Multi-component moulding: integrating functions instead of assembling them
A process chain becomes a single process step
Where a plastic part is meant to seal, damp or sit securely in the hand, conventional design usually ends with two separate parts that have to be assembled. Add to that the handling, the inspection effort and a tolerance chain that ultimately determines the function.
Depending on the part, multi-component moulding replaces this chain with an integrated process: two or more materials are combined in one mould to form a single part. This can reduce assembly steps and joining tolerances and integrate functions that cannot be achieved with a single material.
But not every part is suited to two-component moulding: whether the technology pays off depends on three factors – material combination, tooling concept and production volume.
From the first-shot part to the finished composite
In multi-component moulding – also called two-component moulding depending on the number of materials – a first-shot part is initially produced from the first component. It is repositioned while still in the mould: fully automatically via a rotary table or an index plate, or by hand in simpler concepts. It is then overmoulded or back-moulded with the second component.
There is also a third route: first and second shots as two sequential processes in separate moulds. This reduces the tooling investment and makes the technique accessible for smaller production volumes and ramp-ups as well.
The joint can be designed either through material adhesion or through mechanical interlocking by means of a structural anchoring feature.
Material combinations in multi-component moulding
Typical combinations are hard/soft combinations: a dimensionally stable carrier in PA, PC, PC/ABS or POM with a functional TPE layer for sealing lips, grip zones or damping elements. POM is the most demanding candidate here – on this low-adhesion material, TPE generally only works with specially adhesion-modified grades containing an adhesion promoter.
Hard/hard combinations are equally well established, for example for multi-coloured operating elements. Combinations with widely differing processing windows can be critical; material compatibility, interface temperature and the permissible thermal load on both components are decisive.
For a material bond, the material combination, interface temperature and process control must be compatible. With compatible thermoplastic combinations, the second component can heat the surface of the first-shot part sufficiently for the polymer chains to bond at the interface. If the thermal activation or material affinity is insufficient, no reliable bond is formed.
Adhesion does not happen by chance
Not every material combination bonds reliably. TPE compounds are therefore tailored to specific carrier materials. We work closely with Kraiburg TPE when selecting them. The material combination is pre-assessed on the basis of manufacturer data and experience and then verified in the context of the part and process.
Where material adhesion still cannot be achieved, it must be replaced by mechanical interlocking: undercuts, through-holes or anchoring geometries that mechanically lock the soft component into the carrier.
Two further factors are often underestimated in the design. First, the processing temperatures: the second component has to melt the interface of the first-shot part far enough for a bond to form – without deforming it in the process. Second, the difference in shrinkage: two materials with markedly different shrinkage behaviour build up stresses at the interface that can show up as warpage.
Systematically preventing adhesion failures
Even with a fundamentally suitable material combination, adhesion can fail during processing. Possible causes include residual moisture, contamination, an interface temperature that is too low, unfavourable shear or an unsuitable process window.
Many TPE compounds are shear-thinning. Injection speed, gate dimensions, melt temperature and, where required, drying must therefore be matched to the material – as early as the mould and process design stage.
The mould is what really makes the difference
A multi-component mould is more than two cavities in one mould. It must transfer the first-shot part precisely, keep two runner systems thermally separated and seal the transition geometry between the components cleanly so that the second component does not flow into unwanted areas.
The positioning accuracy required is on the level of precision injection moulding: an offset of just a few hundredths of a millimetre between the stations can result in flash or sealing problems on the finished part.
That also explains the cost structure: depending on the number of cavities and the transfer technique, a two-component mould costs noticeably more than a comparable single-component mould, and the machine requires a second injection unit. The higher investment costs can be offset by eliminating assembly, joining and inspection processes.
Where multi-component moulding plays to its strengths
- Functional integration Sealing lips, damping or grip elements that would otherwise be assembled as separate parts – the sealing function is created directly in the process and is inseparably bonded to the carrier.
- Tolerance-critical assemblies In a two-component part, the relative position of the function-critical areas is created in the mould, which can eliminate an additional assembly tolerance. For leak-tightness and defined tactile response, this is often the more robust approach.
- Production volumes As production volume increases, the economic benefit of eliminating the assembly step grows. From medium production volumes onwards, it can offset a substantial share of the additional tooling costs.
- Cleanliness requirements Every handling and joining step that is eliminated reduces particle ingress and contamination risks – relevant for medical technology and pharmaceutical applications.
When two-component moulding is not the right answer
The economic calculation is essentially a comparison of two process chains. On one side: moulding separate parts, feeding them, assembling or bonding them, inspecting the joint – with every station, the handling effort, the tolerance chain and the scope for error grow. On the other: a higher tooling investment, but a part that comes out of the machine finished.
Just as clear are the cases in which two-component moulding is not the right answer: small quantities that do not justify the investment in a fully automated multi-component mould – although manual transfer or sequential first and second shots lower the entry barrier considerably. Likewise material combinations that neither adhere nor allow a practical mechanical interlock. And wherever a simple assembled solution is technically entirely sufficient.
A feasibility assessment therefore belongs at the start of every multi-component project. It clarifies the material combination, tooling concept and process risks before the mould design is defined.
Assess feasibility technically at an early stage
The approach is similar for any professionally managed multi-component project. It begins with the part function and the material combination: adhesion, temperature window and shrinkage behaviour of the candidates are weighed against one another – this is where unsuitable combinations can be ruled out early, before they cause costs.
Next come the transition geometry between the components, the tooling concept with its transfer technique and runner layout, and the tolerance scheme for the function-critical characteristics. The result is a clear assessment of whether the part can be produced economically as a two-component part, which material combination is appropriate – and whether an assembled solution may still be the better option.
From practice: where multi-component technology makes the difference
The coupling of the Hicoflex® Single-Use Containment System for the closed transfer of pharmaceutical solids is produced in a fully automated two-component injection mould with rotary-table technology: a POM carrier with TPE sealing lips. Material selection, tooling concept and process control are used to tailor the demanding POM/TPE combination to the containment requirements of the pharmaceutical industry.
A second example is the keypad mat for a surgical camera system: it combines a dimensionally stable carrier with a soft operating surface and integrates defined tactile switching response and a sealing function in a single part. This eliminates additional assembly and joining steps.
The same integration advantages apply wherever, in electronics and automotive applications, every additional joint would be a potential failure point.
A look at typical two-component parts from our production
Assess early, then integrate consistently
Multi-component moulding enables extensive functional integration in plastic injection moulding: fewer separate parts, less assembly and fewer additional tolerance interfaces – in return for a more demanding mould and a material combination that has to be right from the outset.
Whether this pays off for a specific part can be clarified early and with manageable effort. Our engineering team assesses feasibility, material combination and tooling concept before any investment is made – contact us.