Bottom layer is smaller than the walls

I have done 3 different prints, all modeled by myself in fusion360 and then sliced in Bambu Studio. All examples are with a “bottom” and then freestanding walls on top. In all three prints the bottom is narrower than the walls and then make a dividing line.
It’s anycubic pla, it’s my only choice of pla at the moment.
What am I doing wrong?

I have this issue as well with any filaments, includes Bambu Basic PLA.

It appears the bottom layer with infill and top surface cool and pulls in the wall. The bottom curve in up as it goes up until the layer doesn’t have infill or surface.

I have opened a support ticket but I am also trying different setting. Maybe higher fan part cooling.



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Yes, I think this is exactly what is happening. Have you tried printing the part in silence mode to see if a speed change helps reduce the issue?

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It makes sense. Would love to hear if you find a solution to the problem, and also Bambu’s respond to your ticket.

I made a test model, one side is 5mm wall and other side is 1.5mm wall.

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The 5mm wall has infill continue from the base and the inward shrink is less or nearly not visible.

But the 1.5mm wall doesn’t have infill continue and actually layer on top of a top layer. The top wall is accurate at 40mm width but the top layer under the wall shrunk to 39.75+/-

My workaround is to avoid making the wall from a base too thin, but that causes many limitations or complications in design.

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Hey all, I got a response back from support; see if you can solve this as well.

Hello,
Thank you for contacting Bambu Lab support, thank you for your patience, and we apologize for the delayed response. We had staff off over the Christmas break and we are doing our best to catch up on the backlog.
My name is Rick, and I will be assisting you.
I am sorry to see that you are experiencing problems with our product. We will do our best to assist you and provide a solution to your problem.

The area on the outside wall, where it swells appears to be a wall bulge. You can see that the bulge corresponds with the floor of the print.

You are also printing in PLA, which is prone to shrinking. During the 3D printing process, plastic undergoes several physical changes that can cause it to change size. These changes are primarily due to the heating and cooling cycles involved in the process:

Thermal Expansion: When the plastic filament is heated to its melting point and extruded through the 3D printer’s nozzle, it expands due to the increased thermal energy. This expansion can cause the material to temporarily increase in size.

Shrinkage during Cooling: After the plastic is extruded and forms the desired shape, it begins to cool down. During this cooling process, the plastic undergoes shrinkage as it loses heat and returns to its solid state. This shrinkage can cause the printed object to be slightly smaller than the intended size.

Regarding the wall bulge - This problem occurs at feature change points where the print has a solid layer, it may be related to the slicer settings or the specific model you’re printing.Here are some suggestions to address the issue:

  1. Check your slicer settings: Ensure that the wall thickness and the number of solid layers are set correctly. Adjusting these settings might help improve the print quality.

  2. Reduce infill/wall overlap: Decreasing the overlap between the infill and walls can sometimes resolve bulging issues. Experiment with reducing this overlap to see if it makes a difference.

  3. Try alternative wall generation: Please try the classic wall generator (you are using Arachne). It could be worth trying it to see if it helps with the issue. Different wall generation algorithms might produce better results for your specific model.

  4. Try turning on detect thin walls and see if it helps with the issue.

As an aside this issue has been around for quite some time on different slicers. If you google “wall bulge prusaslicer” you will see the same phenomenon you have.

If you can control the wall bulge, so that the fit keeps in line with the rest of the walls, you then have the option to change the dimensions of the model. To do this, you can either increase the flow, ratio marginally, and this will make the print slightly bigger (decreasing it will make it smaller). You also have the option to change the size in the XY contour compensation. The following images can be used as a guide on where you go to make these changes.

Another option you can try is to reduce the build plate temperature so that the bottom layers are not contracting as much.

There are a few things to try in this, and hopefully they will influence the print result. If it is a wall bulge issue, it is not easy to rectify this issue with certain models.

I look forward to your response.
Kind Regards
Rick
Bambu Lab Support

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It’s good to see their reply. Ill try their suggestions, maybe one at a time, to see how it results.

Did anyone have any luck with the proposed changes?
I can’t seam to get rid of these.

I tried all of their suggestions but it didn’t get rid of that shift. Basically what I’ve observed is that this happens when you have a floor surface and when it gets to the walls above that surface, that is when the layer shift is really noticable.

My workaround:

I make my floor area very dense (40-60% infill using modifier on the floor portion. or you can set entire model with that high infill percentage if you wanted to) or I make the floor a completely solid floor (increasing bottom shell layers). And I also set arachne and outer/inner. Those settings gets rid of that layer shift.


These photos without the layer shift have very dense infill (yellow) and other has solid floor (bottom shell layers = 11). Also set to use arachne and outer/inner. This has been my solution, no layer shift.

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Would you mind sharing which filament was used in the picture?

Hello, in my photos I was using Bambu PLA Basic in Yellow and also in BlueGray.

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Thanks! Did you print those models in 0.2 layer height? The walls look really smooth!

Yes, that’s with the .4 nozzle and .2 standard layer height. The walls do look really nice, I’ve really liked the bambu pla basic filaments for their print quality. Also, I used a flash on my camera which tends to hide imperfections sometimes, but these did turn out really nice.

They look really great! I’ve been trying to get rid of the wall bulge for months but haven’t found the solution. Thank you for sharing the solution to us. What are your flow rate and max volumetric speed?

Have you tried this using ABS filament?

Those prints in my photo were using Bambu PLA Basic filament profiles which I left unchanged, having flow ratio .98 and max volumetric speed 21 mm3/s. I’ve not tried ABS.