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Long-term loading

Posted by admin on July 25th, 2012

Long-term loading occurs when parts are placed under high external loads, within the proportionallimit, for extended periods of time. This term also refers to parts that must withstand high internal or residual stresses that result from either the molding process or from the following assembly processes:

Press-fit and snap-fit assemblies

Tapered fit between plastic and metal components

Over-stressed joints between mating parts

Thread-forming screws

Counter-bored screw heads

The design rules given below apply to parts that will be subject to long-term loading conditions.

Use Creep modulus

Creep modulus should be used in the design calculations to avoid stress-cracking failure, to maintain the tightness of joints, and to maintain part functionality.

Designing for press-fit and snap-fit assemblies

For Press-fit joints and Snap-fit joints, design snap-fit and press-fit components so that the strain is reduced to the as-molded dimensions after assembly.

Using fasteners

There are several design alternatives you can use for incorporating fasteners into a plastics part. These strategies a discussed in Fasteners.

Design features to avoid over-tightening
Plastic-to-plastic surfaces should be designed to limit the distance that the joint can be closed. Providing stop surfaces can prevent a screw from being over-tightened beyond the design intent or limit the depth of engagement of two matching taper surfaces.


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