Abstract

Carbon fiber reinforced carbon-plastic bipolar plates represent a convergence of polymer processing technology and carbon-based composite science, offering a viable path toward lightweight, corrosion-resistant, and scalable electrochemical cell components. This article provides a comprehensive technical analysis of their material composition, manufacturing considerations, electrochemical performance characteristics, and integration behavior within fuel cell and flow battery stacks. Rather than examining the bipolar plate in isolation, this discussion situates the component within the broader system architecture—addressing how formulation choices propagate through the stack assembly and ultimately affect device-level reliability and service life. Both the inherent strengths and unresolved engineering challenges of this material class are discussed with equal weight, providing a basis for informed selection and deployment decisions.

Target applications addressed include proton exchange membrane (PEM) fuel cell stacks, hydrogen electrolyzers, and vanadium redox flow batteries (VRFBs), each of which places distinct and sometimes competing demands on bipolar plate properties.

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1. Role of the Bipolar Plate in Electrochemical Systems

1.1 Functional Position within the Stack

Within any electrochemical cell stack—whether a fuel cell, electrolyzer, or flow battery—the bipolar plate (also referred to as a flow field plate or separator plate) performs a set of simultaneously demanding functions. It must electrically connect adjacent cells in series, distribute reactant gases or electrolyte uniformly across the active electrode area, manage water or electrolyte transport, provide structural rigidity to the stack, and in most configurations also serve as a thermal management conduit. These functions are not independent: optimizing one often constrains another. For example, increasing resin content to reduce gas permeability tends to reduce electrical conductivity; increasing fiber loading to raise conductivity can compromise impact toughness.

The bipolar plate typically accounts for 60–80% of the total stack mass and 30–50% of the total stack volume in PEM fuel cell assemblies, depending on stack design and active area. This makes material and geometry decisions at the bipolar plate level disproportionately influential on system-level gravimetric and volumetric power density. In stationary and transport applications alike, these metrics matter—not only for packaging and deployment but also for total cost of ownership as raw material inputs scale with mass.

1.2 Material Classes in Context

Historically, the bipolar plate design space has been divided among several material families: machined or molded graphite, stamped metallic plates (stainless steel, titanium, or coated aluminum), expanded graphite composites, and various polymer-based composites. Each class presents a different performance profile, cost structure, and manufacturing trajectory.

Carbon fiber reinforced carbon-plastic composites occupy a distinct position in this landscape. They borrow from the high electrical conductivity and corrosion resistance of graphitic carbon while incorporating a polymer matrix that enables net-shape processing and tunable mechanical properties. Understanding their advantages and limitations requires understanding not just the material in isolation but how it interfaces with the membrane electrode assembly (MEA), gaskets, end plates, and current collector components that make up the complete stack system.

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Table 1: Comparative Property Overview of Major Bipolar Plate Material Classes

Property	Graphite	Metallic	Carbon-Plastic (CF-reinforced)	Pure Polymer	Expanded Graphite
Electrical conductivity	Very high	High	Moderate to high	Low	High
Bulk density (g/cm³)	1.8–2.1	7.9–8.1 (SS)	1.3–1.7	1.0–1.2	0.5–1.2
Corrosion resistance	Excellent	Requires coating	Good–Excellent	Excellent	Good
Mechanical strength	Brittle	Excellent	Good	Moderate	Moderate
Machinability / formability	Difficult, brittle	Stamping feasible	Compression molding	Injection molding	Die cutting
Thermal conductivity (W/m·K)	80–150	15–25 (SS)	10–60 (direction dependent)	0.2–0.5	150–300
Gas permeability	Very low	None	Very low	Moderate	Low
Manufacturing scalability	Low	High	Medium–High	High	Medium
Relative cost index	High	Medium	Medium	Low–Medium	Medium

Values are indicative ranges; actual figures depend on specific formulation, processing conditions, and test methodology.

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2. Material Composition and Microstructure

2.1 Carbon Fiber Types and Their Influence on Plate Properties

The selection of carbon fiber type is among the most consequential decisions in formulating a carbon-plastic bipolar plate. Carbon fibers used in this context are broadly categorized by their precursor material—most commonly polyacrylonitrile (PAN)-based fibers—and by their microstructural orientation, which spans a spectrum from highly turbostratic to near-graphitic crystallinity.

