Hi Nick, so looking at your model more closely it unfortunately does appear to have some little tiny slivery surfaces in it, which tend to create a lot of various problems in translation.

I opened your STEP file in ViaCAD and exported into MoI by IGES format, (turning off 186 type export), and it can import but it is not easy to get it to join up, because there are some little slivery parts like this:





I also zoomed in inside of ViaCAD just to verify that those little slivery surfaces are part of the geometry and not something generated by MoI at import time, these screenshots are from ViaCAD after the import into there:





Those little slivery bits are going to probably make things kind of difficult.

Where did you originally create this object? Do you know if there is any way in that application to produce a cleaner object structure that only has broader surfaces for those fillet areas instead of having a tiny sliver surface near the ends of some of them?

Probably this will have to be joined in strategic pieces, probably trying to join the little slivery parts with their immediate neighbors first before trying to join larger chunks of the model together.

- Michael