Insert molding is when an insert typically made of metal is placed in the cavity prior to plastic molding around it, bonding with the insert to make one final product. Overmolding is quite similar, although the initial substrate is molded first, then, plastic is molded around it to create a two-shot final product.

There are many factors for mold design engineers to consider when designing or constructing a mold for this type of project. Some of the main considerations include:

  1. Evaluating Material Bonding and Adhesion

It is necessary for the over-molded plastic to adhere to the original part, making it imperative to evaluate the bonding properties of both parts. If material bonding proves challenging, designers may consider mechanical bonding to provide a means of connecting the two pieces successfully. Mechanical bonding may be a simple feature, such as a through hole with a counterbore or something more sophisticated, like an elaborate runner system on the bottom of the substrate.

Another option for improving adhesion is the use of heat pretreatment systems that raise the surface temperature of the initial substrate, allowing improved adhesion during the overmolding process.

  1. Maintain Uniform Wall Thickness on Second Shot

There is typically no shrinkage applied to the second shot, considering the molded substrate will not shrink. Maintaining uniform wall thickness in the second shot is necessary, with the same principles applying to injection molding single plastic parts applying to any voids in the substrate. This includes ribs to be 60% of the intersecting wall thickness with generous radii and fillets for optimum flow.

  1. Support Substrate During Second Shot

Injection molding occurs with pressure against the mold cavity walls. During initial part molding, this pressure fills the cavity to mold the perfect part. During overmolding, this pressure can cause molding issues if the substrate is not supported adequately to hold up under pressure. Proper mold design should include support areas on the substrate to help the second molding process. These support features must have built-in clearance to accommodate for part tolerance deviations. Any ribbing should be kept to a minimum in these areas when possible.