Short carbon fibers (typically 50–500 µm in length after compounding) are the predominant form used in compression-molded and injection-molded plates. Their primary advantage is their compatibility with thermoplastic and thermoset compounding processes that allow bulk mixing with graphite powders, conductive carbon blacks, and resin systems. However, short fibers offer limited enhancement to through-plane electrical conductivity because their random orientation in the molded part results in isotropic, but moderately conductive, networks rather than aligned conductive pathways.

Long or continuous fiber reinforcement enables significantly higher in-plane stiffness and, in specific configurations, improved in-plane electrical conductivity, but introduces complexity in flow field forming and requires specialized lay-up or filament winding processes. For most bipolar plate applications, short-to-medium fiber formats remain preferred for their processing flexibility.

The surface chemistry of the carbon fiber, particularly the presence of functional groups introduced by fiber surface treatment (sizing), affects adhesion to the polymer matrix. Poor interfacial bonding leads to microcracking under compressive cycling, which can degrade both mechanical integrity and electrical contact resistance over time. Proper fiber-matrix interfacial engineering is therefore a critical aspect of composite formulation for long-service electrochemical applications.

2.2 Polymer Matrix Selection

The polymer matrix in a carbon-plastic bipolar plate serves as the binder phase that holds the composite together, controls gas permeability, and defines the processing route. Matrix selection is guided by several competing requirements: chemical stability in the electrochemical environment, processability at acceptable temperatures and pressures, compatibility with the conductive filler network, and thermal performance over the anticipated operating range.

Thermoset matrices—primarily phenolic resins, epoxy resins, vinyl ester resins, and furan resins—have historically dominated bipolar plate formulations for PEM fuel cells. Phenolic resins in particular offer a favorable balance of chemical inertness, dimensional stability under compression, and compatibility with high-volume compression molding. Furan resins, though more difficult to process, provide enhanced resistance to the acidic environment inside a PEM cell at elevated temperatures. The cross-linked network structure of thermosets also limits gas permeation more effectively than uncrosslinked thermoplastics, which is advantageous for preventing hydrogen crossover.

Thermoplastic matrices—including polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVDF), and high-performance variants such as polyphenylene sulfide (PPS) and polyether ether ketone (PEEK)—offer different advantages. Recyclability, reprocessability, and in some cases better impact toughness make thermoplastic-based composites attractive where end-of-life material recovery is a design objective. PVDF and PPS in particular provide excellent chemical resistance to sulfuric acid environments that may be encountered in PEM cells or vanadium-based flow batteries. However, achieving sufficiently high electrical conductivity with thermoplastic matrices requires careful percolation threshold management: the filler loading must cross the conductive network threshold without becoming so high that it compromises the melt flow behavior during injection or compression molding.

2.3 Conductive Filler Architecture

In most carbon-plastic bipolar plate formulations, carbon fibers alone do not provide adequate bulk electrical conductivity. A hybrid filler architecture is therefore common, combining carbon fibers with one or more secondary conductive phases. The most widely used secondary fillers include synthetic graphite powders (primary contributor to in-plane conductivity), carbon black or acetylene black (which forms inter-particle bridges that support fiber-to-fiber electron transport), and in some advanced formulations, expanded graphite flakes that create high-aspect-ratio conductive pathways.

The interactions among these filler components are complex. Carbon black agglomeration within the polymer matrix can reduce the effective volume of the conductive network while simultaneously introducing localized stress concentrations. Graphite powder particle size distribution affects both packing efficiency and the surface contact quality at interfaces. The relative proportion of each filler type must be optimized to simultaneously satisfy conductivity targets, meet gas permeability limits, maintain processability, and preserve adequate mechanical strength. This multi-parameter optimization is a core challenge in carbon-plastic bipolar plate development.

The resulting composite microstructure is heterogeneous at the microscale: carbon fibers provide backbone reinforcement and medium-range conductivity paths; graphite particles fill inter-fiber spaces and contribute to a continuous conductive network; and carbon black particles bridge submicron gaps between larger filler particles. The polymer matrix envelops this network, providing binding, sealing, and load transfer. Understanding this microstructure is essential for interpreting performance data and for predicting long-term behavior under thermal cycling and electrochemical loading.

