Thanks everybody!
After removing duplicates and increasing the thickness of the model and uploading my model for the 13th time I finally got the response that it was printable! Not that I actually want to print this model but I learned a lot and now I can finally think about what I actually want to print
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Some responses:
I'm not sure because I had already converted my polygon to a mesh. I could either move a single point or the entire object.
But when I removed duplicates it removed over 19k vertices. I wonder how they got there since blender created the whole thing itself from a fairly simple curve. Or do you think I accidentally duplicated the entire object or something?
Thanks a lot for downloading my model and confirming that the duplicates were the issue. Though my model that worked did have the normals on the inside. And the reason I tried normals on the inside was because I read the last post in this topic:
https://www.shapeways.com/forum/index.php?t=msg&th=2490
Is Mesh Medic something I should have run or does it run automatically when I upload? Because as I said, my accepted model was 'inside out' because of the topic I linked above.
Just before I uploaded the working version, I accidentally uploaded what I think was an 'empty' model. I didn't know I had to select the object before exporting and the file was only 4kB. But again the email didn't say at all what the problem was.
It does tell me to visit www.shapeways.com/mydesign under the header "my errors" but I don't see anything called "my errors" there.
I would have never guessed that!
Now I wonder if it will always find the smallest bounding box. Or can I cheat this check just by rotating my model before I submit it?
Thanks again to everybody!
David