Curious: Model changes from solid (undesired) to hollow (correct) if rotated

[mike_za 0]

In Sketchup I drew a spacer 100mm square shape with edges approx 10mm wide and extruded it to make it about 8mm thick. Solid inspector says "everything is shiny!"

 

If I bring the .stl file into Cura 3.2.1 it lies flat on the build plate and looks right in solid view but showed the print time as 5.5 hours which was unexpectedly high on my Prusa i3.

 

Looking at it with layer view shows that it will print as a completely solid shape.

 

If I rotate the model (even a few degrees) it shows that it will print correctly as a hollow shape and at 90 degrees it will take 2.5 hours (which I would expect). Unfortunately if I try to print it like this it will fall over.

 

I changed various setting on Cura for a previous print (which worked fine) and (as far as I could remember) I have changed them back but would be pleased to hear how to fix this curious problem. 

 

I have done a search on other forum posts but not sure what key words to use so if this problem has been addressed I would be pleased to get a link. 

 

Grateful thanks!

Mike

4x4-spacer5.curaproject.3mf 4x4-spacer5.stl

[burtoogle 513]

Slicing this model using the current Cura doesn't produce a solid result. Perhaps the problem was fixed somewhere along the way?