Well to grind down plastic in small parts you only need a mill. You might think about two rotating coarse stones with two different speeds, that will grind down the plastic into small parts. Its easy to design and construct. Imho.
does that work? Have you perhaps got an example?
Wouldn’t the plastic act like a grease?
here a prototype i found
You would need to be very careful about friction melting / fusing the plastic to itself and the stones.