In today’s manufacturing industry, product design and manufacturing technology continue to evolve. Overmolding, a specialized type of injection molding process, is gradually receiving widespread attention. It brings a lot of innovation and improvement to the function and appearance of the product.
What Is Overmolding?
Over molding is a manufacturing process in which one or more materials are injected onto injection molded parts or substrates to form an integrated part with multiple characteristics.
For example, it is common to cover a rigid plastic or metal part with a soft rubber or plastic material to achieve better feel, slip resistance, water resistance, or other specific functional requirements.
How Does the Overmolding Process Work?
Mold Preparation
The first step is to prepare the mold. The mold is designed with specific cavities and channels to accommodate the overmold material and the shape of the final product. It may have multiple sections or inserts to create the desired geometry and features.
The mold is then cleaned and preheated to the appropriate temperature, which helps the materials flow more easily and adhere properly during the molding process.
Insert Placement
If the overmolding process involves an insert, such as threaded inserts, pre-formed plastic parts, or metal components, it is carefully placed into the mold cavity at the precise location. The insert must be properly positioned and held in place to ensure accurate overmolding and a strong bond between the insert and the overmolded material.
First Material Injection
The first material, which will form the base or substrate of the part, is melted and injected into the mold cavity under high pressure. The injection molding machine forces the molten material through a sprue and into the runners and gates that distribute the material evenly throughout the cavity.
As the material fills the cavity, it takes the shape of the mold and begins to cool and solidify. The cooling process is carefully controlled to ensure the material sets properly and has the desired mechanical and physical properties.
Second Material Injection (Optional)
In some cases, a second material is then injected into the mold to form an additional layer or component over the first material. This can be done immediately after the first material has partially cooled or in a separate injection stage, depending on the specific overmolding process and materials used.
The second material may have different properties than the first, such as a softer or more flexible material to provide a specific tactile feel, better grip, or enhanced sealing capabilities.
Cooling and Solidification
After the injection of all the materials is complete, the mold is cooled further to allow the materials to fully solidify and bond together. The cooling time can vary depending on the thickness and type of materials used, as well as the size and complexity of the part.
During cooling, the materials shrink slightly, and the mold design must account for this to ensure the final part has the correct dimensions and shape.
Demolding
Once the materials have fully solidified and cooled, the mold is opened, and the overmolded part is ejected. Demolding can be done using ejector pins, stripper plates, or other ejection mechanisms built into the mold.
The overmolded part is then removed from the mold and inspected for quality. Any excess material, such as flash or sprue remnants, may be trimmed or removed at this stage.
Post-Processing
After demolding, the overmolded part may undergo additional post-processing operations, such as painting, plating, or assembly with other components to complete the final product.
Quality control checks are performed throughout the process to ensure the part meets the required specifications for dimensions, appearance, and performance.
Overmolding Methods
Two-Shot Overmolding:
In this method, an injection molding machine with two injection units is used, and two different materials are successively injected into the same mold. This process, known as two-shot overmolding or two shot molding, involves first injecting the initial material to form the partial structure of the part.
Then, the second material is immediately injected without opening the mold, combining with the first material in the mold. This method has high production efficiency and tight material bonding, but has higher requirements for equipment and molds.
Pick-n-Place Overmolding:
The part of the first material is produced by the conventional injection molding method, then it is removed and placed in another mold, and the second material is injected for molding. This method is relatively flexible, suitable for some complex shapes or parts that have special requirements for the material combination sequence, but the production efficiency is relatively low, because it involves the middle take-to-place part step.
Overmolding Materials
There are many kinds of materials used in overmolding, including thermoplastics, thermosetting plastics, rubber and so on.
Thermoplastic Elastomers (TPE): Common for overmolding onto rigid parts to provide flexibility, soft-touch feel, or better grip.
Thermoplastics (ABS, Polycarbonate, etc.): Used for the base or primary part, providing strength, impact resistance, and heat resistance.
Rubber Materials: Often overmolded for gaskets or seals. Overmolding is also used in the production of medical devices, where material selection is crucial for creating durable and sterilizable components.
Metals: In some cases, metal inserts are overmolded with plastic to provide added structural strength or conductivity.
