Overmolding

Overmolding is an injection molding process where multiple materials are combined to form a single part. A rubber-like plastic such as TPE(the overmold) is molded over another component(the substrate). The substrate is usually an injection-molded plastic part but can also be made from other materials. The first material(the substrate) is covered partially or fully by a second material(the overmold).

Consider to choose overmolding when your finished product is:

made from thermoplatics, rubber, or a combination of both;
includes multiple layers, materials, or colors;
not require seperation or disassembly.

This article explain overmolding in following aspects, you can continue reading or click the title and jump to the section that you are interested.

Overmolding Process

There are two main overmolding techniques: two-shot molding and pick-and-place molding. In two-shot molding, a single production mold is used, while pick-and-place molding requires two separate molds.

Choosing the right materials for overmolding can be challenging, as the substrate and overmold resins must be compatible to work effectively together. The selection depends not only on the part’s application but also on the specific molding method used. Since overmolding involves a more complex process and outcomes compared to single-shot injection molding, consulting with resin experts can be invaluable in selecting the best materials.

Two-Shot Molding

Two-shot molding uses a single mold to create overmolded parts. It involves molding the substrate material first, then rotating or indexing the mold to inject the overmold material in a second shot. There are three main techniques for two-shot molding:

Transfer overmolding: The substrate part is removed from one mold by a robotic arm and placed into a second mold where the overmold material is injected. This is an efficient method for multi-cavity production.
Rotational overmolding: A two-barrel press with a rotating platen is used. The substrate is molded, then the platen rotates to an overmolding station where the second material is injected. This is highly efficient for high volumes.
Core-back overmolding: The mold has a sliding section that pulls back after the substrate is molded to allow injection of the overmold material. This method is used for parts with specific linear geometries.

Pick-and-Place Molding

In pick-and-place overmolding, multiple substrate parts are molded at once, then placed into a larger second mold where the overmold material is injected around them. The substrate parts need to be warm to facilitate adhesion, especially when using a hard substrate with a soft overmold material like TPE or TPU.

The Role of Bonding

The bonding methods for overmolding primarily focus on how the substrate material and the overmolding material adhere to each other. There are two main types of bonding mechanisms used in the overmolding process: chemical bonding and mechanical bonding.

Chemical Bonding

Chemical bonding occurs when the substrate and overmold materials are compatible at a molecular level, allowing them to form a strong bond without the need for additional adhesives or fasteners. This type of bonding is advantageous because it creates a permanent connection that can enhance the overall integrity and durability of the part.

To achieve effective chemical bonding, it is crucial to select materials that are chemically compatible. For example, materials like thermoplastic polyurethane (TPU) can bond well with various rigid substrates such as ABS and polycarbonate. A strong chemical bond can significantly improve the performance of the final product by reducing the risk of delamination and enhancing resistance to environmental factors.

Mechanical Bonding

Mechanical bonding involves creating a physical interlock between the substrate and the overmold material. This can be achieved through design features such as undercuts, grooves, or textured surfaces on the substrate that allow the overmold material to grip and hold firmly. While mechanical bonding can be effective, it generally does not provide the same level of strength and permanence as chemical bonding. However, it can be a viable alternative when the materials used are not chemically compatible. In such cases, the design must ensure that the mechanical features are sufficient to maintain the bond under stress.

The Benefits and Limitations of Overmolding

Overmolding offers several key benefits while also having some limitations to consider:

Improved Product Performance

Overmolding can significantly enhance product performance. For instance, adding a soft, rubber-like overmold to a rigid substrate can improve grip and shock absorption, making products safer and more comfortable to use.

Benefits of Overmolding

Achieved manufacturing products with multiple materials.
Reduce the need for complex product assembly processes.
Enhancing the visual appeal of products like different colors, textures, and finishes.
Overmolding can add a protective layer which can extend products lifespan

Limitations of Overmolding

Requires additional tooling and setup that increase the initial investment.
Longer cycle times due to the need for two injection cycles and increased product cost.
Incompatible materials can lead to delamination or other issues.
Not suitable for parts with complex geometries or undercut.

