Plastic Overmolding is an injection molding technique that involves combining one material (usually a thermoplastic elastomer, TPE) with another material (usually a hard plastic) by applying heat and pressure. Liquid silicone is used to form the mold in this process. The correctly selected TPE material can form a strong bond with the plastic substrate, ensuring the stability of the product in the end-use environment. This method can achieve an ideal bond between the two materials without relying on primers or adhesives.

Why Use Plastic Substrate Molding?

The core advantage of the overmolding process is that it can significantly enhance the appearance and practical functions of products. This technology can not only give products a colorful coat, but also finely carve products to meet the personalized needs of consumers. Take the screwdriver as an example. Through the overmolding process, it can be equipped with a colorful handle that is both eye-catching and comfortable, which is both practical and pleasing to the eye.

The charm of the overmolding process is far more than that. It can also:

Give a soft "coat" to parts of a single material to provide a comfortable grip experience.

Through the segmentation and matching of colors, give the product a unique visual charm.

Cleverly integrate flexible areas into hard materials to enhance the practicality and comfort of the product.

Greatly reduce the production process and eliminate cumbersome assembly links. In traditional processes, metal tools and plastic handles need to be manufactured separately and then assembled, while the overmolding process can be done in one step, perfectly integrating the two without additional assembly.

Abandoning traditional fasteners and adhesives to achieve seamless connection between components not only simplifies the production process, but also improves the integrity and durability of the product.

There are two main techniques for overmolding—two-shot molding and pick-n-place molding. The key difference lies in the number of molds used: two-shot molding utilizes a single mold for both processes, while pick-n-place molding requires two separate molds.

Choosing materials for overmolding can be challenging. The resin for the substrate and the overmolding material must be compatible to work effectively together. The selection depends on the application and the specific method used to produce the part. Because overmolding is more complex than single-shot injection molding, consulting with material experts is recommended when selecting resins.

Two-Shot vs. Pick-n-Place Overmolding

First, let's examine two-shot molding. In this process, a substrate is molded using one material and then quickly overmolded with a second material. This process is generally highly automated. In contrast, pick-n-place molding involves the production of substrate parts in one mold, which are then manually transferred into a second mold where the overmolded resin is injected to complete the part.

There are three main techniques used in two-shot molding:

1. Transfer Overmolding: A robotic method where substrate parts are lifted from one mold and placed into a larger mold. The overmolded material is then injected to fill the mold, typically while the next substrate is being produced in the first mold.

2. Rotational Overmolding: In this method, the mold moves between two injection stations. This allows both the substrate and the overmold material to be injected efficiently.

3. Core-Back Overmolding: This method is suitable only for specific linear geometries. The mold features a sliding section that moves back after the first material is injected, creating space for the second material.

All three two-shot techniques involve applying the overmolded material to a warm substrate, which enhances chemical bonding. While these methods require specialized equipment and expensive molds, they are highly automated and cost-effective for large-scale production, typically more than 10,000 parts, often reaching 100,000 or more.

On the other hand, Pick-n-Place Molding uses two separate molds. A batch of substrate parts is first molded and allowed to cool. Afterward, these parts are manually placed into the second mold, where the overmold material is injected over the substrate. This process requires less complex equipment and molds, making setup simpler and faster.

There are two primary methods of overmolding: 

Two-shot molding uses a single production mold.

Pick-n-place molding uses two production molds where an entire batch of parts are molded. Then, they are manually placed into a second mold where the overmold resin is injected to produce the completed parts.

Thousands of possible combinations exist for over-molded material. Resins have to be adhesive and compatible with each other in order for the process to work.

If the goal of using overmolding is to enhance grip or increase cushioning in your product, make sure your injection molding company knows those goals. Factors like cushioning, flexibility, and friction will play into the type of resins that are used in the product.

Benefits of Overmolding:

It can provide a soft, non-slip grip to your product.

It acts as an environmental barrier to shock, vibrations, and noise.

