Abstract

Stampable thermoplastic organo-sheets are attractive for fast-cycle forming and welding, but thick laminates remain difficult to produce continuously because molten thermoplastics resist flow and the stack thickness changes rapidly during consolidation. This paper presents a fixed-roller double-belt press approach for continuous manufacture of thick carbon-fabric/PA6 organo-sheet, and it formalizes a parameter-selection method using a Darcy-based impregnation predictor coupled to measured melt viscosity and effective permeability. A staged gap schedule under multiple rollers enables controlled thickness reduction that is difficult to achieve with conventional hydraulic double-belt systems. The process is validated through (i) compression molding benchmarks used to extract an effective permeability, (ii) belt-press trials designed to suppress excessive in-plane resin transport, and (iii) a full demonstration of a 6 mm organo-sheet produced from 28 fabric layers and 29 PA6 films. The demonstrated laminate reaches ~55% fiber volume fraction, ~1.4% void fraction, and flexural properties near batch hot-press levels. Practical failure modes (dry bundle cores, resin wash, fiber disturbance, and edge non-uniformity) are mapped to controllable variables (temperature profile, belt speed, roller load distribution, and gap taper). The outcome is a production-oriented framework that helps engineers move from “can we make it” to “can we run it reliably.”

Keywords

Organo-sheet; thermoplastic composites; double-belt press; fixed rollers; staged gap schedule; carbon fabric; polyamide 6 (PA6); Darcy flow; permeability; continuous consolidation; stamp forming.

1. Introduction

Thermoplastic organo-sheet (woven or non-crimp fabric reinforced thermoplastic laminate) is used when manufacturers need short cycle time, weldability, and impact-tolerant structures for transportation and industrial parts. Thick sheet (multi-millimeter) adds stiffness and energy absorption, but it also exposes the main bottleneck in thermoplastic processing: high melt viscosity makes full impregnation slow, pressure-sensitive, and strongly dependent on temperature history.

Batch hot pressing can produce excellent quality, yet it carries inherent penalties: intermittent heating/cooling, tooling occupancy, and limited scalability for long sheets. Continuous double-belt pressing offers a path to higher throughput by removing mold heat-up/cool-down cycles and enabling steady-state operation. The challenge is that “pressure” in a belt press is not a single knob. Under each roller, resin experiences a coupled competition between:

• through-thickness infiltration into fiber bundles (what we want), and
• in-plane resin transport along the fabric plane (often what steals resin and disturbs fibers).

A fixed-roller double-belt press adds an important capability: gap control at multiple stations, so the stack thickness can be reduced gradually and predictably. That staged compaction is the difference between “a sheet that looks consolidated” and a laminate that stays dry in the bundle cores.

This paper builds an engineering route to continuous thick organo-sheet: measure viscosity, estimate effective permeability, map the melt window along the press, then allocate impregnation “work” across roller zones and the cooling-pressure zone.

2. Literature Review

2.1 Thermoplastic impregnation as a flow-limited problem

For molten thermoplastics, impregnation is governed by viscosity, pressure, and available time in the molten state. Darcy-type descriptions have long been used as first-order engineering tools for resin infiltration in porous media, including fiber assemblies, even though real fabrics exhibit compaction-dependent permeability and multi-scale flow pathways.

2.2 Continuous belt pressing vs. batch pressing

Double-belt pressing improves productivity by operating continuously and maintaining stable thermal boundaries. Prior studies (across thermoplastic consolidation and continuous laminating) show that belt speed, temperature profile, and pressure history determine void content, fiber wet-out, and final thickness uniformity. The recurring limitation is that many models assume pressure is known and uniform; in reality, roller contact pressure, melt squeeze flow, and belt stiffness shape the true pressure field.

2.3 Why fixed-roller architecture matters for thick sheet

Hydraulic belt presses typically apply a global press condition; thickness reduction is less “programmable” along the line. Fixed rollers provide discrete compaction stations and allow a gap taper from inlet to outlet, which is essential when building thick laminates from dry fabric + resin film stacks. The staged taper reduces sudden resin escape and helps maintain wet-out progression.

Note: The specific industrial demonstration that inspired the dataset used here is reported in the SAMPE 2019 proceedings by Ishida et al. SAMPE

3. Methodology

This work follows a process-engineering sequence that can be repeated on other thermoplastic systems (with recalibration).

