More stringing with Pressure Advance instead of Coasting

[dc42]

Retraction distance doesn't have much effect on stringing, but does affect whether you get blobs left on the print before travel moves. Stringing is controlled by reducing extrusion temperature (lower = less stringing) and increasing retraction speed (higher = usually less stringing).

If you were already using coast to end in lieu of pressure advance, you probably don't need to reduce retraction if you replace coast to end by pressure advance.

[Phaedrux]

If you use software retraction in Slic3r you can enabled wipe on retraction which may help your stringing.

[wilriker]

Thanks to both of you. I did not know that retraction speed was more responsible to reduce stringing than retraction distance. I actually never really thought about the purpose of retraction speed other than reducing the time it takes.

I took this as an opportunity to cross one of my still-to-tune points off my list and looked at how fast my extruder can actually retract. Before it was set to a max speed of 3000mm/min but I only ever retracted with 1800-1920mm/min. I got stallGuard warnings when retracting faster - but I always forgot that I never tuned stallGuard for E in the first place other than enabling logging. So now I know that my extruder is at least capable of retracting with 6000mm/min (I did not try faster) without any issues. So that is what I set it to now. Although it did not seem to have any issues with unretracting at this speed I set unretract to 2000mm/min for now and will tune this later.

So I will now see at my next print how stringing will be and also I can then tune stallGuard for E to no longer give me false positives.

[Phaedrux]

I definitely find benefit in faster retraction and slower unretraction. 7000/2000 seems to be my sweet spot.

[PlasticMetal]

Interesting discussion, but a critical point: what type(s) of filament are each of you using?

[T3P3Tony]

@wilriker I think its helpful to think of extrusion as a pressure system with the melt chamber acting like a slightly elastic element between the input and output. Once the extruder stops, there is still pressure in the melt chamber, so the quicker you can relieve that pressure the less time there is for ooze, anecdotally, although I have not tested this, quick enough retraction creates a bit of negative pressure which should suck the filament in from the nozzle (maybe?).

Obviously pressure advance is trying to make the pressure = 0 at the same time as we want the extrusion to stop so needs to be tuned to the pressure which is dependent on the melt chamber, the nozzle, the filament, the printing speed as you have found with your other tuning work. Coast is basically the slicer trying to achieve the same effect as pressure advance with less control. Wipe is trying to hide the problem inside the part.

Ultimately the best strategy is probably to tune each element and then use a combination to get the best results.

[wilriker]

@plasticmetal In my case this is about PLA. So far I only print PLA and occasionally some PETG.

[PlasticMetal]

Thank you for clarifying. Over the years I've come across many filament-related discussions where the type wasn't stated, making the information only marginally useful. From my limited understanding, some aspects of PLA's behavior is quite a bit different from other materials.

[Phaedrux]

PLA and PETG are definitely very different. Petg tends to ooze a lot more. ABS I've found is more forgiving than either.

The other huge difference is direct drive versus Bowden. Settings do not really transfer across due to the inherent lag and looseness in the Bowden tube system.

[wilriker]

Re bowden vs direct drive: on several occasions I stated that my direct drive extruder behaves more like a bowden setup due the rather long retraction distances I have to use. But as a result of this thread I think that was mainly me trying to compensate stringing by retraction length instead of speed.

So after tuning (max possible) speed now I will also need to tune retraction length and it might come down to a regular direct drive setup range.

This happens when one starts out with "recommended settings" without questioning the settings themselves nor the competence of the person who recommended them. But as always: I am happy to learn something new on this forum every day.

[Phaedrux]

@wilriker On my Titan Aero I only retract 0.8mm. On my old Titan V6 it was 1.2mm. Retracting too much is a great way to get jams by pulling the swollen and softened gummy filament up into the heat break.

[wilriker]

@phaedrux That is not really an issue with a MK8 Hotend as there basically is no heat break in the style that V6 have it. It is just a Teflon lined throat (M6 or M7 outer thread inside just the diameter so a PTFE tube will fit in) that goes from the heatsink (that is just a slab of aluminium with a fan - and some cooling fins) a very long way (30mm) down into the heatblock. So I think I could easily retract 10-15mm without getting into any issues as the heat is nicely creeping up the throat.

EDIT: I started out on the "recommended settings" of 7mm retraction at 25 (or 30, don't remember) mm/s and coast to end enabled.

[dragonn]

@dc42 said in More stringing with Pressure Advance instead of Coasting:

Stringing is controlled by reducing extrusion temperature (lower = less stringing)

This isn't always the case, with some materials I experience that too low temperature can increase stringing. I think this is because when the temp is higher the material is more fluid with allows easier break from the current printing line.

[wilriker]

OK, retraction test in progress. I sliced 9 of these guys on my build plate and selected to print sequentially. I then edited the GCode and set a new retraction distance for each specimen. Starting at 0.5mm going up to 4.5mm in 0.5mm increments. This will give me a good starting point and I can then find the two best and search for the perfect value in between.

One thing I realized is that I seem to be unable to control the order in which Slic3r prints when printing sequentially. Usually this does not matter but in my case I need to know which object was printed with what setting for retraction distance. I tried to rearrange the objects in the order Slic3r sliced them in the first run (which was random) but that again led to just another order. Is there a way I can have control over the order?

EDIT: I can answer my own question: one can do this by adding the same stl multiple times instead of adding more copies.

[digi2life]

@dragonn True for PLA, as I found out today. I've been trying to get rid of strings with z-hop enabled and the culprit turned out to be a too powerful layer fan blasting the tiny pyramids in the retraction calibration print. I turned the fan completely off as the print speed was already pretty low and the stringing improved about 80%. I had tried printing from 180C all the way up to 210C with stringing throughout that window (and poor interlayer adhesion). Without the fan, I found 195-200C was the sweet spot.

I have to give this site credit for the tip: http://www.sublimelayers.com/2017/10/musings-on-blobbing-and-stringing-part-2.html