good and bad perimeters in flex prints

Hi,

Ive been printing in flex materials recently and i've observed this weird behavior. The outer perimeter looks pretty bad and patchy but then for a good portion it looks normal.



The perimeter speed is around 10-15 mms and I see no other underextrusion artifacts.
Top surfaces look normal.
The good and the bad portions are the same all around the model and I cant see any reason why this part of the model should be good or bad. The different portions dont have different detail levels or anything.
I tested it with two different flex materials.
Sliced on CURA 4.1
I also printed one in SlicerPE which had the whole print look bad.
I print with .8 nozzle on a volcano with a bondtec QR extruder.

Any ideas whats causing this?

Cheer

Maybe you're trying to print too slow. I use 30-40 mm/sec (with smaller nozzles) for TPU filament and get consistently good results. At 10-15 mm/sec the hot nozzle may be melting the previous layer and distorting it, or maybe the previous layer isn't solidified because of the large volume of plastic due to the large nozzle diameter.



Edited 2 time(s). Last edit at 09/18/2019 11:05PM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

maybe, earlier versions of this model i printed with 25mms and my setup can handle that and it still looked bad
also speed would not explain why only certain parts are bad because the perimeter speed doesnt change


this is screenshot from cura, the outer wall speed looks consistent

What controller board is in the printer?

If it runs reprap firmware you need to tell Cura to use relative extrusion coordinates. Cura defaults to absolute extrusion and that can cause strangeness in prints from machines with Duet boards.

Is the drive gear pressure in the extruder set too high? I typically have to loosen it up when printing flexible filament. If the pressure is too high the filament will squirm as it comes off the drive gear and that may lead to inconsistent extrusion.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Reminds me of some of the benchy pictures we used to see.
Im observing that the good bits are where the outside tends to be vertical. May be its due to the wall thickness. How many peripheries are you printing? What extrusion width? What infill?
It does seem like this occurs with certain geometries so i do wonder if theres some complex combination of dimensions that gives the controller/motor issues. These are stepper motors not analog and theres all sorts of rounding of numbers going on. I just wonder if sometimes it hits that point where it could either be that step to the right or the left and it ends up varying between the 2.

@digital dentist:

that could be something. im using duet and ill do a print with relative extrusion tonight. thanks

update:

turns out its the temperature. I had to lower the temp by 10C and now the extrusion is buttery smooth. dont ask my how the temp apparently dropped so much without triggering runout errors, but now it works.