Resin Casting: One-piece cut molds

One-piece silicone cut molds are a relatively easy way to cast miniatures in resin.

Six years ago we ventured into the world of resin casting to make copies of some of our conversions. It took a lot of trial and error to get something that worked consistently, but we found making a pressure pot was essential. All of the molds we created over the years were two piece molds, which are quite labor intensive. After watching a few YouTube videos by Miscast, where he talks about some of the imaginative models he converted and cast, I started to think about the prospect of creating molds again. What interested me most was trying to make single-piece cut molds, like Miscast favors. These are molds where you pour the silicone all at one time, creating a solid block of silicone surrounding the model. You then use a blade to cut the original model out of the silicone, taking care to cut it just enough to release the model and remain as a single piece. Robert Tolone has some excellent YouTube videos about the process that I encourage you to watch if you are interested. In one of his videos, he used one-piece cut molds to cast some characters from the excellent board game Root. In it, he attaches the models to small plastic cups and pours the silicone into them. This seemed like a relatively straightforward process, so I decided to give it a try, making molds of some old metal Games Workshop models I hope to use as a basis for conversions in the future!

I decided to try to make a mold for this Tallarn Desert Raider. I attached the model to a wedge of resin from an old Forge World model. Having a large sprue/gate allows you to overfill the mold with resin, reducing the possibility of bubbles. It also serves as an easy place to pour the liquid resin into. At any point on the model that air might get trapped in when pouring the resin, I glued a piece of wire to serve as a vent.

After preparing the models, including adding any vents, I superglued them to the bottom of a plastic cup. At this point they are ready to have silicone poured into them.

After mixing and degassing the silicone in a vacuum chamber, I poured the silicone into the cup, taking care to not pour the silicone directly on the model (pouring on the model increases the likelihood of you trapping air between the silicone and the model). I didn't mix enough silicone to completely pour both molds. Rather than mixing more immediately, I let the silicone cure completely (in a pressure pot) overnight (shown in the picture above). Now that it had cured, I mixed more silicone and filled the cup. Silicone can be poured in stages like this without issue.

The poured silicone after it has cured for 6 hours, ready to remove the plastic cup (it can easily be cut off) and cut the mold open.

With the cup removed and the mold inverted, you can see the sprue. I will cut along the sides of this to remove the model and sprue from the silicone mold.

Using an X-acto blade, I cut along the model, pulling the mold apart as I do so to make the process easier. In the beginning I make a jagged cut to ensure the mold can only go back together one way.

I only cut enough so that I can easily pull out the model. By doing as little as possible, you reduce mold lines. With the mold cut, all you need to do is pour liquid resin directly into the large sprue port and put the mold in a pressure pot to cure.

An example of a completed cast. There are a few minor bubbles, but all are easily fixed with modeling putty.

If you have old molds that are no longer good for casting, you can cut the silicone into small pieces, such that when pouring a new mold you can stick them in the mold if you didn't mix enough silicone. Sticking these cured pieces into the mold displaces that volume of liquid silicone and may save you from having to do a second pour. The mold in this picture has pieces of old silicone in it. They are even a different type of silicone (both from Smooth On), but it sticks to the new silicone and works just as well.

It is fun to be exploring the world of resin casting again. I was amazed how much one-piece molds simplify the casting process. They make creating the mold quicker, only requiring you to pour silicone once, and result in less mold lines because it is earlier to align the mold since it is a single piece. Now that I have gotten the process working, I am excited to convert some models and make molds for them!

- Eric Wier