Co-Injection Moulding for Medical Devices: Process, Benefits, and Design Considerations
Co-injection moulding allows a single part to combine different material properties without adding assembly complexity. In medical device manufacturing, this can be useful when a component must meet multiple performance and regulatory requirements simultaneously. In this article, we explain how co-injection moulding works for medical devices, its main benefits, common challenges, and how it compares with other multi-material moulding techniques.
What is Co-Injection Moulding?
Co-injection moulding is a multi-material injection moulding technique in which two polymer melts are injected into a single mould cavity during a single moulding cycle. It produces a layered structure within the wall thickness of the part.
Typically, the first material forms the outer “skin”, while the second forms the internal core.
This technique enables medical device designers to combine surface properties such as biocompatibility, chemical resistance, sterilization compatibility, and regulatory compliance with a different functional core material in a multi-layered part.
Injection Moulding vs. Co-Injection Moulding
| Feature | Standard Injection Moulding | Co-Injection Moulding |
|---|---|---|
| Materials per cycle | Single polymer | Two polymers |
| Internal structure | Homogeneous | Skin–core layered structure |
| Tooling Complexity | Standard mould unit | Specialized runner system |
| Molding Cost | Low | Slightly High |
| Process Control | Standard parameter control | Standard control with layer-sequence control |
| Flow Behavior | Single melt front | Sequential melt front |
A simpler way to understand co-injection moulding is to think of a component with an external medical-grade layer and a functional internal core, each contributing different performance properties within the same part.
The Co-Injection Moulding Process
In multi-material moulding, successful results depend on careful management of melt compatibility, viscosity behaviour, and injection timing to maintain a stable skin-core interface.
Here is the complete breakdown of the co-injection moulding process.
Step 1: Material Preparation
Material pairing determines the functional outcome of the part.
- The skin material is typically a medical-grade resin selected for biocompatibility, chemical resistance, sterilization compatibility, regulatory compliance, or wear performance.
- The core material provides bulk volume and may be a resin selected for stiffness, density control, or other functional requirements, provided it remains compatible with the part’s medical application and performance targets.
Your engineering team and plastic injection moulder should evaluate melt temperature compatibility, viscosity ratio, and interfacial bonding early in development.
Step 2: Sequential Injection
The moulding cycle begins with injecting the skin material into the cavity. As the melt contacts the cooler mould walls, it begins to form a thin outer shell. Before the cavity is fully packed, a second injection unit introduces the core material. The core melt flows through the still-molten center and pushes the skin toward the cavity boundary.
Step 3: Layer Formation
This displacement creates a skin-core-skin structure, where the outer layer remains entirely skin material while the internal volume contains the core polymer. Proper timing ensures full encapsulation and prevents the core from appearing on the surface.
Step 4: Cooling and Ejection
After filling and packing, the part remains in the mould until it reaches its ejection temperature. Cooling time depends on polymer thermal properties, wall thickness, melt temperature, mould temperature, and cooling channel design. Once sufficient rigidity is achieved, the mould opens, and the part is ejected.
Several process variations exist, including sandwich moulding and core-back techniques, but all follow the same principle: controlled distribution of multiple melts within the wall thickness.
Note: Cleanroom injection moulding is required when producing medical components that must meet strict contamination-control and regulatory requirements.
Main Benefits of Co-Injection Moulding for Medical Devices
In the manufacture of medical devices, the decision to use co-injection moulding is rarely based solely on cost. Although co-injection moulding is more expensive than standard injection moulding, it offers numerous advantages.
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Material Efficiency
Co-injection moulding reduces reliance on expensive resins by placing premium material only in the outer skin, while the internal volume uses a lower-cost or recycled polymer. It is well-suited to medical components where the outer layer must maintain regulatory, surface, and sterilization-compatible properties, while the core helps reduce raw material consumption and part weight.
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Performance Enhancement
The layered structure allows engineers and product teams to combine properties that are difficult to achieve with a single polymer. For example, a chemically resistant outer layer can encapsulate a structural core, allowing the part to meet surface-performance requirements while maintaining internal strength and material efficiency for medical use.
