Why is ABS easier to sand than PLA? Which one is harder plastic than the other? Does hardness decide what is more wear-resistant? Or is it density? Something else?

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It’s the temperature, PLA becomes soft at low temperatures 35+ degrees,

Sanding you creates friction/heat this softens the PLA and gets stuck on the grains of the sanding paper, ABS and other higher temperature printing materials have a higher deform temperature.

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hardness. Lead is easy to sand but more dense than steel, which is harder to sand than aluminum. PLA is harder than ABS. Thing is, the item with the lower melting temperature will turn into ooze faster from friction burning. So ABS is just all around easier, it is softer so it scratches away faster, but its somewhat higher melting temperature means it holds its shape while its being sanded instead of turning to goo. When you use a power sander, or a disk, dremel, etc., you will see the PLA turn on you within a couple of seconds. But all the hand sanding in the world won’t feel good enough with PLA.

I don’t know what carbon fiber or nylon and the like will be for sanding, but I’m not looking forward to finding out.

Generally speaking, a combination of low hardness and high glass transition temperature.
-Jesse