Why plastic looks like plastic

Put a plastic Space Marine next to a resin one from a small studio and the plastic one looks softer. Not worse, softer. Edges are gentler, deep recesses are shallower, and the crisp undercut where a cloak lifts away from a leg is somehow not quite there.

For a while I assumed that was a quality gap. It is a manufacturing constraint, and once you know what it is, you can read it off any model on the shelf.

The part has to come out of the steel

Molten plastic fills a steel cavity and cools. All thermoplastics shrink as they cool, and that shrinkage grips the tool hard enough that a straight-sided part will not release. So every face running parallel to the direction the mould opens gets a slight taper, which the industry calls draft.^*

Without enough of it the part drags on its way out, and drags leave scrape lines, bending, warping, or a part that simply will not eject.^*

The published minimums are small but they are not zero: half a degree on any vertical face, one to two degrees in most cases, three at minimum where there is light texture, five or more where the texture is heavy.^*

Texture costs the most, which is the opposite of what I expected

That last figure is the interesting one. Texture on the tool wall would lock the part in place if it were not for draft.^*

Read that the other way round: the rougher a surface is meant to look, the more it has to lean. Chainmail, fur, bark, rubble, the stone slab a model is standing on. Every one of those is a negotiation between looking rough and being able to leave the steel, and the model always loses a little.

That is a large part of why plastic looks like plastic. Not the material. The exit.

And every part carries the seam

The mould’s two halves have to meet, so the finished part carries a line where they did. It traces the path along which a tangent to the surface runs parallel to the opening direction, and it cannot be removed from the process, only placed.^†

So the mould line is structural. The related marks each have their own name too: flash where material oozes into the seam and needs trimming, sink marks where a thicker section cooled unevenly into a dimple, knit lines where two flows of plastic met and left something that can look like a crack.^‡

What resin and metal do differently

Games Workshop publishes its own desk-level guide for the older materials, and the useful thing about it is that the clean-up is different rather than absent.

Both resin and metal arrive with spurs from casting, which GW also calls flash. Metal carries mould lines to shave. Resin carries “little shims in gaps between details”, which is its own kind of nuisance. Resin also needs washing and gentle scrubbing in warm soapy water before you assemble or paint it, both materials want a mask when you cut or file them, and neither takes plastic cement, so it is superglue for both.^§

GW’s own remastering account adds the other half: transferring a shape from the original sculpt to a cast master loses a little detail, and someone had to cut those details back in by hand.^** Copying a shape costs fidelity, in any medium.

What to do with this at the desk

Nothing, mostly. That is the point. You cannot file draft off a model, and the seam was never going to be absent.

What changes is what you go looking for. A soft edge on a textured surface is a manufacturing decision rather than a mould that has worn out. A line that looks like a crack may be a knit line and not a gap to fill. A dimple in a thick panel is cooling, not your sanding. And a resin kit is not automatically better made, it is made in a process with a different set of costs, one of which is a bowl of soapy water before you start.