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Material shrinkage pulling in vertical side walls  

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Brad
 Brad
(@brad-2)
Trusted Member
Material shrinkage pulling in vertical side walls

This is a problem that I often run into (on both my printers) and I would be interested to know how you guys deal with it. I have here a large tray with a bottom surface and [what should be] vertical side walls. I assume that the problem is caused by the large flat surface shrinking and pulling in the bottom section of the sides (by over 0.5mm in places but slightly less at the corners). This model is a good example of the problem and is the full size of the MK3 bed and printed in Prusament PLA (9+ hours).

Posted : 30/06/2021 5:41 pm
bobstro
(@bobstro)
Illustrious Member
RE: Material shrinkage pulling in vertical side walls

Are you referring to the change in the vertical walls? Do you get the same variations when printing a perfectly vertical surface like a cube? If you see variations in solid prints like a cube, it could be a hardware issue (e.g., bearings causing layer shift). I don't think that's the case here. There is a common phenomenon that you'll encounter as vertical surfaces vary ever so slightly during a print. This was given the unfortunate name of "buldge" in earlier posts. Deviations in wall thickness can occur as layers transition:

  • Between solid infill and sparse infill. (I think this is what you're seeing.)
  • Between sparse infill and gap fill
  • When the number of perimeters increases or decreases between layers.
Happily, the "buldge" misspelling in that earlier post has made the problem very easy to search for. It's not truly a bulge but can give that appearance in some circumstances. There are several threads you might want to look through:
Unfortunately, no single fix has been identified that will work in all cases, but a few things that help:
  • Be sure you're not just seeing the effect of minor warping/lifting/curling along edges or in corners. This can really throw troubleshooting off. Rotate the print and verify the problem occurs in the same place.
  • Slow down external perimeter speeds (and all speeds in general). If the nozzle is moving a bit too fast, you get slight under extrusion on some layers. These are apparent as adjacent layers print with slightly different extrusion rates. I use 25mm/s for external perimeters when appearance is important.
  • Calibrate your extrusion multiplier for each filament. Any slight over or under extrusion can produce very small but noticeable variations in layers with different features (e.g. infill, gap fill, top solid infill). The closer your slicer settings match your actual printer and filament, the more accurate the gcode will be.
  • Calibrate linear advance (LA) for each filament. LA adjusts the flow of filament to compensate for acceleration and deceleration. If it's not right, you may see artifacts even away from features such as bumps or hole on the same layer. In some cases, a hole on one wall causes imperfections on the far side of the print.
  • Add an external perimeter if vertical walls allow it. The thicker combined perimeter allows the filament flow to even out.
  • Tweak perimeter extrusion widths. The problem can appear when the slicer switches between gap fill, sparse infill, and solid or top infill. If you can find a multiple of perimeter widths that minimizes these transitions, it can work for a specific print.
  • If you are the part designer, make vertical free-standing walls thicker. IME, at 1.5mm thick, the problem is less noticeable. You can try for a multiple of extrusion widths, although be aware the PrusaSlicer does some internal calculations for overlap between extrusions that can throw you off. PrusaSlicer gives hints about optimal thin (free standing) wall thicknesses for different print parameters.
  • Check cooling. Variations in fan speeds can cause changes in print appearance.
Unfortunately, with the current state of FFF consumer-grade 3D printing, we still have to do some hand-tuning for specific prints. I've gotten to the point that I can usually eliminate the effect to my satisfaction. Dig through those threads for examples. Contrary to some theories, this is not exclusively a PrusaSlicer thing
 
In your specific example, slice the part and spend some time in preview mode checking to see if the defect aligns with any shifts in infill, solid layers, transition from perimeters to sparse infill or gap fill, etc. If you'd save your part & settings off in a 3MF project file, zip it up, and attach it to a reply here, we may have better suggestions.
 
Looking at your pictures, it appears (to me) that you might be able to tune this issue out a bit, but pictures under specific lighting conditions -- particularly from above -- will highlight surface inconsistencies. Sanding may be required if you need a truly smooth surface. You might want to follow the Cura slicer development. They've recently released an Alpha of their Arachne slicing engine that dynamically adjusts perimeters based on wall thickness. It's still early alpha, and not yet producing ideal results, but it's worth a look. There's a lot of cross-fertilization between slicers, so something similar may evolve for PrusaSlicer in the future.
 
