Your guide to double shot plastic injection molding and overmolding

Double shot injection molding and overmolding are two high value-added techniques used in plastic injection molding. Both methods are used to create complex parts with multiple colors, materials, or textures, often eliminating the need for additional post molding operations. While they’re often thought of in the same category, though, these techniques differ substantially in terms of process and have their own unique benefits and drawbacks.

When determining which method is right for your project, you’ll need to understand how both processes work, as well as the benefits and challenges of each. In this guide, we’ll outline the most important considerations when you’re deciding whether double shot injection molding or overmolding is a better fit for your needs.

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chair backs made with overmolding

What are double shot injection molding and overmolding?

Double shot plastic injection molding, also known as 2K injection molding, involves injecting two different materials into a single mold to create a single part. This method is ideal for producing parts with multiple colors or materials without requiring additional assembly. It allows for complex designs, reduction of manufacturing time and costs, and access to a broad range of injection molding applications.

Overmolding, on the other hand, involves molding an additional layer of material over an existing part or substrate. Unlike double shot injection molding, where two materials are injected into a mold in sequence, overmolding often involves a separate molding operation on an already-formed part. This process can involve various techniques and is often used to add functionality, aesthetics, or protection to an existing part.

While similar, the processes differ somewhat significantly when you look at the details. Let’s look more closely at the processes step-by-step to enable a proper comparison.

The double shot injection molding process

2k injection molding involves only one mold and production cycle, in which the primary substrate is injected first, followed by a secondary injection of the overmold material.

  1. Primary injection. The first step involves injecting the primary substrate into a mold cavity. This substrate is typically selected for its strength and rigidity.
  2. Mold rotation/shift. After the primary substrate is injected, the mold rotates or shifts to align with the secondary injection station.
  3. Secondary injection. The secondary material, often a softer plastic, is injected onto the primary substrate. The overmold material can be a different color, texture, or hardness to achieve the desired effect.
  4. Curing and bonding. The part is left to cure, allowing the primary and secondary materials to bond. Proper curing ensures a strong and durable connection.
  5. Quality control and ejection. After curing, the part is ejected from the mold, and quality control checks are performed to ensure correct alignment, bonding, and consistency.

The overmolding process

In contrast, overmolding is a two-step process that involves molding an additional layer onto an existing part, which can be plastic, metal, or other materials. Overmolding is often used to add protective coatings, soft-touch grips, or other functional enhancements to existing products.

  1. Primary part preparation. The primary part is produced separately, often through traditional plastic injection molding or another manufacturing process.
  2. Part placement in mold. The existing part is placed into the overmolding mold, ensuring proper alignment.
  3. Injection of overmold material. The overmold material is injected, creating a layer over the primary part. The overmold can cover specific areas or encase the entire part, depending on the design.
  4. Curing and bonding. The overmold material cures, bonding to the primary part. This bond's strength depends on material compatibility and other design considerations.
  5. Quality control and ejection. After curing, the part is ejected from the mold, and quality control checks are performed to ensure proper bonding and consistency.
razor handles made with multiple injection molding materials
pet hair brushes made with 2k injection molding

Advantages and disadvantages of double shot plastic injection molding and overmolding

Because of the process differences, double shot/2k injection molding and overmolding have their own benefits and drawbacks that must be considered when choosing between them:

Process

Advantages

Disadvantages

Double shot injection molding

  • Complex designs. Double shot injection molding enables intricate designs with multiple colors or materials in a single part.
  • Cost-effective. By combining multiple manufacturing steps into one, it reduces labor and assembly costs.
  • Strong bonding. The bond between the primary substrate and overmold material is strong and durable.
  • High-volume production. Ideal for large-scale production runs, making it cost-effective for high-volume projects.
  • Specialized equipment required. Double shot injection molding requires specialized injection molding machinery and tooling, which can be costly.
  • Design limitations. While versatile, the design must account for the specific sequence of injections and alignments.

 

Overmolding

 

  • Versatility. Overmolding allows for a broader range of applications, including molding onto existing parts or integrating electronic components.
  • Functional enhancements. It can add protective coatings, soft-touch grips, or other functional features.
  • Compatibility with other materials. Overmolding can be used with metal, plastic, or other substrates, offering more design flexibility.

 

  • Separate manufacturing step. Overmolding often requires an additional manufacturing step, which can increase production time and cost (especially if the original part needs to be hand-loaded into the secondary mold).
  • Additional quality control. Given the additional manufacturing step, rigorous quality control is needed to ensure consistent results, as well as proper compatibility and adhesion between the substrate and overmold materials.

Special considerations

Understanding the more detailed aspects of 2k injection molding and overmolding helps engineers select the best approach for their manufacturing needs, ensuring high-quality products with unique characteristics and functionalities. Some of these special considerations include the following:

Primary substrate/overmold material considerations and compatibility

The primary substrate serves as the foundational layer onto which additional materials are applied. The choice of substrate largely depends on the required properties of the final product, such as strength, rigidity, and thermal resistance. Common primary substrate materials in double shot injection molding include ABS (acrylonitrile butadiene styrene), PC (polycarbonate), and PA (polyamide). These materials are valued for their structural stability and compatibility with a variety of overmold materials. With overmolding, substrates can be even more diverse, including metals and other plastics.

