Mold Making & Casting, Part 1

A pictoral guide to setting up your mold,
pouring the rubber and cutting it open.

By Ro Annis

Introduction: In this series of articles I will go through the process, step by step, on how I make rubber molds which can be used to cast resin parts. I have broken the process down into bite sized nuggets. Parts one and two take you through the process using photographs and consequently be very light on theory. In part three I willl explain the logic as to how and why each thing was done the way it was. By seeing it first, then explaining what you saw, it will hopefully make more sense.

One other note of explanation: I am lucky enough to have access to many casting amenitys, such as various RTV’s, resins and pressure pots. I’m just going to go ahead and explain how I usually do it, understanding that most modelers do not have access to this kind of stuff. The theory, however, is still quite relavant to any kind of casting. I also intend to write a part 4, in which I will cast a part using no special tools (such as pressure pots).

Ok, lets pour some rubber!

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Illustration 1: Here are two typical parts (or masters) to cast. These are quarter inch scale Focke wolf wheels that Jim just has to have (apparently he has a basement full of Focke’s that need accurate wheels). So casting is the perfect option, as scratch building them individually is way, way, way too much work.

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Illustration 2: This is plastic stock that will become sprues for the parts to be cast (usually sprues are made out of rod or strip stock). The wheels have a delicate tread pattern, its important to attach the sprues in such a manner that the detail is not disrupted.

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Illustration 3: This is why I cut the sprues the way I did. They are connected between the tread pattern. When the sprue is removed, the tread pattern remains. Where to put your sprues is always an issue.

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Illustration 4: Here is the whole thing mounted to a small square of plastic. This will be hard to grasp at first, but everything is set up upside down. The bottom (the plastic) will become the top of the mold. The cone shaped thing is left over resin from an old pour, from the funnel. I use these as the “pour in” spot. The longer sections of flat sprue (on the bottoms of the wheels) will become the vents to let air escape. This will become clear, later on.

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Illustration 5: Here is everything boxed up in plastic sheet. Ready to pour.

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Illustration 6: This RTV 1000. The big gallon container is the silicone rubber, the smaller bottle is the catalyst. A system like this is around a hundred bucks, has a shelf life of about 8-10 months and will make at least a dozen small molds.

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Illustration 7: The rubber gets mixed in a cup 10:1 (ten parts silicone to one part catalyst). Mix well. Take your time and try not to stir in any air bubbles.

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Illustration 8: Pour rubber into mold.

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Illustration 9: Put mold in pressure pot. This is like the big pressure cooker your great aunt used to cook meat in. Basically, it will compress all the air inside making any air bubbles in your casting smaller (the trick is not to have any air bubbles in the first place….)

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Illustration 10: Put the lid on, clamp it down, turn on the air valve to 40 pounds of pressure…..let sit over night.

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Illustration 11: Remove mold (make sure it kicked first). Pull off the plastic.

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Illustration 12: Remember, the bottom is now the top. RTV 1000 is pale blue.

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Illustration 13: Pull out the exposed vents and “pour in”. Use your left hand and bend your knee. Balance a book on your head. Act like a chicken….

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Illustration 14: Time to cut this baby open. This is a “Slit” mold, which is a way to pour a two-part mold in one pour. Also, you don’t have to mess with clay.

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Illustration 15: Basically we are going to cut straight down to the part.

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Illustration 16: The cut is made like a Ruffles brand potato chip, not straight and flat. These ridges register the two mold halves together.

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Illustration17: Open it up as you go. Follow the sprues down to the part.

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Illustration 18: Cut half way down to the part then stop! The bottom of the mold acts as a hinge. You can open it up to get your casting out. It also holds it together (along with some rubber bands) to pour.

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Look at the time! See you in part two, where we pour some resin into this mold.