Meaning
Flow resistance inside a shearing fluid determines the processing behavior of thermoplastic materials during high pressure shaping. Polymer melt viscosity governs barrel pressures, filling balances across multi cavity tools, and molecular orientation during injection and extrusion. Extrusion lines and injection moulding machines set this parameter through barrel temperature profiles and shear rates applied at the screw tip.
Thermal degradation occurs when barrel temperatures exceed polymer stability limits, causing molecular chain scission that reduces resistance and drops finished part mechanical strength. Crosslinking agents increase resistance unexpectedly, leading to machine stall and excessive motor torque demands.
Shear Rate Sensitivity
Pseudoplastic fluids thin under mechanical stress as entangled chains align with flow directions. Viscosity drops during rapid injection phases through narrow gate geometries, lowering clamping tonnage requirements while increasing molecular orientation in the skin layer of moulded articles. Shear heating compounds this reduction locally, generating thermal gradients across thick walled sections that cause differential shrinkage and internal stress concentrations.
Moulders adjust screw speeds to alter shear rates without changing thermal inputs, balancing flow length against molecular degradation risks. Viscosity stability depends heavily on maintaining uniform residence times within the plasticising unit, since prolonged residence under high heat breaks down susceptible resins.
Resin Specification
Material datasheets report melt flow rates measured under standardised dead weight conditions, providing a preliminary screening tool for incoming pellet lots. Virgin polyethylene and polypropylene grades arrive with certified ranges that match processing equipment capability. Regrind introduction alters the molecular weight distribution, lowering viscosity and requiring compensatory adjustments to injection velocity profiles and holding pressures to prevent flash formation.
Contamination from moisture or higher melting point polymers creates localized viscosity spikes, plugging hot runner tips and halting production runs until purged. Part specifications demand consistent viscosity to ensure dimensional repeatability, preventing sink marks and warping in complex geometries.
Thermal Control
Melt temperature dictates the baseline viscosity before mechanical shear takes over inside the mould cavity. Heaters mounted along the barrel transfer thermal energy to the resin, and thermocouples feed temperature data back to proportional controllers to minimize fluctuations. Excessive barrel heat lowers viscosity past the threshold needed for adequate packing, producing parts with voids and reduced tensile strength.
Insufficient thermal input raises resistance, creating short shots where the cavity fails to fill completely before freeze off occurs. Accurate thermal management prevents the viscosity drift that causes dimensional variation across long production shifts.
