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Combining colors and materials in one product: 2k injection molding

21 April 2021

What is 2k injection molding?

‘Two shot’ injection molding, also known as 2k injection molding, is a manufacturing technique of combining two different polymers or two different colors into one plastic part. The 2k injection molding machine gives Rompa Group the possibility to offer one part made of two different colors or materials. For example a combination of soft and hard material, like your toothbrush which is made with a soft grip.

The bonding of the two components happens because they merge into one another or because the first component is designed in such a way that the second component bonds mechanically. This process enables many options to be integrated into an injection molded part. The chemical connection between the two materials provides an excellent bonding. All this without the need to chemically pre-treat the thermoplastic part.

You can find insert molding in various industries. Some of the industries that use 2k injection molding a lot are the automotive sector, medical industry, electronics, aerospace and many other markets.

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Why use double injection molding?

When designing for assembly, an assembly step can be eliminated with the 2k injection moulding process. This means less cycle time, a consistent process and avoidance of manual insertion. Because of these advantages choosing a 2 component injection molding process is cost saving, but it also provides advanced design options and aesthetic possibilities. To give you a clear view of the advantages, below a list why 2k injection molding could be the right technology for a part or product:

  • Assembly step can be eliminated.
  • The parts are strongly bounded and fitted so the product becomes more durable.
  • A wide choice in appearance and customization due to the many colour options.
  • The manufacturing procedure is completed with one injection molding machine, this means less pre- and post-production processes.

Overmolding vs 2k injection molding

What is overmolding?

Overmolding is a technique used to combine multiple materials into one single part or product. The first material, also referred to as the substrate, is in most cases partially or fully covered by subsequent materials during the process. Plastic overmolding is seen in many industries such as automotive, household appliances, medical devices and electronic devices. The reason to implement overmolding in the manufacturing process is to add a soft touch to a part or product.

Overmolding is suitable for smaller scale production volume in comparison to 2k injection molding which is more suitable for a larger production scale. During the overmolding process, the material that is used to mold must cool down before removing the part. With 2k injection molding the polymer is in most cases still hot and flexible, which enables us to do additional operations.

2k injection molding is popular for producing compact and rather small products or parts. Overmolding is used for producing bigger items in general. In general it takes more time to make a product with overmolding, but it is a great solution for rather big products or parts.

Rompa Groups offers both solutions and we will always take multiple technologies in consideration to achieve the best end result for the product or part.

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What materials can be used for overmolding?

Choice of materials is one of the factors to determine which technology is best. But, which materials will be used depends on different requirements such as adhesion, compatibility, thickness, the softness, etc. Below some of the materials used for overmolding:

  • HDPE, High-density polyethylene. This material has a high strength-to-density ratio and is recyclable.
  • POM, Polyoxymethylene. The best option for precision and is high in stiffness.
  • PMMA, Polymethyl methacrylate. Very suitable for lightweight products or parts.

What materials can be used for insert molding?

For 2k injection molding the elements to take in consideration are mostly the same as overmolding, but 2k injection molding gives the possibility to use different materials. Below a list of materials that can be used:

  • Thermoplastics such as polyethylene, polystyrene and nylon. 
  • Thermosets such as phenolic and epoxy.
  • Elastomers. A great option for elastic products or parts.
  • Natural Rubber.
  • Polyurethane.

Is 2k injection molding suitable for your idea?

In some cases it is not recommendable to use 2k injection molding, in general it is advisable to only use the 2k injection molding process for high volume of production. For this technology you need a custom mold for the 2k injection moulding machine, which in some cases would not be profitable. We at Rompa Group strive to offer you the full package and we will always choose the most suitable solution for any production. Maybe the two shot injection molding is not suitable for your idea but we offer many other solutions and technologies. Make sure to see all our technologies that we offer here.

Double injection molding at Rompa Group

2k injection molding is just one of the many techniques Rompa Group offers. You can contact us for a wide range of injection molding techniques. Think of thin-walled injection molding, high precision injection molding, overmolding and in-mold labeling. Rompa has a large machine park, with injection molding machines with a clamping force of 25 to 4800 tons. In addition, we have a modern cleanroom production of medical articles and an ESD room for assembly.

We are always happy to answer all of your questions regarding these technologies, so do not hesitate to contact us!