Ejection systems

The method of ejection has to be adapted to the shape of the molding to prevent damage. In general, mould release is hindered by shrinkage of the part on the mould cores. Large ejection areas uniformly distributed over the molding are advised to avoid deformations.

Several ejector systems can be used:

Ejector pin or sleeve
Blades
Air valve
Stripper plate

When no special ejection problems are expected, the standard ejector pin will perform well. In case of cylindrical parts like bosses a sleeve ejector is used to provide uniform ejection around the core pin.

Blades are poor ejectors for a number of reasons: they often damage parts; they are prone to damage and require a lot of maintenance. Blade ejectors are most commonly used with ribbed parts.

Blade ejectors.

A central valve ejector is frequently used in combination with air ejection on cup or bucket shaped parts where vacuum might exist. The air valve is thus only a secondary ejection device.

A high-gloss surface can have an adverse effect on mould release because a vacuum may arise between cavity wall and the molding. Release can be improved by breaking the vacuum with an ejection mechanism.

A stripper plate or ring is used when ejector pins or valves would not operate effectively. The stripper plate is often operated by means of a draw bar or chain.

Three-plate molds, as shown in the figure below, have two parting lines that are used in multi-cavity molds or multiple gated parts. During the first opening stage automatic degating takes place when the parts are pulled away from the runners.

Three plate mold with two stripper plates for ejection.

The opening stroke is limited by adjusting bolts, which also operate stripper plate A.

Runners are stripped from slightly undercut cores at the injection side. Then, the mold is opened at the main parting line. Stripper plate B ejects the parts.The following equation can be used to calculate the require ejection force: