There are a few inherent guidelines to vacuum forming that must be understood. First of all, vacuum forming is more of an art than it is a science. This means that no two jobs are alike and every part presents its own unique set of challenges to overcome. There are many “tricks of the trade” to be considered when approaching a new job, so sometimes it may take some time to get a part right.
In terms of mold design, undercuts are not desirable and require special mold inserts which tend to greatly effect the tooling costs. A draft is also needed in the mold so parts will pull off. Male molds require a draft ranging from 1½ degrees to 3 degrees, depending on the material. Female molds, on the other hand, usually require no draft if the mold is smooth and ½ degrees if textured. Ideally, parts should also have the same general cross section.
The most common limitation is with regards to part depth when utilizing a female mold. As a rule, the depth of a part cannot exceed 75% of the width or length, which ever is less.