I work in 3D printing. One of the materials we use is polyethylene terephthalate glycol-modified, or PETG. It is a pretty good material, both in strength and appearance. It is melted and pushed through a "hot end" nozzle, typically either brass or hardened steel, at about 230 degrees C.
Unfortunately, it is rather sticky, and tends to accumulate on the hot metal surfaces, where it slowly degrades due to the heat. The resulting residue is black, and somewhat hard to remove. I have tried blowtorching it, which works with polylactic acid (PLA), but that just results in the same black substance.
I have seen recommendations to use either acetone, dichloromethane, or phenol. Other substances like toluene are recommended - some say they work, some say they don't. However, all of these seem to be recommended for dissolving/welding the plastic in its original form. There doesn't seem to be any advice for dissolving the residue after it has degraded from heat, at which point its chemical composition has changed.
From my research, the residue may be composed primarily of acetaldehyde. I was not able to find information (that I can understand as a lay person) about how to dissolve that.
So far, I've tried putting a brass nozzle caked with PETG residue into a 50C acetone bath for an hour. A little of it came off, but not much. The ideal solvent would attack the residue so completely that it would come off without too much force. The inside bore of the nozzles (the melt zone) is typically on the order of 1-2 millimeters, so the best option is to use an abrasive cleaning filament. Something tougher, like a wire brush, is not possible in that confined space, as far as I know.
If anyone has a good answer, I'll spread it around the 3D printing community, so you won't just be helping me. Thanks in advance!