The Polyformer is a 3D printer filament extruder created for people living in 3rd world countries with limited access to affordable filament. It allows you to recycle PET plastic from beverage containers by cutting them into a ribbon and feeding the ribbon through a heated extrusion nozzle.
I've seen some videos where paper shredders have been modified for this purpose. I don't know how consistent the filament diameter was, though.
I would think the best way would be to first melt the plastic into a wider intermediary filament and then pull that through the desired final extruder to get a more consistent result.
I've been working on an openly documented recycling solution that would make use of litter from my area (mostly less commonly recycled plastics like polystyrene and LDPE), but I'm still in the identification/sorting phase. Finding an infrared spectrometer or sensor for the 1000-2300nm wavelengths to properly identify plastics has been difficult, but I think I'm close.