pouring until your part is completely covered by at least 1/4″ over the top of it. Check to make sure the base isn’t leaking anywhere. If it is, you might have to build a quick support wall, or put a piece of tape over the location. If some has leaked out, pour more in the top. The rubber will thicken within a few minutes, so you won’t have to worry about this for long.

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The mold will need 30 minutes to cure. Then  you can begin pulling off Lego pieces until the mold is completely free. Flip it upside down, and cut a hole around the base with a hobby knife. Use your fingers to pull and stretch the rubber away from the part – this base will be able to withstand quite a bit of stretching. Then you should be able to pull your part out.

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What you will be left with is a rubber mold with a hole in the top for pouring. This mold will be able to make 50-60 copies before it starts to break down. Keep it out of direct sunlight and keep it at room temperature.

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This item is relatively simply, so it only needed a one part mold. This means that the part has a flat base with no overhanging parts, and I can just pour in the liquid resin in the hole and easily pull out the final product. The place where the resin settles at the top will be the flat base. If you have a part with no base, or a complex part, you will need to make a 2 part mold. This means that the rubber mold is in 2 pieces, as show by this diagram:

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When you make a 2-part mold, you will have to suspend the item in the middle of the enclosure, as if it were floating. I usually use a long lego piece to do this. The lego piece would act as the pour hole when the mold is done. After curing, you cut the entire mold in half. Each time you cast a part, you hold it together with rubber bands, and then take it apart separately once the part has cured. If a 2-part mold is something that you would like a full tutorial on, please let me know.

Stay tuned for Part 2, where I will cover casting the part in resin!

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5 Responses

  1. fj says:

    Excellent, now I don’t have to annoy you with asking you about molding making every few months!

    -fj

  2. Lobsterman says:

    Have there been specific conditions with which you’ve had luck degassing the mold compound? I haven’t tried that brand; does it bubble up a lot or degas itself pretty well?

  3. Mnemosis says:

    I just de gassed MYself!

    Very helpful tutorial, sir!

  1. June 20, 2015

    […] we last left off in Molding and Casting Part 1, we had a one-part rubber mold that was ready for pouring casts. Here in Part 2, it’s time to […]