Or in this case, the angle between the two faces must be greater than 2*arctan(0.5), or about 53 degrees. For the model I submitted, I now see that the angle is a little smaller, at 48.7 degrees.

Here is the relevant part of the image:

The measurements are both on the triangular prism with triangular cross-section having side lengths of 2.625mm, 2.625mm and 3.464mm.

This was from an email to

service@shapeways.com rather than an attempted printing of an uploaded model, because I wanted to check that I had fixed a previous problem. Joost wrote back: "I have checked the attached models but unfortunately still some thin walls (see capture).

Please make the walls at least 0.7 mm thick otherwise the model(s) will get rejected."

As I said above, this reason doesn't make sense to me for a triangular prism. But I see that the airplane wing condition (or angle condition) makes sense. If this really is the condition then the only problem is that I didn't get that information from the email.

The triangular cross-section isn't a necessary part of my design, so I will be bevelling off all of the edges, which should remove the angle problem.

Thanks all!