Insert molding

Insert injection molding

What is Insert molding?

Insert molding is a variation of injection molding where pre-made inserts, typically made of metal but can also be of other materials, are placed into the mold before the injection of plastic. 

Once the molten plastic is injected, it encapsulates the insert, creating a single, integrated part.

There are numerous factors contributing to the popularity of this technique, with its affordability being the most significant advantage over alternative insertion methods.

Additionally, it offers a secure and sturdy connection for inserts, ultimately enhancing the overall durability of the component.

1. Process

A pre-made insert (or multiple inserts) is manually or robotically placed into the mold cavity. These inserts can be anything from metal screws and rods to electronic chips or even other plastic parts.

The mold is closed, securing the insert in place.

Molten plastic is injected into the mold cavity, filling the spaces around the insert.

The molten plastic begins to cool and solidify, adhering to the insert and integrating it into the final plastic part.

Once the plastic has fully solidified, the mold opens, and the combined plastic and insert part is ejected.

2. Advantages

Eliminates the need for post-molding assembly, saving time and labor costs.

The mechanical bond formed between the insert and plastic is often stronger than if the components were assembled post-molding.

The process allows for the simultaneous creation of complex parts, reducing the number of manufacturing steps and increasing overall efficiency.

Inserts, especially metal ones, can provide enhanced strength, electrical conductivity, or other functional attributes to the plastic parts.

3. Applications

Insert molding is commonly used for parts like connectors, where a metal component is required for conductivity within a protective plastic casing.

Surgical instruments that need a combination of plastic and metal, or devices that embed a metal core within a sterilizable plastic exterior.

For components that require the durability of metal but benefit from the versatility and design flexibility of plastic, such as knobs with metal threads.

Tools with a metal core and a comfortable plastic grip, or protective casings for electronic components.

4. Considerations

The insert material and the plastic need to be compatible. For instance, they should have compatible thermal expansion rates to avoid issues during cooling.

Depending on the design, there might be a need for specialized tooling or fixtures to keep the insert securely in place during the molding process.

The mold must be designed to accommodate the insert and ensure even flow of plastic around it.


In summary, insert molding is a versatile technique that combines different materials into a single, integrated component, enhancing the functional attributes of the molded part. It offers both manufacturing efficiencies and design advantages, making it a preferred choice in various industries.

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