Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : November 28 , 2025

Abstract

This study investigates the development, processing, and mechanical assessment of thermoplastic composite (TPC) sandwich structures designed for structural helicopter components. The work explores a fully thermoplastic architecture in which carbon fibre-reinforced PEEK skins are joined to a PEI foam core by melt-bonding (fusion bonding) supported by a PEI interlayer film. The aim is to reduce manufacturing cycle time, improve recyclability, and maintain adequate structural performance compared with established thermoset sandwich systems such as prepreg-Nomex® honeycomb and RTM-PMI cores. Process modelling based on intimate contact and molecular interdiffusion is combined with controlled hot-press experiments to establish suitable processing windows. Mechanical characterisation—including flatwise tension, peel, shear, edgewise compression, CAI, and flexural loading—provides a comprehensive assessment of bond quality and failure modes. Results demonstrate that optimal skin temperature (≈300 °C), limited core heating, a compaction distance of ~2 mm, and a 125 µm PEI interlayer yield strong, cohesive skin-core joints without foam collapse. Although absolute strength remains lower than thermoset honeycomb systems, the thermoplastic panels show excellent bond integrity, competitive damage tolerance, and significantly shortened cycle times. These characteristics suggest clear potential for rotorcraft structures and selected aerospace, rail, and automotive applications where manufacturing throughput, environmental performance, and impact resistance are decisive.

Keywords

thermoplastic composite; CF/PEEK laminate; PEI foam core; fusion bonding; melt-bonded sandwich; hot-press consolidation; intimate contact model; foam-core stability; aerospace sandwich panel; helicopter structure; peel resistance; post-impact behaviour

1. Introduction

Sandwich construction has long served high-performance lightweight structures in aerospace, rotorcraft, marine and mobility sectors. The principle—a stiff composite facing bonded to a low-density core—offers high flexural rigidity at minimal weight. Historically, the aerospace sector has relied on thermoset prepreg skins paired with Nomex® honeycomb cores, a combination that provides high stiffness-to-weight ratios but involves extended cure cycles, limited repairability, moisture concerns, and end-of-life disposal challenges.

Growing pressures related to operating cost, turnaround time, and sustainability have raised interest in thermoplastic composite (TPC) architectures. High-performance thermoplastic matrices such as PEEK and PEI offer rapid heating–cooling cycles, weldability, and recyclability. Their melt-based processing reduces autoclave dependence and permits fusion bonding rather than adhesive bonding. For rotorcraft structures, where maintenance, impact events, and operational wear create demanding service conditions, the potential value of a tough, damage-resistant thermoplastic laminate is considerable.

Foam-core sandwiches are also evolving. Polyetherimide (PEI) foams, with their isotropic cellular structure, thermoformability, and lower moisture uptake, present a promising alternative to traditional honeycombs. Combined with carbon/PEEK skins, they allow a fully thermoplastic assembly that can be hot-pressed in minutes rather than hours.

This study focuses on the development and evaluation of CF/PEEK-PEI foam sandwiches manufactured via non-isothermal compression moulding. The goals are to establish robust processing parameters, quantify mechanical response, compare performance with the state of the art, and evaluate suitability for helicopter structural use.

2. Literature Review

2.1 Thermoplastic Composite Skins

PEEK-based laminates are widely recognised for outstanding chemical resistance, thermal stability, and fracture toughness. Their melt-processability enables welding, tape-laying, and rapid tooling cycles. Previous research has highlighted the potential of CF/PEEK for primary aircraft structures, yet integration with lower-cost cores remains challenging due to the narrow thermal window between PEEK melting (343 °C) and PEI foam softening.

2.2 Foam Cores versus Honeycomb Cores

Nomex® honeycomb retains unmatched stiffness-to-weight performance but introduces drawbacks: moisture uptake, complex adhesive filleting, limited out-of-plane shear uniformity, and difficult recycling. Polymeric foams—particularly PEI and PMI foams—provide isotropic mechanical properties and easier forming for double-curved parts. However, their compressive strength typically lags behind honeycomb, imposing limits on load-critical regions of aircraft structures.