It creates colorful, visually attractive surfaces.

It reduces the number of secondary steps and costs associated with them, in turn reducing the complexity of assembly.

It can provide adhesion between different materials and eliminate the need to assemble different materials by hand.

Thingyfy has partnered with many OEM customers over the last 10 years, providing us with invaluable overmolding experience in a wide array of applications. In manufacturing overmolded plastic parts, Thingyfy has been able to reduce labor and assembly costs for our customers, while adding quality and durability to the end product. There are numerous advantages associated with overmolding design, including:

• Plastic overmolded components ensure proper alignment, prevents loosening and the plastic resin can provide improved resistance to vibration and shock

• Improved part strength and structure

• Enhanced design flexibility and multi-material components (Custom plastic overmolding allows for production of parts molded of multiple thermoplastic materials)

• Reduced secondary operation, assembly and labor costs (Molded as one assembly)

• Eliminates bonding step in the manufacturing process (Improved component reliability)

1. Consumer Electronics

• Tool Handles: Overmolding is used to create ergonomic, non-slip grips on tools like screwdrivers, pliers, and power tools.

• Toothbrushes: The handles of toothbrushes are often overmolded with soft, comfortable materials for better grip and usability.

• Kitchen Utensils: Overmolding provides heat resistance and comfortable grips on spatulas, knives, and other utensils.

2. Automotive Industry

• Steering Wheels: Overmolding is used to create soft, comfortable grips on steering wheels.

• Gear Shifts: Overmolded gear shifts provide better tactile feedback and durability.

• Seat Components: Overmolding is used to create padded and ergonomic seat components.

• Connectors and Sensors: Overmolded plastic protects electrical components from moisture, vibration, and other environmental factors.

3. Medical Devices

• Surgical Instruments: Overmolding provides soft, non-slip grips for precision tools.

• Hearing Aids: Overmolding is used to create custom-fit, comfortable ear molds.

• Syringes and Handles: Overmolded grips improve usability and comfort for medical professionals.

• Prosthetics: Overmolding is used to create comfortable, durable, and lightweight prosthetic components.

4. Industrial Equipment

• Handles and Grips: Overmolding is used to create ergonomic handles for industrial tools and machinery.

• Seals and Gaskets: Overmolded plastic provides sealing and insulation for industrial components.

• Control Panels: Overmolding enhances the durability and usability of control panels and buttons.

5. Electronics and Electrical Components

• Cable Connectors: Overmolding provides strain relief and protection for cables and connectors.

• Switches and Buttons: Overmolded switches and buttons are more durable and user-friendly.

• Enclosures: Overmolding is used to create protective casings for electronic devices.

6. Sporting Goods

• Golf Clubs: Overmolding is used to create comfortable grips on golf clubs.

• Bicycle Handles: Overmolded grips improve comfort and reduce hand fatigue.

• Fitness Equipment: Overmolding is used to create ergonomic handles and grips for dumbbells, resistance bands, and other equipment.

7. Household Products

• Appliance Handles: Overmolding is used to create comfortable, heat-resistant handles for appliances like irons, kettles, and pans.

• Storage Containers: Overmolded lids and handles improve usability and durability.

• Cleaning Tools: Overmolded grips on mops, brooms, and brushes enhance comfort and control.

8. Toys and Recreational Products

• Toy Handles: Overmolding is used to create soft, child-safe grips on toys.

• Game Controllers: Overmolded grips improve comfort and durability for gaming devices.

• Outdoor Gear: Overmolding is used to create durable, weather-resistant components for camping and outdoor equipment.

9. Packaging

• Bottle Caps: Overmolding is used to create soft, easy-to-open caps for bottles.

• Containers: Overmolded handles and seals improve the functionality and durability of packaging.

• Enhanced Ergonomics: Improves grip and comfort.

• Durability: Adds strength and resistance to wear and tear.

