You can use the PET Bottle Cutter to cut sprips from the PET bottles with selected width. The PET strip can be then used in machine which converts it to filament for 3D printers. This cutter is using sharpened bearings, they works like scissors. The width of the strip can be set by the knob.
I'm using this bottle cutter for some time and it works very well. You will gain the skill after a few bottles.
Here you can see how I cut a bottle using this cuter: