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.
Ejector Drag Stress
Meaning
Mechanical resistance measured against part demoulding force quantifies the frictional shear imparted on polymer features during tool opening. High core adhesion generates ejector drag stress when ejection pins push against unsupported internal bosses or tall rib intersections. Excess friction along vertical mold walls opposes pin forward motion, bending or stretching hot plastic features before complete mold release.
The mechanical loading stops once the component fully clears the mold core.
Shear Gradient
Draft angle and pack pressure define the magnitude of frictional resistance experienced during the ejection stroke. Insufficient draft forces pin faces to push against localized plastic volume while opposing walls scrape steel cavities, elevating ejector drag stress across thin-walled geometries. Applying polished core surfaces or low-friction nickel-PTFE coatings reduces demoulding friction.
Increasing pin contact area distributes force evenly, preventing localized shear failure.
Thermal Influence
Ejection timing and mold coolant temperature dictate resin flexural modulus during demoulding. Early ejection while the polymer core remains warm amplifies ejector drag stress, causing pin push-through or surface stress whitening on polyolefin parts. Allowing extended cooling increases part rigidity, enabling the plastic to withstand ejection forces without structural distortion.
Over-cooling increases volumetric shrinkage onto cores, raising friction on deep ribs.
Part Distortion
Excessive ejection shear yields permanent dimensional defects, including pin indentations and warped base plates. Production lines face higher scrap rates when drag forces exceed resin yield strength.