• Aesthetics: Allows for customizable colors, textures, and designs.

• Cost Efficiency: Reduces the need for assembly and additional components.

• Vibration Dampening: Absorbs shocks and vibrations in tools and machinery.

When designing for overmolding, choosing the right materials is essential to ensure optimal performance, durability, and functionality of the final product. Overmolding involves combining two different materials—one for the substrate (the core) and another for the overmold (the outer layer)—so material compatibility is a key factor. Here are the key considerations for selecting materials for overmolding:

1. Material Compatibility

• Adhesion: The substrate material and the overmold material must bond effectively during the molding process. Material compatibility is crucial for ensuring a strong, long-lasting bond between the two materials.

• Thermal Properties: Both materials should have similar thermal expansion rates to avoid delamination or warping during cooling. The overmold material needs to conform well to the substrate while also being able to withstand the temperature conditions without losing its properties.

• Chemical Compatibility: The materials should not react with each other negatively during the injection process or over time. The overmold material should also resist environmental factors like UV exposure, chemicals, and wear.

2. Substrate Material

The substrate (the core material) is typically chosen based on the part's primary function and required strength. Common materials used for substrates include:

• Metals (e.g., aluminum, steel): Used when strength, conductivity, or heat resistance is required.

• Plastics (e.g., ABS, polycarbonate, polypropylene): Preferred for lower cost, lightweight, and molded parts.

• Thermoplastics: Often used when flexibility, ease of manufacturing, and recyclability are needed.

3. Overmold Material

The choice of overmold material depends on the properties required for the final product:

• Soft Materials: Thermoplastic elastomers (TPE), thermoplastic polyurethane (TPU), and silicone are commonly used for overmolding. They offer flexibility, cushioning, and impact resistance.

• Hard Materials: Engineering plastics such as polycarbonate (PC), ABS, and nylon can be used when the product requires hardness or higher structural integrity.

• Rubbers and Elastomers: These materials provide excellent grip, cushioning, and flexibility, often used for ergonomic features and tactile surfaces.

4. Application Requirements

• Mechanical Properties: Consider the strength, flexibility, and durability required by the end product. Overmolding can enhance grip, improve aesthetics, or provide electrical insulation, depending on the material selection.

• Surface Texture: The surface finish of the overmold material can provide additional features such as soft-touch surfaces or enhanced aesthetics.

• Environmental Resistance: Materials for overmolding need to be chosen based on their ability to resist harsh environments, UV radiation, moisture, and chemicals that the part may be exposed to during its use.

5. Manufacturing Process Considerations

• Moldability: Ensure that both the substrate and overmold materials are easy to mold and can be processed in the chosen molding method, whether it's two-shot molding, insert molding, or pick-n-place molding.

• Cost: Material cost and processing cost are also crucial factors. While high-performance materials may offer better durability, they may also increase the overall cost of production.

6. Design Flexibility

• Geometrical Compatibility: The shape and size of the substrate and overmold should align with the molding process. Complex geometries or intricate details may require specific materials that are easier to mold or that flow better in the mold.

• Thickness and Layering: The thickness of the overmold layer should be considered. Thicker layers can provide more durability but may complicate the molding process or affect the final product’s weight.

7. Testing and Prototyping

Before finalizing material selection, it’s essential to prototype and test the overmolded parts under real-world conditions. This helps to identify potential issues with material compatibility, adhesion, and performance.

Common Material Pairings

• Thermoplastic elastomers (TPE) with ABS: TPE offers flexibility and comfort, while ABS provides structural strength.

• Silicone with Plastic substrates: Silicone’s flexibility and durability combine well with the stability and molding ease of plastics.

• Polyurethane (PU) with Metal or Plastic cores: PU’s resistance to abrasion and chemical exposure is a good match for many industrial applications.

Material selection for overmolding is a critical part of the design process. The right combination ensures that the final product meets performance, durability, and cost requirements, while also optimizing the molding process.