A Complete Guide to Overmolding – Process & Design Guide

Overmolding is a process that combines the strength and durability of polymer with the aesthetics and design of plastic. It’s an economical way to create a one-piece part that meets your functional needs.

Overmolding is used to create complex shapes or designs that are difficult to produce using traditional molding methods. The process involves injecting molten plastic into an outer shell (the “overmold”) that has been previously molded from thermoset plastic or metal. The result is a seamless, finished product.

The Basics of Overmolding

The first step in the overmolding process is to create a mold for the outer shell. This can be from thermoset plastics or metal alloys, depending on your needs. The mold come into true using injection molding, die casting or investment casting processes.

Once you have your mold, you need to fill it with molten plastic. This can make by placing molten resin inside the mold cavity, pouring molten resin over the top of your part or injecting it through channels in your part (depending on your design).

After filling your mold with molten resin, you must cool it down quickly so it doesn’t deform while cooling down. You can do this by placing it in a freezer, putting it in a large vacuum chamber or using a special cooling fluid. Once your part has cooled down to room temperature, you can remove it from the mold.

Why overmold?

Overmolding is an efficient way to create one part that incorporates two different materials. The first material, known as the core or core material, is molded first and then placed into the cavity of the second material, known as the sleeve or jacket material. After both pieces have cooled down enough to be touched safely (often around 60°C), they are joined together using heat and pressure at high temperatures (typically around 230°C). The melted jacket flows into the empty spaces between the core and outer shell, creating one piece with multiple features.

Application advantages

Overmolding has many advantages over other types of assembly, including:

Smoother surface finish. If your product has an external surface that needs to be smooth, overmolding can give you a look and feel similar to injection molding.

Improved strength. If your product needs increased structural integrity, overmolding provides it through the combination of multiple materials with different properties.

Improved electrical insulation. If you need added protection against electrical shorts or interference, overmolding can provide it.

Improved chemical resistance or corrosion resistance. This is especially true when one material used in overmolding is a plastic such as nylon or polycarbonate that has good resistance to water or oil exposure.

Important Design Considerations for Molded Parts

Designing parts for overmolding is an art. And, like all arts, it requires practice to perfect. By carefully considering the design of your part, you can ensure a successful overmold experience. This article covers the most important design considerations for molded parts.

The following are some of the most important design considerations for molded parts:

1) Surface Finish — The surface finish of the product must be smooth enough to allow proper adhesion between the rubber and plastic components during the bonding process.

2) Draft — Draft came on any sharp edges or corners of your part so that these features can get out of the mold tooling with relative ease. This will help ensure that your part comes out of the mold in one piece without excessive force to remove it from a given mold cavity.

3) Walls — Thin walls can provide structural support while keeping weight down and reducing material costs, but they also create challenges when making molds. Thin walls can make it difficult to achieve adequate parting lines and gates when making molds using traditional injector pin technology or other types of injection molding machines (such as single or multi-cavity tools).

Overmolding Process Overview

Before we dive into the details about designing for overmolding, it’s important to understand how the process works. Overmolded parts are from joining two separate injection molds with one or more surfaces in contact with each other. The first mold name is the core mold while the second mold name is cavity mold. These molds join together with a third component called an indexer plate (not shown in figure below). The core and cavity molds share a common parting line, which separates them once they eject from their respective runners.


This short guide intension is a one-stop shop for designers and engineers looking for information regarding overmolding. The guide will begin with the basics of the process, and then go into more detail outlining some best practices, common tools used during the process, and will even go so far as to include a hardware and software reference guide.

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