Undercuts

An undercut is any inward or outward projection of the nominal wall of a formed part which prevents stripping of the part in a direction perpendicular to the open face (deck) of the forming die. Undercuts are one of the plastic product designer's favorite design details. They are widely used for snap fit assemblies and other locating and fastening applications. Undercuts on thermoformed parts as well as on closed mold parts have always been discouraged. Undercuts invariably complicate the tooling and increase its initial cost and in some cases, the expense of ongoing maintenance.

The normal material shrinkage that takes place as the formed part is cooling helps pull small undercuts (Figure 17 A) or minor reverse drafted walls (Figure 17 B) free of their cavities on parts formed in a female die. Unfortunately, this same material shrinkage tends to cause undercuts formed on a male portion of the die to be seated even tighter into the cavities that formed the undercut. Small undercuts can be stripped out of their cavities on male dies (Figure 17 C). However, female dies are always capable of allowing the stripping of larger undercuts.

The size of strippable undercuts is dependent upon the size and shape of the part as well as the plastic material which is being used. Tough materials like impact styrene will be easier to strip than a strong material like polycarbonate. Ductile materials such as ABS will be able to flex to allow stripping of the undercut. A brittle material such as acrylic may be overstressed or even cracked by the same amount of flexing.

Undercuts on thermoformed parts have traditionally been limited to soft, smoothly blended contours with relatively large radii due to the process' limitations. Thermoforming had never been able to produce the sharply detailed undercuts of the type that are possible with closed mold processes such as injection molding. Refinements in the Thermo Pressure Forming process have changed this long-standing situation. The normal use of female dies and the higher forming pressures combine to allow today's designer the opportunity to specify detailed shallow undercuts with sharply defined details.

Deep undercuts that cannot be stripped from the die can be Thermo Pressure Formed (Figure 17 D). These undercuts however, do require side-acting, hand take down, lay in or hinged details in the die. These tooling details add to the cost and maintenance expenses associated with the die. These added costs detract from one of thermoforming's primary advantages which is low cost dies. However, if an undercut is required, it can be provided by the Thermo Pressure Forming process.

The added cost of incorporating undercuts in Thermo Pressure Forming dies is a one-time charge that is only a fraction of the cost of providing the same detail in the more complicated tooling of the type used for injection molding or structural foam.