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TPE in multi-component injection molding

Thomas Renner, Head of Application Engineering, describes the benefits of processing thermoplastic elastomers in multi-component injection molding. What factors influence the adhesion of different materials, and how is the bond evaluated? Read more here about the possibilities of two-component injection molding.

TPE in Multi-Component Injection Molding

Multi-component injection molding has become a key technology in the production of functional and design-oriented plastic components. Thermoplastic elastomers (TPE) in particular open up a wide range of possibilities here—from sealing functions to ergonomic surfaces. But what makes this material class so special in two-component injection molding?

Thermoplastic elastomers combine the properties of traditional elastomers with the processability of thermoplastics. They can be melted, molded, and recycled — a clear advantage over cross-linked materials such as rubber or silicone. TPEs are classified according to ISO 18064. Within these groups, TPS (thermoplastic styrene block copolymers) are considered particularly versatile—both in terms of mechanical properties and adhesion. You can read more about the classification in our previous post.

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Multi-component applications with TPE

In 2-component injection molding, materials are combined in a single process—typically a hard/soft combination. Thermoplastic elastomers often perform the elastic function, such as sealing or damping. Possible applications include functional seals in housings, overmolded grips on tools or medical devices, or elastic components such as those found in snap fasteners. The major advantage: The process is automated, efficient, and is increasingly replacing traditional solutions involving inserted, cross-linked materials.

Adhesion as the Key to Success

A central issue in multi-component injection molding is the adhesion between TPE and the hard component. Modern TPE compounds now enable bonds with a wide variety of materials. Depending on the formulation, TPE compounds can bond with polyamides (PA 6, PA 6.6, PA 12—with or without glass fiber) or with PC, ABS, and combinations thereof. Specially developed materials even allow bonding to POM, PBT, PARA, or EPDM. Polypropylene-TPE combinations are considered an established standard.

What factors affect adhesion?

For a stable adhesive bond in multi-component injection molding, the precise interaction of several influencing factors is crucial. Material compatibility plays a central role here: The TPE used must be specifically matched to the respective hard component. To this end, specific material series are developed that are designed for defined substrates and have proven themselves in practice. Equally important is temperature control during processing. The surface of the rigid component should be sufficiently warm so that the polymer chains remain mobile and allow for good bonding. At the same time, the melt temperature of the TPE must be carefully adjusted - too high temperatures can lead to warping, while too low temperatures impair wetting and thus adhesion.

Another key factor is the surface quality of the component to be overmolded. This should be free of release agent residues and additive migration to ensure a reliable bond. Lightly textured surfaces can further support mechanical adhesion, while high-gloss surfaces—depending on the material system—may have a rather detrimental effect.

Finally, component design and mold layout also significantly influence adhesion performance. Aspects such as wall thicknesses and flow paths must be designed with the plastic in mind; the gate should be precisely positioned, and precise mold temperature control is necessary to ensure stable and reproducible process conditions.

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Trend

Multi-component processes such as 2K injection molding are increasingly replacing traditional solutions with manually inserted seals made of cross-linked materials. The primary reason lies in the higher efficiency and process reliability: post-processing steps are eliminated, as is the complex chemical cross-linking.

By integrating multiple functions into an automated process, cycle times can also be reduced. At the same time, thermoplastic materials offer the advantage of recyclability. New design possibilities also open up, as TPEs can be flexibly colored and their surfaces can be specifically customized. Stable and reproducible processes are crucial, particularly in sensitive sectors such as medical technology or the toy industry—requirements that 2K injection molding reliably meets.

How is adhesion assessed?

The peel test according to DIN ISO 813-2 is used to determine the adhesive strength between a soft material, such as TPE, and a hard component. In this process, a composite test specimen is produced in which the TPE is applied to a substrate. During the test, the elastic material is peeled away from the rigid substrate under defined conditions. The force required for this is measured continuously. The result is expressed as peel force per width (e.g., N/mm) and provides insight into the quality of the adhesion.

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For this purpose, the fracture pattern should also be evaluated (adhesive vs. cohesive fracture).

Conclusion

Multi-component injection molding with thermoplastic elastomers is a complex interplay of material chemistry, design, and processing. Stable adhesion does not occur (-> is not achieved) by chance but is the result of the targeted coordination of all influencing factors.

Those who consider the material combination, geometry, and process parameters early in the design phase lay the foundation for durable, functional, and cost-effective components.

TPE is thus far more than just a “soft material”—it is a highly developed system material for modern plastic solutions. What experience have you gained in multi-component injection molding? We look forward to your questions and comments below.

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