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After printing some parts in TPU, I encountered continuous clogging after returning to PLA. Not right out of the bat, but after about an hour or such, all prints I started since the swap back clog after about an hour. I use an Ender-3, Bowden Style, and print my PLA generally at 200 °C. The TPU had been listed as 220-240 °C on the roll, so I printed at 230 °C.

How can I regain normal printing behavior?!

Trish
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1 Answers1

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What's the cause of the problem?

The problem is the dissimilar printing temperatures:

  • TPU is printed at around $\pu{230 °C}$
  • PLA is printed at around $\pu{200 °C}$

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.

Problem solution

To fix the clogging, I took the following steps after the very first time I encountered it:

  • 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 $\pu{215 °C}$ 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.

Refined problem solution

Since the problem occurred first, I managed to refine my procedure to prevent clogs in the first place.

  • Heat the nozzle to ca. $\pu{230 °C}$
  • Pull the TPU Filament without cooling
  • Load the PLA Filament
  • Manually push filament until the about 10 to 15 mm are extruded from the nozzle
  • Order the extruder to extrude 10 cm of Filament
  • Set print temperature down to to $\pu{200 °C}$

While the extruder pushes the PLA through at an elevated temperature, it clears the whole path while cooling down, and can cycle right into the next print. It can help to use a different color PLA than the TPU to have a visible confirmation of the last residue being gone.

Another helpful indicator is the cooling down filament's flexibility: as long as TPU is in the mix, the extruded string is bendy but becomes stiff as soon as there is almost no more residue in it.

Trish
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