Advantages of Overmolding
| Advantages | Description |
| Improved Durability | Overmolding creates a stronger bond between materials, enhancing the overall durability of the part. |
| Multi-functionality | It allows for combining materials with different properties, such as flexibility and strength, in a single part. |
| Ergonomics | Soft-touch overmolding (using materials like TPE) can make products more comfortable to hold, such as handles, grips, or buttons. |
| Cost Efficiency | Overmolding can reduce assembly time and costs since multiple materials are combined in a single molding step. Selecting the right manufacturing partner is crucial for the success of any injection molding project, including overmolding. |
| Design Flexibility | Complex geometries and material combinations can be achieved, allowing for more innovative product designs. |
Disadvantages of Overmolding
| Disadvantages | Description |
| Increased Complexity | The process is more complex than traditional injection molding, requiring specialized equipment, molds, and precise material control. |
| Higher Costs | For small production volumes, overmolding can be more expensive than traditional methods, especially when using multiple materials or complex molds. |
| Material Compatibility Issues | Some materials may not bond well together, leading to adhesion failure or reduced part performance. |
| Limited by Mold Design | The design of the mold must accommodate both materials, which may limit the types of products that can be overmolded. |
Applications of Overmolding
Consumer electronics products:
such as mobile phone shell, headset, game console handle, etc., over-molding can provide these products with comfortable feel, good anti-slip performance and beautiful appearance, while also integrating some functional components, such as antennas, keys, etc.
Auto parts:
Widely used in automotive interior parts, door handles, shift rods and other parts, which can improve the comfort and beauty of the parts, but also enhance its durability and functionality, such as waterproof, dustproof, noise reduction.
Medical Devices:
In tools, handles, or implants, overmolding can combine materials to ensure both comfort and durability.
Tools and Household Products:
Ergonomic handles, grips, and non-slip surfaces can benefit from overmolding.
Overmold Design Guide
Parting surface design:
Reasonable design of the parting surface of the mold to ensure that during the overdie molding process, the two materials can accurately fill their respective cavities, and will not cause damage to the formed parts during the release of the mold. The position of the parting surface should be selected as far as possible in the inconspicuous part of the product or in the place that has less impact on the appearance and function.
Gate design:
According to the fluidity of the material and the structural characteristics of the product, design the appropriate gate location and number. The position of the gate should ensure that the material can evenly fill the cavity to avoid defects such as insufficient filling and welding marks. For double shot molding, it is also necessary to consider the relative position and order of the gate of the two materials to ensure a good combination between the materials.
Cooling system design:
Optimize the cooling system of the mold to ensure that the two materials can be cooled and cured quickly and evenly after forming. The cooling speed of different materials may be different, and it needs to be adjusted according to the characteristics of the material to prevent internal stress, deformation and other problems due to uneven cooling.
Ejector mechanism design:
A reasonable ejector mechanism is designed to ensure that the over-molded part can be removed from the mold smoothly during the demoulding process without causing damage to the product. The ejection position should be evenly distributed to avoid excessive ejection stress on weak parts or key parts of the product.
Overmolding Design Considerations
When designing for overmolding, ensuring good functionality and a seamless integration of the base and overmold materials is crucial. Below are the considerations for functionality, shut-offs, and recommendations on shut-offs to avoid:
Approach for Good Functionality
Design shut off to minimize potential for edge peeling of TPE
General geometry needs to be a very sharp transition area between molded TPE edge and supporting substrate.
Resulting TPE geometry must be designed to vent the cavity properly
Shut-Off Recommendations
Provide interference fit of 0.002 to 0.004 in (0.050 to 0.101 mm) – somewhat dependant on cosmetic needs
Specific plastic substrate ductility
Heat treat shut off steel to a minimum 54 Rockwell hardness
Consider substrate edge design to “hide” TPE edge from consumer.
Pre-dry hygroscopic substrate and TPE pellets to avoid porous surfaces nearest substrate interface
Where appropriate, have the actual shut-offs employed as inserts (helps downstream injection mold maintenance)
Shut-Offs to Avoid
Avoid using rounded/radiused shut offs
Component designs where TPE geometry lies high above the substrate (e.g., cliff wall)
Placing shut offs directly in the mold base
Building the tool without developing a clear shut off strategy first
Placing the vent directly at shut off edges – can actually encourage flashing
Best Practices
To ensure successful overmolding and insert molding projects, it’s essential to follow best practices:
Material Selection: Choose compatible materials that bond well together to ensure a strong and durable final product.
Design for Bonding: Incorporate proper surface features, undercuts, and interlocking structures to enhance the bond between materials.
Proper Wall Thickness: Maintain consistent and appropriate wall thickness to prevent defects and ensure structural integrity.
Draft Angles: Include draft angles in the design to facilitate easy part ejection from the mold.
Avoid Sharp Corners: Use rounded edges and corners to prevent stress concentration and potential failure points.
Venting: Integrate venting features in the design to allow gases to escape and prevent defects.
Overmolded Material Placement: Position the overmolded material’s injection points away from critical features to avoid compromising the part’s integrity.
Tool Design: Design molds with appropriate gating, runner systems, and cooling channels to ensure efficient and effective molding.
Consider Shrinkage: Account for material shrinkage differences between the injection-molded part and overmolded material to maintain dimensional accuracy.