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Design Flexibility
By separating surface and structural functions, designers can create parts with multi-functional performance. For example, co-injection moulding for medical devices supports multi-layered structures such as rigid housings with elastomeric interfaces, integrated seals, or ergonomic instrument grips.
Co-injection enables engineers to balance cost, performance, and sustainability within a single moulded structure.
Common Medical Co-Injection Challenges
Co-injection moulding can offer important advantages, but it also introduces technical challenges that must be addressed during development and production. Material behaviour, flow dynamics, and tooling design all influence part quality and process stability. In medical device manufacturing, these factors are especially important because they can directly affect part integrity, consistency, and compliance.
Delamination
It occurs when the skin and core polymers fail to bond properly. This typically arises from incompatible materials, mismatched melt temperatures, or incorrect viscosity ratios between the layers. Weak interfacial adhesion causes separation under stress.
The fix is careful material pairing and controlled processing windows so the polymers fuse during filling. Maintaining compatible viscosities also stabilizes the skin-core interface and prevents layer separation.
Flow Imbalance
Flow imbalance appears when the core penetrates unevenly or breaks through the skin layer. This often results from poor gate placement, unstable pressure profiles, or uneven cavity filling. Engineers and moulding partners can mitigate this issue by using mould-flow simulation and prototype trials. Also, validating flow paths before tooling release for moulded precision plastic medical components.
Cost Factors
Co-injection tooling is more complex because dual material delivery and hot-runner control systems are required. However, design for manufacturability analysis often reveals lifecycle savings through part consolidation, improved performance targeting, and more efficient use of high-value medical-grade materials.
When properly developed and supported by the right moulding expertise, co-injection moulding can turn complex multi-material requirements into scalable manufacturing solutions for high-performance medical components.
Comparing Co-Injection vs. Two-Shot vs. Overmolding
| Parameter | Co-Injection Moulding | Two-Shot Moulding | Overmolding |
|---|---|---|---|
| Material arrangement | Layered skin–core structure within wall thickness | Two materials positioned side-by-side or regionally | Second material moulded over a pre-formed substrate |
| Manufacturing sequence | Two melts are injected sequentially into one cavity | Two injections occur in one automated moulding cycle | The base part is moulded first, then transferred for overmolding |
| Equipment requirement | Multi-injection unit system with controlled melt sequencing | Specialized two-shot machine and rotating/index mould | Conventional injection machine plus secondary mould |
| Tooling complexity | High due to hot-runner and flow control requirements | Very high due to rotating tools and precision alignment | Moderate because tooling is simpler but involves two steps |
| Cost profile | Higher setup but lower material cost through core substitution | High initial tooling and machine investment | Lower upfront tooling but higher handling or cycle costs |
| Applications | Medical device housings, fluid-management components, and layered functional parts | Medical components requiring distinct functional regions, soft-touch zones, or integrated user-contact surfaces | Medical device grips, seals, ergonomic handles, and protective interfaces |
Conclusion
For engineers, product teams and medical device manufacturers evaluating co-injection moulding, the key lies in balancing material compatibility, process control, and tooling investment. When these factors are carefully engineered, complex medical components with layered structures can be produced reliably at scale. Understanding the process limitations and design considerations ultimately determines whether the technology delivers long-term manufacturing value.
Precikam offers precision plastic injection moulding services to medical device manufacturers across Canada and around the world. If you are considering co-injection moulding for a new medical component, or evaluating whether two-shot moulding or overmolding is more appropriate for your device design, please contact us. Our team will evaluate your project and recommend the most appropriate process for it.
About Jack McDonald
Jack McDonald is the President of PreciKam, a leading North American precision plastic injection molding manufacturer based in Baie-d’Urfé, Quebec. With over three decades of industry experience, Jack is dedicated to producing quality precision molded plastic parts crucial to health and safety in the medical, automotive, and food sectors.
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