Researchers push 3D printing beyond its perimeter of comfort to make incredible discoveries, and this latest paper from Aubrey Woern and Joshua Pearce out of Michigan Technological University is likely to make a few makers uncomfortable.
Their invention is called a 3D Printable Polymer Pelletizer Chopper and it chops 3D printer filament into pellet-sized pieces that can be used in a fused granular fabrication (FGF) 3D printer or re-extruded as a new filament spool that can be used on a standard fused deposition modeling (FDM) 3D printer.
Those familiar with FGF know that one of its big advantages is the lower cost of raw materials as it uses plastics in pellet format that are significantly cheaper than spooled filament. Even though the authors of the paper point out that there exists a “markup on commercial filament over base commercial polymers, which is currently about five to 10 times the cost of the raw plastic pellets,” they go on to invent a device specifically designed to convert already-spooled filament (the kind with the big markup) into pellets, which in most cases could have been purchased for much cheaper in pellet format from the start. It’s unclear why the cost-saving advantage of FGF 3D printing was mentioned in the introduction of this paper when that advantage is negated by their invention.
That’s not to say their filament pelletizer doesn’t have its benefits, which the authors do discuss near the end of the paper; cost savings just isn’t one of them. Where this invention shines is its ability to easily and cheaply create custom polymer blends on a small scale. The bill of materials to build the chopper is only $185, mostly because many of the specialized parts are 3D printed. That means nearly any maker (or researcher) with a filament extruder, of which there are multiple affordable options, can make their own filament blends from off-the-shelf spools. Most polymers can already be purchased in pellet format, but this machine allows makers to pelletize proprietary blends of filament that can then be mixed with other proprietary or virgin polymers.
Woern and Pearce discovered that the pellet size can be controlled by adjusting the RPMs of the motor and that two or more filaments can be chopped simultaneously; this is useful for setting the ratios of specific pellet blends. With two motors, the pelletizer has an output of 1kg per hour. Several plastics were tested, including PLA, ABS, coffee-filled PLA, PP, PETg, and NinjaFlex, and only the NinjaFlex didn’t work due to its flexibility.There is as much as a 10% degradation in the mechanical properties of thermoplastics during the first cycle of extrusion, so plastic can only be cycled a few times, though adding some virgin pellets in with the mix helps maintain mechanical properties. Filament that was extruded from roughly chopped (recycled) plastics can also be pelletized and then re-extruded to improve the quality of the filament. The researchers hope that the technology will be used to incorporate more waste wood into biopolymer composites. All of their designs, STL files, and build instructions are open source and freely available, so it's probable their hopes will be realized.