Home 3D printing en mass requires ecological consideration, particularly for popular 3D printing plastic ABS. Polylactic Acid (PLA) has a much better reputation as it is derived from starches available in crops such as maise. Recycling plastic at home using devices such as the Filastruder, Filabot and Lyman Filament Extruder is a solution that has emerged in recent years. An interesting and fun idea – for maker enthusiasts at home and perhaps teachers in the classroom – for grinding down old plastic for use in these machines is using liquid nitrogen, to cool PLA plastic below its brittle transition temperature.
Most plastic extruders available convert small plastic granules into plastic filament, but the granules can be difficult to make. By using liquid nitrogen to cool a polymer below its brittle transition temperature, a common household blender can be used to grind down plastic for filament extruders. The ideal source would be old 3D prints to perpetuate a sustainable production loop, also many other sources of household plastic waste can be used such as plastic bottles. This also has the benefit of reducing the price of input plastic for your home 3D printer, which averages around USD$40 per kilogram, though can be found more cheaply, and also at much more expensive prices.
Cold polymers can be easily chopped and blended into sizes compatible with filament extruders. The target brittle transition temperature for PLA is approximately -20˚C to -40˚C commonly available dry ice has a temperature of -78˚C but is nothing compared to the blisteringly cold temperatures of liquid nitrogen -196˚C and even some Walmart locations sell dry ice. The following is a process that originally appeared at the University of British Colombia’s website.
The blender used here is a 250W Magic Bullet – weaker than many common household blenders, thus your own will almost certainly be of use. The first step is to cool the plastic by submerging it wholly in liquid nitrogen. Once cold, the liquid nitrogen is carefully strained, and the plastic can be placed in the blender.
Grinding the plastic down will take around five minutes for a fine consistency to be achieved. Best results so far have come from filtering out the larger pieces from the smaller every now and again until everything is the right size for filament extruders.
The final product is shown against commercially available granules for comparison. Note that while many of the green pieces are even smaller than the red granules, there are some larger pieces that would need to be filtered out and blended again prior to use in a filament extruder. In this example to show the ease of the simplest approach to the process, everything was grinded in a single run without stopping to filter out the smaller pieces, but you can achieve better results by following the process as described regarding taking out the larger pieces.