Is it really possible to recycle 3D printer thermoplastics?
All FFF 3D printer operators collect massive amounts of waste material from failed prints, support structures and other scraps. Everyone has a secret fantasy of being able to collect them together and somehow recycle them into usable filament.
There are even several solutions on the market and in open source that are systems to chop, remelt and extrude scraps into supposedly useful filament.
I’ve written in the past how challenging it is to produce usable filament with desktop equipment. Today’s 3D printers require input material of very precise accuracy, and that’s extremely difficult to achieve on desktop extrusion systems.
Another challenge is the type of material. Your 3D printing scraps are no doubt composed of different colors and different material types. Is that black support tree PLA or ASA? Who knows? Recycling scraps requires using identical materials. Even materials from the same provider are sometimes slightly different in color and chemistry.
But there’s yet another fatal flaw in the recycling concept that few talk about: the materials themselves are not designed for recycling.
When you print a material it is subjected to high temperatures, and these can change the molecular structure of the polymer. Extruded PLA is not the same as fresh PLA.
What’s happening? PLA (and PET and PETG) are polyesters, which suffer something called “chain scission” during heating. Polyesters are long chains of molecules connected by ester bonds (hence the name). It turns out that during heating, the presence of water can cleave some of these bonds. In addition, the forces pushing the material through a nozzle can also cause rips in those bonds.
If you were to recycle that extruded material, the ester bonds do not reappear. This leaves the resulting material weaker and more brittle, even though it might visually appear the same. The shorter molecules (because they were cut) flow more easily, making the viscosity lower, which messes up print profiles.
In other words, recycled PLA (and PET and PETG) cannot be as good as fresh, original material.
The same effect happens on other types of 3D print material. Styrene materials, like ABS and ASA, also have a similar problem, although it is caused by oxygen exposure, not moisture. This causes the material to become more brittle and discolor.
TPUs are a type of polyurethane, which when recycled suffers reduced density and viscosity, changing the mechanical behavior.
These effects become compounded if repeated. If a material is recycled once, it is worse than before. If it is recycled again, it degrades even more.
Is there a way to overcome this? Maybe. There’s a process known as chain extending, in which additives react with the dangling molecular ends and partially restore them. This rebuilds some of the molecules, increasing their weight and potentially restoring some original properties.
However, desktop recycling systems would certainly not involve this level of chemistry. All desktop recycling systems simply grind up scraps and re-extrude them into filament form. That’s absolutely going to result in poorer quality filament.
There are not many sources of recycled filament, and this is yet another reason why. It’s a shame, really, because without proper recycling 3D printers will generate massive amounts of plastic waste, which ultimately gets into the environment as microplastics.
