Meaning
Dynamic mechanical analysis establishes polymer viscoelasticity by applying a controlled oscillatory deformation to a sample while measuring the resulting stress response. A strain sweep measures the linear viscoelastic region of a resin by holding frequency and temperature constant while stepping the deformation amplitude upward until molecular deformation exceeds the linear threshold. Tooling designers and polymer engineers rely on this specific rheological profile to determine the maximum amplitude a material withstands before its internal structure breaks down irreversibly during processing.
Processing technicians use the resulting data during compounding trials to set oscillation limits that protect polymer chains from thermal and mechanical degradation during extrusion. When practitioners run this test incorrectly, the material undergoes structural destruction, yielding false linear limits that cause excessive flash, weld line weakness, and dimensional warpage in moulded parts. Virgin resin typically maintains a wider linear viscoelastic range than regrind material because thermal history shortens polymer chains and alters molecular weight distribution.
Datasheet values derived from laboratory rheometers often assume ideal sample preparation, whereas production injection moulding subjects the melt to high shear rates that reduce the actual operational window.
Shear Boundary
Molecular chain slippage occurs when the applied deformation exceeds the threshold defined by the test, transitioning the polymer from elastic recovery to permanent viscous flow. Extrusion barrels and injection moulding nozzles generate intense shear forces that can push a resin past its linear boundary if barrel temperatures drop below optimal processing windows. Resins with broad molecular weight distributions exhibit wider linear limits because smaller chains lubricate larger entangled molecules during deformation.
Elastic Plateau
Storage modulus remains constant throughout the initial portion of the oscillation test, representing the purely elastic energy stored and recovered by the entangled polymer network. Loss modulus measures the viscous energy dissipated as heat through chain friction during the oscillatory cycle, remaining lower than storage modulus across the stable region. Fillers and glass fibers increase the stiffness of the compound, raising the height of the elastic plateau while narrowing the strain limit where structural breakdown begins.
Stress Limit
Internal molecular orientation shifts abruptly when the storage modulus drops by ten percent from its initial plateau value, defining the absolute upper limit of safe processing deformation. Structural parts moulded beyond this threshold suffer from anisotropic shrinkage, internal voids, and brittle fracture under mechanical load because the polymer chains fail to relax uniformly. Moulders prevent these defects by keeping machine stroke profiles strictly below the measured breakdown point, ensuring the injected melt retains its molecular integrity throughout the cavity filling phase.
