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Double Injection Molding: Injection Molding Techniques & Design Guidelines

August 18, 2023
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Injection molding has revolutionized the manufacturing industry, enabling the mass production of complex and precise plastic parts. Among the various injection molding techniques, double injection molding, also known as 2-shot molding or multi-material injection molding, stands out as a highly versatile and efficient method. In this blog, we will delve into the intricacies of double injection molding, its benefits, and how manufacturer ACE has mastered this advanced technology.

Understanding Double Injection Molding: What is Double Injection Molding?

Double injection molding, also known as 2-shot molding or multi-shot molding, is a specialized injection molding process that involves the simultaneous injection of two different materials or resins into a single mold. By combining two materials in a single production cycle, manufacturers can create complex parts with enhanced aesthetics, improved functionality, and reduced assembly costs. The process can be accomplished using various techniques, such as over-molding, core-back, or rotational.

Materials and Resins in Double Injection Molding:

1. Material Selection:

The success of double injection molding lies in choosing compatible materials with distinct properties and functionalities. Manufacturers typically use two different thermoplastic materials, ensuring that one is compatible with the other during the molding process. Commonly combined materials include thermoplastic elastomers (TPE), polypropylene (PP), acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and various engineering-grade resins. The versatility of these materials allows designers to create components with diverse characteristics, such as soft-touch grips, rigid cores, and UV resistance, expanding the range of applications significantly.

2. Material Properties and Strengths:

Each material used in double injection molding possesses unique properties and strengths. For instance, TPEs offer excellent flexibility, tear resistance, and weatherability, making them ideal for comfortable grips and seals. On the other hand, ABS boasts exceptional impact resistance and surface finish, suitable for providing structural support and aesthetic finishes. By combining materials with complementary properties, manufacturers can develop products that deliver enhanced performance, reduced assembly steps, and improved user experiences.

Applications of Double Injection Molding:

1. Automotive Industry:

The automotive sector has been a major beneficiary of double injection molding technology. Manufacturers can produce complex components such as gear shift knobs with TPE outer layers for a comfortable grip and a rigid core made of ABS or PC to ensure durability. Additionally, over-molding allows for the integration of various functional elements like buttons, grips, and textures into a single component, streamlining the assembly process and improving the overall aesthetics.

2. Consumer Electronics:

In the consumer electronics domain, double injection molding has revolutionized the design and functionality of devices like mobile phone cases and remote controls. With this process, manufacturers can create sleek and ergonomic products by combining soft and hard materials seamlessly. This not only enhances the device's appearance but also provides better impact resistance and protection for internal electronic components.

3. Medical Devices:

Medical device manufacturers have embraced double injection molding for its ability to produce components with a soft, hypoallergenic surface, which ensures patient comfort during prolonged usage. By using materials like TPEs in combination with antimicrobial engineering-grade resins, medical devices can maintain high hygiene standards while meeting strict regulatory requirements.

Benefits of Double Injection Molding:

a. Improved Product Aesthetics: Double injection molding enables the creation of visually appealing products with intricate designs, textures, and multi-color patterns, eliminating the need for additional assembly steps or secondary operations.

double injection molding

b. Enhanced Functionality: By integrating dissimilar materials, manufacturers can optimize product performance, such as improving shock absorption, providing a soft-touch feel, or enhancing chemical resistance.

c. Cost-Efficiency: Although the initial tooling costs may be higher for double injection molding, the overall production cost can be reduced as it eliminates the need for separate assembly processes.

d. Tight Tolerance and Precision: The process ensures precise and consistent molding, resulting in parts with tight tolerances and minimal dimensional variations.

Double Injection Molding: Design Guidelines for Optimal Part Creation by ACE

1. Draft Angles: Ensuring Smooth Ejection and Functionality:

Draft angles are critical in double injection molding to facilitate the removal of the part from the mold. ACE recommends incorporating draft angles into the design to allow the part to release smoothly without causing damage to the mold or compromising the part's functionality. Adequate draft angles ensure minimal friction during ejection, reducing the risk of defects and improving production efficiency.

2. Wall Thickness: Striking the Right Balance:

Proper wall thickness is essential for achieving both structural integrity and efficient production. In double injection molding, designers must carefully consider the varying wall thicknesses of the two materials being used. ACE advises maintaining uniform wall thickness to prevent sink marks, warpage, and other defects that can compromise the part's performance and aesthetics.

3. Part Complexity: Harnessing the Full Potential:

Double injection molding allows the creation of intricate and highly functional parts with varying textures, colors, and materials. Designers must capitalize on this capability to optimize part performance and aesthetics. However, it is crucial to strike a balance between complexity and manufacturability. ACE recommends thoroughly evaluating the design's feasibility and ensuring that it aligns with the manufacturing capabilities to avoid unnecessary complications and cost overruns.

4. Material Selection: Matching Compatibility and Performance:

Selecting the right materials for double injection molding is paramount to achieving desired properties in the final product. Engineers should consider factors such as material compatibility, adhesion, and shrinkage rates to ensure successful bonding between the two materials. ACE offers an extensive range of materials, and their expertise can assist designers in choosing the best combination for the intended application.

5. Overmolding vs. Co-molding: Understanding the Difference:

In double injection molding, two common techniques are over-molding and co-molding. Overmolding involves adding a second material over a base part, while co-molding simultaneously injects two materials to form a unified structure. Each technique has its advantages, and the choice depends on specific application requirements. ACE provides guidance on the most suitable technique based on the part's complexity, material compatibility, and cost considerations.


Double injection molding is a game-changer in the world of plastic manufacturing, offering unmatched versatility, improved aesthetics, and enhanced functionality. As we have seen, ACE, a leading manufacturer in this domain, has elevated the standards of double injection molding through its dedication to innovation, quality, and customer satisfaction. As technology continues to evolve, double injection molding will undoubtedly play a pivotal role in shaping the future of the manufacturing industry.

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