3k or 3-shot moulding, also known as triple shot injection moulding.

Multi-shot plastic injection moulding

Plastic injection moulding, as we’ve already covered in some of our other blogs, is an ideal manufacturing process for many types of plastic components used in electronic devices, medical equipment and other general industrial applications. There are, however, also different methods of injection moulding.

We’ve previously touched on over moulding as a solution for bonding types of materials and colours to form a single component but multi-shot injection moulding is also an option for this. Multi-shot moulding offers great benefits for complex part designs giving high precision repeatable results for volume manufacture.

What is multi-shot injection moulding?

Multi-shot moulding is a multi-material injection moulding process where a product is formed by injecting two or more different materials one after the other but as part of a single moulding cycle. This is a value-added process that requires specialist machines.

The most common forms of multi-shot moulding are:

2K or 2-shot moulding which is also sometimes called twin shot or double injection moulding. This process can produce complex components with two grades of plastic chemically or mechanically bonded together. Not only does this reduce assembly work further down the production line, it gives a more resilient bond between the two different materials with chemical bonds. Application examples include the addition of a window in a housing, rubber grips or complex form seals. It is different to over moulding because it is a process that is carried out within the cycle of the mould tool without the need for external input from operators or robots and so greatly reduces processing time.

3k or 3-shot moulding, also known as triple shot injection moulding. This is similar to 2-shot but uses three different materials often to reduce the required assembly even further. It can also reduce time and the number of operations required to produce complex components using multiple materials. This process is even more complex and also requires an experienced injection moulding company with specialist equipment. 

How does multi-shot moulding work?

The first shot (base material) is injected into the cavity and solidified, half the tool is then rotated 180° by a rotating platen so that the first cavity is shut off and the second cavity can then receive subsequent shots of material. This forms a molecular/mechanical bond between the different polymers and the part is then cooled and ejected from the mould.

2 and 3-shot moulding is a highly skilled process that requires technical expertise and knowledge about the different material properties, compatibility, cool rates, weld strengths and tolerances.

The benefits of 2 and 3-shot moulding

  • Greater design freedom and complexity
  • Part consolidation:
    • less need for post-production assembly
    • reduced validation costs as it produces a single part
  • Better quality and accuracy
  • Improved adhesion and stronger bonds
  • Improved efficiency from fewer production stages

2-shot moulding v overmoulding

Overmoulding takes place over two separate injection moulding processes. The base material is injection moulded first, the part is then cooled, ejected and inserted within a separate mould by a technician for overmoulding with the second material. Overmoulding uses simpler moulds and is easier to set-up using a standard injection moulding machine but takes longer and requires an operator or a robot.

2-shot moulding is a single automated process that has greater repeatability and delivers superior bonding between polymers. The best process to use therefore depends on what is required of the part being manufactured but some applications for multi-shot moulding are listed below.

Applications for multi-shot moulding

There are many design considerations to take into account when designing a product that will require multi-shot moulding. Getting your injection moulding partner involved at the earliest stage possible will save time and they will be able to advise where, with design tweaks and through considering material choices, cost savings can be made.

Speak with one of our technical design engineers about your next project.

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