DISCLAIMER: Please don’t view this as a tutorial or guide. This particular venture is really just showing what I’m working on right now. It’s most certainly not the best approach but I do it this way because I’m a bit weird sometimes.
So I wanted to make a mold of the torso and legs of a Primarch conversion I’m currently working on. I’ve done similar molds with models of mine in the past with the initial intent of using them to cast in resin. However, my casting skills with resin are even poorer than my mold making skills, so I quickly adapted their use to casting in Green Stuff. I don’t recommend this approach to others, but I figured I’d show how I do it anyway. For some reason it does seem to work for me.
I start by deciding the size of the mold. I bought a selection of lego bricks (direct from the online store) that are perfect for building the box on which to mold in. In this case I have the torso and legs as one piece and a single shoulder pad with trim. One I know the dimensions of the box I’ll build it up and fill the bottom with plasticine. Plasticine is all I have on hand but I’m wondering if something softer like Play Dough might work better.
I continue to add Plasticine until the model is embedded half way in it. I ended up using a clay shaper to get as clean a line around the edge of the mini as possible. The pad was just laid flat on the plasticine and the underside filled in. Once it is all cleaned up I used the handle of the clay shaper to push small holes all over. These will help to locate the two side of the rubber mold together properly later.
After spray the inside of the box with a release agent (nasty toxic stuff) I then filled the rest of the box with RTV Silicone Rubber (approximately 120grams of it). I couldn’t get any photos whilst pouring at this stage, though I did for the second half which uses the same method. The box is held with one of the corners held lower down than the rest. The rubber is mixed and poured slowly into this corner allowing the whole box to fill up from that point. This helps reduce the likelihood of air bubbles forming. I spent a few minutes tapping the underside of the box to help loosen and potential air bubbles and then left if overnight (about 12 hours) to cure.
Once it has cured the box is easily disassembled and the plasticine removed leaving the parts in place. A little bit of cleaning up is required at this point just to take off some residual plasticine. Upon examining this half of the mold I found only one air bubble on the edge of the shoulder pad, certainly nothing that would cause any issues. I then rebuilt the box placing this half of the mold at the bottom and respraying release agent into it.
Again holding one corner lower than the others I poured rubber into that corner and allowed the box to fill up slowly.
Once it has cured (again, left overnight) the box can be disassembled and the two halves of the mold pried open. This didn’t work out as well as I’d hoped as it seems I’d not used enough mold release prior to the second half being poured. Whilst most of the two halves came apart relatively easy an area around one leg of the mold was stuck and ended up ripping. Note to self: in future use lots more release agent (in fact I’m tempted to try a coating of vaseline on the actual mold, not the model though). This rip isn’t as bad as it sounds as the two halves still fit together perfectly.
If this had been made for casting in resin I’d have used plastic rod to create flow channels for the resin to move through. I’ve not had a lot of success with resin though, so I only intend to jam Green Stuff in these. I start by mixing a large amount of Green Stuff at once and estimating how much to use to fill the space on each half. The Green Stuff has to be really pushed in (with lubricated tools so as not to pull it out again) to make sure all the detail is picked up. Once the space on each half is filled I then add a bit of extra Green Stuff. Too much will create some flash round the mold line, too little will leave gaps.
Both halves are then wedged together and left overnight for the Green Stuff to cure (preferably with a heavy weight on top to help apply force).
At the moment of writing this I have no idea how the cast will turn out. It might be a horrible tradegy, or it might be a reasonable cast. We’ll have to wait and see in the morning….