Abstract

The transition from thermoset-based sandwich structures to thermoplastic composite systems in aerospace applications is driven by the need for reduced cycle time, improved recyclability, and lower life-cycle cost. However, the industrial implementation of thermoplastic sandwich panels remains constrained by the incompatibility between high-performance skin materials and thermally sensitive core structures. This study presents a systematic investigation into carbon fiber reinforced polyetheretherketone (CF/PEEK) skins combined with polyetherimide (PEI) foam cores, joined through a fusion bonding approach enabled by an interfacial PEI film layer. A combined intimate-contact and molecular healing model is applied to define the bonding mechanism, followed by experimental validation through non-isothermal compression moulding. The results identify a narrow but stable process window (skin temperature ≈ 300 °C, compaction distance ≈ 2 mm, interlayer thickness ≈ 125 µm) that enables strong interfacial bonding while limiting foam collapse. Mechanical characterization confirms that interface strength exceeds core strength, shifting the structural limitation to the foam itself. The study concludes that while fusion bonding resolves the interface challenge, future development must focus on core reinforcement and coupled thermo-mechanical modelling to achieve full structural equivalence with conventional aerospace sandwich systems.

Keywords

Thermoplastic composites; Sandwich structures; CF/PEEK; PEI foam; Fusion bonding; Compression moulding; Intimate contact; Polymer healing; Aerospace structures; Process window

1. Introduction

Sandwich structures have long been recognized as one of the most efficient load-bearing configurations in aerospace engineering. Their ability to achieve high bending stiffness at low weight makes them indispensable in helicopter components such as floor panels, fairings, and secondary structural elements. Traditionally, these structures rely on thermoset prepregs combined with honeycomb cores. While mechanically efficient, such systems suffer from long processing cycles, complex manufacturing steps, and limited recyclability.

Thermoplastic composites offer a fundamentally different pathway. Their ability to be reheated, reshaped, and rapidly consolidated makes them attractive for high-rate manufacturing. However, the transition from monolithic laminates to sandwich structures introduces a new challenge: reliable bonding between thermoplastic skins and lightweight cores under constrained thermal conditions.

In high-performance systems such as CF/PEEK skins and PEI foam cores, this challenge becomes particularly severe due to mismatched processing temperatures. The skin requires temperatures approaching its melting point, while the foam core risks structural collapse under similar conditions. Therefore, the key question is not material selection alone, but how to engineer a process window that enables bonding without compromising core integrity.

This study addresses this challenge through interface engineering and controlled compression moulding, bridging the gap between material capability and industrial feasibility.

2. Literature Review

Previous research in thermoplastic composite manufacturing has primarily focused on impregnation technologies and laminate consolidation. Techniques such as film stacking, commingled yarns, and hot-melt impregnation have successfully enabled high-quality unidirectional tapes and organo-sheets. Continuous processes, particularly double-belt press systems, have further demonstrated the feasibility of producing thick thermoplastic laminates at industrial scale.

However, sandwich structures introduce additional complexity. Unlike laminate consolidation, sandwich manufacturing requires:

• Controlled heat transfer across thickness
• Simultaneous management of two materials with different thermal limits
• Stable interface formation under pressure
• Prevention of core deformation

Conventional joining methods rely on thermoset adhesives, which are incompatible with thermoplastic processing advantages. Fusion bonding, based on polymer chain diffusion, has emerged as a promising alternative. Studies on polymer welding mechanisms have established that bonding strength depends on two key phenomena: intimate contact and molecular healing.

Despite these advances, most existing models neglect the behavior of foam cores under thermal and mechanical loading. As a result, there remains a gap between theoretical bonding predictions and actual sandwich performance. This study aims to close that gap by integrating interface physics with process constraints.

3. Methodology

3.1 Material System

The selected sandwich configuration consists of:

• Skins: Carbon fiber reinforced PEEK laminates
• Core: Closed-cell PEI foam
• Interface layer: PEI film (50–250 µm)

The interfacial PEI film serves as a compatibility layer, reducing the mismatch between skin and core materials and enabling effective fusion bonding.

