Meaning
Frictional mechanical resistance encountered during initial part release characterizes localized release behavior in precision injection moulding. At the instant of mold opening, micro-ejection force quantifies the peak friction resistance overcome by core pins and ejector sleeves as they break mechanical contact with solidifying polymer features. Standard macro-ejection force measurements average resistance across the entire mold face, concealing localized stress concentrations that cause pin push-through or surface stress whitening.
This mechanical interaction terminates once initial static friction breaks and sliding kinetic friction takes over during the remaining ejection stroke.
Core Shrinkage Stress
Volumetric shrinkage causes cooling polymer to clamp tightly around core pins and micro-textured cavity surfaces. Elevated micro-ejection force occurs when cavity holding pressures overpack micro-features, driving resin deep into surface microscopic asperities. Molders adjust hold pressure and cooling duration to prevent part distortion during early release phases.
Surface Polish Impact
Cavity surface finish directly modulates static friction thresholds during part demoulding. High micro-ejection force leads to surface drag marks or pin impression defects on optical components and textured cosmetic parts. Incorporating permanent nickel-polytetrafluoroethylene coatings on core pins reduces release resistance significantly.
Adding regrind resin increases friction variance due to inconsistent additive dispersion and polymer molecular weight degradation.
Pin Diameter Ratio
Small diameter ejector pins concentrate ejection load over tiny surface areas on thin-wall moulding geometries. When micro-ejection force exceeds local compressive yield strength, pins puncture or deform the plastic part before full separation occurs.
