Abstract

Thermoplastic composites have gained increasing attention due to their recyclability, high toughness, and rapid processing capabilities compared to thermoset systems. However, their industrial adoption is limited by challenges in achieving effective fiber impregnation due to high melt viscosity. This study investigates the production and processing of continuous fiber-reinforced thermoplastic composites through a direct melt (hot-melt) impregnation approach. A laboratory-scale tape production line was developed and optimized to produce unidirectional (UD) prepreg tapes. These materials were compared with powder-based towpregs and subsequently processed via pultrusion and compression molding. The study also introduces a viscosity estimation model, a consolidation monitoring method using displacement sensing, and an improved calcination technique for carbon-fiber composites. Results demonstrate that melt-impregnated tapes achieve higher fiber wet-out, lower void content, and improved mechanical performance compared to towpregs. The findings confirm that prepreg architecture and impregnation strategy are critical factors in determining downstream processing efficiency and final composite properties.

Keywords

Thermoplastic composites; Melt impregnation; UD tape; Towpreg; Pultrusion; Consolidation; Fiber wet-out; Composite processing

1. Introduction

The demand for lightweight, high-performance, and recyclable materials has accelerated the development of thermoplastic composite systems. Compared to thermoset composites, thermoplastics offer advantages such as improved impact resistance, shorter processing cycles, weldability, and recyclability. Despite these benefits, their application in continuous fiber-reinforced structures remains constrained by the difficulty of achieving effective impregnation.

Unlike thermosets, which exhibit low viscosity in their uncured state, thermoplastics require melting before processing and retain significantly higher viscosities. This creates challenges in fully wetting fiber bundles and minimizing void content. Consequently, the development of efficient impregnation methods and optimized intermediate materials is essential.

This study focuses on the design and validation of a melt impregnation process for producing thermoplastic prepreg tapes, aiming to improve both material quality and downstream manufacturability.

2. Literature Review

Existing thermoplastic impregnation methods can be broadly categorized into:

• Film stacking (alternating layers of fibers and polymer films)
• Commingled fibers (pre-mixed fiber and polymer filaments)
• Powder impregnation (towpreg production)
• Direct melt impregnation

Among these, powder-based towpregs have been widely studied due to their relative simplicity and adaptability. However, they often result in non-uniform polymer distribution and require additional consolidation during processing.

Direct melt impregnation offers a more controlled approach by introducing molten polymer directly into fiber bundles. However, it requires precise control of temperature, pressure, and fiber architecture to overcome viscosity limitations.

Previous research has highlighted the importance of fiber spreading, residence time, and pressure distribution in achieving high-quality impregnation. Despite this, there remains a gap in developing scalable, cost-effective systems capable of producing consistent thermoplastic prepregs at laboratory-to-pilot scale.

3. Methodology

3.1 Materials

Reinforcements included glass fibers and carbon fibers. Thermoplastic matrices investigated were polypropylene (PP), polyethylene terephthalate (PET), recycled PET (rPET), polyamide (PA), and polycarbonate (PC).

3.2 Tape Production System

A continuous melt impregnation line was developed, consisting of:

• Fiber unwinding system
• Mechanical fiber spreading unit
• Melt impregnation chamber with heated die
• Pulling mechanism
• Winding system

Two impregnation die prototypes were designed and tested to evaluate the effect of geometry on impregnation quality.

3.3 Characterization Methods

Material and process characterization included:

• Thermogravimetric analysis (TGA) for thermal stability
• Melt flow rate (MFR/MVR) for rheological behavior
• Scanning electron microscopy (SEM) for microstructure
• Mechanical testing (tensile and flexural)
• Fiber content analysis via calcination

3.4 Processing Techniques

Prepared prepregs were processed using:

• Pultrusion for continuous profile production
• Heated compression molding for laminate fabrication

A consolidation monitoring system using a displacement sensor was implemented to analyze thickness evolution during molding.

4. Results

4.1 Tape Production Performance

The second-generation melt impregnation prototype significantly improved impregnation quality, reducing void content from approximately 30–40% to below 5%. SEM analysis confirmed uniform polymer distribution and effective fiber wet-out.

4.2 Comparison with Towpreg

Melt-impregnated tapes exhibited:

• Higher fiber volume fraction (50–60%)
• Improved impregnation uniformity
• Reduced consolidation requirements

Towpreg materials showed incomplete wetting and localized polymer agglomeration.

4.3 Pultrusion Results

Tape-based composites demonstrated:

• Higher mechanical strength (15–30% increase)
• Improved processing stability
• Ability to operate at higher pulling speeds

Optimal processing temperatures varied by polymer type, with polycarbonate systems achieving the highest mechanical performance.

4.4 Compression Molding Results

Consolidation time decreased significantly with increased pressure. Tape-based laminates achieved near-complete consolidation with low void content, although deviations from theoretical predictions were observed due to fiber alignment and flow effects.

5. Discussion

The results confirm that impregnation quality plays a decisive role in both processing efficiency and final composite performance. Melt-impregnated tapes provide a more uniform starting material compared to towpregs, reducing the required consolidation distance during downstream processing.

However, the study also highlights that optimal viscosity conditions differ between processing stages. Lower viscosity benefits impregnation but may reduce pressure build-up during pultrusion, affecting final consolidation.

The improved die design in the second prototype demonstrates that equipment geometry is critical in controlling resin flow, residence time, and pressure distribution. This reinforces the importance of integrating material behavior with process design.

Furthermore, the consolidation monitoring approach provides a practical method for optimizing cycle times, reducing reliance on trial-and-error in industrial settings.

6. Conclusion

This study demonstrates that direct melt impregnation is a viable and effective method for producing high-quality thermoplastic prepreg tapes. Compared to powder-based towpregs, tapes offer superior impregnation, improved mechanical properties, and enhanced process efficiency.

Key conclusions include:

• Melt impregnation significantly improves fiber wet-out and reduces void content
• Prepreg architecture directly influences downstream processing performance
• Process optimization requires balancing viscosity, temperature, and pressure across stages
• Real-time consolidation monitoring can enhance manufacturing efficiency
• Recycled thermoplastics can be successfully integrated into structural composite applications

Overall, melt-impregnated tapes represent a promising intermediate material for scalable thermoplastic composite manufacturing.

References

Esfandiari, P. (2022). Production and Processing of Pre-impregnated Thermoplastic Composites.

ASTM D1238 – Standard Test Method for Melt Flow Rates of Thermoplastics.

Gibson, R. F. (2016). Principles of Composite Material Mechanics.

Mallick, P. K. (2007). Fiber-Reinforced Composites: Materials, Manufacturing, and Design.

Strong, A. B. (2008). Fundamentals of Composites Manufacturing.