What makes it impossible to release a vaccuform?

The correct answer pertains to the concept of undercut surfaces in the context of vaccuforming. An undercut plug surface can create a situation where the material being formed cannot be easily released from the mold. When a plug has undercuts, it means that part of the plug extends beneath the surface, preventing the sheet plastic from pulling away smoothly once it has cooled and conformed to the shape of the plug. The complexity in this design can "trap" the formed material, making it physically impossible to separate it without damaging the piece or the mold itself.

In contrast, excessive trimming could lead to problems with the finished product but doesn't inherently make the release impossible; it generally just affects the fit and detail. A smooth plug surface typically allows for easier release since there are fewer obstacles for the formed material to adhere to. Additionally, a flexible material can sometimes aid in the release process, as it may allow for bending or manipulation that helps in removing the formed part from the mold. Thus, undercut surfaces are a key issue specifically because they create the physical barriers that impede removal after vacuum forming.