Choosing the right overmold material is crucial for achieving a successful bond with the primary substrate and ensuring overall product durability. In double shot plastic injection molding, compatibility between the two materials is key. Common overmold materials include TPE (thermoplastic elastomer), TPU (thermoplastic polyurethane), and various types of softer plastics. Overmolding allows for greater versatility in overmold material selection, enabling the use of materials that might require additional processing or specific conditions to bond effectively with the substrate. Material selection also plays a significant role in adding tactile features, aesthetics, or protection to the product. Common overmold materials in overmolding include silicone rubber PP (polypropylene), and transparent acrylic-based polymers.

In both applications, the overmold material should be selected based on desired characteristics such as flexibility, texture, and color. Compatibility is not only about the physical properties but also the chemical interaction between the substrate and overmold. Proper bonding is essential to ensure the durability and integrity of the final product.

For more on resin selection, check out our plastic injection molding material selection guide.

Design considerations

To ensure proper alignment and bonding, both processes require detailed planning in the design phase. Proper mold design is critical to ensure successful manufacturing in both processes.

In the case of double shot injection molding, the mold design has to account for both the primary and secondary injections to ensure the mold can accommodate the sequence of operations. Complex geometries, wall thickness, and material flow paths must be carefully engineered to prevent defects like warping or incomplete fills.

With overmolding, design considerations must be made to achieve a proper fit between the existing substrate and the overmold. This often involves creating features like grooves or undercuts to enhance bonding or facilitate secure placement of the primary part in the mold.

cooling system for injection molding machinery

Curing and bonding mechanism and control

To achieve a strong and durable connection between materials, both 2k injection molding and overmolding require a proper curing and bonding process. Of course, quality control is vital in both processes—thorough testing is required to ensure proper curing and bonding. Factors such as surface preparation, injection speed, and curing time play significant roles in achieving a successful bond.

Because double shot injection molding takes place in one production cycle, curing occurs at the conclusion of that cycle. Precise control over temperatures, pressures, and injection timings is necessary, and proper curing temperatures and duration ensures proper chemical and mechanical adhesion. In overmolding, the bonding mechanism often involves additional bonding agents or coatings to enhance adhesion.

Special tooling and equipment

While both double shot injection molding and overmolding require specialized tooling and equipment to ensure successful manufacturing, double shot plastic injection molding requires entirely different injection molding machinery. A machine equipped for 2k injection molding typically includes a rotating or shifting mold to allow for the secondary injection. This type of equipment demands precise control over multiple injection parameters, such as temperature and pressure, as well as additional controls for proper mold rotation and alignment.

In overmolding, the equipment needs to support the placement of existing parts and provide reliable mechanisms for securing and overmolding them. This might require additional fixtures or custom-designed molds to accommodate varying shapes and substrates, but rarely requires new equipment altogether. The cost/availability of the right equipment may be the deciding factor for an engineer considering double shot plastic injection molding vs. overmolding.

Applications

Both double shot plastic injection molding and overmolding find applications in various industries, including consumer electronics, automotive, medical devices, and other consumer products like toothbrushes and kitchen utensils. However, they are often selected for specific applications due to their distinct strengths.

Because of its ability to create intricate multi-material designs within a single production cycle, 2k injection molding is ideally suited for crafting electronic device casings with built-in buttons or grips. With double shot injection molding, manufacturers can incorporate both hard plastic for durability and soft rubber-like material for tactile buttons or grips. This seamless integration within a single part enhances the user experience without requiring separate assembly steps, offering an efficient way to create complex designs with high precision. The automotive industry also benefits from this technique, where dashboards and control panels can feature different textures and materials to enhance both aesthetics and functionality.

Overmolding is often used to add a protective or functional layer over an existing substrate. For example, one common application is in the production of power tool handles, where the base handle is molded from a rigid plastic, and a softer, shock-absorbent rubber is overmolded for added comfort and grip. This approach is also suitable for integrating electronics, where a circuit board or metal component can be securely encased with a protective plastic layer to shield it from environmental factors. The versatility in dealing with different substrate materials provided by overmolding allows for additional customization and flexibility in design, making it ideal for applications where the base part requires a unique coating or enhanced functionality through an additional layer.

Choosing the right manufacturer

Like selecting any manufacturing partner, finding the right manufacturer for double shot plastic injection molding and overmolding is critical. Here are some things to consider as you evaluate potential partners:

  1. Experience and expertise. Look for a manufacturer with a track record in both double shot injection molding and overmolding. Experience ensures they understand the complexities of each technique.
  2. Material knowledge. A manufacturer should have deep knowledge of injection molding materials and be able to recommend the best options for your project.
  3. Quality control systems. Ensure the manufacturer has robust quality control measures to guarantee consistent results.
  4. Tooling and equipment. The manufacturer should have the appropriate injection molding machinery and be able to accommodate the required tooling adjustments for your selected injection molding method.
  5. Collaborative design approach. A good manufacturer will work closely with you during the design phase to ensure compatibility and address potential bonding issues.
  6. Customer references. Ask for references or case studies to gauge the manufacturer's success with similar projects.

Double shot plastic injection molding and overmolding are powerful techniques. By combining multiple materials, colors, or textures in a single part, these methods offer a high degree of design flexibility and efficiency. While both methods present unique challenges, their benefits make them valuable tools for engineers seeking innovative solutions for plastic manufacturing.

Nelson Miller Group is your go-to partner for injection molding and other thermoplastics manufacturing. We’ve got the expertise, capabilities, and equipment to make your project a success. Contact us today to discuss how we might be able to help you with your next plastic project.

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