I got close to biting off more than I could chew recently in the process of making a Secret Santa gift. With Mata as my 'victim' I decided to make some little action figures of the Mittens and Snowdrop characters from one of his animations.
The initial plan was to just build and paint the figures, but partway through I decided to cast them in resin. This has the advantage of being stronger than the master model (depending on materials) and as a bonus let me make a set for myself.
First up, here's the finished product:
To start off with I decided to have some fun, and ended up building a mini lathe to make the basic body shape. This allowed me to use cheap 3/4 inch balsa dowel as stock material instead of wasting (slightly more) expensive putty, it also made getting the body shape smooth and uniform a lot easier.
The lathe is very light duty but works well to shape a piece of balsa with sandpaper or a craft knife. There's a separate blog post on how I made the lathe.
Once I had the body shape I started adding details. Mittens' mouth was just cut out with a razor saw and then the throat drilled out. After that, a lip and uvula were sculpted with Milliput epoxy putty. Eyes and feet were added in a similar fashion over several days to allow the putty to cure and prevent accidental damage to bits of detail that were still setting.
More complex features such as Mittens' hands and the ears were made by rolling out sheets of putty and cutting it to shape with a sharp sculpting tool. In the case of the ears they were rolled onto a curved bit of clay and cut to shape then allowed to cure and trimmed down to size before being attached with fillets of putty.
Once the masters were finished I brushed them with polyurethane varnish to ensure they'd release cleanly from the mould. I should have used some filler at this point to get rid of the woodgrain finish, but since I didn't think about it at the time I had to clean this up later.
I ordered some RTV (Room Temperature Vulcanising) silicone rubber online in order to make the mould. The correct way to make a two part mould is to build a box and fill the bottom half with clay, then press the masters into the clay and pour silicone over. Unfortunately the clay I had dried up, so I ended up using salt dough and surprisingly it worked well though it was very soft.
I build the mould box out of cardboard protected with parcel tape. The base is some 0.5mm sheet styrene I had lying around. The card was just formed into a box and taped closed I packed the bottom half with salt dough. I added more dough around the box to seal the edges. The master models were then pressed into the dough making sure there was a tight enough seal all around. I then mixed up the silicone thoroughly and poured it from as high as possible. It's best to pour into the corner of the mould and allow it to flow over the models of its own accord to prevent air bubbles and voids in the mould.
To make it easy to line up the two mould halves it's worth pressing the handle of a paint brush or similar into the clay to make some small alignment nubs in the first mould half. These will translate to matching holes in the second half. I made once larger than the others to make it obvious which corners should match up.
After leaving this overnight I removed the box and washed off the salt dough from the models. A downside of the salt dough is that it gets very sticky but it wasn't difficult to remove. Although it's advised against I cracked part of one of the models out of the mould to check that it released properly.
Before pouring the second half of the mould I painted a thin layer of Vaseline over the rubber (but not the models) to act as a mould release and avoid the two halves bonding together. I realised at this point that I hadn't added any vents or pour channels into the mould so I had to hack them in with some clay. These would have been much easier to add when making the first mould half as they could be set into the clay. The Vaseline didn't help either as everything was very slippy at this point.
The second mould half was then poured in a similar fashion to the first. A day later once it had cured I cracked open the box and managed to separate the two mould halves with relatively little hack and slash with the craft knife.
Once the mould was complete I mixed up some resin and did a test pour. The results were quite promising but highlighted a few flaws in the mould. The pour channels had to be opened up a bit with the craft knife to let more resin in. Additionally since the feet were above the point where the pour channel hits the body they trapped pockets of air. I solved this by skewering the silicone with a cocktail stick through to the tips of the feet and inserting some 'biro inner' style nylon tube to prevent the channels from closing up. The resin doesn't stick to the nylon so they can be easily removed with some pliers, leaving a little stick of cast resin which easily breaks off (useful for parts that can't be aligned with the mould line).
I used some more sheet styrene and elastic bands to hold the mould shut. It doesn't need too much pressure as the resin is quite thick and you can warp the mould.
It's worth noting that the resin and silicone I used required mixing in roughly 10:1 ratio with a catalyst, which required some very accurate scales. It's possible to get stuff that can be mixed 1:1 by volume which is a lot less hassle if you can get it conveniently.
Once the resin was cured and cracked out of the mould I trimmed off the flashing where the mould halves joined and primed them with grey spray on primer. At this point I noticed the woodgrain had transferred through from some parts of the bodies and other parts were not so smooth. To fix this I used some filler and a few rounds of sanding/filing and re-priming until I was happy with the finish.
Finally I painted the models with a mix of Citadel and Vallejo paints I had to hand and varnished them with some EzeCote polyurethane.