Why inner walls on the first layer?

[vaar]

Before getting the A1 mini I was using PrusaSlicer. When I slice this model:

I got this first layer, which is what I would expect since the bottom is flat:

However Bambu Studio splits the first layer with an inner wall:

Why? That inner wall serves no purpose. Which setting affects that? I can remove the wall by enabling Quality → Only one wall in first layer, but I want all the perimter inner walls, just not that inner wall.

BTW, I know I can remove the wall by lifting that part above the rest in OpenSCAD, but I would rather not change ALL my models, which until now printed ok in PrusaSlicer, because of this.

[MalcTheOracle]

Without the model to review this is guess work, but, I believe this is the reason.

All walls are printed over the top of structural support (not traditional support).

The base is printed with diagonal lines, this would mean the wall printed above would be supported by those diagonal lines.

Bambu Studio uses the technique of providing structural support a couple of layers earlier. In this case the straight line is the start of that structural support by proving an even support underneath the start of the coming wall.

You will likely find the same thing occurs if you raise the start of the wall height for the same reason.

The same thing happens if you add a cylindrical hole in a model, the base of the cylinder begins a couple of layers earlier than the cylinder begins.

All of these structural items aim to give the eventual model more strength.

[vaar]

Ok, simplified example, in OpenSCAD:

cube([20,20,0.2]);
cube([10,10,0.6]);

Bambu Studio first layer:

PrusaSlicer first layer:

I can get this in Bambu Studio by doing:

cube([20,20,0.2]);
translate([0,0,0.2]) cube([10,10,0.4]);

But obviously I’d rather get the same results as PrusaSlicer by touching the settings instead of all my models… Is it possible to remove that structural support, if this is indeed caused by that?

The funny part is that the first layer is done correctly in one sweep (i.e., one big square, not one little square + one L-shape) but it’s printed over that yellow line, which b0rks the first layer).

[MalcTheOracle]

@vaar

cube([20,20,0.2]);
cube([10,10,0.6]);

Do me a favour and union these two.

I do not know if it will solve it as I am not near my computer right now (on iPad). But, it might because of what that command does.

union() {
    cube([20,20,0.2]);
    cube([10,10,0.6]);
}

This should treat the resulting shape as a single entity rather than two shapes occupying the same space.

[vaar]

No, it generates exactly the same stl file.

FWIW, the problem only appears with a single layer cube. If I increase the larger cube to 2 layers (z=0.4mm instead of 0.2mm) the problem no longer appears: the first layer is a single smooth plane.

The second layer is as expected:

This is different from the first layer in the previous model: now it prints the smaller square and then an L shape around it, instead of a single square OVER the inner wall.

But I want a continuous single layer…

Oh well. Since the problem is specific for single layers I’ll modify the model. The other models are not affected by this bug (because IMO it is a bug; it should treat the first layer as it does in the 0.4mm version).

[MalcTheOracle]

@MalcTheOracle

union() {
    cube([20,20,0.2]);
    cube([10,10,0.6]);
}

Can you share the code you used to avoid the issue you described?

I am in front of my computer for a short while and I would like to experiment.

[vaar]

Sorry, I must have been mistaken. Now when I try:

cube([20,20,0.2]);
translate([0,0,0.2]) cube([10,10,0.4]);

which I though resolved the problem, I get exactly the same as with:

cube([20,20,0.2]);
cube([10,10,0.6]);

which means I cannot print what I want: a single layer square with an object above it. I thought it was the intersection that caused the problem, but it isn’t, because in the first example it’s just a cube above the other.

(edit) You can also try with two layers to make the first layer ok (assuming layer height of 0.2):

cube([20,20,0.4]);
cube([10,10,0.6]);

but obviously it’s not the same. I’m putting leds behind that layer, and I want it as thin as possible.

[MalcTheOracle]

Yeah

I tried all variations and had the same outcome as you originally noted.

Which is why I wanted to see what changed. Nothing it seems.

I still believe this is a structural support issue.

I created two cubes and merged and stacked them.

[vaar]

Yeah, PrusaSlicer does something similar with a thick cube, but still does not interfere with the first layer with the single layer square. Bambu Studio should do the same. As it stands, it’s impossible to print something over a smooth, thin layer, at least with an stl generated in OpenSCAD.

[blub]

Hey, I’m having a similair problem, did you manage to find a way to stop the slicer from inserting these walls into the bottom layer?

[vaar]

Nope. I was going to report a bug but I forgot. I ended up slicing in PrusaSlicer.