odd vertical imperfection - bending outward layer (Prusa CORE One)
Hey everyone. I have a very challenging problem that I've never had before. This design has a middle bottom a few millimeters above the print bed. See the section view of the design (2 versions):
When I print these on the Prusa CORE One, the print has always an odd vertical imperfection that results in a bending outward layer exactly at the height of that middle bottom (see image below):
Print settings on the CORE One:
- PLA - tried various high quality filaments from well-known brands.
- Standard print settings for 0.2 layer (speed and structural)
- Only modification 3 perimeter (does not make a difference in the result if 2 or 3 perimeter)
- Vent grill and door is open
- PrusaSlicer 2.9.3
Despite several year of experience with FDM and various printers, I couldn't figure out the root cause. I've already tried the following with no difference in the results:
- other filaments
- other printer
- different settings like speed, structural
- change the seam position e.g. to rear
- changed the nozzle of the nextruder
My observation and conclusion so far of the root-cause: this odd imperfection seems to be caused by some stress of this middle bottom in conjunction with the infill perimeter. If I remove this bottom completely then the quality is perfect. Or if I would use concentric infill, that has almost no connection between the bottom and the infill structure. But this infill lacks stability of the part. Strange in addition is the fact that the print result is much better on a Prusa i3 MK3S with much less visible outward / imperfect layer at the bottom height.
Any other ideas or recommendations? You may download the 3mf.file here and print with other settings. Very curious if anyone get better results or has ideas on how to improve.
RE: odd vertical imperfection - bending outward layer (Prusa CORE One)
A bit late to the party, but I just downloaded the 3mf and had a look:
Usually, these "benchy hull line" issues are caused by:
- fan speed changing
- layer time changing
- inaccuracies due to heavy acceleration (look for weird patterns in certain layers etc.)
- speed changing
What I can spot for your 3mf:
- fan speed is different for that area
- layer times is different for that area
- the infill does some very different movements towards the walls for a single layer in that area due to "bridge infill" being used in that layer too.
Solutions:
- print slower (use stealth mode, which lowers max. speeds for everything from 350 mm/s to 160 mm/s and lowers max. acceleration values a lot)
- use rounded edges for the center area (fillet). If you can't due to fitting issues -> use negative fillet => "pipe" + 2x fillet for the edges of the pipe.
- set the min fan speed to 100% for PLA. It's at 85% for the Buddy3D PLA preset
Also, Grid infill sadly isn't great in my experience.. Coming from the mk3s, I wondered what that "brrrlrlrlrlrlrlr" was, while printing infill.
Grid infill is printing into its own lines in a 90° angle, hitting the already laid down filament. Usually it's not an issue, but it can cause issues like skipping belt teeth, building up filament around the nozzle, clogging the nozzle, wearing down the nozzle etc.
If no real stability from the infill is needed -> zig zag.
If a bit of stability is needed but speed is important too -> aligned rectilinear.
If speed isn't that important and you want really good stability -> honeycomb
RE:
Earlier, we've had some long talks about this issue. It is present in nearly every printer when using Prusa Slicer. Other slicers seem to handle the issue better. I'm too lazy to search for that old thread, but it's still out there in the Mk3/Mk4 forums. Images may be gone because of the server changes a while back. But look for BobStro's and "-" comments, too. A few others were part of the discussion and had their theories, too.
But, printing external perimeters first was one easy "improvement" ... reducing infill-flow and/or infill extrusion width was another. And as Raz said, ensuring print head speed is consistent across all layers also helps. Cooling settings can affect print speed. Also, directionality of the head movements...