3.2 Process Design

A non-isothermal compression moulding process is employed:

1. Preheat CF/PEEK skins to near-melting temperature
2. Maintain PEI foam below collapse threshold
3. Assemble skin–film–core stack
4. Apply controlled pressure and compaction
5. Hold for bonding development
6. Cool under pressure

3.3 Bonding Model

The bonding mechanism is described using two coupled variables:

• Intimate contact (Dic): dependent on pressure, temperature, and deformation
• Healing (Dh): governed by polymer chain diffusion

The resulting bond strength is expressed as:$$\sigma = \sigma_\infty \cdot D_{ic} \cdot D_h$$

This model is used to predict the influence of process parameters on bonding performance.

3.4 Experimental Evaluation

The process window is evaluated through:

• Skin temperature variation (280–320 °C)
• Compaction distance (1–4 mm)
• Interface thickness (50–250 µm)

Mechanical testing includes:

• Drum peel tests
• Interlaminar shear strength
• Compression after impact
• Core compression and shear

4. Results

4.1 Process Window Identification

Optimal bonding conditions were identified at:

• Skin temperature: ~300 °C
• Compaction distance: ~2 mm
• Interface thickness: ~125 µm

Below these values, insufficient contact limits bonding. Above them, excessive heat and pressure lead to foam collapse.

4.2 Bonding Performance

The fusion bonding process achieved:

• Peel strength exceeding 0.8 N/mm
• Failure predominantly within the foam core
• Strong and consistent interface formation

This confirms that the interface is no longer the limiting factor.

4.3 Core Behavior

The PEI foam exhibited:

• Local cell collapse at elevated temperatures
• Reduced structural stability beyond 310 °C
• Sensitivity to compaction-induced deformation

The core becomes the dominant failure mode.

4.4 Model Validation

The bonding model accurately predicted trends within the optimal temperature range. However, it overestimated performance when core degradation occurred, highlighting the need for coupled thermal-mechanical modelling.

5. Discussion

The results demonstrate a fundamental shift in thermoplastic sandwich engineering:

5.1 Interface Problem Solved

The use of a PEI interlayer successfully enables fusion bonding between incompatible materials. This represents a transition from adhesive-based joining to material-integrated bonding.

5.2 Core Becomes the Bottleneck

Once bonding is achieved, structural performance is governed by:

• Foam compressive strength
• Shear resistance
• Stability under thermal load

This shifts research focus from interface chemistry to core architecture.

5.3 Process Window is Narrow but Controllable

The viable process window is constrained but stable. It requires:

• Precise thermal control
• Controlled compaction
• Balanced heat input

This reinforces the importance of process engineering over material selection alone.

5.4 Implications for Industrial Production

Compared to thermoset systems:

• Cycle time can be reduced by approximately 50%
• Manufacturing steps are simplified
• Recyclability is improved

However, further development is required to achieve full structural equivalence.

6. Conclusion

This study demonstrates that thermoplastic composite sandwich structures based on CF/PEEK skins and PEI foam cores can be manufactured using fusion bonding within a controlled process window. The introduction of a compatible interfacial layer enables strong bonding without adhesives, marking a significant advancement in thermoplastic sandwich technology.

However, the findings also reveal that the primary limitation has shifted from the interface to the core material. Future developments must therefore focus on:

• Reinforced foam architectures
• Coupled thermal-mechanical modelling
• Process integration for shaped components

The transition to thermoplastic sandwich structures is not limited by bonding technology anymore—it is defined by system-level optimization of materials, process, and structure.

References

1. Grünewald, J. (2018). Thermoplastic Composite Sandwiches for Structural Helicopter Applications. University of Bayreuth.
2. Köhler, R. (2017). Thermoplastic impregnation methods for continuous fibers.
3. Ishida, O. et al. (2019). Continuous manufacturing using double-belt press.
4. Ageorges, C., Ye, L., Hou, M. (2001). Advances in fusion bonding of thermoplastic composites.
5. ASTM D1781 – Drum Peel Test Standard.
6. Gibson, R.F. (2016). Principles of Composite Material Mechanics.