What is Overmolding

Definition of Overmolding and Its Advantages in custom plastic parts

Overmolding is a process where a rigid plastic or metal substrate is first injection molded, followed by an overmolding step where one or more layers of elastomers or functional plastics are molded over or around it. For custom plastic parts, Overmolding brings several benefits:

• Function Integration: Rigid structural support and soft protection within the same part

• Enhanced Appearance and Comfort: Soft-touch layers improve ergonomics and anti-slip performance

• Reduced Post-Processing: Eliminates gluing, assembly, or secondary bonding steps

• Optimized Cost: Lower per-unit cost in high-volume production compared to manual assembly

How Overmolding Enables High-Volume Multi-Material Injection

In mass production, Overmolding utilizes several types of automated systems to achieve continuous, efficient, multi-material injection molding:

1. Rotary Platen Systems

The most common industrial solution, ideal for large-scale plastic parts manufacturing:

• The mold consists of two cavities: one for base material and one for overmolding

• The mold rotates 180° after each injection cycle to switch between cavities

• Dual injection units operate alternately, ensuring high efficiency and low energy consumption

• Final parts are completed within a single mold, significantly improving cycle time and consistency

  2. Transfer Molding Systems

  Perfect for medical plastic molding applications requiring high precision and complex geometry:

  • Robotic arms or automatic loaders transfer the base component to a second mold

  • Accurate overmolding is performed in the second cavity

  • Both molds may share the same injection machine or cooling system

  • High precision and flexible production, suitable for mixed production lines

  3. Dual Injection Machine Coordination

  Used in high-end industrial scenarios like complex medical device molding or advanced electronics:

  • Two injection machines each inject different materials

  • Robots manage the automatic transfer and overmolding

  • MES systems enable traceability and defect control

  • Widely applied in production lines for premium custom plastic parts
    

    Origins of Overmolding and Its Growth in medical device molding

    Overmolding originated in the mid-20th century alongside two-shot injection molding technology. It gradually developed in Europe and North America as a key method for integrating complex functions into single components. Its use in medical manufacturing began in the 1990s, when advanced medical-grade materials and mold precision enabled its adoption.

     

    Today, Overmolding is widely used in medical devices, personal care instruments, and biocompatible components. Many global medical plastic molding companies now incorporate Overmolding as a core production capability.

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    Overmolding Process & Mold Design Considerations

    Overmolding involves a multi-step injection molding workflow. The main steps include:

    1. First Injection (Substrate Molding)
       A rigid material (e.g., PC, ABS, PA66) is molded to provide structural support.

    2. In-Mold or Robotic Transfer
       The molded part is rotated or transferred to the overmolding station.

    3. Second Injection (Overmolding Material)
       Soft materials (e.g., TPE, TPU, silicone) are injected to form outer layers or seals.

    4. Cooling, Ejection & Inspection
       The part is cooled, ejected, and inspected for dimensional accuracy and bonding strength.

     

    Mold design for Overmolding is more complex, requiring multi-cavity layouts, precise alignment systems, and dual-material runner control. These are essential in high-end plastic parts manufacturing projects.

  • Typical Applications of Overmolding

    Overmolding serves numerous precision industries. Here are common use cases:

    Medical Applications (medical device molding / medical plastic molding)

    • Surgical handles, anti-slip grips, hemostats with overmolded areas

    • Soft-seal connectors for catheters

    • Integrated housings for ventilators and injection systems

    • Analytical device parts with color codes and flexible seals

    Industrial and Consumer Electronics (custom plastic parts)

    • Industrial buttons, waterproof switches

    • Oil-resistant handles, power tool grips, flashlight housings

    • Dual-layer insulation for electronic connectors

    Automotive and Home Appliances

    • Gear shifters, soft-touch door handles

    • Remote control and appliance bezels

    • Shock-absorbing housings for high-end electric tools

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    Conclusion: JSJM Provides Complete Overmolding Support for Your Projects

    Overmolding enables significant breakthroughs in materials integration and structural design, especially for custom plastic parts, medical plastic molding, and medical device molding. At JSJM, we offer advanced injection platforms, custom mold development, and strict quality control to deliver efficient, reliable Overmolding solutions.

     
     

    Contact JSJM today to receive professional Overmolding consultation and a personalized quote.