Tooling Surface Polish Impact on First Article Inspection Accuracy
Uncalibrated optical scans on mirror-polished plastic introduce up to fifteen micrometers of false elevations, ruining First Article Inspection accuracy.
Mold Boundary Shear
Meaning
Frictional drag between flowing polymer melt and the static metal cavity wall generates localized velocity gradients during the injection stroke. Establishing mold boundary shear controls skin layer formation and molecular orientation in thin-walled injection moulded parts. High shear rates near cold steel walls lower melt viscosity for non-Newtonian resins, allowing complete cavity filling under lower injection pressure.
The phenomenon applies during fluid flow and ceases once fluid movement stops during the packing phase.
Rheological Behavior
Shear-thinning polymers like polypropylene experience sharp viscosity drops when subjected to high flow velocities through tight gates and thin sections. Velocity gradients near cold cavity steel drive mold boundary shear to its peak magnitude near the frozen layer interface. Excessive shear stress strips polymer chains, causing mechanical degradation and localized discoloration in sensitive resins.
Maintaining shear rates below material degradation limits preserves resin molecular weight.
Processing Influence
Injection speed and mold wall temperature directly govern boundary shear rates. Higher injection speeds increase mold boundary shear, reducing bulk flow resistance but raising frictional heating at gate entrances. Cold mold temperatures thicken the frozen skin layer, narrowing the active flow channel and intensifying localized shear gradients.
Defect Risk
Uncontrolled shear stress generates core-skin orientation imbalances that promote post-moulding part warpage. Parts molded under extreme boundary shear display surface jetting halos and premature stress cracking under load.