Plastic Design - Which materials are suitable for bi-injection?
Plastic Design – Which materials are suitable for two-component injection moulding?
Two-component plastic injection moulding, also known as bi-injection, is a manufacturing technique that involves combining two different polymers or two different colours into a single plastic part.
L'two-component plastic injection, also called bi-injection 2K injection is a manufacturing technique that combines two different polymers (or two distinct colours/textures) into a single part in one production cycle. The choice of materials is the most critical element of the project: it determines adhesion, watertightness, durability, and the final cost.
A classic example: the toothbrush with a rigid PP handle and soft TPE grip zones.
See also: Our page on the two-shot moulding process To understand the operation of the rotary mould and the cycle stages.
Why use two-component injection moulding?
the high-performance product design implies a process Design for Manufacture and Assembly (DFMA). This approach determines the future industrial life of the product: it is at this stage that costly assembly and manufacturing phases are reduced or eliminated.
Multi-material injection allows for the elimination of an assembly step or the use of direct labour to join two different materials. This means a cycle time shorter, better quality control, and significant savings on manufacturing costs.
The main reasons to choose bi-injection for your part:
- Elimination of gluing, welding, screwing steps
- Perfectly fitted and secured parts, therefore more durable over time
- Wide choice of appearance and customisation thanks to the colour and options materials
- Process carried out on a single machine, thus less handling and quality risks
The material selection is above all linked to the application of the product. The functional specification will provide an initial series of answers on the types of thermoplastic"thermoplastics to use. The designer then sizes the part according to the constraints, but also based on Desired material combination.
Chemical adhesion or mechanical interlocking: understanding the two mechanisms
Literally, All thermoplastic materials can be used in two-component injection moulding. (if it makes functional sense). But some families naturally embrace it, others not at all.
Two mechanisms ensure the consolidation of materials:
| Chemical adhesion | Mechanical hook |
|---|---|
| Principle Molecular bonding at the interface between the two polymers | Principle Geometric locking by keying shapes in the mould |
| Conditions Chemical compatibility of the two families (TPE-V/PP, TPE-E/PA, TPU/PC…) | Conditions Specific mould design with reliefs (grooves, dovetails, holes) |
| Advantage No geometric relief necessary, perfect aesthetic. | Advantage allows almost all combinations, even chemically incompatible ones |
| Limit requires affine materials (limited range) | Limit More complex and expensive mould, design constraints |
| Examples: PP+TPE-V, PA+TPE-E, PC+TPU, ABSTPU, opaque PC + diffusing PC | Examples: PP+PA, PC+POM, overmoulded metal, decorative combinations |
1. Chemical adhesion
Certains polymères bond naturally thanks to their compatible chemical structures. TPVs (PP-based), TPEs (PA-based) and TPUs are designed to adhere to thermoplastics within their family. This is the ideal option: no geometric relief required, perfect aesthetics, high peel strength.
2. Mechanical attachment
When materials are not chemically compatible, a geometry is created that mechanically fastens the parts together: grooves, dovetails, anchor holes, undercuts. The flexible injected material fills these shapes and is mechanically locked. This approach allows for almost any combination, at the cost of a more complex mould design.
Bi-injection Material Compatibility Table
Use the interactive tool below to check the compatibility of two materials in bi-injection. Click on a material in the first column or use the filter to display only one line.
Materials Compatibility Chart
Limited compatibility
Poor compatibility
No compatibility
Uninformed
If your materials do not adhere naturally, no problem: our teams design bespoke mechanical fixings to guarantee long-lasting adhesion. Contact our design office To validate your combination.
