Choosing materials for two-component injection moulding: which combinations work?

The success of 2K injection moulding depends entirely on choosing the right materials, as not every combination of plastics bonds together naturally.

Chemical bonding: the basis for a strong bond

The strongest bond between two materials in a two-component system is achieved through chemical bonding: the two polymer chains bond at a molecular level during the injection moulding process. This is possible when the materials are chemically related or have a high affinity. Polypropylene as the base material with a TPE-S as the soft component is one of the most common combinations, precisely because the chemical affinity yields excellent bonding results. Combinations of chemically unrelated materials, such as PC and TPU, require greater attention to process parameters or the addition of mechanical anchoring.

Commonly used and proven material combinations

In practice, there are a number of combinations that are widely used and have demonstrated well-documented bonding results. The choice always depends on the functional requirements of the end product, but the combinations listed below provide a good starting point for selecting materials.

• PP + TPE-S (TPS/SEBS): excellent adhesion, suitable for a wide range of applications, including handles and seals

• ABS + TPU: good adhesion, suitable for wear-resistant soft areas

• PC + TPU: moderate chemical adhesion; mechanical anchoring recommended

• PA + TPE-A (TPA): strong adhesion in technical automotive and industrial applications

• PBT + TPE-E (TPC): resistant to higher temperatures, suitable for electronics

Process parameters as a determining factor

Even with a chemically compatible combination of materials, the bond may be unsatisfactory if the process parameters are incorrect. The temperature of the first component at the moment the second shot is injected is of great importance: if it is too cold, the bond will be weak; if it is too hot, the first component will warp.

The mould temperature, injection pressure and injection rate of the second component also influence the final bond strength. Proper process development, including bond tests and tensile tests on the interface, is therefore a standard part of a thorough 2K validation process.

Colour, transparency and aesthetics as an added dimension

In addition to mechanical and chemical considerations, aesthetics also play a role in 2K moulding. Colour combinations with sharp contrasts require a tight mould contact along the parting line. Where one of the two components is transparent, additional requirements apply regarding material purity and processing temperature to prevent lens clouding.

As an experienced specialist in both TPE processing and the manufacture of 2K products, De Beer Plastics is the ideal partner for developing challenging projects. Involve us at an early stage in your ideas to avoid wasting time on solutions that won’t work.