PrusaSlicer: How do I preserve the location of (interlocking) parts in a muliti-part model exported from OnShape / Blender?

Question

How do you import a multi-part object into PrusaSlicer without losing the position of the parts? For example, I want to print a two-material interlocking structure like this:

I've tried exporting this from my CAD software (OnShape) as a mesh (STL or OBJ) but when I import it into PrusaSlicer (v2.7.0) the result is either a single object with no internal walls:

or (3MF) two objects placed on the build plate individually, so they no longer interlock:

If I use a STEP file it at least preserves the position of the parts within a single object, and there are some internal walls, but no internal top and bottom layers which I want for structural reasons (in this case the dovetail will simply crush in the XY plane if the part is tensioned in the Z axis):

What's a good way to import multiple parts and have them "print in place", like they were in the original design?

Answer 1

I've found a way to do this, which I'll document here in case it helps someone else in the future.

• Export your parts as a single STEP file
• Import that STEP file into PrusaSlicer
• Set Print Settings > Multiple Extruders > Advanced > Interface shells
  • (You need to be in Expert mode to see this option)

The STEP file adds the internal walls, and this interface shells option adds the internal top and bottom layers I was missing:

Answer 2

Despite how featureless the STL mesh looks when imported, all the data is there and you can split it to parts using the right click menu. I feel silly for now having spotted this before .

The result is exactly as I had hoped:

My take away is that PrusaSlicer does not always prompt you to split your important mesh into multiple parts; so its always worth attempting to split to parts using the menu after import.