One topic that recently came up on a discussion group was whether to pre-heat spin-casting moulds for wargames figures before running them. Nowadays, I always pre-heat, leaving moulds in an oven at 100 deg C for at least 30 minutes before casting. Moulds that have thin items such as horses’ reins are often heated to 120 deg C for the same time before casting.
The main reason for doing so is to improve metal flow and surface finish. Better flow helps cast fine parts of a figure. And the better surface finish results by eliminating the bubbling on the surface caused by the temperature difference between molten metal and rubber. Some casters will tell you this bubbling is caused by metal being too hot; it’s also caused by a mould being too cold. More accurately, it’s a matter of reducing the temperature difference between rubber and metal to improve surface finish.
Of course running hot moulds requires gloves to be worn at all times.
How much a mould has to be heated also depends on the casting metal. I find lead-free pewter is trickier to cast in organic black rubber moulds than the leadier alloys I’ve previously used. It is lower density and therefore lower mass, producing less force behind it when poured. Leadier alloys may only need a mould to be gently warmed.
Running a hot mould will also allow a lower metal temperature to be used. This stands to reason because the metal will lose less heat flowing through a hot mould.
The other benefit of heating the mould is that it reduces figure shrinkage. A cold mould is smaller than a hot mould, and you can demonstrate this by simply dropping a cold mould back into the moulding can and looking at the difference in diameter. Depending on rubber, a mould may be 2mm to 5mm less in diameter when cold, translating to a 1% to 2.5% shrinkage. This shrinkage naturally affects the size of mould cavities.
I heated up natural black rubber and silicone moulds to compare them. The black rubber shows more of a difference, i.e. there is more shrinkage on a cold black rubber mould.
Shrinkage from silicone moulds also appears to be dependent on the vulcanization temperature. I use low temperature cream silicone from J Coker of Faversham vulcanized at 90 deg C for two hours. Vulcanizing it at the recommended higher temperature for just an hour appears to result in more shrinkage, and I switched to lower temperatures and longer times years ago.
This is backed by the data sheet for Nicem’s low temperature silicones. Nicem’s light green or orange low temperature silicones boast a 0.3% shrinkage when vulcanized at a very low temperature, but shrinkage increases as curing temperature increases. I’m waiting on samples to check what is possible.
Shrinkage caused by mould material is in addition to compression shrinkage caused by pressure on the mould. Casting with a hot mould will by and large reduce shrinkage to that caused only by compression. By comparison, a metal figure cast in a cold mould will lose height, length and thickness because the mould is smaller, and thickness because the mould is also compressed by the casting machine.