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3. Advantages of Carbon Fiber Reinforced Carbon-Plastic Bipolar Plates

3.1 Low Density and Gravimetric Efficiency

One of the most practically significant attributes of carbon-plastic bipolar plates is their low bulk density, which typically ranges from 1.3 to 1.7 g/cm³ depending on the specific resin and filler combination used. This compares favorably with metallic alternatives (stainless steel: ~7.9 g/cm³; titanium: ~4.5 g/cm³) and is broadly comparable to pure graphite (1.8–2.1 g/cm³) while offering improved mechanical toughness relative to machined graphite.

At the stack level, the weight reduction achieved by using carbon-plastic plates instead of metallic plates can be substantial. For a 100-cell PEM fuel cell stack with 200 cm² active area per cell, the difference in bipolar plate mass between a metallic and a carbon-plastic design can exceed 10–15 kg—a meaningful contribution to system-level specific power (kW/kg) for transportation and portable power applications. In grid-scale flow battery installations, where hundreds of cells may be arrayed in a single stack module, the cumulative weight reduction from composite plates simplifies structural support design and reduces installation complexity.

This gravimetric advantage also has secondary effects. Lighter stacks impose lower mechanical loads on compression hardware, reduce vibration-induced fatigue stress in mobile applications, and simplify handling during assembly and maintenance. The benefit propagates through the system design in ways that pure material property comparisons do not fully capture.

3.2 Corrosion Resistance in Acidic Environments

Carbon-plastic bipolar plates demonstrate inherent electrochemical stability in the acidic, humidified environments characteristic of PEM fuel cells and PEM electrolyzers. The carbon-based filler phases—graphite, carbon fiber, and carbon black—are thermodynamically stable under typical PEM operating conditions (pH 2–4, 60–80 °C, in the presence of fluoride ions from membrane degradation byproducts). The polymer matrix, provided it is selected from chemically inert resin systems, adds a passivation layer that further limits ionic leaching.

In contrast, metallic bipolar plates, even those fabricated from austenitic stainless steels or titanium alloys, are susceptible to surface oxidation and ion release under the combined effect of humidity, elevated temperature, and electrochemical potential. Metal ion contamination—particularly iron, chromium, and nickel ions from stainless steel—is a well-documented mechanism of membrane and catalyst layer degradation in PEM fuel cells, reducing proton conductivity and catalyst activity over time. Carbon-plastic composites, by their nature, do not introduce these ionic species into the cell environment.

For vanadium redox flow batteries, the chemical environment is even more aggressive: the electrolyte contains concentrated sulfuric acid (typically 1.5–2 M H₂SO₄) and vanadium ions in multiple oxidation states, including the strongly oxidizing V(V) species present at the positive electrode. Carbon-plastic plates based on PVDF or PPS matrices show good stability in this environment, with minimal matrix dissolution and acceptable carbon phase stability over extended cycling.

3.3 Near-Net-Shape Processing and Manufacturing Flexibility

The ability to form carbon-plastic bipolar plates by compression molding or injection molding into near-net-shape parts with integrated flow field channels is a manufacturing advantage that differentiates this material class from both machined graphite and some metallic options. Machined graphite requires stock material production followed by time-consuming multi-axis milling or grinding to define flow channels—a process that is inherently slow, generates significant graphite waste, and scales poorly beyond research and small-volume production contexts.

Compression molding of carbon-plastic compounds, by contrast, can produce a complete bipolar plate—including serpentine, parallel, or interdigitated flow field geometry—in a single press cycle of 2–10 minutes. The mold geometry directly defines the channel dimensions, landing widths, and inlet/outlet manifold features without secondary machining. This near-net-shape capability reduces material waste, shortens cycle time, and enables geometric complexity that would be cost-prohibitive in machined materials.

For high-volume production scenarios—such as automotive PEM fuel cell stacks where tens of thousands of plates may be required annually—compression molding of carbon-plastic compounds can be adapted to multi-cavity tooling and automated material handling systems. While cycle times for thermoset systems are longer than for thermoplastic injection molding, the achievable part quality and flow field fidelity with thermoset compression molding are generally superior for thin-wall plates with high-aspect-ratio channel features.