Materials Common Used in Overmolding

Proper material selection ensures strong bonding between the substrate and overmold materials, enabling them to function together effectively without delaminating or failing under stress. The chosen materials should possess the necessary properties, such as flexibility, hardness, temperature resistance, and chemical resistance, to meet the specific requirements of the intended application.

Common Substrate Materials

Substrate materials are the base components that provide structural integrity to the part. Commonly used substrate materials include:

Acrylonitrile Butadiene Styrene (ABS)
Polycarbonate (PC)
Polyamide (PA, Nylon)
Polypropylene (PP)
Acrylonitrile Butadiene Styrene/Polycarbonate (ABS/PC)
Polyoxymethylene (POM)
Glassfiber (GF)

Common Overmold Materials

Overmold materials are typically softer and are applied over the substrate to enhance grip, comfort, and aesthetics. Common overmold materials include:

Thermoplastic Elastomer (TPE)
Thermoplastic Polyurethane (TPU)
Thermoplastic Rubber (TPR)
Silicone

Design Considerations for Overmolding

Bonding

As we mentioned before, the two primary bonding methods are chemical bonding and mechanical bonding. A strong bond between the substrate and overmold materials is critical. Designers must ensure that the materials are compatible and that the bonding methods are effective.

Tips:

Substrates with a higher surface polymer tend to bond better.
Ensure there is enough vent on the mold.
Pre-drying of the overmold resin before application.
Adoption of mechanical interlocks in the mold design.
The thickness level must be appropriate to avoid delamination.

Material Compatibility

The substrate and overmold materials must be physically, chemically, and thermally compatible for successful overmolding. Incompatible materials can lead to weak interfaces, delamination, or failure under stress, compromising the part’s durability and performance. Since many resins including both thermoplastics and thermosets can have a range of characteristics, it may be useful to consult experts in choosing the right resin grade for a specific application.

Tips:

Choose materials with similar thermal expansion rates to avoid warping or cracking from heat stress.
Make sure the overmold material has a lower melting point than the substrate for proper bonding.
For cushioning, the thickness of the material matters as much as its softness. If thin layers under 10mm may feel hard, so many products use ribs to feel thicker, use less material, and stay flexible.
For better grip, focus on how sticky the material is. TPEs grip well because they aren’t slippery. The hardness of the material doesn’t always mean it will grip better.

Moldability Principles

While overmolding follows the same basic principles as standard injection molding, there are a few additional considerations:

Maintaining proper draft angles, uniform wall thickness, and smooth transition lines in both the substrate and overmold
Limiting the overmold thickness to be less than or equal to the substrate thickness
Texturing the substrate surface to enhance adhesion and the overmolded surface for improved grip and aesthetics
Designing the overmolded surface to be even with or slightly below adjacent substrate surfaces for a seamless finish
Incorporating mechanical interlocks like undercuts or holes in the substrate when chemical bonding is not feasible

Common Applications of Overmolding

Overmolding is utilized across various industries due to its ability to enhance product functionality, aesthetics, and user experience. Here are some key applications:

Consumer Electronics

Smartphones and Tablets: Overmolding is used to create soft-touch grips and protective casings that improve ergonomics and provide shock absorption.
Remote Controls: The overmolding process adds a rubberized exterior for better grip and improved tactile feedback.

Automotive

Dashboard Components: Overmolding provides a soft-touch surface on dashboard panels, enhancing comfort and reducing glare.
Handles and Controls: Ergonomic grips for door handles, gear shifters, and steering wheels are often overmolded to improve user comfort and control.

Appliances

Kitchen Gadgets: Overmolding is used to create comfortable, non-slip handles on utensils and appliances, improving usability.
Small Appliances: Devices like blenders and mixers often feature overmolded grips for better handling and safety.

Outdoor Equipment

Waterproof Cameras: Overmolding creates seamless, watertight seals that protect sensitive electronics from water damage.
Outdoor Lighting Fixtures: Overmolding is used to encase electronic components, providing protection from the elements.

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