Meaning
Fluid resistance varies with the shear rate applied to the material rather than remaining constant at a given temperature and pressure. Non-Newtonian viscosity describes the internal friction of polymers as molecules align or disentangle under the mechanical stress of injection moulding. High molecular weight resins exhibit more pronounced changes in flow resistance as the screw speed increases.
This property dictates the pressure required to fill a cavity and defines the cooling cycle duration.
Flow Dynamics
Molecular chains within the melt undergo orientation when forced through narrow gates or runners during the processing phase. Non-Newtonian viscosity drops as the shear rate rises, a phenomenon known as shear thinning that allows high speed filling with reduced injection pressure. Tooling designs depend on this behaviour to balance cavity pressure across complex part geometries.
Stable production requires consistent resin rheology because variations in batch additive levels can shift the flow curve away from the nominal machine setting.
Quality Control
Datasheets typically report a melt flow rate that serves as a single point of reference for comparison rather than a complete map of flow behaviour under load. Non-Newtonian viscosity measurements obtained via capillary rheometry provide the true mapping needed for injection simulations and gate placement. Moulders verify this profile to avoid short shots or flash that occur when material flow deviates from expected parameters.
Regrind addition often alters the shear thinning profile compared to virgin resin, necessitating adjustments to the injection velocity profile.
Cost Impact
Scrap rates increase when process windows tighten because the material flow resistance does not match the stored injection parameters. Managing non-Newtonian viscosity reduces the frequency of part defects like jetting or sink marks that arise from uneven filling patterns. Effective oversight of resin rheology improves overall throughput by preventing machine downtime spent revalidating settings for every new lot.
Reliable performance hinges on the ability of the system to maintain a constant shear environment during the high speed injection stage.
