Short-term loads are those imposed during handling and assembly, and during usage where the load is applied occasionally with short durations. The following suggestions apply to parts that will be subject to short-term loading conditions.
Use proportional limit in stress-strain curve
Designers should consider the stress-strain behavior of the plastic material when designing parts for bearing short-term loads. The proportional limit should be used as the maximum allowable stress in the design calculations to avoid permanent deformation of the part and possible loss of function.
Use stiffeners and fiber reinforcements
Stiffeners, such as ribs and gussets, are often used to increase the part strength. Fiber reinforcements, oriented in a favorable direction, can also increase the part strength. You should
consider using ribs for parts with large spans. Increasing the rib height and/or decreasing the spacing (span) between the ribs also improves part strength.
Long-term loading occurs when parts are placed under high external loads, within the proportional 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
Counter-bored screw heads
The design rules given below apply to parts that will be subject to long-term loading conditions.