And What About Polycarbonate?
recycling
polymers
filament extruder
No one asked that question, so let me: What about polycarbonate?
It’s a thermoplastic (✅ possible to 3D print), prints at typically 275°C (✅ reachable with typical 3D printers), is horribly hygroscopic (❌ humidity in a melting filament makes vapour bubbles), and can warp a lot (❌ enclosure required for any bigger pieces)
The two red crosses can partially be circumvented though, so I gave it a try.
Shredding #
It all starts with shredding the polycarbonate into granules. Well, once the big pieces (which are all of them in this case) are cut smaller on the woodworking bandsaw.
The only material source of PC are the prusament spools (their sides, not contents) our makerspace at SPH uses. I got some for this experiment, and they have also lots of holes which makes the material thickness max 2mm everywhere, so not too big of a load on the cutting blades. This is good, given they start to struggle with anything thicker than 10mm massive, and that’s just for PLA/PETG; PC is much stronger.
Not staged at all, but every spool side has the recycling symbol and polymer stamped into it at the factory. Very useful for recycling and identifying the plastic type.
Pictured are the too-big granules that did not get through the sieve, so back in the kitchen mixer with them.
Extrusion #
The extruder can run with pure polycarboante! It has a diameter of 1.67mm: still needs some process tuning, but we’re not even that far from 1.75mm.
Given how PC is strongly hygroscopic, there are no signs of humidity when feeding it directly out of an oven set to dry the granules at 115°C (that’s the thermostat temperature. the granules themselves are maybe 95-100°C).
The melt/extrusion temperature is 255-260°C, which gets close to the maximum designed limit of the machine around 275°C. The extruder also has a thermal fuse in case of a thermal runaway, but I “just” need to maintain that 255-265°C temperature bracket in the meltzone in a stable way.
Purging #
I need to clean the extruder’s insides of the PC after finishing this experiment: for use with lower temperatures like PLA and PETG again. This time I tried just purging the material out, instead of disassembling the machine and cleaning it by hand.
The inside volume of the extruder is reasonably small, maybe 40mL.
But when some bits of PC remain in there, the next PLA extrusion at 180-190°C will just have flakes of PC in there and clog the nozzle, simply because of the PC melting temperature being very far from 190°C.
To purge, I go to the max temperature and feed in PLA, transparent in this case to see said flakes of PC. then the extruder runs and moves new material instead of the old one: The output changes from the black pure PC to a grey, then to a lighter and lighter much runnier plastic. At 260°C, PLA has a very low viscosity (runny) but only gets damaged when it spends a long time at that temperature.
Then I lower the temperature progressively, to also catch the solidifying flakes of PC in the molten PLA. Lowering slowly makes sure any big flake is still viscous enough to be torn and bent through the nozzle instead of clogging it.
That’s pure enough now, it falling down in circles also shows an even mass flow rate.
This recycling project is supported by