My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 30/06/2021 6:05 pm
Brad
 Brad
(@brad-2)
Trusted Member
Topic starter answered:
RE: Material shrinkage pulling in vertical side walls

@bobstro

Thank you very much for your detailed reply. I have read through your comments and followed the links that you provided. I had not previously seen the info from Prusa on their research into the "Benchy Hull Line" and I am sure that this is my issue.

Do you know if the Benchy Hull Line and the "Buldge" issues are the the same issue, two different issues or two different but potentially related issues?

Thanks again

Posted : 01/07/2021 4:06 pm
bobstro
(@bobstro)
Illustrious Member
RE: Material shrinkage pulling in vertical side walls
Posted by: @brad-3

[...] Do you know if the Benchy Hull Line and the "Buldge" issues are the the same issue, two different issues or two different but potentially related issues?

I'd put this in the category of "things to be aware of". There are a LOT of different things that can cause similar-looking problems. The "buldge" category -- at least as I'm defining it -- is really about minute extrusion deviations within a layer. If you transition from, for example, sparse infill to solid, or if there's a change in the speed or direction it takes to print a layer (e.g., there's a hole in a wall), the small changes in speed and extrusion can create very small (sub-mm) but perceptible variations in wall appearance. Each layer is calculated independently by the slicer, so there can be very small but noticeable variations between layers depending on what else is going on within the layer. A print feature (opening, floor, etc.) on one side of a print can shift a flat wall on another side.

There are other problems that can look very similar:

  • Temperature deviations can cause variations in wall thickness. If you print in a drafty area, this can be a problem.
  • Z binding can cause variations in vertical walls. These will tend to show repeating patterns (e.g., every X mm a bulge) regardless of the underlying print shape.
  • Belt tension can cause irregular patterns. These may cause patterns independent of the print shape, or cause dramatic layer shifts once in a while.
  • Loose belt and idler screws.
  • Cats.

They key characteristic of the "buldge" issues (it's a stupid name, but it sure is easy to search for) is that irregularities show in relation to the print itself. You'll see odd lines on the side opposite an opening or transition to solid infill for example.

Don't get hung up on the name. It's just a silly shorthand way of saying "deviations in vertical print surfaces based on internal features" or something like that. 😀 

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 01/07/2021 4:38 pm
Brad
 Brad
(@brad-2)
Trusted Member
Topic starter answered:
RE: Material shrinkage pulling in vertical side walls

@bobstro

Thanks again.

As you say - cats most certainly can cause a deviation. My daughter’s 3D printer deviated to the dustbin via the floor (she could no longer level the bed after the cat had knocked it off of the table so she threw the printer out). Fortunately, not a Prusa though!

Posted : 01/07/2021 5:00 pm
ssill2
(@ssill2)
Noble Member
RE: Material shrinkage pulling in vertical side walls

@bobstro

Why the hate on cats 🙂  I'm sure dogs could do the same!   I do have to keep my rubbish bin with a top on it because the cats like to get in and pull the skirts from my prints out and play with them.   I keep my printers in enclosures and the cats seem to steer clear lol

Posted : 01/07/2021 5:56 pm
bobstro
(@bobstro)
Illustrious Member
RE: Material shrinkage pulling in vertical side walls

Suggest we use the term "caht" to make feline-induced print anomalies easier to search for. Or "catdge"?

One evening I tossed a whiffle ball for my 65lb bulldog to chase, not thinking much about it. I rounded the corner to my office to catch him as he was preparing to jump up on my desk to fetch it. I think that'd be a different category of print defect.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 01/07/2021 6:09 pm
Brad and ssill2 liked
ssill2
(@ssill2)
Noble Member
RE: Material shrinkage pulling in vertical side walls

@bobstro

you were lucky.  it would be a shame to have to wait months to get a new printer!

Posted : 01/07/2021 6:13 pm
Brad liked
bobstro
(@bobstro)
Illustrious Member
RE: Material shrinkage pulling in vertical side walls
Posted by: @ssill2

[...] you were lucky.  it would be a shame to have to wait months to get a new printer!

This guy has caused a couple of layer shifts...

 

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 01/07/2021 9:22 pm
Brad
 Brad
(@brad-2)
Trusted Member
Topic starter answered:
RE: Material shrinkage pulling in vertical side walls

@bobstro

I'm sure he has!

Posted : 01/07/2021 10:37 pm
ssill2
(@ssill2)
Noble Member
RE: Material shrinkage pulling in vertical side walls

@bobstro

Looks like trouble 

Posted : 01/07/2021 10:43 pm
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