Shredding is not the issue for recycling plastic. The issue is extruding the ground up pellets to product a uniform diameter filament. This requires both the pellet extruder output a uniform rate of plastic through a round nozzle, and that the extruded cylinder is pulled at a constant rate while cooled at a constant rate. Pulling will result in a diameter change and the goal is to have the OD harden at the same filament diameter.
In comparison shredding is very simple.