Why does Cura think my models are completely solid?

[sim69 0]

I have no idea what is going on with this, no idea what to look up to find answers, so I'm going to ask by myself. 

I am relatively new to 3D modelling, and made a mount for a motor using Sketchup. I know, Sketchup isn't great for 3D printing, but it is user friendly and more importantly: free. When I loaded in this small model to print and sliced it, it said it would take 40 minutes, so I previewed it only to realize it has made the model completely solid with infill all the way up to the top. I can't just set infill to 0 for obvious reasons, but, like, why? Is it my fault, Cura's fault, or Sketchup's fault? Posting this to a Sketchup forum too. Running ver. 4.6.0 for Mac. 

 

Attached are images of 1) The model, showing what it's supposed to look like, 2) The infill filling up what's supposed to be hollow and 3) The outer shell

Not pictured is the hole in the bottom, which also is completely ignored.

[gr5 2,333]

So sketchup specifies which part of every surface faces air and which side faces plastic.  This information is stored in the STL file as something called a "normal".  Your normals are backwards inside that part.  In sketchup you can tell because the surface is gray instead of white.  Right click on that surface and select "reverse faces".

 

More sketchup tips for future models: https://i.materialise.com/blog/3d-printing-with-sketchup/

 

If you are new to sketchup I suggest you switch to a solid modeling CAD versus sketchup which lets you create infinitely thin planes.  Normal CAD software does not let you do that.  Sketchup makes it work to seal up all solids - people often forget to seal in the bottom for example.  But if you are a sketchup expert then definitely read the advice in the above link.

[sim69 0]

Ah thank you. On a sketchup forum I discovered it was actually the file format it was exported in. The faces were already right-side out, but I appreciate the link you provided.