Meaning
Fluid velocity gradients across a channel cross-section describe the spatial and temporal shear deformation rates experienced by molten polymers during processing. The shear rate dynamics inside injection mould nozzles, runner channels, and restrictive gates govern local melt viscosity, shear heating generation, and molecular chain alignment. Molders and tool designers calculate these gradients to ensure processing occurs within the shear-thinning window without exceeding the critical shear degradation limits of the polymer.
The concept applies exclusively to flowing fluids and viscoelastic melts, ceasing to be relevant once the material solidifies into a rigid solid state.
Velocity Gradient
Molten polymer adheres to stationary cold mould walls while traveling at maximum velocity along the center of the flow channel, creating a steep velocity gradient. In high-speed injection operations, shear rate dynamics reach values exceeding one hundred thousand reciprocal seconds inside narrow pin gates and thin-wall part sections. This intense shearing causes molecular chains to disentangle and align with the flow vectors, dropping melt viscosity by several orders of magnitude.
Capturing these localized shear variations is essential for sizing hot runner nozzle tips and predicting required injection pressures accurately.
Viscosity Response
Non-Newtonian pseudo-plastic melts respond to elevated shear rates by flowing with substantially reduced flow resistance. Process engineers utilize fast injection velocity profiles to lower the effective viscosity, enabling the melt to fill long, thin cavity sections before thermal conduction freezes the material. If shear rate dynamics exceed the thermal threshold of the resin, excessive shear heating degrades sensitive polymer backbones, resulting in discoloration, mechanical embrittlement, and outgassing defects.
Balancing injection speed against shear degradation limits protects material properties while maximizing flow length.
Molecular Alignment
High shear forces orient polymer backbones and reinforcing glass fibers along the direction of cavity flow, inducing pronounced anisotropic mechanical properties in the final component. When the skin layer freezes rapidly under high shear, aligned molecules freeze in place, creating high tensile stiffness along the flow path but lower strength in the transverse direction. Unbalanced shear gradients across opposing walls induce asymmetric frozen skin layers that cause severe post-moulding warpage upon ejection.
Controlling filling speed and shear profiles ensures uniform material properties and dimensional stability throughout the moulded part.
