Machine Design

Screw Techniques for Vinyl Components

Engineers have two options when it comes to assembling plastic components with screws: self-tapping screws and threaded inserts. There are two basic types of self-tapping screws used with plastics: thread forming, such as ASA type B, and thread rolling, such as Camcar Plastite. Either type works well with properly designed plastic components. Thread-rolling type screws provide the best long-term performance for vinyl assemblies because they induce less hoop stress in bosses and the resin cold flows around the threads after assembled, which helps prevent loosening.

General design criteria for assembling vinyl with self-tapping fasteners include:
• The molded or drilled pilot hole should be 0.8 times the nominal screw diameter.
• Boss OD should be twice the nominal screw diameter. An increase in boss length is recommended to compensate for a reduced boss diameter, which may cause sink marks on the part surface. • Boss height should be a minimum of twice the boss OD. The hole depth should be slightly longer than the screw length to make room for chips as they accumulate.
• Torque screwdriver speed should be held no greater than 500 rpm.
• Repeated assembly and disassembly is not recommended.

Threaded inserts used along with standard machine screws are an alternative to self-tapping screws, particularly when screws are repeatedly removed and reinserted. Inserts provide high strength with excellent pullout and torque retention.

For high-speed/high-volume applications, an insert with a lead-in at each end is recommended. This helps eliminate the need for orientation during automatic assembly. Straight holes are best when using threaded inserts, but a 0.5° taper can be used when straight holes aren’t possible. However, tapers shouldn’t exceed 0.0015 in. above the hole diameter and holes shouldn’t be countersunk or chamfered.

Information for this article provided by the Geon Co., Avon Lake, Ohio.

© 2010 Penton Media, Inc.

TAGS: Fasteners
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