Meaning
Structural design calculations for plastic components must modify the nominal strength values of the material to account for real-world service conditions. This reduction is achieved by applying a derating factor that accounts for temperature, chemical exposure, and mechanical stress. The coefficient scales down the published laboratory data to a safe working stress level.
It does not apply to short-term impact calculations where the loading is nearly instantaneous.
Safety Margin
Laboratory test results represent optimized conditions that are rarely sustained in service. Incorporating a derating factor compensates for the degradation that polymers undergo when exposed to heat or harsh chemicals. This multiplier directly reduces the allowable stress used for the component.
Moulding Influence
Processing conditions during injection moulding can alter the strength of the part and require a lower multiplier. Weld lines, high residual stresses, and regrind usage all weaken the plastic matrix below the datasheet baseline. If the injection speed or melt temperature is too low, the resulting part will require a larger reduction in its design limits to avoid premature failure.
Design Application
Engineers apply this coefficient to prevent long-term failure under sustained loading. For example, a polypropylene pipe carrying hot water requires a much lower multiplier than a similar pipe carrying cold water because heat accelerates molecular slip. This design practice ensures that the part remains below its yield point and does not suffer from creep or stress cracking during its service life.
Using an incorrect coefficient can lead to catastrophic bursts in pressurized systems or sagging in structural supports, which increases warranty costs for the manufacturer.
