A recent development for reducing costs in industrial manufacturing is called design for manufacturing. In a nutshell, a designer plans an object in such a way that it can be manufactured as cheaply as possible. To make this feasible, industrial CAD software needs to interactively provide feedback about manufacturing options. A common manufacturing process for plastic parts is injection molding. Liquid plastic is injected under pressure into a cavity formed by two steel shells that are clamped together. After the plastic has solidified, the shells are opened and the plastic part is ejected. The difficulty is to assure that the part is not broken by the opening of the steel cavity. A CAD-system should be able to test this, based on the geometric model, and to warn the designer when the model is changed in a way that makes it harder to manufacture the part. We have designed geometric algorithms to perform such tests, considering injection molding and several related manufacturing techniques, such as iron casting.