Abstract

The continuous production of thick thermoplastic organo-sheets remains a major challenge in composite manufacturing due to the high viscosity of molten thermoplastics, the slow heating of thick laminate stacks, and the difficulty of achieving uniform impregnation without excessive cycle times. This paper examines the use of a fixed-roller double-belt press for the continuous manufacture of thick carbon fiber/polyamide 6 (CF/PA6) organo-sheets. Unlike conventional hydraulic hot presses, the fixed-roller architecture enables controlled, stepwise thickness reduction during processing, which improves impregnation and minimizes resin leakage. A Darcy-based impregnation model is used to relate permeability, viscosity, belt speed, and roller force to impregnation behavior. Experimental results demonstrate that a 6 mm thick organo-sheet can be continuously produced with good wet-out, low void content, and mechanical properties approaching those of batch hot-pressed laminates. The study confirms that the fixed-roller double-belt press is not merely a laminating system, but a continuous impregnation and consolidation platform for thick thermoplastic composites.

Keywords

double-belt press; organo-sheet; thermoplastic composites; continuous impregnation; fixed rollers; carbon fiber; PA6; Darcy law

1. Introduction

Thermoplastic composites are increasingly used in automotive, aerospace, and industrial applications because they offer short processing cycles, recyclability, weldability, and good impact resistance. However, their wider adoption in thick structural laminates is constrained by one persistent difficulty: impregnation.

Unlike thermoset systems, thermoplastic matrices must be processed in the molten state, where viscosity remains high compared with reactive liquid resins. As laminate thickness increases, the time and pressure required for resin to penetrate the reinforcement also increase substantially. This makes batch hot pressing reliable but slow, while continuous processing remains technically demanding.

One promising solution is the fixed-roller double-belt press. By combining continuous heating, staged compression, and controlled cooling, this equipment makes it possible to process multi-layer textile and film stacks into thick stampable organo-sheets in one pass. The process is especially relevant for carbon fiber woven fabric and thermoplastic film systems, where the target is not only consolidation, but also full impregnation of the textile architecture.

This paper investigates the process logic, experimental validation, and predictive modeling of thick organo-sheet production using a fixed-roller double-belt press.

2. Literature Review

Thermoplastic composite manufacturing routes can be broadly divided into two categories: processes that begin with already impregnated feedstocks, such as UD tapes and prepregs, and processes that impregnate dry reinforcements during consolidation.

Hot-melt impregnation, commingled yarns, and powder-based prepregs reduce the impregnation burden before final forming. These methods are especially effective when the goal is to produce thin, highly tailored laminates or automated tape layups. In contrast, film stacking remains widely used for woven thermoplastic laminates because it is simple, scalable, and compatible with organo-sheet production.

Traditional film stacking is generally carried out in hydraulic hot presses. Although reliable, this approach suffers from low productivity, long heating and cooling times, and limited flexibility in handling large or thick laminates. Double-belt press technology has therefore emerged as a continuous alternative.

Previous double-belt systems have often been used for pre-consolidation of already impregnated stacks. Far fewer studies address the full impregnation of dry textile and film stacks in a continuous belt press, especially for thick laminates above 5 mm. The present work addresses that gap by focusing on a fixed-roller double-belt system designed specifically for thick, stampable organo-sheets.

3. Methodology

3.1 Materials

The investigated laminate system consisted of carbon fiber woven fabric and polyamide 6 (PA6) film. For full-scale organo-sheet production, 28 layers of carbon fabric and 29 layers of PA6 film were used to achieve a final laminate thickness of 6 mm.

3.2 Machine Configuration

The process was carried out on a fixed-roller double-belt press with the following main characteristics:

• Two heating zones, each with eight fixed rollers
• One cooling zone with hydraulic pressure
• Steel belt width of 600 mm
• Effective product width of approximately 490 mm
• Hot-air heating with temperatures up to 380°C

A key feature of the system is the ability to preset roller positions at the inlet and outlet of each zone, creating a progressive reduction in gap thickness along the machine.

3.3 Experimental Approach

Three types of experiments were conducted:

3.3.1 Pressure Measurement Under Rollers

A force sensor embedded beneath the laminate path was used to measure real pressure peaks under the rollers. These experiments helped characterize how pressure evolved as the laminate thickness decreased during processing.

3.3.2 Hot Press Reference Tests

A hydraulic hot press was used to establish a baseline impregnation behavior under simpler, more controlled conditions. Carbon fabric and PA6 film stacks were processed at 245°C and 0.78 MPa for varying holding times between 30 and 480 seconds. Cross-sectional microscopy was used to measure impregnation progress.

3.3.3 Double-Belt Press Trials

Continuous trials were first conducted on thinner stacks at belt speeds of 0.4, 0.6, and 0.8 m/min to validate the impregnation model. Full-scale trials were then performed on the 6 mm thick laminate at 0.2 m/min.

