The problem is the dissimilar printing temperatures:
- TPU is printed at around ${230\ °\text C}$
- PLA is printed at around ${200\ °\text C}$
As a result, when the PLA is molten and well printable already, residue of TPU in the hotend is at an awkward spot: it is molten enough to seep down along the filament path with molten PLA, but it is not soft enough to get easily extruded from the nozzle. This is what leads to clogging.
To fix the clogging, I took the following steps:
- Swap the nozzle to reduce the residue still in the machine
- Do a cold-pull with the PLA, taking away a quite good chunk of the residue that still might remain in the heatbreak.
- Finally, do a purge print at an elevated temperature. For me, about ${215\ °\text C}$ did work to get the last traces of residue from the heartbreak out.-
There you go! One restored printing behavior!
Technically, the nozzle swap and cold pull were overkill, but reduced the amount of TPU that needed to be purged out of the nozzle.