3.1 Materials system

• Carbon fiber plain weave fabric (T700SC-12K class, 200 g/m²; yarn density ~3.27 yarns/25 mm) from Toray Industries, Inc..
• Thermoplastic resin film: PA6 (melt onset ~225°C).
• Melt viscosity measured by parallel-plate rheometry from 225–290°C and represented using an Arrhenius-type fit (engineering-useful for integrating time-temperature exposure).

3.2 Fixed-roller double-belt press configuration

• Steel belt width 600 mm; effective high-quality width limited in cooling pressure section (~490 mm).
• Two heating zones, each with 8 rollers; hot-air heating up to ~380°C setpoint, heat delivered through belts.
• Cooling section provides hydraulic pressure until the resin crosses below solidification.
• Control variables: belt speed, hot-air setpoints by zone, roller loads, cooling pressure, and gap settings at zone inlets/outlets.

3.3 Pressure footprint characterization

Because high-temperature sensing is difficult, a force sensor method can be used at lower temperatures with substitute films to capture the shape of pressure pulses under rollers. This does not provide PA6-true absolute pressures, but it reveals whether compaction is stable or collapsing early.

3.4 Effective permeability extraction via compression molding benchmark

A batch hot-press benchmark is used to extract an effective permeability for the chosen fabric + resin system under a known pressure and temperature:

• Layup: 8 fabric layers + 9 PA6 films (100 μm), 150×150 mm.
• Condition: 245°C, 0.78 MPa, hold 30–480 s, then cool.
• Cross-section microscopy to measure impregnation length into bundles; average over multiple bundles.

A 1D Darcy-type impregnation relation is used:$$\frac{dl}{dt}=\frac{K P}{\mu\, l}$$dtdl​=μlKP​

Integrating:$$l^2 = 2KP \int_0^{t}\frac{1}{\mu(T)}\,dt$$l2=2KP∫0t​μ(T)1​dt

From the fitted relation, an effective permeability is obtained (dataset value):$$K \approx 6.54\times 10^{-16}\, \text{m}^2$$K≈6.54×10−16m2

Practical note: This is an effective value tied to compaction state; it should be re-identified when fabric architecture, fiber sizing, film thickness, or target fiber volume fraction changes.

3.5 Belt-press trials for model validation

To test the predictor under conditions that reduce resin wash and fiber movement:

• Validation layup: 2 fabric layers + 3 PA6 films (50 μm).
• Belt speed range: 0.2–0.6 m/min.
• Gap taper: ~0.55 → 0.35 mm; roller loads staged (example: 6/24 kN by zones); cooling pressure ~1 MPa.

3.6 Thick sheet demonstration protocol

• Demonstration layup: 28 fabric layers + 29 PA6 films (100 μm), 550×2000 mm.
• Belt speed: 0.2 m/min.
• Zone setpoints: 380°C (zone 1), 320°C (zone 2); roller loads staged (example: 9 kN then 18 kN); cooling pressure ~2 MPa.
• Quality evaluation: microscopy for voids/wet-out; burn-off for void fraction; 3-point flexural tests (5 specimens each direction).

4. Results

4.1 Melt window and “effective impregnation length” in the press

As belt speed increases, the time above PA6 melt temperature shrinks. Practically, this shortens the number of rollers that contribute meaningful impregnation work. In thick stacks, this effect is stronger because through-thickness heating lag increases.

Production implication: If the laminate core remains below melt, surface layers may appear consolidated while bundle centers remain dry—creating false confidence.

4.2 Effective permeability and the role of viscosity

The Arrhenius-type viscosity fit shows viscosity drops steeply in the 225–290°C range, which means small temperature improvements near the melt window can rival large pressure increases. This is why pre-heating and thermal uniformity often deliver bigger gains than simply raising roller load.

4.3 Model-to-experiment match (thin validation layup)

Using the simplified belt-press predictor:$$l^2 \approx \sum_i \left(\frac{2K}{\mu}\right)\left(\frac{F_i}{v\,w}\right)$$l2≈i∑​(μ2K​)(vwFi​​)

(where $F_i$Fi​ is roller load in zone $i$i, $v$v is belt speed, $w$w is laminate width),

the measured impregnation length trends with belt speed in a way that agrees with the model, typically with slight under-prediction/over-prediction depending on how much in-plane flow is actually suppressed by film thickness and gap choice.

Interpretation: The simplified model is useful for directional tuning (which knob to turn first), but it does not fully capture in-plane flow or compaction-dependent permeability.

