New Project: DIY Casting

I’m taking a break from 28mm models to work on 15mm stuff once again. As All Quiet on the Martian Front has gained new life, I thought I’d dust off the models, but set the next Martian invasion in the 1960s while America was busy planning to fight on the nuclear battlefields of a Cold War gone hot. Part of the project is bringing US Army vehicles from the era into the AQMF rules and upgrading the Martian invaders. All that will be left for another post, but I wanted to include some of the bizarro weapons of the era like atomic tanks, specifically tanks like the TV-1 and the Chrysler TV-8. Unfortunately, no company currently produces anything like these models in 15mm. Since I’ve been playing with casting for my soap project, I decided to try building my own resin TV-8 tanks and silicone molds.

 

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1:48 scale Chrysler TV-8. The entire hull was mounted within a watertight turret, including crew, ammo, and engine. The tank was intended to swim thanks to a think outer hull trapping air around a hardened inner hull. The  tank was never produced but the space age appearance and alternate atomic engine has long since captured the imagination of many treadheads. 

 

Step 1: The Master

To being the project, I needed something to cast in the first place. I used Sculpey oven-hardened clay and two-part modeling epoxy to craft a basic egg shape. The TV-8 makes a great experiment for casting as the design had very few extreme details. The size is based on 15mm (1:100 scale) M60 Patton and M41 Walker-Bulldog tanks. The TV-8 was intended to be a medium tank so it would have been larger than the M41.

 

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The upper hull/turret master. 

Step 2: Making the Mold

Silicone casting materials can be expensive to experiment with, especially while perfecting your molds. I found several instructables for using caulking to make cheap molds. Necessary ingredients are 100% silicone caulk and dish-washing liquid. Glycerine in the soap acts as a catalyst for the silicone, greatly reducing curing time. Make sure to use only 100% silicone caulk, not fast-drying caulking or anything with additives. Also, I’ve used both white and clear caulking. I prefer white as it seems to be a little harder when it cures and doesn’t smell quite as hideous.

You should probably do this somewhere well ventilated. The silicone smells like rotten vinegar as it cures, and the smell will permeate whatever room the molds are in.

Making a mold

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I used a bath of dishwashing soap and cold water, pumping the caulking directly into the pot. The water prevents the caulking from sticking to the pot or your hands. I don’t have an exact recommendation for soap-to-silicone ratios. I use about 1/4 of a bottle of liquid soap to one tube of caulking. The caulking has a minimum setting temperature of 40 degrees F, so use colder water rather than hotter to give yourself time for working.

 

 

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The caulking comes out in a string. Swirl it around in the bath and let it sit for a couple minutes to be fully exposed to the glycerine. When it’s not sticky, knead it into a ball like bread dough. I should probably be wearing gloves…

 

 

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Keep kneading the ball until you’re ready to place it in your molding container. 

 

Step 3: Prep Your Mold

Before you start with the silicone and water, prep the container and master. I’m using a thin layer of petroleum jelly as mold release. It’s adequate, but be sure to use just a little so that there aren’t clumps or deposits in corners that may inhibit details in the mold. Some people recommend using Legos to build walls for their containers. Legos are expensive unless you buy them second-hand. For now I’m using small cardstock boxes and silicone molds I have for my soap-making project.

If you’re going to do a 2-part mold, be sure to all some sort of guide to help keep the halves aligned. On my original mold I used small channels cut into the side of the surface. That worked well enough, but this time I’m trying out spare dice to make much more prominent guides. Lay your silicone in layers to create the bottom of the mold, and place your master onto it. Gently push the master into the silicone, shaping the edges where you want the seam (on a 2-part mold).

 

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Blurry from trying to work the cellphone one-handed while covered in Vaseline and silicone. I’m using a silicone baking tray that will easily peel off of the mold once it has set. My fill point on this mold is from the bottom where the turret attaches to the tracks. I pushed that flush to the baking tray. When I pulled the mold out, that will become the top section. 

 

Step 4: Casting with Resin

I’ve tried casting several times before, all with little luck. This time I bought a simple quick-cure 2-part resin from Michael’s crafts, labeled simply Amazing Quick Cure Resin. No MSDS or chemical content on the packaging. This kit has a 10-minute cure time, which is very nice as I was impatient to see if the mold had been successful or not.

After pouring I carefully but vigorously shook each mold. I swirled the 2-part turret mold to make sure the resin flowed into every crevice, and lightly pounded it on the table to remove air bubbles. Careful pour can mitigate most bubbles, but you really will want to shake or pound for a minute to get as many out as possible. You can also use a pin to pop visible bubbles.

 

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The original 2-part turret mold. The register channel helped to keep both halves aligned and prevented resin from leaking. I also used rubber bands to keep the sides secure while spinning and pounding. 
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Final product. The resin cast looks much like the original. It will need light sanding and finishing, but overall it was a success. 

Final Thoughts

Well, for $5 per tube of caulking silicone, it’s hard to go wrong. I cast several small pieces in the 15mm scale as one-piece molds as well as my TV-8 turret. The small pieces did not hold details, likely because caulking silicone is too thick while pouring to really get into those areas. A liquid pour-silicone would probably offer greater success. Latex may also work, however, avoid using petroleum products with latex.

Wear gloves or your hands will smell like horrid vinegar. Be careful cleaning up– dried silicone peels off flat surfaces very easily but you don’t want to pour any down the drain or you may end up with plumbers’ bills.

As for finished product, I need to finish building the TV-8 tracks and make a mold. I’ll post the project as it goes to show how the final piece looks after it’s painted.

 

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