Meaning
Viscosity behavior characterized by changes under stress determines how a polymer fills a cavity. Non newtonian melt shear governs how a plastic flows and how it reacts to different gate sizes or injection speeds. It applies to almost all thermoplastic resins under typical processing conditions.
Shear Thinning
Most polymers exhibit shear thinning as a result of non newtonian melt shear where the viscosity decreases as the flow rate increases. Long molecular chains align themselves in the direction of the flow which reduces internal friction. This property allows thick resins to flow through thin gates and into complex mold details.
Gate Design
Engineers must account for non newtonian melt shear when sizing the gates and runners of a tool. A small gate increases the shear rate which can lower the viscosity but may also cause material degradation due to frictional heating. Balancing these effects is necessary to avoid burnt plastic or poor surface finishes.
Process Stability
Variations in non newtonian melt shear during a production run can lead to inconsistent part quality. Small changes in melt temperature or resin moisture content alter the way the chains respond to shear. Monitoring the injection pressure provides an indirect measurement of these rheological changes.