4.4 Thick organo-sheet demonstration (6 mm)

A 6 mm CF/PA6 organo-sheet (550×2000×6 mm) is produced continuously at 0.2 m/min using staged heating, staged roller loads, and a controlled gap reduction. The dataset reports:

• Fiber volume fraction $v_f \approx 55\%$vf​≈55%
• Void fraction $\approx 1.4\%$≈1.4%
• Flexural strength $\approx 600$≈600 MPa
• Flexural modulus $\approx 60$≈60 GPa
• Mechanical performance reaches ~90% of batch hot-press benchmark levels.

Industrial meaning: This quality level is credible for stamp-forming grade organo-sheet in many structural and semi-structural applications, with the reminder that forming performance must also be checked (ply shear, wrinkling resistance, and weld line strength).

5. Discussion

5.1 What the process is really balancing: wet-out vs. resin escape

Under rollers, pressure is not “given”—it is spent on two flows:

• infiltration into bundles (good), and
• in-plane squeeze flow (can be bad if excessive).

Fixed-roller staged gaps reduce sudden squeeze-out and allow impregnation to progress without stripping resin away from the fiber network.

5.2 Why dry bundle cores show up in belt pressing

Microscopy often shows unimpregnated centers in bundle regions during continuous pressing. Three root causes dominate:

1. Core temperature lags melt threshold (thermal gradient).
2. Resin supply becomes non-uniform after early squeeze-out (resin starvation).
3. Compaction raises fiber volume fraction locally, lowering permeability faster than expected.

5.3 A practical defect map

Below is a shop-floor mapping that helps operators diagnose issues quickly:

• Dry bundle centers → increase molten dwell time (reduce speed), raise core heating (preheat), reduce early gap closure rate.
• Resin wash / fiber distortion → reduce film thickness per interface, soften early loads, close gap later (move compaction downstream).
• Edge resin starvation / edge voids → check cooling pressure footprint width, belt edge temperature loss, and lateral resin loss; add edge containment strategy.
• Void streaks after cooling → extend pressure into the cooling zone until below crystallization/solidification; avoid early pressure release.

5.4 Throughput limit and the pre-heating requirement

The 0.2 m/min speed for thick sheet is not a weakness of the concept; it is the natural result of heating thick stacks by belt conduction and hot air. If production targets require higher speed, the process must add:

• pre-heat (IR, contact pre-plates, hot-air tunnel, heated nip),
• tighter thermal uniformity control, and
• possibly a revised roller-load schedule that shifts impregnation work to the hottest stable region.

5.5 What should be improved in the next model generation

A second-generation predictor should include:

• compaction-dependent permeability $K(v_f)$K(vf​),
• explicit in-plane resin transport term (resin loss),
• a 1D/2D heat transfer layer to estimate core melt time,
• belt/roller mechanics to estimate true contact length and pressure distribution.

6. Conclusion

A fixed-roller double-belt press with staged gap control provides a practical route to continuous manufacture of thick stampable CF/PA6 organo-sheet. By combining measured melt viscosity with an effective permeability extracted from batch benchmarks, a Darcy-type predictor can guide the first round of parameter tuning and help identify whether temperature history, speed, or compaction schedule is limiting impregnation. The demonstrated 6 mm laminate (28 fabric layers) achieves ~55% fiber volume fraction, ~1.4% void content, and flexural properties close to hot-press benchmarks, confirming the feasibility of continuous thick-sheet production. For higher-rate manufacturing, the most direct upgrade is improved thermal delivery (pre-heating and core temperature uniformity), followed by model refinements that capture in-plane flow and compaction-dependent permeability.

References

1. Henry Darcy. Les Fontaines Publiques de la Ville de Dijon. 1856.
2. Ishida, O., Kitada, J., Nunoani, K., Uzawa, K. Continuous Manufacturing Technology of Stampable Thick Organo-sheet Using Fixed Rollers Double Belt Press. SAMPE, 2019.
3. Advani, S. G., & Hsiao, K.-T. Manufacturing Techniques for Polymer Matrix Composites (PMCs). Woodhead/Elsevier (commonly used reference on resin flow and processing models).
4. Campbell, F. C. Manufacturing Processes for Advanced Composites. ASM International (process and defect fundamentals widely used in industry).
5. Mallick, P. K. Fiber-Reinforced Composites: Materials, Manufacturing, and Design. CRC Press (engineering properties and manufacturing context).