Meaning
Non-Newtonian fluid behavior causes melt viscosity to decrease dramatically as applied shear rates increase during processing. Process of polymer shear thinning enables long, disentangled macromolecular chains to align parallel with flow field velocity vectors inside runners and gates. The viscosity reduction effect ceases when shear rates drop back to zero or when flow velocity slows inside wide cavity geometries.
Material datasheets report single point melt flow values under static loads, masking the steep viscosity drops that occur at processing shear rates above ten thousand reciprocal seconds. Injection moulding relies on high shear rates to fill thin wall sections without exceeding machine pressure limits.
Chain Entanglement
At rest, random polymer coils interlock with adjacent chains, offering high resistance to mechanical motion. Increasing shear forces untangle molecular structures, allowing chains to slide past one another with reduced frictional resistance.
High Shear Flow
Small gate dimensions convert ram velocity into extreme local shear rates, drastically lowering local melt viscosity during cavity entry. Lower viscosity reduces required clamp force and prevents premature thermal freezing in thin part walls.
Process Optimization
Engineers adjust injection velocity profiles to maximize shear thinning effects without generating thermal degradation from excessive viscous heating. Operating in the shear thinning regime stabilizes cavity filling against minor batch variations in resin viscosity.