2.3 Bonding Mechanisms in Thermoplastic Sandwich Panels

Adhesive films have traditionally been used to join thermoplastic skins to cores. However, melt-bonding through an interlayer film of compatible polymer offers improved long-term reliability. Bond formation can be described by two mechanisms: (1) intimate contact, where surface asperities flatten under pressure, and (2) healing, driven by chain interdiffusion. Numerous studies confirm that interlayer thickness, temperature gradient, and pressure profile dictate the quality of the joint.

2.4 Manufacturing Technologies

Compression moulding remains attractive for thermoplastic sandwiches because of its short cycle times and ability to consolidate multiple layers simultaneously. Double-belt press systems offer continuous processing but require stringent temperature control to protect the foam. Hot-press forming with controlled skin heating offers flexibility for prototypes and short-run aerospace components.

2.5 Mechanical Testing and Performance Benchmarks

Peel strength, CAI capacity, and shear behaviour are decisive measures for bonded sandwich structures. Thermoset systems with honeycomb cores often deliver high initial stiffness but show brittle behaviour under impact loading. Thermoplastic core-skin assemblies, although softer in compression, often exhibit better damage tolerance due to ductile matrix behaviour and more uniform load transfer across the interface.

3. Methodology

3.1 Material Systems

Skins: Unidirectional and fabric-based CF/PEEK laminates (Victrex AE250) with ~58% fibre volume fraction.
Core: Closed-cell PEI foam (Airex R82) with densities between 60–110 kg/m³.

3.2 Interlayer Design

A 125 µm PEI film (“Thermabond”) was placed between the laminate and the core to bridge the disparity in processing windows and promote melt-bonding.

3.3 Processing Approach

Sandwich panels were produced using non-isothermal compression moulding:

• Skin temperature, TSkin: 290–320 °C
• Core temperature, TCore: 23–200 °C
• Compaction pressure: ~0.2 MPa
• Compaction distance: 2–5 mm depending on core thickness
• Heating and cooling rates established from heat-transfer modelling

Temperature gradients were arranged such that the PEEK matrix reached sufficient melt flow while the PEI foam remained below collapse threshold.

3.4 Process Modelling

Interfacial strength was predicted using established models for:

• Intimate contact, assessing surface flattening as a function of temperature, viscosity, and pressure.
• Healing, evaluating polymer chain mobility and interdiffusion.

Heat-transfer analysis ensured the foam remained structurally stable during the process.

3.5 Mechanical Characterisation

Standards employed:

• Flatwise tension: DIN 53292
• Climbing drum-peel: DIN EN 2243-2
• Core shear: DIN 53294
• Edgewise compression: ASTM C364
• Compression after impact (CAI): AITM 1-0010
• 4-point bending: DIN EN 6061

Each configuration used at least five specimens. Failure modes were documented according to DIN EN ISO 10365 to distinguish adhesive, cohesive, and boundary-layer failures.

4. Results

4.1 Influence of Skin Temperature

Raising the skin temperature from 300 °C toward 320 °C substantially improved melt-bond formation. Peel tests showed that higher TSkin promoted deeper chain interdiffusion into the foam’s cell walls, resulting in cohesive foam failure. Maximum failure depth approached ~950 µm under optimal parameters.

4.2 Core Integrity and Compaction Distance

Compaction distances beyond ~2 mm risked local foam densification. Excessive core heating (>180–200 °C) led to collapse zones. The optimal balance occurred with skins near 300 °C and the core close to ambient during bonding.

4.3 Interlayer Thickness Effects

The 125 µm PEI film provided consistent performance. Thinner films reduced flow availability; thicker films created unnecessary resin pockets and local stiffness discontinuities.

