This was not what I was trying to print, but it’s what I found on the printer this morning.
Today, as a result, I replaced the thermistor, heater cartridge, throat, nozzle, heater block, and throat lock screws on my printer. I also replaced the cheap 24V fan with a 12V noctua and a stepdown converter while I was at it. Getting hardened PETG off apparently requires THF which I don’t have on hand, so I just threw out the heater block, throat, and nozzle that I couldn’t get clean with just heat.
The thermistor pulled partway out of its can, and the surface of both the heater cartridge and thermister can were coated with melted PETG; it apparently injected everywhere under pressure.
The replacement heat block has a silicone sock instead of being wrapped in layers of kapton tape and I expected to have to pid tune again, but the temperature already holds better even without a pid tune cycle, so I’m back to printing. I hope this one works better.
Why would it catch fire? The temperature sensing is inside the blob and the blob insulates it, so it will pump less and less heat into it as the blob grows, and unlike the [censored] designers at Anet I always build firmware with thermal runaway protection. I was printing this PETG at 260°; the boiling point or PET is 350° so I would expect it to be fine at 260° indefinitely. 260° is the melting point of PET; the glycol lowers the melting point but I think doesn’t lower the boiling point, or at least not meaningfully. So it was an annoyance, but I don’t think particularly dangerous.
(Yes, there is a smoke detector with fresh batteries in the room.)
I can’t see the part number on the chip in the converter, and it gets warm enough that I think it’s actually a programmable linear regulator. Oops. I have some LM2675 stepdown converters handy; they are just a bit larger and I was trying to use the smallest thing available. I taped it to a heat sink while I finished my first print, and now I’ll swap that out before doing more printing.
