The Advantages and Disadvantages of Overmolding

If you are an engineer, you may face the need to make a product with two or more different materials. Here lies the question: should you use traditional methods (gluing or screwing materials together) or manufacture the parts as a single piece using a technique like overmolding?

In this article, we will go over what overmolding is, how it compares to insert molding, and considerations to keep in mind when using it.

What is Overmolding?

Overmolding is a type of injection molding process that combines multiple materials into a singular part or product. Overmolding can be used to combine similar materials of a different color, or to combine materials for functional purposes, such as giving a product a soft grip.

If you are a more visual learner, linked is our YouTube video, explaining what overmolding is.

(See examples of overmolding below)

Overmolding has many benefits over the traditional assembly processes and can provide products with:

• Better adhesion between components
• More compact design
• Lower labor costs during production
• Noise and vibration reduction
• Protection from UV and corrosive chemicals
• Waterproofing
• Electrical insulation

However, these advantages come with a cost. They require a more complex and expensive setup process for manufacturing. With that said, overmolding is often reserved for specialty parts and high volume production.

What is the Difference Between Insert Molding and Overmolding?

Insert molding is when plastic is injected into a mold with a pre-placed insert. This insert helps shape the part. Insert molding is very similar to standard injection molding; the only difference is the pre-placed insert inside of the mold. This insert can be made of metal or various types of plastic. Overmolding, on the other hand, is when a singular part or product is made by combining two or more materials.

The part of the image above where the substrate is injected over the bolt is considered insert molding. The added rubber layer around the nail is called overmolding, which is a good way to improve grip for parts.

Overmolding Design Considerations

When designing products for overmolding, there needs to be a thorough plan in place because of the complexity of the 2-step process:

1. Have both molds ready to go.

-1 mold for the substrate

-1 for the final over-molded product

2. Load the machine with substrate parts and unloaded completed parts with overmolding.

Here are some things to consider:

• Avoid thin edges: avoid edges that taper to a fine point. During injection, slowing of the flow and subsequent cooling can have adverse effects on the adhesion between the substrate and overmolding.
• Maintain uniform thickness of the overmolding: the overmolding should maintain a constant thickness between 1.5mm to 3mm (0.060–0.120")
• If you need sections of different thicknesses, avoid abrupt changes in thickness and smooth out surfaces as much as possible.
• Due to the elastic nature of many overmolding materials, you may incorporate smooth bumpoffs into the design.
• To help avoid warping, keep the overmolding layer thinner than the underlying substrate.
• Account for shrinkage of the substrate to ensure proper fixation in the overmolding cavity.

Material selection is another crucial consideration to be made during the design process. Overmolding is suitable for use with a wide range of materials. The substrate is usually composed of a harder plastic or a blend of plastics to achieve desired properties.

Some typical materials used for the substrate are:

• Polycarbonate
• ABS (Acrylonitrile Butadiene Styrene)
• Polystyrene
• PETG (Polyethylene terephthalate)
• Nylons

Overmolding is most commonly made of Thermoplastic Elastomers (TPEs) because they have desirable physical properties:

• High-temperature, wear and impact resistance
• Can be hard or soft
• Flexibility & resilience
• Easy to bond with other plastic materials

They also have a wide array of compatible substrates. Material compatibility is a complex but vital part of the design for overmolding. Material suppliers should be consulted during the material selection process to ensure compatibility.

Material Bonding

An essential part of the overmolding process is ensuring the two materials are adequately bonded. There are two types of bonding that ensure the parts do not separate: chemical and mechanical.

Chemical

Chemical bonding refers to the attraction between the materials at a molecular level and the primary means by which overmolding is bonded to the substrate. For this reason, the substrate and the overmolding materials must be chemically compatible. An adhesive can be applied to the substrate in specific applications to aid the bonding during the overmolding process.

Mechanical

Mechanical bonding is the use of the geometry of the product to help prevent separation of the overmolding. Mechanical bonding relies on creating geometry in which the overmolding material would have to deform significantly to be released from the substrate. You can enhance mechanical bonding by geometries that use the natural shape of the product to provide fixturing of the overmolding.

If additional bonding is required, you may add more interlocks. An interlock is a local geometry added explicitly to the product. Interlocks allow the overmolding to grip the underlying substrate.

Conclusion

Overmolding can be a potent tool in product design and requires meticulous care and attention during the design process. Although overmolding can be more expensive during the design and development phase, it increases the product value and can reduce long-term manufacturing costs.

Here at Jaycon, we have a team of professionals to help you incorporate overmolding in both your design and manufacturing phase. If overmolding is out of your price range, we are happy to help you find another avenue with similar results.