3.4 Tunable Electrical and Thermal Properties

Unlike monolithic graphite or metallic plates, carbon-plastic composites offer formulation latitude to adjust electrical conductivity, thermal conductivity, and mechanical stiffness by varying the type and proportion of conductive fillers. This tunability is a meaningful engineering advantage when designing for specific application requirements.

For instance, a flow battery bipolar plate prioritizing corrosion resistance and dimensional stability at the expense of peak electrical conductivity can be formulated with a higher polymer matrix fraction and moderate fiber loading. Conversely, a high-power-density PEM fuel cell application may warrant a higher graphite and carbon fiber content to minimize ohmic losses at high current densities, accepting some trade-off in gas permeability margin. This formulation flexibility—absent in metallic plates and constrained in pure graphite—allows carbon-plastic bipolar plates to be positioned across a range of applications without fundamental material platform changes.

Thermal conductivity in the in-plane direction, which governs heat removal from the active area to the stack cooling channels, can be enhanced by incorporating high-conductivity graphite flakes or by aligning short fibers during the molding process. This directional thermal management capability is important for maintaining temperature uniformity across large active areas, a factor that becomes increasingly critical as cell sizes increase for electrolysis and stationary storage applications.

3.5 Low Gas Permeability

Gas crossover through the bipolar plate—migration of hydrogen from the anode side to the cathode side, or oxygen in the reverse direction—represents a safety and efficiency concern in PEM fuel cells and hydrogen electrolyzers. Carbon-plastic bipolar plates, when properly formulated and molded, achieve bulk hydrogen permeability values well below the threshold specifications typically used in fuel cell design standards. The polymer matrix phase, which is largely impermeable to hydrogen, acts as the primary barrier, while the carbon filler network provides conductive pathways through the composite without forming connected macroscopic pores.

This low permeability is achievable across the range of molding processes applicable to carbon-plastic composites. Proper process control—particularly mold temperature, applied pressure, and resin cure profile for thermosets—is necessary to minimize void content in the finished plate. Voids or incomplete consolidation are the primary causes of elevated gas permeability in composite plates and can originate from volatile evolution during cure, insufficient mold closure, or inadequate material flow into thin channel regions. Quality control by helium or hydrogen leak testing of finished plates is standard practice in production environments.

3.6 Compatibility with Multiple Electrochemical Architectures

Carbon-plastic bipolar plates are not confined to a single device type. With appropriate formulation adjustment for chemical environment compatibility, they are applicable to PEM fuel cells, PEM water electrolyzers, alkaline electrolyzers (with suitable polymer matrix selection), and redox flow battery stacks. This application breadth is commercially relevant for component suppliers and for end users developing multi-technology energy portfolios.

In redox flow batteries, bipolar plates perform the additional function of ionic isolation: preventing electrolyte mixing between the positive and negative half-cells. The sealing provided by the polymer matrix phase—both within the plate body and at the gasket-to-plate interface—is important for long-term stack integrity in systems that may operate for thousands of cycles over 10–20 year lifetimes.

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4. Disadvantages and Engineering Challenges

4.1 Electrical Conductivity Below Metallic and Pure Graphite References

The primary performance limitation of carbon-plastic bipolar plates is their electrical conductivity, which, while acceptable for many applications, remains lower than that of pure graphite or metallic plates. Typical in-plane bulk resistivity values for carbon-plastic composites fall in the range of 5–50 mΩ·cm, compared to 0.5–2 mΩ·cm for dense machined graphite and sub-0.1 mΩ·cm for metallic materials. Through-plane resistivity, which is the more operationally critical direction for bipolar plate performance, is generally higher still, owing to the preferential in-plane orientation of flat graphite particles and carbon fibers during molding.

In high-current-density applications—such as electrolyzers operating above 2 A/cm² or high-power automotive fuel cells—this elevated ohmic resistance manifests as measurable voltage loss across the bipolar plate, reducing system efficiency. The contact resistance between the bipolar plate surface and the gas diffusion layer (GDL) or porous transport layer (PTL) contributes additionally to this ohmic budget and is strongly influenced by surface finish quality, landing width geometry, and assembly clamping pressure.

