Meaning
Time-dependent release of internal mechanical forces locked into plastic components during shaping occurs as polymer chains realign toward equilibrium configurations. Over extended storage or service, residual stress relaxation causes dimensional changes, warpage or altered mechanical properties in molded parts. The relaxation process proceeds continuously at rates dictated by temperature and plasticizer content, decelerating as internal stress approaches zero or structural equilibrium is reached.
Chain Mobility
Molding operations force polymer molecules into non-equilibrium orientation during cavity filling and rapid cooling. Driven by thermal energy, residual stress relaxation involves localized segmental motion of polymer chains, allowing frozen-in orientation to recoil over time. Amorphous polymers like polystyrene exhibit rapid stress decay near their glass transition temperature, whereas semi-crystalline polymers relax through both amorphous segment movement and lamellar reorganization.
Part geometries with non-uniform wall thickness retain localized stress gradients that accelerate warpage as relaxation occurs.
Dimensional Drift
Uncontrolled stress relief in service leads to post-moulding dimensional changes that compromise assembly tolerances. As residual stress relaxation occurs, optical components suffer shifts in birefringence, while structural housings undergo post-mold distortion days or weeks after production. Moulders utilize post-molding annealing treatments at elevated temperatures to accelerate stress relief under controlled conditions before final machining or assembly.
Adding regrind resin can alter molecular weight distribution, modifying relaxation kinetics and producing unpredictable dimensional movement across lots.
Creep Interaction
Internal stresses combine with externally applied mechanical loads to drive localized yield or environmental stress cracking. Understanding residual stress relaxation enables engineers to predict long-term load-bearing capability in components subjected to sustained static forces. Over time, internal stress relief reduces the driving force for cracking, but initial peak stresses can exceed critical thresholds before relaxation completes.
Standard mechanical tests conducted immediately after molding fail to capture the altered stress state of components aged prior to installation.
