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What is a Retainer Plate?

Paul Scott
Paul Scott

A retainer plate is part of the internal mechanism of injection molding used to reinforce the mold and secure other internal parts such as bushings and ejector pins. Retaining plates are typically two-part assemblies consisting of a backing plate and the actual retainer plate. Supported parts such as guide and ejector pins pass through holes in the retainer plate and are held in place against it by the backing plate. The plate is located at the rear of the mold between the mold core and the ejector mechanism. The plate assembly is often equipped with internal channels to circulate heating or cooling fluids.

Injection molds are fairly complex devices consisting of a laminar sandwich of components all brought together to form the closed mold. This collection of parts consists of the two halves of the product mold which are secured between two heavy support plates. The mold halves are in direct contact with the front support plate which is equipped with the sprue or channel through which the molten material is forced into the mold cavity. The area behind the mold halves is occupied by several components including the retainer plates, ejector mechanism, and rear support plate. The retainer plate assembly, which is in contact with the rear of the mold halves, serves to reinforce the mold and supports several pieces of additional equipment essential to the molding process.


The retainer plate assembly consists of two flat plates which are positioned back-to-back behind the mold halves. The plate in contact with the rear mold half is known as the retainer plate and the second as the ejector plate. These two plates support additional mold parts such as the guide pins, ejector pins, and their related bushings. The guide and ejector pins typically pass through bushings in the retainer plate and are held in place by the ejector plate. Located behind the retainer plate assembly is the ejector box and finally the rear support plate.

When molding is complete, the front support plate is removed along with the front mold half. The ejector mechanism then forces the retainer plates and ejector pins forward to push the part out of the mold. Retainer plates are often also used as rear cooling or heating elements in the mold depending on the specific application. This function is achieved by incorporating channels or cores into the plate assembly through which steam or chilled water is circulated.

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