Meaning
Rheological behavior describes how non-Newtonian polymer melts decrease in dynamic viscosity when subjected to increasing shear rates during fluid flow. Polymer characterization labs evaluate shear sensitivity to predict how resin formulations flow through runners and thin-wall mould cavities. The property dictates processing window boundaries and energy requirements during melt conveyance.
Scope covers pseudo-plastic thermoplastic melts, terminating at low shear plateaus where Newtonian behavior resumes.
Molecular Alignment
Entangled polymer chains align parallel to the flow direction under high shear stress applied within narrow channels. Disentangling molecular coils reduces inter-chain friction, allowing polymer layers to slide past each other with reduced resistance. Higher molecular weight polymers exhibit greater chain entanglement, producing sharper viscosity drops as shear rate increases.
Removing shear stress allows chains to re-coil, restoring initial melt viscosity.
Processing Response
Injecting highly shear-sensitive resins through narrow gates lowers injection pressure requirements without elevating barrel temperatures. High flow velocity reduces melt viscosity inside thin walls, improving mold filling while minimizing thermal degradation risks. Excessive shear rate generates viscous dissipation heating, which causes localized thermal degradation if screw speeds or injection velocities exceed thermal thresholds.
Viscosity Control
Controlling shear response ensures uniform cavity filling across multi-cavity moulds. Processing engineers adjust injection speed profiles to optimize shear thinning during the fill phase. Consistent viscosity prevents sink marks or flash formation across production cycles.
