Once the molded part has cooled sufficiently in the cavity, it has to be ejected.
This is done mechanically by KO pins, KO sleeves, stripper plates, stripper rings or compressed air, used either singly or in combination.
The most frequent problem in new molds is with ejection.
Because there is no mathematical way of predicting the amount of ejection force needed, it is entirely a matter of experience.
Since ejection involves overcoming the forces of adhesion between the mold and the plastic, the area provided for the knockout (KO) is an important factor.
If the area is too small, the KO force will be concentrated, resulting in severe stresses on the part.
As a result, the part may fail immediately or in later service. In materials such as ABS and high-impact polystyrene, the severe stresses can also discolor the plastic.
Sticking in a mold makes ejection difficult. Sticking is often related to the elasticity of steel and is called packing.
When injection pressure is applied to the molten plastic and force it into the mold, the steel deforms; when the pressure is relieved, the steel retracts, acting as a clamp on the plastic.
Packing is often eliminated by reducing the injection pressure and/or the injection forward time.
Packing is a common problem is multicavity molds and is caused by unequal filling.
Thus, if a cavity seals off without filling, the material intended for the cavity is forced into other cavities, causing overfilling.