Ejection Force

Frictional Resistance

Thermal contraction causes cooling resin to grip male tool cores tightly before the mould opens. The normal contact pressure exerted by the shrinking polymer multiplied by the steel-polymer coefficient of friction determines basic sliding resistance. Core depth, draft angle and surface roughness scale this mechanical resistance directly.

Polycarbonate and other amorphous resins exhibit high adhesion to polished steel, whereas self-lubricating polyethylenes require lower release thrust.

Process Influence

Packing pressure, cooling time and mould temperature govern the magnitude of the stripping force during each machine cycle. Excessive pack pressure over-compresses polymer into cavity micro-textures, creating mechanical interlocks that spike demoulding force. Insufficient cooling leaves the part soft, causing ejector pins to push through part walls even under moderate thrust.

Inadequate venting generates vacuum pockets behind deep draw components, compounding mechanical resistance with atmospheric pressure resistance.

Maintenance Factor

Toolmakers install load cells behind ejector plates to monitor pin push forces in automated production cells. Rising force trends indicate flash buildup around ejector pins, galling on guide pins or surface texture degradation. Mould releases provide short-term relief, but permanent fixes require polishing draw faces, increasing draft angles or adding mechanical air poppet valves.

Processing engineers optimise pin quantity and surface contact area to keep localized demoulding stress below the yield strength of the resin.