A1 mini, PLA Basic, 0.4 nozzle, 0.2 mm layer height. Flat overhanging surface supported by tree hybrid / normal support.
The problem: several layers immediately above the support interface are visibly narrower than the rest of the wall, creating a trapezoid/staircase profile on the vertical face. The model has a sharp 90 degree corner with zero chamfer - no bevel is in the design.
Key observations:
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The slicer preview shows clean vertical walls with no stepping
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The staircase appears only in the physical print
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Tested with both tree hybrid and normal support - identical result
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Tested with overhang slowdown on and off - no difference
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Tried adjusting xy distance, Z distance, interface spacing, bridge flow, bridge speed - no change
Current support settings: Top interface layers 2, interface pattern Rectilinear, interface spacing 0.5 mm (also tested 0), Top Z distance 0.21 mm, xy distance 0.15 mm.
I am attaching test print images. I understand the test model could simply be rotated to avoid the issue, but the actual model I am working with is more complex and reorienting is not a practical solution.
Question: what causes the first several layers above support to print narrower than the rest of the wall, and how do you fix it?
All your edges look overly rounded, especially on that blue one. Recheck pressure advance/k-factor/flow-dynamics/same-thing
Beyond that, the lower your Top Z Distance, the more crisp that first line will be. There is an air gap there, that the machine is hovering over, while dispensing it’s tube of goo. at 0.2x gap it doesn’t get to flatten out and smush, it just kinda rolls over on top of the support surface and stays there. It gets rounded because thats what would happen to you too if you tried to dispense rope around a corner. At low 0.1x gap, there’s now enough filament being dispensed to get squished and make contact, so it might have a hope of getting fused down and held into place. But its fusing to support, so it’ll be hard to get off 
tune carefully!
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Thank you for the quick response! I’ll recalibrate pressure advance and will carefully test a lower Top Z distance - yeah, the bonding issue is real, been there before. On the overall edge rounding, good catch - will check that too, though the test model is pretty small at 15x14x5 mm so some of it might just be the scale.
And that actually makes sense about the Z gap - if the filament isn’t getting squished down it would naturally sit narrower and rounder rather than spreading out and making solid contact. So lower Z distance might address both the stepping and the edge quality at the same time. Will test carefully and report back.
Quick update on my testing. I went through all the suggestions - ran Flow Dynamics calibration (K 0.051 for my SUNLU PLA matte), reduced Top Z distance to 0.1 mm, tried both tree hybrid and normal support, tested inner/outer/inner wall order, adjusted small perimeter threshold to 2 mm, tweaked bridge flow and bridge speed. The staircase effect improved only slightly with the lower Z distance, everything else made no noticeable difference.
Interestingly, a friend printed the same model on an A1 (not mini) using Bambu PLA Basic matte with default settings and got a perfect result - clean vertical face, no stepping at all.
So the variables between our prints are: A1 mini vs A1, and SUNLU PLA matte vs Bambu PLA Basic matte. I suspect my filament moisture level might be a factor since it hasn’t been dried recently. Has anyone seen filament moisture affect overhang and support transition quality this significantly? And is there any known difference in how A1 mini handles this type of geometry compared to the full A1?
Correction: A1 prints exactly the same way as A1 mini – same issues.
Just to be clear, before you reach a dead end: Have you been looking at the problem from only one angle? In fact, there are three basic approaches that should be applied simultaneously.
- Optimizing printer: Calibrate your printers, adjust settings, clean. Choose the right printer plate, etc. Trying different support settings.
- Optimizing filament: Different brands, different materials, different supports, two different materials for the support and the object. Drying filament. Keep the ambient temperature in mind: Open windows and doors are your enemy.
- Optimizing 3D model: Often neglected: alignment, rounding, and making more printer-friendly.
This kind of thing often fails because people only consider the problem from one angle when, in fact, you should optimise all three aspects. The real work actually begins with making the 3D object printer-friendly - that’s essentially the foundation.
I suspect that you’ve become so fixated on printer optimisation that you’ve seriously neglected the other two aspects.
