Meaning
Time-dependent decrease in stress within a polymer held at a constant strain. In the design of snap-fits and plastic seals, viscoelastic relaxation determines the long-term holding force and sealing integrity of the joint. When a plastic part is deformed and held in that position, the polymer chains slowly reorganize and slide past one another to relieve the internal stress.
This behavior means that the initial force exerted by the snap-fit or seal will decrease over time, which can eventually lead to loose connections or leaks if not properly accounted for in the initial design.
Molecular Mechanism
Reorientation of polymer chains is the driving force behind this reduction in stress over time. Under an applied strain, the coiled molecules are initially stretched out of their equilibrium conformations, which generates a high restoring force. Over time, the thermal motion of the chain segments allows them to slide and find new, lower-energy configurations that accommodate the deformation.
This molecular rearrangement is a characteristic feature of all thermoplastic materials, distinguishing them from purely elastic solids which maintain a constant stress indefinitely under fixed strain.
Process Influence
Molding parameters that alter the crystallinity or residual stresses within the part can significantly affect this time-dependent behavior. Parts molded with high internal residual stresses will relax more rapidly because the molecules are already in a state of high tension. High melt and mould temperatures can reduce these molded-in stresses by allowing the polymer chains more time to relax before the part solidifies.
This optimization can reduce the rate of subsequent stress decay and improve the long-term reliability of the component in service. In contrast, using lower melt temperatures can freeze in high levels of molecular orientation, leading to an accelerated decay of stress in the flow direction when the part is held under load.
Temperature Influence
Elevated temperatures accelerate the rate of relaxation by increasing the thermal energy and mobility of the polymer chains. This acceleration can be modeled using the time-temperature superposition principle, which allows long-term performance to be predicted from short-term tests conducted at higher temperatures. Designers must utilize these predictive models to ensure that critical molded components do not lose their required holding force during their expected service life, particularly in demanding environments like under-hood automotive applications.
