Bonding between materials in two-component injection moulding: chemical vs. mechanical

A two-component system is only as strong as the bond between its two materials. And that bond requires a carefully considered choice between chemical bonding, mechanical anchoring, or a combination of both.

Chemical bonding: molecular affinity as the foundation

Chemical bonding occurs when two polymers form a bond at the molecular level during the injection moulding process. This requires the materials to be chemically compatible: they must have similar polarity, similar surface energy or direct chemical reactivity. With well-matched material combinations, such as PP and TPS, the chemical bond is so strong that, under load, the component will break elsewhere rather than at the bonding zone. This type of bond is reliable, invisible and resistant to cyclic loading, but requires careful material selection and validated process parameters.

Mechanical anchoring: geometry as a binding agent

Where chemical bonding is insufficient or impossible – for example, when combining chemically incompatible materials – mechanical anchoring offers an alternative. In this method, the design of the first component is provided with openings, undercuts, ribs or passages through which the second material flows, forming a physical interlock.

The strength of this connection depends on the geometry and the filling of the anchor structures. Mechanical anchoring is less dependent on the choice of material but requires greater design effort and a mould capable of accurately forming the anchor geometry.

• Openings through which the second material flows through the first

• Undercuts and T-profiles: the material interlocks mechanically

• Ribs and studs: increase the contact surface and grip

• Combine mechanical and chemical methods for maximum bond strength

Surface treatment as an interim solution

In some cases, surface treatment of the first component can improve chemical bonding without the need for a different material combination. Plasma treatment, flame treatment or laser roughening of the material can increase the surface energy of the substrate, allowing the second material to bond more effectively. These techniques are commonly used for materials that naturally have low surface energy, such as polyolefins. However, this processing step does introduce a handling stage between the two injection moulding phases, which complicates process automation.

Validation of the joint: bond tests and fatigue tests

Regardless of the bonding strategy chosen, validation of the bond is an essential part of the qualification process. Peel tests, tensile tests and shear tests provide quantitative data on bond strength under static loading. For applications involving dynamic or cyclic loading, fatigue tests are also relevant. Thermal cycling is a third test dimension: materials contract and expand at different rates, which leads to stresses in the joint zone when there are large temperature differences. A robust test programme prevents bonding issues from only becoming apparent in the field.