Achieving low and stable contact resistance over the service life of the stack is a known challenge for carbon-plastic composites. The polymer-rich surface regions of a compression-molded plate may exhibit higher resistivity than the bulk material due to resin-rich surface layers that form during molding. Surface treatment processes—such as controlled abrasion, plasma treatment, or thin carbon coatings—are sometimes employed to reduce surface resistivity, but each introduces additional process complexity and cost.

4.2 Thermal Conductivity Anisotropy and Through-Plane Limitations

Thermal management in electrochemical stacks depends critically on the through-plane thermal conductivity of the bipolar plate, which governs heat transfer from the active reaction zone to the coolant channels integrated into the plate structure. In carbon-plastic composites, through-plane thermal conductivity is typically 10–20 W/(m·K) for well-formulated systems, compared to values of 100–150 W/(m·K) for machined graphite in the same direction and 15–25 W/(m·K) for austenitic stainless steel.

While the absolute value for carbon-plastic composites is not necessarily inadequate for moderate power densities, the anisotropic nature of the thermal conductivity—where in-plane conductivity may be two to five times higher than through-plane due to particle and fiber orientation—introduces asymmetry in the heat flux paths within the stack. At high power densities, this can result in elevated temperature gradients across the thickness of the active area, potentially contributing to membrane dryout at the anode or flooding at the cathode in PEM fuel cells.

Addressing through-plane thermal conductivity limitations requires either the use of high-conductivity filler materials with favorable out-of-plane orientation (difficult to achieve in standard compression molding) or system-level thermal management design that accommodates the lower plate conductivity through more densely distributed coolant channels or active cooling architectures.

4.3 Mechanical Behavior Under Freeze-Thaw and Thermal Cycling

Carbon-plastic bipolar plates based on thermoset matrices generally exhibit brittle fracture behavior under impact or bending loads. While their compressive strength is adequate for typical stack clamping pressures, their resistance to tensile cracking and delamination under thermal cycling conditions is lower than that of metallic alternatives. This becomes particularly relevant in automotive fuel cell applications, where the stack must survive multiple freeze-thaw cycles (operating environment: -40 °C to +80 °C and above) over the vehicle lifetime without developing cracks that compromise gas sealing or structural integrity.

During freezing, water retained in the flow field channels and GDL pores expands volumetrically. If the bipolar plate material cannot accommodate the associated stress—either by elastic compliance or by controlled microcracking without loss of hermeticity—seal integrity may be compromised. Thermoset-based composites have limited elongation to failure, typically less than 1–2%, which constrains their ability to absorb freeze-thaw stress without cracking. Thermoplastic-based carbon-plastic composites generally offer better fracture toughness in this regard, but may sacrifice some chemical stability and dimensional stability at elevated temperature.

Long-term cyclic mechanical loading, even at relatively low stress amplitudes, can lead to progressive interfacial degradation at the fiber-matrix interface within the composite. This manifests as a gradual increase in contact resistance and potentially as subtle changes in flow field channel geometry due to creep, particularly in phenolic-based systems at temperatures above 80 °C.

4.4 Anisotropy from Fiber Orientation

The electrical and mechanical properties of carbon-plastic bipolar plates are inherently directionally dependent due to the preferential orientation of short carbon fibers during molding flow. In compression molding, fibers tend to align parallel to the plate surface (in-plane), resulting in higher in-plane conductivity and lower through-plane conductivity. In injection molding, fibers may show more complex orientation distributions dictated by the flow front geometry, leading to property gradients across the plate that can be difficult to predict without dedicated process simulation.

This orientation-induced anisotropy is not inherently problematic—for in-plane heat spreading and in-plane electrical transport, it can be beneficial. However, it introduces variability in through-plane properties, and in large-format plates (>400 cm² active area), achieving uniform fiber distribution and orientation across the entire plate face requires careful attention to gate placement, mold filling simulation, and compound rheology. Non-uniformity in fiber distribution translates directly into non-uniformity in electrical resistance, which manifests as uneven current density distribution across the active area—a factor that accelerates localized catalyst and membrane degradation.

