Meaning
Time-dependent decay curves measure the decline of shear stress in a polymeric material maintained under a constant shear strain state at fixed temperatures. The property governs transient viscoelastic behaviour during melt processing, structural creep response under mechanical loading, and internal stress dissipation during component solidification. Data points quantify both the instantaneous elastic stiffness and the progressive disentanglement of molecular chains over logarithmic time spans.
The property reaches its measurement boundary when the stress fully dissipates to zero in uncrosslinked melts or reaches a stable rubbery equilibrium plateau in crosslinked elastomer networks.
Relaxation Mechanics
Imposing an instantaneous step shear strain forces macromolecular chains to deform away from their preferred thermodynamic equilibrium configurations. The immediate shear modulus relaxation reflects the initial resistance of entanglements, molecular bonds, and intermolecular interactions opposing the applied strain field. Over time, Brownian motion drives molecular chains to slip past entanglements through reptation and Rouse motion, gradually dissipating internal shear stresses.
In semi-crystalline materials, crystalline spherulites anchor amorphous chains, sustaining a higher intermediate plateau modulus than purely amorphous polymers above their glass transition temperature.
Rheological Testing
Dynamic mechanical analysis and rotational parallel-plate rheometry quantify this decay spectrum across broad time scales. Generating master curves through time-temperature superposition extends experimental data across operational decades otherwise impossible to measure directly. Processing engineers evaluate these curves to establish the minimum cycle dwell times needed for stress relief within hot moulds.
Virgin resins exhibit clean, predictable relaxation spectrums with distinct molecular weight transition zones. Degraded resins or high-percentage regrind lots display accelerated stress decay rates, caused by severed molecular backbones and shortened chain lengths.
Tooling Consequence
Inadequate stress dissipation during injection cycles leaves extensive molecular orientation aligned along flow paths inside the mould cavity. Once the component ejects from rigid steel tooling, these unresolved internal shear forces drive uneven dimensional contraction and severe structural warpage. Regions around narrow sub-gates experience high shear rates during cavity filling, demanding sufficient thermal dwell time to allow internal stress decay before ejection occurs.
Premature ejection interrupts relaxation, causing post-mould distortion, lower optical clarity, and accelerated environmental stress cracking when the component contacts aggressive solvent environments.
