Multiple joined components in one STL generate shell walls between components

[Aubs 0]

I don't normally ask questions on boards, because most of the answers are already out there, or I figure it out myself.  This one has stumped me.  I like Cura and don't want to change.

 

I have already asked about this on the Fusion 360 board (https://forums.autodesk.com >> multiple-components-to-stl-have-shell-walls-between-components), the response was:

Quote

It's your slicer, not Fusion 360.  Check Cura settings to see if it has any options on how to handle multiple bodies.  I have Cura, but don't use it. Prusa Slicer works better for me.  It handles multiple bodies the way your are expecting.  The screencast shows how it handles your two stl's as well as one I created.

 

For completeness, I've included the information from that post:

 

I've been struggling to work out how to export multiple components to STL, and I hope someone can help me.  I've searched extensively online and the only results come up with how to export multiple components, but none appear to relate to the inner shell walls.

 

If I turn on only the components I want, then export Save As STL from the FileName at the top of the browser, import to Cura, I get the first image, which has inner shell walls between each component. As you can see, there is an overlap of the wall which creates a bulge in the printed piece. [Plus the inner/outer walls between the two components]

 

If I merge the components to a single new component and Save As STL, it all looks as one piece with the infill correcly flowing between components and only a single outer wall where there should be one.

 

Any assistance to point me in the right direction would be appreciated.

 

 

I know there are several similar posts, but neither Union Overlapping Volumes nor Remove Mesh Intersection resolve it, I've also tried Binary and ASCII exports from Fusion 360 and same.

 

If I export a new STL from Cura (Binary or ASCII) and open that (as per the old thread above), it is still the same.

 

 

Attached

Images of the uncombined and combined

Binary STL of the uncombined and combined from Fusion 360

3MF export from Cura 4.9.0,

Binary STL exported from Cura 4.9.0

 

Configuration

Windows 10 (19041)

Fusion 360, Personal 2.0.9937

Ultimaker Cura Version 4.8.0 [Tested today on 4.9.0, only difference is white 'Starts']

TestOverlap v1(uncombined).stl TestOverlap v1(combined).stl CE3_TestOverlap v1(uncombined).3mf CE3_TestOverlap v1(uncombined).stl

[nallath 1,125]

It's not the slicer really. If you don't combine the parts, there will be a surface between the two objects. Since STL doesn't actually define what is inside or outside (it only defines surfaces) you get weird results like that.

I've taken a screenshot of the uncombined version of the file to show you what the issue is:

 

As you can see, there is a small gab between the two objects. This gap is caused by the tessalation (eg; It needs to create flat surfaces, so a round surface is approximated by multiple flat ones). This process will pretty much always introduce inaccuracies, which can lead to gaps like this.

The result that you see is basicly your slicer doing exactly what you ask it to do; There are two components and they are printed as two components.

[Aubs 0]

Thanks @nallath for your advice, I now understand what it's doing and the rationale behind it.

 

As much as I've got to like Cura's simplicity and ease of use (even in Expert mode which I am certainly not!), it's a shame it doesn't have similar capability as to what PrusaSlicer appears to do, that is, see both components (it calls them parts) in the single STL and when combined, slices them both as one.