Comprehensive Guide to Overmolding: Understanding the Process, Design, and Applications

Overmolding is a specialized injection molding process used in the plastics industry to manufacture parts. This technique can enhance the functionality, uniqueness, and aesthetics of components according to the different product requirements, making it popular and widely applied across various industries. Given its significance, any industry looking to elevate their parts to a higher level must understand what overmolding is. Through this detailed explanation, you will gain a deeper understanding of the value and contributions of overmolding to various sectors.
Overmolding is a specialized injection molding process used in the plastics industry to manufacture parts. This technique can enhance the functionality, uniqueness, and aesthetics of components according to the different product requirements, making it popular and widely applied across various industries. Given its significance, any industry looking to elevate their parts to a higher level must understand what overmolding is. Through this detailed explanation, you will gain a deeper understanding of the value and contributions of overmolding to various sectors.

Basic Concepts and Process of Overmolding

Overmolding utilizes specialized injection molding machines that separately load the base material (metal or plastic) and the overmolding material (hard or soft plastic or rubber). By employing either a sequential or simultaneous injection method, these two materials are combined into a single part or product.

Once the materials are combined through the overmolding process, it can eliminate (or improve) the typical drawbacks associated with common plastics and rubbers, such as a cheap appearance, poor aesthetics, and unpleasant tactile feel. This process effectively enhances the part or product's thermal resistance, chemical resistance, insulation properties, mechanical strength, appearance, and practicality. As a result, overmolding extends the application of plastics and rubbers to a broader range of fields, making it an indispensable technology in the plastic injection molding industry.

6 Characteristics of Overmolding Materials

The key advantage of overmolding lies in its ability to bond and encapsulate two different materials, requiring them to have high affinity and compatibility, specifically thermal compatibility. The materials used in overmolding are categorized into substrates and overmolding materials. The substrate's melting point should be slightly higher than the overmolding material's melting point or, at the very least, be able to withstand the overmolding material's molten temperature. Common substrates include metal, PP, PC, ABS, ABS+PC, and NYLON, while overmolding materials include TPE, TPR, TPU, and Silicone. When selecting materials, the following characteristics should be considered:

High Tensile Strength
- The material needs sufficient tensile strength, which can be evaluated based on its tensile strength, elongation, fracture strain, and tensile modulus. These properties help assess the impact of the injection position and plastic flow direction on the strength performance.
High Temperature and Pressure Resistance
- Overmolding involves high pressure and temperature injection, so the materials used must withstand these conditions without damage or deformation over long-term use.
Low Coefficient of Friction
- The coefficient of friction relates to the anti-slip properties of the final product. Different parts and products require varying coefficients of friction. A lower coefficient can help reduce wear and increase the durability of the final product, thus lowering maintenance costs.
Hardness
- Hardness indicates the material's ability to resist indentation and is also related to tactile feel. Different materials have different hardness levels, so it's crucial to ensure the hardness of the two bonded materials is compatible. The flexural modulus can also be considered in this context.
Flexural Modulus
- The flexural modulus measures the material's resistance to bending and is a standard for evaluating its flexibility and fracture propensity. Understanding the maximum bending stress a material can withstand is essential to ensure it meets the final product's requirements.
Material Thickness
- Material thickness affects not only bending performance but also vibration absorption. Thicker materials generally offer better vibration absorption and flexibility, while thinner materials provide weaker vibration absorption. Therefore, changes in material thickness will impact the vibration performance and overall functionality.

💡 Additionally, it's important to note that even within the same type of substrate, products from different brands may not necessarily pair well with the same overmolding materials. Therefore, consulting with material suppliers for recommendations and pre-testing the compatibility between the substrate and overmolding material is advisable to ensure they meet the requirements.

