A 1970s ad from Lego played up how kids don't see a problem making their beautiful truck out of whatever pieces were available.
I would have been a color snob, but all I had in 1979 were red bricks.
1. The texture on the top slope of roof bricks is a standard mold finishing (acid etch?) that works fine with mold release.
2. Headlight grille bricks in the 70s used to have texture, along with printing. I'm not in the know but I suspect the texture could be a byproduct of the printing machine that was used then. (I've only seen 2000s era trade machines that used inkjet printing post-moulding.)
3. The 1x2 brick with vertical grooves on one face, horizontal grooves on opposite face... this has been common since the 80s! 1979's Galaxy Explorer had two of them (I think). "Corrugated Steel" or spaceship "greebling" texture.
Trying to puzzle out that mold, I imagine you need a moving insert textured with the horizontal grooves.
Some sets have had dozens or hundreds of that brick (Star Wars), without a noticeable impact on price/piece?
2 - I'm not sure what piece you mean :/
3 - That's a very good point that I hadn't thought of. That's piece 2877, and I'm not sure how that would be manufactured... You could potentially have one part on the "bottom" of the mould extend out with the horizontal grooves, and have the vertical grooves on the "top" half of the mould. When the mould separates, the horizontal grooves are locked into the bottom half of the mould, but the vertical ones slide out. Then, when the moulds are fully separated, the part can fall off the bottom half sideways. I hope that makes sense, I don't know how better to describe it. However, that only works if you can cleanly separate the moulds and then have the piece fall off, if the texture was on all sides then you wouldn't be able to separate the mould halves or extract the piece without a complex separating assembly
I've seen a machine that makes 64 of them in one go. Inside the cap is a collapsible core, because the molded screw is all undercuts... the mold opens (horizontally), the cores collapse, and 64 caps fall into the bin. On top of that, it's two materials, over-molded!
You could try to make the tops of the bumps textured, but that's where Lego puts their trademark, and I don't think they'd compromise on that, since its another protection against fake bricks that claim they're Lego but are worse. I also don't know how well you could feel textural differences in an area that small
With some exceptions (80s plates with sprue on short end) I expect to find the sprue mark on a corner stud.
Anyhow I'm imagining after-market ways to add texture.
I imagine an aftermarket machine to heat-stamp this on. It would have to be very precise. Pressure would displace plastic and easily change the clutch power.
