Nozzle path is ok but some sections are not printed
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The is a bit of an odd problem that has me stumped. Let me set the stage here:
0.8 mm nozzle, 0,4 mm layer height, PLA, 210C, Volcano hot end, BMG extruder clone. 50 mm/sec for normal print speed.
The model has been checked and double checked, layer by layer.
The issue shows up on the solid layers on the bottom or the top.!
The picture shows the first layer of the model looking with the print laying upside down. (being first ayer, this was printed at 25 mm/s and 0.3 mm layer height)
You can see that there is one or two lines of print missing.
It has been verified that the nozzle traverses the section where the print is missing. It's just not extruding anything. There are three instances of missing lines in this picture.
You can see that everywhere else the print is looking quite nice. I have verified pressure advance because that was my first possible thought even though it didn't really look like a pressure advance problem (btw, around 800 mm worth of bowden tube).
Now here is the big clue .... as a solid layer is laid down in zig zag fashion, eventually the nozzle will run into a feature that prevents the zig zag from continuing. It goes to a different area and carries on. Eventually, the nozzle will want to fill in any areas that weren't filled originally because of obstructions. This is where the problem occurs ... and ONLY at these spots and only at the beginning of these areas.
Material is laid down perfectly (relatively speaking) everywhere else but when the printer fills in the spots of the current layer that were missed, it will miss one, two and sometimes even three lines of the zig zag.
This can happen on any solid layer (so not a first layer adhesion issue), I have not tried a different fill pattern.Anybody run into this issue before ?
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Here is another picture that might give more hints:
Looking at the picture, in the center you can see material laid down at the edge and then you see the material flow decreasing until it peters out about 2/3 along the way.
I do not know if the laid down material starts slowly or if it tapers out slowly (which could just be drool from the large nozzle volcano)
I suspect that what I am seeing is a slow start of extrusion (edit: confirmed that it's a slow start to extrusion) .... thinking about why this might be, is it possible that what I am seeing is a partial blocked nozzle that the BMG is just doing it's darnedest to push through ?
Currently retraction is 1.4 mm at 30 mm/sec for retraction and prime speed.(Guess I have to either drop the nozzle a tiny bit or maybe add 5% to my first layer flow - didn't notice the gaps before)
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@jens55
We are using the same extruder/nozzle combo.
My PA setting is 0.05, the retraction 1.8mm @60mm/s.
With my BMG clone, I had to use a longer thumbscrew, because the spring was pretty big and the screw would only have one turn in the brass insert. With the strong spring, it was almost impossible to adjust idler pressure, so I replaced it by a softer one. Maybe there's your problem, you are killing the motor with the prime move? -
Thanks for chiming in. You must be using a direct drive system rather than a long bowden tube or else you mis-typed your PA setting. I am about 10 times higher on the PA at 0.48 ....
My idler pressure is set with the knob about 90% turned in so pretty hard. -
An update:
I have reduced nozzle height by 0.1 mm which helped a tiny bit but did nothing to the underlying problem. I reduced retraction from 1.4 to 0.9 mm - no change. Turning combing from 'not in skin' to 'off' seems to have eliminated the problem at the cost of some errand ironing lines on the top surface and some stringing.I am really no closer to understanding what exactly is happening.
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Rather than zipzap try lines?
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I am just finishing a print with 'concentric' at the top.
I will try 'lines' for both top and bottom for my next try.Update: based on the first layer, the problem is present with 'lines' as well.
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@jens55 I'm thinking it's related to PA and the long bowden tube combined with the fat nozzle and volcano. Perhaps in this combo pressure advance is backing off the pressure too much too soon.
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As mentioned in the beginning, this is a 0.8 mm nozzle. What I did not say was that line width is set to 1 mm.
Could this somehow be related to nozzle or line width size?
Using 'lines', the filament feed stops at the end of a line, the head moves over one line width and extrusion starts again. There is no problem with the stop/start of the filament so how could the start of the extrusion on a surface fill, after the combing moves, be any different.
I am starting to wonder if throwing in a 0.4 mm nozzle might be worthwhile.
