In two-component moulding, different plastics can be combined directly in one part. Typical applications are hard-soft bonds, integrated seals, grip surfaces and functional operating elements. If the material pairing does not match, the soft component either never adheres properly or comes loose in service – usually noticed late, once mould and part are already designed.
Whether a durable bond forms between the components does not depend on the polymer designations alone. The specific grades and formulations, part geometry, surface conditions and processing parameters are decisive. Material manufacturers therefore offer special TPE compounds for adhesion to defined substrates such as PP, PC, ABS, acetal or polyamide.
The key points at a glance
- Check material compatibility. For direct adhesion, the hard and soft components must match chemically and in processing terms. Manufacturer data and adhesion matrices are an important basis.
- Mechanical anchoring as an alternative. Where sufficient direct adhesion cannot be achieved, the bond can be supported or created by design – through openings, undercuts or anchoring geometries.
- Account for shrinkage and warpage. The components can differ in shrinkage and thermal expansion. These differences must be considered in part and mould design.
- Process conditions affect adhesion. Melt and mould temperature, injection speed, pressure, moisture, cleanliness and the thermal history of the substrate all influence bond strength.
- Decide the material early. The material combination should be defined before final mould design and, where necessary, validated by trials.
How the bond forms
With suitable material pairings, direct adhesion can form at the interface between the two plastics during overmoulding. The achievable bond strength is influenced by several factors and cannot be derived from the short designations of the material groups alone. Influencing factors include:
- chemical affinity of the polymer or compound systems
- additives and formulations
- surface condition and contamination
- moisture content and material preparation
- temperature of the substrate
- melt temperature of the second component
- pressure, injection speed and flow conditions
- contact area and part geometry
Mechanical bonding through part geometry
If the materials do not develop sufficient direct adhesion, mechanical anchoring can be provided. Possible design elements are openings, undercuts, grooves, serrations or encapsulations. Demoulding and mould technology have to be taken into account: undercuts may require additional slides or other moving mould components.
Typical material systems
- PP with adhesion-modified TPE: a frequently used combination, for example for grip zones and seals in consumer goods and mechanical engineering. What matters is a TPE compound designed for polypropylene.
- PC, ABS or PC/ABS with suitable TPE or TPU: specially formulated overmoulding grades are available for these engineering plastics. Actual adhesion should be confirmed by manufacturer release or trials.
- Polyamide with suitable TPE: adhesion-optimised TPE grades are available here too. Moisture content, conditioning and the specific PA formulation can affect behaviour.
- Acetal (POM) with suitable overmoulding grades: the bond is demanding, but specially developed TPE systems exist. A blanket statement that POM can only be joined mechanically would not be correct.
How adhesion can be tested
Whether a material pairing holds is not shown by the data sheet but by the part. Two approaches are common and complement each other:
- Peel test: the soft component is pulled off in a defined manner and the force required is measured. What matters is less the absolute value than the failure pattern – if the elastomer itself tears, the interface is stronger than the material.
- Function and ageing tests: temperature cycling, media contact and sustained load show whether the bond also holds after months. For seals in particular, this determines the service life of the part.
It is advisable to test parts from the production mould rather than hand-made samples. Only there do temperature control, transfer time and surface condition match the later process.
Preparing the bonding zone by design
A large share of adhesion problems originates not in the material but in the geometry. These points belong in part design before the mould is built:
- Sufficient contact area: narrow sealing lips without a bonding surface come loose first. Where possible, the soft component is bonded across a wider area.
- No sharp transitions: edges act as notches and initiate delamination. Radii distribute the stress.
- Matched wall thicknesses: strongly differing thicknesses cool unevenly and create residual stress in the interface.
- Consider demoulding: undercuts for mechanical anchoring require slides or jaws – this increases tooling effort and belongs in the cost calculation early.
Material choice and mould concept belong together
The material combination influences mould temperature control, gating and flow path, cavity concept, parting lines, shrinkage and dimensional design, possible mechanical anchoring, demoulding and the process window. The combination should therefore be settled early and validated technically before major tooling investment. Where a moulded part is combined with further components, our subassembly assembly takes over the following steps.
Two-component moulding at Schröder + Heidler
We produce multi-component parts as well as hard-soft and hard-hard combinations in two-component moulding, including rotary platen, transfer and core-back processes. Together with the customer we assess the requirements for part, material combination and manufacturing process. On that basis, the appropriate mould and process concept for reproducible series production is defined.
Frequently asked questions
Which plastics can be bonded in two-component moulding?
What matters is not the polymer group alone but the specific compound. Material manufacturers offer adhesion-modified TPE grades for defined substrates such as PP, PC, ABS, polyamide or POM. The manufacturer’s adhesion matrix is the first reference point but does not replace a trial on the actual part.
What can be done if two materials do not adhere?
The bond is then created by design: through openings, undercuts, grooves or encapsulations that anchor the soft component mechanically. This considerably widens the material choice but increases mould effort, because undercuts require moving components.
Why does the soft component come loose only in service?
Often because the interface holds but is under stress. Differing shrinkage, uneven wall thicknesses or a substrate surface that is too cold during overmoulding create residual stresses that only release under temperature cycling or sustained load.
When does the material combination have to be settled?
Before final mould design. The material choice influences temperature control, gating and flow path, cavity concept, parting lines and shrinkage allowance. Changing it after mould design means modifying a finished mould.
Can a two-component part be replaced by two separate parts?
Technically usually yes – economically rarely sensible. Two separate parts mean two moulds, two production steps and an assembly operation. Two-component moulding replaces exactly that assembly, which is why the higher tooling investment often pays off across the series.