What is insert molding?

Typically insert molding, or overmolding, requires accurately placing part(s) made from metal or other suitable materials (referred to as the insert) into the mold, closing the mold and then injecting the plastic into and/or around the insert(s).  A typical application would be electrical connectors that incorporate machined metallic pins or sockets into a molded plastic housing.  Most insert moldings utilize metal parts, including handles, threaded inserts or electrical contacts.  At Rebling Plastics, insert molding can be done with any of our molding processes, including thermoplastic injection molding and thermoset injection, transfer and compression molding.  Rebling Plastics uses both vertical clamp and shuttle type plastic injection molding presses for insert molding applications and has experience overmolding circuit boards, wires and harness assemblies, as well as connectors.  Plastics materials insert molded at Rebling Plastics include thermosets such as epoxy, phenolic and DAP and thermoplastics such as nylon, acetal , PBT, polycarbonate, Polyphenylene oxide, Polyphenylene sulfide and others.

We also specialize in wire overmolding, where we have molded plugs and strain reliefs over bare and insulated wire of various sizes.

Special design considerations for insert molding

The design of both the insert and the end product are critical for successful insert molding. At Rebling Plastics, we have several engineers on staff to help our customers with product design and material selection. For example, the suitability of non-metallic parts for overmolding must be evaluated from a temperature and stress standpoint.  A solder joint using standard 60/40 Tin/Lead may not be able to withstand the high mold temperature needed for processing a thermoset or high temperature thermoplastic injection molding material.  Since the part will be subjected to high pressure caused by the flow of plastic material in the cavity, it is important that the insert be restrained from movement.  Rebling Plastics has implemented several innovative mold designs to accomplish this.

Undercuts, thru holes, and knurling on the insert are common ways to rigidly secure the insert in plastic. However, it is important to avoid sharp corners on the inserts when molding with notch-sensitive plastic materials such as acrylic. For thermoplastic injection molding, the plastic part must be designed with a uniform wall thickness to avoid sinking and warping. It is important to balance the wall thickness with the adhesion of the plastic to the insert.

Mold design is particularly important for this type of molding. All of the toolmakers used by Rebling Plastics have extensive experience designing tools for insert molding. Whenever possible, we mistake-proof the insert and mold designs to prevent press operators from incorrectly installing parts in the mold.  Our mold designers take into account the maximum material conditions of the insert on one hand and still insure a flash-free molding when inserts are received at less than the maximum material condition.  Frequently, Rebling Plastics uses its molding experience and expertise to make recommendations and incorporate them into the product design.