Getting Lulzbot Taz Pro to work with PVA

Anyone had luck setting up PVA-support dual extrusion?

I have a reel of PolyDissolve S1 and FilamentsCanada PVA and both are failing hard.

I’m using Lulzbot Cura 3.6.20, the current stable version.

I have PLA on extruder 1, the PVA on extruder 2.

Extruder 2 standby temp 170C, print temp 200C, speed 40mm/s.

There are 2 main problems.

  1. After the Taz Pro purges the PLA and PVA, it retracts then blurps a bit of PLA but not the PVA. So when it’s time for PVA to print, there’s nothing coming out for a good XXmm distance!

  2. When the PVA starts printing, some of it doesn’t stick to the bed and it curves and splits, creating tangles on the printbed. Most of the PVA actually doesnt stick! It seems like the curvesplitting also happens when the nozzle stops printing, due to backpressure and temperature (nozzle does not cool down instantly to 170C of course).

I’ve tried printing at different temps with the S1 (220, 210, 200), only 200C with the FilamentsCanada PVA.

What’s weird is that when I manually set the temp to 180 or 200 or 220 then manually extrude, the PVA comes out fine.

To solve problem 1, I’m going to set a long skirt for extruder 2 and get back to you guys.

I appreciate any help/guidance.

In many circumstances, the PVA gave me a lot of problems. I can kinda get it to behave but I learned that once the nozzle goes idle, you can’t trust the PVA in the nozzle the next time the nozzle goes active… it needs to be purged.

I turned on the ‘prime tower’ option in Cura and I think I increased the amount of filament purged (I think I set it to 20 or 30mm^3).

Print very slowly with it.
Avoid retractions that aren’t necessary. I increased some minimum retraction distances to 5mm (so that short moves don’t retract … they’re just stringy but it’s not like it matters on the supports.). I usually need to inspect how it prints a part and have to tweak this to avoid the stringing from coming across areas printed of the part.

The PVA doesn’t like to stick to the bed without PVA glue-stick being down.

I have not (yet) toyed with fan settings … but wondered if it would stick better if I reduced fan speed.

UPDATE 1: TheVirtualTim is right. Once the nozzle goes idle the filament develops air pockets which is worsened by retraction movements. I started using a prime tower and it has been helping get rid of the “bad PVA” and the “no PVA output” issues. I barely got a decent PVA test print at 220C and 210C at 30mm/s speed, 30mm^3 prime tower pre-output. At these higher temperatures the oozing and curling is worse. I want to make 200C work but the PVA fails to stick to the PLA. These UPDATE1 tests were all conducted on FilamentsCanada PVA, which seems discontinued from their website, so I will perform more tests with the PolyDissolve S1 filament and get back to you guys.

The key issues so far:

  1. No PVA output. If the temperature is too low the filament will be too thick inside the nozzle for the gear to push out. If the temperature is too high the filament will crystallize. If the nozzle of extruder 2 is not equal Z-height as extruder 1 then PVA can be blocked by the current printing surface. If the reel is incorrectly positioned or the filament runout detector overly tightened the added friction will stop extrusion. Retractions cause no output for the first Xmm post-retraction extrusion. The complete solution is making sure all of the above are checked and fixed.

  2. Bad PVA output. For various reasons (ex. retraction, temp fluctuations, aeration, fan disturbance), the extruded PVA will excessively curl, ooze, be thin-thick, contain small/big air bubbles. A partial solution is using the Prime Tower setting that will dump the bad PVA elsewhere then resume the normal print with good-extruding PVA.

  3. PVA does not stick to the print surface, whether on the bed or print object. This is my current focus.

UPDATE 2: Had an OK quality print (print result was good but there was a tangle of PVA on the bed) with PVA interfaces using 215C, normal retraction, 20mm/s speed, 100% support interface density, 30mm DIA prime tower with 60mm3 priming volume positioned further (like 5-10cm) from the print object.