The same model printed on a CR10 with the stock controller, a 0.4 mm nozzle and Marlin seems to not have this issue. (the issue occurs on a CR10-5 with Duet)
For completeness sake, I should mention that the cr10-5 is set up with two extruders but the model is only using one extruder.The extruder and hotend are running at their limit as I get occasional skipped step noises but that does not apply with the first layer that shows the issue as well. I will be printing the next model at 30 mm/s instead of 50 mm/sec to eliminate over-taxing the hot end and extruder.
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Do you happen to have coasting enabled in cura?
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@Phaedrux said in Nozzle path is ok but some sections are not printed:
@jens55 I'm thinking it's related to PA and the long bowden tube combined with the fat nozzle and volcano. Perhaps in this combo pressure advance is backing off the pressure too much too soon.
While it could be related to the items you mention, I can't agree with the 'pressure advance is backing off too soon'. The issue happens after the nozzle has traversed a bunch of the model (to stay within the print) and it happens at the beginning of the new extrusion starting.
Further, PA seems to be right on the spot as the 'lines' pattern that starts and stops constantly shows nothing unusual.
Now it is quite possible that my fundamental understanding of PA is flawed in which case it would be nice to understand why a short stop of the filament flow followed by a start works ok but a stop flow followed by a bunch of non printing moves followed by an extrusion start would cause a delay.As mentioned earlier, I can only explain this in my mind if the filament in the nozzle had a chance to cool and expand a bit in the retracted position and the extruder had to work against the resistance of breaking that not-quite blockage.
I obviously am missing some magic incantation here someplace .....
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@Phaedrux said in Nozzle path is ok but some sections are not printed:
Do you happen to have coasting enabled in cura?
No.
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I just saw it first hand .... printing fine, a number of non printing moves and then, when it starts to print again, there isn't enough filament coming out of the nozzle.
This does not seem to happen if there are only a couple of short non-printing moves. It is definitively related to the inactivity of the nozzle.I will turn retraction off on my next print test ......
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Last test for now ... RETRACTION DISABLED, print speed 30 mm/s 15 mm/s for first layer.
I just saw it again .... after doing a number of long non printing moves because of combing, there is no filament being extruded when the printing moves 'should' be starting again. I just saw three lines (about 3/4" long each) that should be laying down material but behaving as if the nozzle was not primed. A tiny drip here and there before a line starts forming. Exactly the same thing that occurs at a bigger scale when the print first starts and it prints the skirt in order to prime the nozzle. This particular example was more of a missing print area then before. I am not sure why it went on for three lines before recovering and starting to extrude again.?????????????
Edit: The only thing that seems to fit is that there is enough material drooling out of the nozzle during the non-print moves that there is no material in the nozzle to print with.
There is no obvious excess material where the nozzle passed during it's non printing moves but it could very well be squishing/distributing so thin that I have not noticed it. -
Slow down the unretract speed?
I've never had good results with combing. I usually run with it off.
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@Phaedrux said in Nozzle path is ok but some sections are not printed:
Slow down the unretract speed?
I've never had good results with combing. I usually run with it off.
The last test was with retract disabled so I assume 'unretract' speed doesn't apply.
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With retraction disabled I'd imagine the problem would be worse.
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I think that this explains why the missing filament on the last print was worse (because of no retraction). Maybe I should try increasing the retraction ?
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No change with going to 2.0 mm retraction. I am now back to 1.4 mm.
I am now printing with no combing and 0.5 mm z hop to eliminate the stringing and so far things are actually looking pretty darn nice!
Although I don't have absolute proof, I am assuming that the liquid filament drools out of the hot zone during the combing moves and then, when it comes time to print again, there is no filament available because it leaked out.
I welcome suggestions how to fix this if there is a fix.Although I am currently printing in PLA and using Z hop is working ok, it generally creates a mess when printing in PETG and most of my prints are in PETG.
@Phaedrux, you indicate that you mostly print with combing off. Do you use Z hop for PLA to reduce stringing? What do you do when printing PETG - still without combing? Z hop ?
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I don't really have a stringing problem but I am using a 0.6mm nozzle and V6 titan aero. So less of a drool problem. 1mm retraction. Same for PLA and PETG. No z hop. No combing.
The reason I suggested lowered unretract speed is that I sometimes notice a bit of flow hesitation at line start with some filaments.
