Undercuts and Inserts

Avoid undercuts in excess of 0.020 inches (0.51 mm). If undercuts are necessary, design the mold with a collapsible core or a split body. 

Undercut sections can be formed using hinged mold sections, cammed sections, and loose pieces in the mold, such as removable split rings. An example undercut with a removable split ring is shown in the figure below.

Inserts

In some designs it is desirable to form an undercut and/or to reinforce the formed unit in a certain section. In such a case, an insert — generally a metal strip or bar — is placed around it. The metal section thus becomes an integral part of the molding. This must be done with great care, however, because the residual stress may cause crazing, cracking, and even part failure. Plastic having a much higher coefficient of thermal expansion (CLTE) than metal shrinks around the insert and becomes stressed at the interface due to the restriction imposed by the insert. 

• Introduction
• Mechanical Properties
• Thermal Properties
• Environmental Considerations
• Design
  • Thermoforming
  • Fillets & Radii
  • Draft Angles
  • Depth of Draw
  • Undercuts and Inserts
  • Shrinkage
• Assembly
• Machining
• Finishing