Avoiding stringing with flexible filament
I have some flexible PLA filament (https://www.amazon.com/dp/B00VKSSA4E/, presumably a mix of PLA with some platicizer) that's supposed to be easy to print with settings similar to regular PLA. I've seen recommendations to disable retraction, and indeed I get huge failures to extrude at all for a while after retraction if it's enabled. But with retraction disabled, I get stringing all over the place, and since the material isn't brittle, it's really difficult to remove.
I'm using a bed temp of 60 and print temp of 220, increased from 210 for normal PLA since I had trouble getting it to adhere at lower temp. Printer is Creality Ender 3. Using CuraEngine for slicing. The extruder is feeding the material fine; there's no kinking going on or anything.
Where should I start trying to improve this? Might retraction work with a really really slow print speed or greatly reduced retraction distance? Or are there other ways to avoid stringing?
OK, this turned out to be really idiotic. The main source of the problem was Cura's custom start gcode for my printer (Ender 3), maybe duplicated for other printers too: it crams a massive amount of filament through the extruder to prime it. With flexible filament, this doesn't actually extrude most of it; rather, it just builds up as pressure in the bowden, which makes the first few [tens of] layers ooze like crazy, and then the hot nozzle picks up the ooze and drags it all over the place, and yay, stringing everywhere!
A previous version of this answer (see edit history) described a lot of options I was using to try to improve things, some of them rather dubious. Really, the core of the matter, which can manifest in lots of ways, is that if pressure builds up in the extruder due compression of the material being easier than forcing it through the nozzle, it will ooze during travel, or as a glob prior to travel during ineffective retraction, and either way it will eventually lead to catastrophic stringing. So to solve this, I needed to address all the ways it could happen.
First was the start gcode. I dropped the extruded material amount over 200mm for the priming from 15mm to 9mm (about 90% nominal material needed rather than 150%) and edited the custom end gcode to revert all but 1mm of the retraction it does, after homing. This reduces the need for wacky over-priming at print start, and makes the state after printing roughly match the state after loading filament, so that print results are independent of whether filament was just loaded or not.
Second, pressure can build up again if the extrusion rate is too high for the material, nozzle size, and temperature. At 30 mm/s print rate and 0.3 mm layers, I found I need a ridiculously high temperature like 235 °C to keep it flowing. This in turn produced a lot of sagging of overhangs, so really 0.3 mm layers seem to need significantly slower printing, defeating the purpose. 0.2-0.25 mm seems to flow marginally ok at 30 mm/s and 215°C, which is better behaved with regard to overhangs, and fine at 225 °C.
I'd noticed previously (in the previous version of this answer) a problem with layer adhesion with temperatures lower than 225 °C. This was not actually a layer adhesion problem but rather an underextrusion problem, the same one causing stringing.
The retraction amount still needs to be sufficient to account for the compression of the material in the bowden, plus enough to get it backed out of the hotend. I seem to need at least 15 mm, compared to 5 mm for plain PLA. Note that retraction min travel needs to be disabled (set to 0) to prevent skipping retraction for short moves, where stringing will be the worst.
Higher retraction speeds seem to work better, and without them the 15 mm retraction is miserably slow. However, I found that my printer's default limits on extruder feedrate and jerk were very low. Adding
M205 E25to the custom start gcode made it honor my requests for faster retraction, and doesn't seem to have any ill effects.
In summary, the settings that work for me are:
- All print speed settings: 30 mm/s or less
- Travel speed: 250 mm/s
- Retraction amount 15 mm
- Retraction speed: 150 mm/s
- Retraction combing: all
- Retraction hop: disabled
- Retraction min travel: 0 mm
- Temperature: 235 for 0.3 mm layers, 215 for 0.2 mm layers
And the results: