What is Two-Shot Injection Moulding?

Hymid specialises in technical injection moulding, whether that be for single or two-shot moulded components. The knowledge and experience required for the complexity of two-shot mouldings has positioned Hymid as an industry leader in the technical aspects of the injection moulding process.

What is Two-Shot Injection Moulding?

Two-shot injection moulding, sometimes referred to as Twin-shot, Multi-shot or 2K Moulding, is the process of injecting two polymers during a single cycle of the injection moulding process.

Two-shot moulding consists of a substrate polymer, which is injected first, followed by a second shot which is moulded over the top which bonds to the substrate. The second shot is injected quickly after the first shot, which enables the plastics to bond together strongly; chemically bonding together as if they were part of the same material.

With two-shot moulding, different polymers can be used to achieve different colours or textures. A common use of two-shot moulding involves combining a hard plastic, which acts as the substrate, with a softer plastic, which is moulded over the top to give a rubbery feel to the product for enhanced grip. The hard polymer provides the rigidity and defines the product shape, whilst the softer polymer provides enhanced ergonomics and aesthetics.

Another popular use of two-shot moulding is to incorporate windows or buttons into a product design. Rather than moulding the parts separately and combining them through a manual process with seals, or a secondary over-moulding process, two-shot injection moulding enables the incorporation of windows or buttons as part of a single waterproof component.

A good example of a two-shot part is this autoinjector case for one of our medical customers, Owen Mumford, whilst the same polymer was used for both shots, the first shot is clear and the second is coloured. This design utilised different colours to make identifying the strength of the drug much quicker, allowing for users to be more confident they’re choosing the right vial. Two-shot is a popular injection moulding method for the medical and healthcare industries.

How does Two-Shot work?

In contrast to over-moulding, where two distinct tools are required, so that the first polymer can be moulded over in a secondary process, for two-shot moulding only one mould tool is required, as the process is done all in one go.

The first polymer (the substrate) is injected into the first-shot tool cavity, creating the initial shape of the product. The mould tool then separates whilst retaining the first shot and rotates 180 degrees, closing with this first shot lined up with the second cavity of the tool. The second polymer is injected into the second-shot cavity, over the top of the substrate. The tool is then rotated through 180 degrees back to its original start point and the completed moulding is pushed off the mould tool.

This is depicted in the diagram below:

Tool Design for Two-Shot Moulding

Due to the complexity of two-shot moulding, the associated tool design is also more complex, compared with single-shot. On top of the multitude of design considerations involved with any mould tool, such as shape and size of the product, quantity of parts to be produced, tool temperature, and surface finish, there are additional mechanical considerations with two-shot tools to ensure precise alignment of the cavities.

Two-Shot moulding tool
This image shows a two-shot tool, focusing on the moving half. This is the half that will rotate 180 degrees to place the substrate into the second shot cavity.

One of the biggest factors that determines tool design is material selection. There are many different materials that can be used, each with their own characteristics and properties. Each polymer has a different shrinkage rate, cooling time and flow behaviour and these all need to be accounted for when designing the tool. When considering materials for two-shot moulding, it’s also important to consider the interaction between the two polymers and how strongly they bond together.

You can read more about material selection here.

Machine set-up for Two-Shot Moulding

Single-shot injection moulding only requires one feed point and a single hopper filled with material. By contrast, two-shot injection moulding requires two separate feed points and separate hoppers to provide material to each feed point. The feed points are commonly positioned at 90 degrees to each other, providing material for the first and second shots from different angles as shown in the setup below:

 

Arburg Allrounder 470s with Two-Shot configuration.
Arburg machine with two-shot configuration, the red circles highlight the feed points for the first and second shots.

 

Why choose Two-Shot Moulding?

Two-shot moulding has a number of advantages compared with the manual fitting of single-shot components or over-moulding:

  • Improved aesthetics of a single seamless component
  • Increased product strength due to strong chemical bonding between polymers
  • Enables incorporation of windows or buttons into a single waterproof component
  • Faster production times
  • Lower total component prices

Why choose Hymid for you Two-Shot needs?

Hymid is more than just a manufacturing partner, we aim to add value at every stage of your project; through development and into production. We have a huge amount of experience in two-shot moulding, as well as single-shot and over-moulding. Our extensive knowledge and expertise in all areas of injection moulding ensures that your product will not only be optimised for production, but will also look and function at its best.

Give us a call or email for a friendly and in-depth chat about your project to see if we can help.