Meaning
Polymer rheology and solid mechanics define the threshold stress level at which a solid resin or high-viscosity melt transitions from reversible elastic deformation into irreversible plastic flow under applied shear forces. This shear yield marks the boundary where intermolecular secondary bonds slip, initiating macromolecular alignment along the direction of flow. In solid moulded parts, the value sets the load capacity for snap-fits, press-fit pins, and torsional torque transmission features.
In the molten state, the threshold characterizes yield-stress additives, highly filled compounds, and cross-linking polymers requiring minimum shear stress to begin flowing through runner channels. The mechanical parameter ceases to govern deformation once stresses exceed tensile failure limits or when high thermal degradation destroys polymer chain entanglement entirely.
Rheological Transition
Highly filled resin systems require a minimum applied stress to overcome internal particle networks and begin fluid motion. Reaching the critical shear yield enables smooth melt flow through narrow gating and hot runner tips without nozzle freeze. Stresses falling below this barrier leave the material immobile.
Injection barrels stall under insufficient pressure.
Viscous Dissipation
Frictional interactions between polymer chains under high shear rates convert mechanical machine energy into thermal energy, altering effective melt viscosity. Passing the critical shear yield allows pseudoplastic thermoplastics to undergo shear thinning, dropping apparent viscosity and facilitating rapid filling of thin-walled geometries. If shear stresses spike excessively within runner channels, severe shear heating degrades heat-sensitive polymers like polyoxymethylene or polyvinyl chloride, compromising finished part strength.
Regrind compounding alters yield behavior because shortened polymer chains from prior heat cycles lower the yield threshold, causing inconsistent cavity fill rates compared to virgin resin feeds. Virgin resins display consistent shear-stress curves, allowing predictable tool fill simulations and repeatable screw recovery times. Process engineers optimize injection speed profiles to maintain shear stresses safely above the yield threshold while avoiding thermal degradation.
Uncontrolled shear heating generates cosmetic burn streaks and volatile gas splay across cavity surfaces.
Moldability Threshold
Part designers account for shear behavior when sizing structural interference fits, bosses, and retaining latches subjected to torsional loads. Operating components below the mechanical shear yield prevents permanent plastic creep and loose mechanical assemblies over the operating product lifetime. Datasheet shear strengths assist engineers in calculating safety factors for structural gears and threaded inserts.
Regrind content weakens mechanical yield limits, risking component failure under unexpected impact or high-torque conditions. Reliable mechanical performance demands that operational shear stresses remain safely beneath calculated yield limits across all service temperatures.