3.4 Theoretical Model

Impregnation inside the fiber bundle was modeled using Darcy’s law for one-dimensional through-thickness flow:$$\frac{dl}{dt} = \frac{KP}{\mu l}$$

where $l$l is impregnation length, $K$K is permeability, $P$P is pressure, and $\mu$μ is viscosity.

After integration:$$l^2 = 2KP \int \frac{1}{\mu} dt$$

For the roller-based process, a simplified form was used in which local viscosity was assumed constant under a roller, leading to a relation between impregnation and roller force, belt speed, and material width.

4. Results

4.1 Pressure Distribution

Pressure measurements revealed bell-shaped peaks under each roller. The peak pressure decreased along the processing direction at elevated temperature because the laminate thickness gradually reduced as impregnation and consolidation progressed. This confirmed that pressure in the machine is not constant, but evolves with the material state.

4.2 Hot Press Reference Impregnation

Cross-sectional observations from the hot press showed that impregnation began at the resin–fabric interface and gradually advanced toward the center of the fiber bundle. Nearly complete impregnation was observed after 480 seconds. These data were used to estimate a permeability value of:$$K = 6.54 \times 10^{-16} \text{ m}^2$$

for the investigated carbon fabric/PA6 system.

4.3 Temperature and Viscosity Behavior

Material temperature increased progressively through the two heating zones and dropped sharply in the cooling section. The effective impregnation region was limited to the zones where the polymer temperature exceeded the melting range and pressure was still applied. As expected, higher temperature significantly reduced melt viscosity and improved impregnation kinetics.

4.4 Belt Speed Effect

At belt speeds of 0.4, 0.6, and 0.8 m/min, cross-sections showed progressively larger unimpregnated central regions as speed increased. This demonstrated that belt speed directly affects impregnation quality by reducing both the heating time and the pressure interaction time.

4.5 Full-Scale 6 mm Organo-Sheet

A continuous organo-sheet measuring approximately 550 mm × 2000 mm × 6 mm was successfully produced at a belt speed of 0.2 m/min. Process parameters included:

• Heat zone 1: 380°C, 9 kN
• Heat zone 2: 320°C, 18 kN
• Cooling pressure: 2 MPa
• Thickness reduction from 8.5 mm at inlet to 6 mm at outlet

The final laminate exhibited:

• Fiber volume fraction: 55%
• Void content: 1.4%
• Bending strength: approximately 600 MPa
• Bending modulus: approximately 60 GPa

These properties reached nearly 90% of the values obtained from conventional hot press processing.

5. Discussion

The results show that the fixed-roller double-belt press functions as more than a continuous laminator. It acts as a staged impregnation and consolidation system in which pressure, heat, and gap design work together to determine laminate quality.

5.1 Impregnation as a Coupled Flow Problem

Under each roller, resin flow occurs both through the thickness and in-plane. This means that not all nominal pressure contributes directly to useful fiber impregnation. Some pressure is dissipated by resin escaping along the laminate plane. As a result, effective impregnation pressure is lower than nominal roller load.

5.2 Importance of Gap Scheduling

The progressive reduction in belt gap is a major advantage of the fixed-roller design. It allows thickness to be reduced gradually, maintaining useful pressure as the laminate consolidates. This is especially important for thick stacks, where a single compression step would risk leakage or unstable flow.

5.3 Speed Versus Quality Trade-Off

The study clearly confirms that belt speed is not only a productivity parameter but also an impregnation parameter. Increasing speed shortens residence time in the effective heating and pressure zones, leading to incomplete impregnation. For thick laminates, the speed penalty is particularly severe because the thermal penetration into the core becomes the limiting factor.

5.4 Comparison with Hydraulic Pressing

Hydraulic hot pressing still provides the highest degree of control and remains a benchmark for laminate quality. However, the double-belt press approaches comparable performance while enabling continuous production. This makes it highly attractive for industrial applications where throughput and continuous handling outweigh the last increment of property optimization.

6. Conclusion

This study demonstrates that a fixed-roller double-belt press can be used for the continuous production of thick, stampable thermoplastic organo-sheets with good impregnation quality and mechanical performance.

The main conclusions are as follows:

1. The fixed-roller architecture enables staged thickness reduction, which is essential for thick laminate impregnation.
2. Impregnation in the double-belt press is governed by coupled pressure, viscosity, speed, and flow-path effects rather than simple compression alone.
3. Darcy-based modeling provides a practical framework for estimating impregnation behavior and optimizing process conditions.
4. Belt speed has a direct and strong effect on laminate quality, especially for thick stacks.
5. A 6 mm carbon fiber/PA6 organo-sheet can be continuously produced with low void content and mechanical properties close to those of batch hot-pressed laminates.

Overall, the fixed-roller double-belt press represents a viable industrial route for the continuous manufacturing of thick thermoplastic laminates, particularly where high-volume stampable sheets are required.

References

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