I am trying to 3D print an object from Shapeways. Confidentiality prevents me from sharing the actual STL files publicly, but what I can say is that it's a shape about 2 cm tall x 4 cm wide x 4 cm deep, and where the design specifies a vertical hole 5 mm in diameter, the hole comes out fine, but where the design specifies multiple holes 3 mm in diameter, passing diagonally through the shape (at a 45° angle with the ground), the holes seem to collapse (i.e. I can't see through them, and if I push a straightened paper clip through the hole, it's blocked). This has happened repeatedly (e.g. Shapeways offered to re-print some shapes for free when they came out defective, but this happened the second time too).
I am new to 3D printing and trying to figure out:
Is the hole more likely to "collapse" if it's diagonal rather than straight up and down? Is this because when the shape is being 3D printed, it's oriented exactly as shown in the design, and so before the 3D printing materials have fully solidified, the holes are more likely to be collapsed by gravity? Or does it have more to do with the diameter of the holes, and if I made the shape with 5 mm diagonal holes, they might not collapse?
The material I'm picking from Shapeways's options is "Black Natural Versatile Plastic". Are there other materials that are less likely to have this problem?