4.4 Mechanical Performance

Representative values for the optimal configuration (referred to as “300-125-2”) include:

• Peel strength: >1.2 N/mm
• Shear strength: ~1.3 MPa
• Edge compression: dominated by foam capacity (~1.0 MPa)
• CAI behaviour: intermediate relative to PMIs and Nomex® composites
• 4-point bending: lower stiffness than honeycomb but robust post-impact response

4.5 Comparison with Reference Systems

Property	TPC (300-125-2)	RTM63 (Epoxy/PMI)	Prepreg-Nomex®
Tensile strength	Lower	Higher	Highest
Peel strength	Highest	Lower	Lower
CAI	Intermediate	Lower	Highest
Failure mode	Cohesive in foam	Interface	Interface

The TPC sandwich exhibited superior bond toughness, improved post-impact stability, and reduced process time, though not matching honeycomb-based stiffness.

5. Discussion

5.1 Bond Quality and Failure Mechanisms

The dominance of cohesive foam failure demonstrates effective chain entanglement at the interface. This differs markedly from traditional adhesive-bonded thermoset sandwiches, which often fail at the adhesive interface. The interlayer approach successfully mediated the thermal mismatch between PEEK skins and PEI cores.

5.2 Mechanical Limitations of Foam Cores

Despite impressive bond quality, foam cores inherently display lower compressive capacity than honeycomb cores. Edgewise compression and shear results confirm that performance is governed by core strength rather than bond integrity. Reinforcement strategies—such as pin-reinforced foams—show promise, with trials indicating improvements up to 40%.

5.3 Damage Tolerance and Impact Behaviour

The thermoplastic matrix contributes noticeably to improved damage tolerance. CAI results reveal lower knockdown factors than comparable thermoset PMI sandwiches. For rotorcraft structures subject to tool drops, maintenance events, or random impacts, such behaviour is valuable.

5.4 Manufacturing Throughput and Process Robustness

The cycle time for RTM-PMI sandwiches often exceeds three hours due to layup, vacuum preparation, and curing. In contrast, the TPC hot-press cycle is measured in minutes. This difference becomes critical for fleet-level cost reduction and production scaling.

5.5 Forming of 3D Structures

Trials with shaped panels indicate that skins and core should be pre-formed separately, then joined. Direct forming risks core densification and uneven pressure distribution. The demonstrated shear frame panel confirms feasibility for non-planar aerospace components.

6. Conclusions

This work demonstrates that CF/PEEK skins melt-bonded to PEI foam cores create a fully thermoplastic sandwich system with strong, cohesive interfaces and competitive damage tolerance. Although the foam core imposes mechanical limitations relative to honeycomb cores, the system offers compelling advantages:

• Short processing cycles
• Weldable, recyclable structure
• High interfacial toughness
• Stable post-impact behaviour
• Lower moisture sensitivity
• Cost-effective processing

For rotorcraft components where impact resistance, maintainability, and manufacturing throughput are priorities, this architecture is an attractive alternative. Further gains are expected through hybrid cores, pin reinforcements, and automation. With refinement, this class of sandwich structures has the potential to serve not only helicopter applications but also rail, automotive, and industrial lightweight structures.

References

1. Grünewald, J. Thermoplastic Composite Sandwiches for Structural Helicopter Applications. PhD Dissertation, University of Bayreuth, 2018.
2. Hufenbach, W., et al. “Fusion bonding of thermoplastic composites—Mechanisms and modelling.” Composites Part A, 2012.
3. Osswald, T., & Menges, G. Materials Science of Polymers for Engineers. Hanser, 3rd Ed.
4. Carlsson, L., Kardomateas, G. Structural and Failure Mechanics of Sandwich Composites. Springer, 2011.
5. ASTM C364 – Standard Test Method for Edgewise Compressive Strength of Sandwich Constructions.
6. AITM 1-0010 – Airbus Compression After Impact Standard.
7. Gardiner, G. “Thermoplastic composites in aerospace.” CompositesWorld, 2018.