I thought I had posted this before. But I didn’t find it, so here I go again… The author build a simple and cheap shredder for plastic. Why do this? Well if you want to melt it into a mold, its easier in small pieces. Or if you want to turn it into filament, you need it pretty small too. I have seen the planer for $25 (after coupons) at Harbor Freight, so you can keep the cost down pretty good too. I like the stick that goes with this to “shove” the plastic down. I have looked at commercial solutions, and the cheapest would run me around $150. So $30-40 total cost isn’t too bad.
http://www.instructables.com/id/Low-Cost-Plastic-Shredder/