Has anyone made their own custom LEGO bricks?
People with 3d printers have made their own LEGO bricks. The plastic used is ABS.

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Interestingly, 3d printers can be constructed out of LEGO parts - you could have it self replicating!

Of course, you would need a very expensive and high quality printer to make good pieces. Cheaper ones can only make hollow bottoms and the banding from manufacture is highly visible.