Oyumaru is a rubbery plastic material that has been the subject of a lot of chatter among modellers lately. It's a great resource for casting. When placed in hot water it becomes soft and pliable, so that it can be wrapped around a miniature. After a few moments it cools to room temperature, becoming firm again (about as tough as rubber). Filling this mould with resin or milliput and waiting a short while rewards you with a copy as good as your casting skills - and most important of all - you can then reheat and reuse the oyumaru to start the process all over again, instead of filling your garage with boxfuls of casts.
After five minutes in freshly boiled water, I pressed the oyumaru over the miniature. I made sure the oyumaru cast was square so that it would fit into the lego case I used later. Many people also add a single-pip lego block to a corner or two of their moulds to keep the two pieces correctly aligned in the casting stage. I'll do this in future.
I did not use a release agent, as oyumaru does not stick to resin. Another note; heat generated by the curing resin isn't hot enough to warp the detail of the oyumaru mould.
When it dried, I cut around the edge of the miniature with a scalpel to free it.
I lay both sides of the cast open and filled them with resin before it started to get thick. In total, I used a teaspoon of material for the one miniature - half a teaspoon from bottle a, and half from bottle b. Use two separate teaspoons! I keep a couple of little 'bits' moulds nearby so I can put the leftover mixed resin to good use. I watched the resin until it looked like treacle and then pressed them together and put them in the lego mould case (see next stage).
When filling larger moulds be sure to fill the deepest recesses of the mould first - otherwise, air bubbles may form. I used Smooth-Cast 300; it costs slightly more, but it has the slowest curing time of the resins I could find. This is important because it allows precious time before it becomes too viscous for bubbles to escape.
Sitting snug inside the lego mould case, flat on the top and on the bottom. Inside, the resin would have been oozing (slowly) into places I didn't want it to, so I needed to fix it in place for the remaining 3 minutes.
I held down this block reasonably firmly for about 3 minutes while the resin set. Then left it alone and I gave the miniature another 10 minutes curing time. watch the excess resin in your mixing pot - so you can get a clue as to the condition of the resin inside your mould.
After ten minutes, I popped the oyumaru mould out of the lego case, and carefully opened it up. I was careful to treat the sword gently as I have heard stories about the fragile nature of resin, but at this stage the miniature still had some flex to it. No problems came about from not using a release agent.
The model feels light and cheap to hold, even moreso than plastic. The detail is near perfect - the little pips on the armour are there when I compare it to the original. Before painting a resin model, I'll need to wash it - it 'sweats' for a while. Sadly, I won't be painting this one, because I failed to check how carefully I'd made the mould, and messed up her face (air bubble). Check your mould!
Overall, I am very pleased with how easy it was to use oyumaru and resin. This was my first attempt at using a lego box to hold the mould and keep it firmly fixed, and I'd highly recommend it - using your hands to hold it all in place just doesn't do the trick.