+1 Melt and reuse is the only reasonable thing to do, though personally I’d use it to make armour instead of weapons. Remoulded ABS probably won’t hold a point or edge particularly long, but I’m not sure most Neolithic weapons would be effective against a one piece Lego-derived helmet.

Great idea. However do you have any evidence that cauldrons (presumably iron) could have been available in a neolithic society? Can you suggest a neolithic substitute?

P.S. What are the moulds made of?

@chasly-reinstateMonica Good point. Wikipedida divides neolithic up into various phases by how advanced their pottery got. ABS melts below both clay and ceramic, so I'd expect one of them to survive.

I would've guessed first Two flat chunks of stone with a matching triangle cut out? But clay or ceramic might do it now I've looked it up. Wood doesn't autoignite until 300 degrees, so a tree stump with the shape carved into it could maybe do in a pinch.

@chasly-reinstateMonica Wood burns at 600C, which is both below the melting point of ceramics, and above the melting point of ABS. A good ole campfire will do.

ABS holds an edge well enough to make arrows, you could even make composite long bows using it.

@John - Are we certain that Legos are made of ABS? The bricks seem quite brittle. I don't know if you could make a flexible bow from the plastic.

Since 1963, Lego pieces have been manufactured from a strong, resilient plastic known as acrylonitrile butadiene styrene (ABS) - en.wikipedia.org/wiki/Lego

note heating abs in boiling water keeps you from burning it which is a real risk. cast them into tiny pyramids like d4 dice and they should work a lot better as caltrops.

@JoeBloggs ABS mixed with hair (or any fiber) would make amazing armor, you can even shape it by hand while cooling. even by itself it would be pretty good.

@John - I think you probably have enough ideas here to post a supplementary answer that includes moulded artifacts not suggested by Ash.

@John: now all I can picture is a boar pelt coated in molten minifigure heads...

Notice that as well that the warriors routinely go barefoot. They do not have the delicate feet of the shoe-wearing but tough, calloused feet that can handle a lot more things than ours can -- to increase the difficulty of using them as weapons by themselves.

Doesn't ABS stink, though? I remember a 3-D printing enthusiast telling me that it isn't used often in printing because it makes an awful smell. Then again, neolithic-style humans aren't really known for their intolerance of bad odors.

Yeah it has a smell, you want to 3D print in a ventilated area. But outside melting it around a fire the smell isn't overpowering.

"or just some minesweeper guy going through with a broom." this alone gives a whole new meaning to the phrase "minesweeper"

The point about a differing stride is important here, because any primitive culture is likely to naturally have a similar stride because it helps when hunting too (you just don't put weight on your leading foot until you know what's under it, this trivially deals with caltrops or LEGO bricks, but it also makes sure you don't step on something noisy when hunting or sneaking around).

Regarding melting and shaping it, ABS becomes ductile at around 80 degC. Simply putting it in boiling water will let you reshape it for a short period. Putting it in hot animal fat will give you a longer working time. No moulds needed - it can be manipulated the same way blacksmiths shape iron.

Related to Graham's point, you can't really "pour" abs into a mold - it really needs to be pressed / squished into the mold. Something like forging is probably easier.