Sudden appearance of previous color while printing
I had printed something with deep blue Prusament PLA, and then started printing a mini temperature tower with some new orange silk PLA+ I got from a Taiwanese company called Lee Fung. The MMU2S did its thing, retracting the blue, feeding in the orange silk, and making a purge line along the edge of the build plate. Then the temperature tower started printing from the highest temperature (230°) first.
As the temperature changed to 215°, suddenly it turned blue:
I'm thinking that this happening at a temperature change point may just be a coincidence.
It's hard for me to believe, however, that some un-purged blue filament was stuck in the nozzle all the way from 230° down to 215°. I had done a cold pull prior to the previous print and it came out clean. And I've never had Prusament PLA leave junk behind.
The best I can figure is that maybe a string of blue PLA was caught in the MMU2S and got dragged by the orange filament down the feed tube. That blue Prusament does sometimes produce long stringy ends, although it doesn't string while printing. So it's plausible that's what happened.
I'm glad it was just a temperature tower, and not some elaborate long-duration print.
By the way, that Lee Fung orange silk PLA+ turned out to be a really nice filament to work with. It printed smoothly over the entire temperature range without issue although the bridges at 200° and below didn't bond together and remained separate strands. That contrasts with some Lee Fung non-silk red PLA+ I have (refill coil, not a spool) that is quite finicky to work with and jams below 215°.