• Guide for Self-Tapping Inserts
• Choosing Self-Tapping Screws

• A coarse external thread is recommended for unfilled thermoplastics. This provides adequate room for material being displaced in the part, reducing the possibility of cracking.
  
• Finer threads are recommended for thermosetting or glass-filled thermoplastic materials.
• If a boss is not used, the wall thickness of the part should be at least one insert diameter.
• Inserts should be located at least one insert diameter from the part edge. Hole centers should be at least two insert diameters apart.
• The outside diameter of the boss should be at least 2 times the outside diameter of the insert.
• The depth of the molded-in hole in the boss should be 1.2 to 1.5 times the length of the insert.
• A countersink at least equal to the insert diameter is recommended to prevent chipping or splitting at the hole edge.

Choosing Self-Tapping Screws

Self-tapping screws fall into two categories: thread-forming and thread-cutting. When used with plastics, the material’s flexural modulus is the best indicator as to which category is most suitable for use with that material.

Thread-Forming Screws
• Are usually less expensive than thread-cutting screws and have the highest resistance to backout. The threads are formed in the plastic part by forcing the plastic to deform around the metal screw.
• Can cause high, concentrated hoop stress in the part.
• Most suitable for materials with a flexural modulus up to 200,000 psi (1400 Mpa), with the exception of crystalline nylons.
• May be used with most materials having a flexural modulus of 200,000 to 400,000 psi (1400 to 2800 Mpa), except for some acetals and nylon 6/12. A special thread form, such as Trilobe or Hi-Lo, should be considered to reduce radial pressures in the part.

Thread-Cutting Screws
• Form the threads in the plastic part by removing material from the part. These screws typically have higher thread engagement and can withstand higher clamp loads.
• Produce lower residual hoop stress in the part.
• Most suitable for materials with a flexural modulus of 400,000 to 1,000,000 psi (2800 to 6900 Mpa), including glass-filled acetal and glass- or mineral-filled nylon.
• May be used with materials having a flexural modulus greater than 1,000,000 psi (6900 Mpa), but a T-type fine thread is recommended.
• If frequent removal and reinstallation of the screw is required, the boss should be large enough to allow replacement of the screw in the next larger size.