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.
This is clever - I bought a filistruder for a local makerspace awhile back (with the idea that I'd be able to recycle my own prints/supports as well as prints from folks in my town), but ran into trouble finding a good way to shred/granulate big prints. There were a few articles about attempts to shred PLA using blenders, electric woodchippers, etc, none of which seemed very effective. There were a couple custom made shredders/kits you could assemble still around on ebay, but they were getting kind of rare. The extruders were designed around plastic pellets but it didn't seem like there had been much development over the next years on getting the plastic into pellet sizes (unless you get one of the big professional granulator/extruders). That's kind of where we stopped for the moment.
Starting with thinner (inconveniently common) material is smart.
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.