Overmolding Design and Key Considerations

Overmolding allows for the combination of two mold designs, using a two-shot injection molding machine to inject two different plastics simultaneously. Alternatively, the mold can be divided into two parts, each formed using a different injection machine. The considerations for mold design depend on the complexity of the final product. Although overmolding mold design is not significantly different from standard plastic injection mold design, the following points require special attention:

Mold Position

 Base Material Mold Adjustment

The base material mold must first be adjusted to achieve stable dimensions and quality. The parts produced with the base material mold are then transferred to the overmolding mold for the final molding process.

 Interface Between Base and Overmolding Molds

An appropriate amount of safety steel (usually 0.03-0.05mm) should be reserved at the interface between the base material and overmolding molds to facilitate the molding operation. Additionally, the wedge surfaces need to be properly cleared to avoid excessive pressure on the base material parts.

Uniform Wall Thickness

Consistency

The wall thickness of both the base material and the overmolding material should be kept uniform. This uniformity aids in the even flow of materials and prevents issues such as shrinkage marks and voids that can occur due to inconsistent wall thickness.

Process Control and Optimization of Overmolding

Overmolding eliminates the need to separately produce two components, effectively shortening manufacturing time. However, the overmolding process is complex. Here are some practical suggestions to ensure stable production efficiency and high-quality output:

✔️Fit of the Joint Surface

Design the joint surface of the two materials to be as orthogonal as possible to the draft angle to avoid irregular joint surfaces or burrs. If it is unavoidable to be near the draft direction, increase the angle of the joint surface to ensure good sealing. This can help avoid abnormal mold wear and reduce the need for frequent mold maintenance. Adding surface texture and undercut designs to the joint surface can also enhance the bonding strength between different materials.

✔️High Precision Machining and Bonding Quality

The hallmark of overmolding is the complete bonding of two materials. Therefore, ensure the compatibility of the materials to avoid issues like color mixing or poor bonding that could lead to part separation. Additionally, the interacting surfaces of the two molds (cavities) should have high-precision machining quality. Minimize manual polishing to improve bonding strength.

✔️Precise Injection Points and Sealing

The injection point location affects the appearance, so the material must accurately flow into the target area of the part and effectively seal the surface of the substrate. This ensures clean edges between the substrate and the overmolded component.

Overmolding Applications and Recommendations

Overmolding can seamlessly combine multiple materials into a single part or object to meet the needs of different industries and create unique parts and products. Overmolding not only has a good appearance and touch, but also can be used to achieve anti-vibration, anti-slip, sound absorption, waterproof, electrical insulation and other properties, and these characteristics can extend the service life of the products and parts, so it has been favored by customers in various industries, you can learn more about it through the following three cases.

All-Weather Satellite Phone Case with Waterproof, Shockproof, UV-Resistant, and Low-Temperature Resistance

Base Material	ABS+PC (Bayblend T65 XF by Bayer)	Provides excellent waterproofing, shock resistance, UV resistance, and low-temperature durability. It protects the phone from environmental conditions, offering all-weather protection.
Overmolding Material	TPE (Desmopan DP 9370AU, also by Bayer)

Barcode Scanner Case with Shockproof, Waterproof, and Comfortable Tactile Feel

Base Material	ABS+PC (CYCOLOY C1200 by GE)	Offers shockproof and waterproof capabilities with a comfortable tactile feel and excellent operability, suitable for use in various work environments.
Overmolding Material	TPE (Softflex 0615 by Network Polymers)

Non-Slip, Secure Jacuzzi Equipment Step

Base Material	PP (TAIRIPRO K8025 by Formosa Chemicals)	Features non-slip functionality with sturdy treads, enhancing safety during jacuzzi use.
Overmolding Material	TPR (Elastone 2190N by Min Yi Plastics)

If you want to learn more, we recommend visiting Gateway for more Overmolding case studies. With over 40 years of experience and achievements in molding technology, we are familiar with the characteristics of various materials. We can use the Overmolding process to flexibly extend into products that meet expectations, making products stand out in the market.

Feel free to contact us immediately to conduct research and development and customized production according to your needs. Using the Overmolding process, we can reduce production costs and create products with diverse shapes and powerful functionalities.

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