The Center for 3D Visualization Hardcopy (C3VH) was initiated in 1995 as a web-accessible facility to create 3D physical models. By making such prototyping hardware easy to access and the geometry files easy to check and correct, we have been able to drive the model-making process into the less traditional manufacturing area of scientific visualization. But, there have been requests for many copies of a single model. As “rapid” prototyping is actually quite slow and unsuitable for mass production, these requests have required us to enter the world of molding for mass production. As part of this, we have needed to quickly determine how feasible it is to mold a particular model. This paper shows an interactive OpenGL method that we developed for determining the optimal orientation for a model's parting line. By using a novel OpenGL stencil buffer technique, the user can reorient the model on the screen, and see from the colors in the display how vertically convex it is in that orientation, and in what parts of the model it is not vertically convex.