4.5 Long-Term Contact Resistance Stability

The contact resistance between a bipolar plate and the adjacent porous transport layer (carbon paper, carbon cloth, or sintered titanium felt in electrolyzers) is a dynamic rather than static property. It evolves with operating time, stack clamping force distribution, temperature history, and electrochemical environment. In carbon-plastic composites, the primary concern is surface oxidation of the carbon phase under the electrochemical potential and temperature conditions of operation, which can progressively increase surface resistivity.

At the cathode of a PEM fuel cell, carbon oxidation is thermodynamically favored at operating potentials above approximately 0.7 V, a condition that occurs during start-up and shutdown transients as well as during open-circuit hold periods. While the polymer matrix phase provides some barrier to oxidative attack, the exposed carbon fillers at the plate surface are susceptible. Over thousands of operating hours, this can result in measurable increases in interfacial resistance, contributing to performance degradation that is difficult to separate from membrane or catalyst degradation during field diagnostics.

In flow battery applications, the electrochemical potential window is generally less extreme than in PEM fuel cells, but the continuous contact with vanadium electrolyte introduces a different oxidative pathway, particularly at the positive electrode half-cell. Carbon fiber and graphite surfaces can catalyze vanadium ion oxidation and reduction reactions, which may alter surface chemistry over long-term cycling.

4.6 High-Temperature Operation Constraints

Increasing the operating temperature of PEM fuel cells above 100 °C—a strategy pursued to improve CO tolerance of platinum-group metal catalysts and to simplify water management by enabling operation without liquid water condensation—places additional demands on bipolar plate materials. Conventional phenolic or epoxy-based carbon-plastic composites may experience matrix softening, accelerated hydrolysis, or increased gas permeability at temperatures approaching 120–160 °C, the range targeted by high-temperature PEM (HT-PEM) designs using phosphoric acid-doped polybenzimidazole (PBI) membranes.

For HT-PEM applications, the polymer matrix must maintain dimensional stability and chemical resistance in the presence of phosphoric acid vapors at elevated temperatures, which eliminates many standard thermoset systems. Specialty high-temperature thermoplastics such as PEEK or modified polyphenylsulfone (PPSU) offer better thermal stability but introduce significant formulation and processing complexity, and their cost is substantially higher than commodity thermoset systems.

4.7 Recycling and End-of-Life Considerations

Carbon-plastic bipolar plates based on thermoset matrices present end-of-life challenges that are not present for metallic plates. Metallic plates can be recovered and recycled through established scrap metal processing streams. Thermoset composites, by contrast, cannot be remelted and reprocessed due to their cross-linked molecular network. Current options for thermoset carbon composite recycling include mechanical grinding (yielding low-value filler material), pyrolysis (recovering carbon fibers of reduced quality), and solvolysis (chemical decomposition of the matrix, recovering higher-quality fibers but at higher process cost and energy input).

As regulatory frameworks governing battery and fuel cell system end-of-life management develop in major markets, the recyclability of bipolar plate materials may become a selection criterion. Thermoplastic-based carbon-plastic composites offer a partial solution, as the matrix phase can in principle be remelted and reprocessed, though recovering the full composite for reuse as bipolar plate material remains technically demanding.

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5. Manufacturing Process Considerations

5.1 Compression Molding

Compression molding is the most widely used manufacturing process for thermoset-based carbon-plastic bipolar plates. In this process, a pre-weighed charge of compound—typically a bulk molding compound (BMC) or sheet molding compound (SMC) containing carbon fibers, graphite powder, resin, and process additives—is placed in the open mold cavity and compressed under controlled temperature and pressure to achieve resin flow, consolidation, and curing.

The process variables critical to plate quality include mold temperature (typically 150–180 °C for phenolic systems), applied pressure (commonly 5–20 MPa for thin plates), cure dwell time, mold surface finish, and compound flow characteristics. Mold release agent management is important to avoid surface contamination that can impair subsequent bonding or surface treatment steps. Plate-to-plate repeatability in electrical resistance, thickness uniformity, and flow channel fidelity are monitored in production as key process indicators.

5.2 Injection and Transfer Molding

Injection molding, applicable primarily to short-fiber thermoplastic composites, offers shorter cycle times than compression molding and is better suited to high-volume production of smaller-format plates. However, the injection process subjects the compound to high shear rates during flow, which can break down fiber length and disrupt