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I'd call it "monotonically expanding" along some particular direction vector.

In die casting, you have to do a little better than the vertical line test and stay within the specified draft angle. A vertical wall would be a 0 degree draft angle, and not suitable for casting most materials.

I would call it self-occluding.

Convex perhaps?

There are non-convex shapes that don't have undercuts. I think convexity implies that you don't have undercuts, though.