The most common combinations in production
Beyond the interactive whiteboard, here are the most frequently used combinations in series production:
| Rigid material | Flexible/secondary material | Membership | Typical application |
|---|---|---|---|
| PP | TPE-V | Chemical | Tool handles, flexible joints |
| PA (PA6, PA66) | TPE-E | Chemical | Technical Components, Automotive |
| ABS | TPU | Chemical | Waterproof electronic enclosures |
| PC | TPU | Chemical | Outdoor equipment, EV charging |
| PC opaque | PC diffuser | Chemical (same family) | LED facade lighting |
| ABS/PC | PC transparent or diffusing | Chemical | Mobility, consumer electronics |
| POM | POM | Chemical (same family, different colours) | Two-tone technical components |
| PA | POM | Required mechanics | Bi-functional technical parts |
| PP | PA | Required mechanics | High-stress sector parts |
| Elastomer | Elastomer (different formulations) | Chemical (compatible formulations) | Technical insoles, multi-density joints |
| Thermoplastic | Textile technique | Chemical or mechanical according to textile | Insoles, workwear, medical |
How to choose your two subjects in practice
5 criteria to systematically validate before finalising the combination:
- Chemical compatibility or need for mechanical anchoring — First filter. If chemical, design simply; if mechanical, design the mould with anchors
- Thermal compatibility — The injection temperatures should be close (ideally with a difference of less than 80°C) to avoid degradation of the first material when injecting the second
- Withdrawals nearby — If the material removal rates for the two materials differ too greatly (by more than 0.5%), the workpiece will warp Cooling. Prefer families with comparable withdrawals
- Desired end properties — Rigidity + flexibility / opaque + transparent / insulating + conductive / aesthetic + technical
- Cost and availability — Certain specialised TPEs have significant MOQs or long lead times. To be validated in advance
Hybster Recommendation: A pre-production adhesion trial is strongly recommended for any new or high-risk combination. The test involves measuring the peel strength between the two materials and validating that it remains above the application threshold after thermal and chemical ageing.
To go further
- Two-component injection process
- Technical plastics comparator
- All Hybster subject sheets
- Glossary of Plastic Injection Moulding
Your materials don't seem to be adhering? Our Hybster design office Analyse your combination and propose the optimal bonding solution (chemical, mechanical, or hybrid). Request a compatibility analysis
❓ Frequently Asked Questions
What is two-component injection moulding in plastics processing?
Two-component injection, also known as bi-injection or overmoulding, is a technique that combines two distinct polymers or two colours in a single part. It is carried out in a single operation on a multi-component press or in two operations with a rotary mould. It provides several functions without requiring subsequent assembly.
Quels matériaux ont une bonne adhérence l'un sur l'autre lors de la bi-injection ?
The main compatibilities in bi-injection are: PP with TPE-V, PA with TPE-E, ABS with TPU, PC with TPU, PA with polyacetal POM. Chemical compatibility is essential for adhesion. If it is not natural, mechanical retention (grooves, anchor holes) is created between the two materials to ensure holding strength.
Quelle est la différence entre la bi-injection et le surmoulage?
Bi-injection injects two polymers in a single operation on a multi-material press. Overmoulding injects the second polymer onto a first moulded part, which is placed manually or robotically into a new mould. Bi-injection is more productive, while overmoulding is more flexible for small production runs.
How to choose the two materials for a two-material part?
The choice relies on chemical compatibility for adhesion, compatible injection temperatures, similar shrinkage rates to avoid deformation, and the desired final properties (rigid structure with a flexible surface, insulator with conductor, opaque with transparent). A pre-production adhesion test is highly recommended.
Are TPEs (elastomers) compatible with PP in bi-injection?
Yes, TPE-V (thermoplastic vulcanisates based on PP) are designed to bond naturally with PP. Chemical adhesion is excellent without preparation. This is the most commonly used combination for soft grips, integrated seals, and grip zones on large-scale PP structural parts.
Pourquoi associer un thermoplastique rigide et un thermoplastique souple ?
This combination incorporates several functions into a single part: structural rigidity on one side, softness or anti-slip on the other. Frequent applications include: tool handles, housings with integrated seals, toys, and medical devices. Two-shot moulding avoids subsequent assembly, reducing costs and potential product failure points.
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