Hard Polystyrene
Does anybody know what grades are used? Does the material contain chalk or other fillers to help painting?
Dunno about grades. Fairly sure there's no fillers used (the idea of chalk or suchlike seems very strange to me, and seems like it'd have a very negative effect on the plastic's durability). Different kit makes do use different formulas of some kind though, as hardness and flexibility among other things varies from manufacturer to manufacturer and sometimes from kit to kit.
Paintabilty of polystyrene is usually quite good regardless, as long as it's been cleaned of mold releases and skin oils. Only exception I've found is Plastruct's scratchbuilding stock, which is actually quasi-ABS formulated for re-usability, so it's actually deliberately designed to reject glue and paint to a degree.
Evergreen kind of owns the scratchbuilding stock market, and their stuff is plain white and takes glue and paint very well.
Clear polystyrene tends to be a lot more brittle than white or other opaque colors. I don't think that's the dye though: I think it may be something about the molecular structure that enables it to be clear that does it. Though I have on rare ocassion seen kits with clear parts that didn't have the typical brittleness (The Jupiter 2 kit from the 1990's Lost in Space move had clear plastic that was delightful to work with) , so I know it's possible to formulate clear styrene so it isn't brittle, but I don't know why that isn't common.
Polyethylene
Traditional injection moulded 20mm or 1/72 plastic figures are polythene (polyethylene).
This is cheap and common but soft and bendy and prone to flash.
Also a pig to glue and paint, due to it's waxy everything-phobic surface. I tend to avoid it when at all possible.
Reaper Bones
This is a mystery. The material feels like PVC that was used for toys when I was a kid. However it has very little smell so I don't think it is PVC.
I think it is some form of TPE (Thermoplastic Elastomer) but I have no idea which one.
I have just received a lot of Reaper Bones from their Kickstarter and the material is quite nice to use.
Does anybody know what it is?[/i]
My understanding is it is a thermoplastic vinyl of some kind. Not all Vinyls are PVC, and not all PVC formulations have a smell, so that's not really an indicator.
My experiences with Reaper Bones have not been as encouraging. Stuff I received was unusably bendy and did not scrape or file well, so seams were a a big problem. Testimonials I've read have varied so wildly that I strongly suspect they have manufacturing consistency issues.
Resin
There are a lot of resins but I dont understand what they are.
As far as I understand most are thermoset polyesters but some usable in spincast machines so may be a thermoplastic.
Finecast is very brittle and I avoid it. Mantic and Spectre have polymer plastics that appears to be tougher and slightly flexible.
Can anybody explain what they are?
Polyurethane resins, actually, not polyester. Polyester resins are only used by dire cheapskate manufacturers and recasters. They are brittle, have poor dimensional stability, and are more caustic to silicone molds, so although they are cheap, they are not considered desirable.
The difference between the Mantic/Spector castings and others you may have bought is most likely just one of them using a different formula of polyurethane from a different supplier. There are numerous suppliers/makes of polyurethane casting resins on every continent, and they are not all equal. When getting started in resin casting, it pays to test out resins from different manufacturers/suppliers to find the best for your application, but many don't do that and just get whatever's cheapest or easiest, so a lot of resin stuff, especially in the gaming mini market, uses mediocre resin.
I dunno what "finecast" was specifically, but if their marketing was to be believed it was something oddball specifically formulated for GW. The examples I've seen were so horribly cast, full of flash, bubbles, short runs, and peppered with torn bits of mold rubber, that it kinda doesn't matter what the resin was, as it was clear someone must have "hired" their 13 year old nephew to do the casting instead of getting, well, ANYONE who even had the SLIGHTEST resin casting knowledge/experience. Even a halfway intelligent beginner would have been able to avoid doing such a bad job on their very first try, so I'm not inclined to credit any apologetics on GW's behalf. Anyone who's done any resin casting could immediately tell there was a truly epic level of not-giving-a-s*** going on there.
Given that, I find it hard to believe they actually spent any time or money testing resins or having something specifically formulated for them. Unless there was similarly epic levels of right-hand-not-knowing-what-the-left is-doing involved, it seems most likely they just bought something dirt cheap off the shelf, and the whole "finecast" story was pure marketing BS.
Thermoplastics are rare on the garage kit and small manufacturer level, as they usually require injection molding, which is EXTREMELY costly and so only profitable on a mass-market scale. They often require much greater casting pressures than metal or resin, so spin casting and/or rubber molds isn't really a thing with them.
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