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Technical Document | Study on Permeability of Reinforcement for Composite RTM Process
Jun.09.2026
Resin Transfer Moulding (abbreviated as RTM) is a composite manufacturing process. Liquid thermosetting resin and curing agent are separately pumped out from storage drums by metering equipment (resin pumps), evenly blended via a static mixer, then injected through an injection gun into a sealed mould pre-loaded with glass fibre reinforcement materials. Finished products are obtained after curing, demoulding and post-processing.
The RTM process boasts multiple merits including high molding efficiency, low consumption of raw materials and energy, low investment in equipment and moulds, compatibility with computer-aided design for mould and product development, and minimal emission of volatile substances during molding, which benefits physical health and environmental protection. Therefore, it is widely adopted in aerospace, automobile, electrical and electronic, construction, fitness equipment and other sectors, and has achieved rapid growth in recent years.
Given the relatively short development history of the RTM process, challenges and unresolved issues inevitably remain. The common difficulties at home and abroad fall into three main categories:
- Insufficient impregnation of fibers by resin, leading to high void ratio and dry fiber areas in finished parts;
- Relatively low fiber content in products (typically 50%);
- Unbalanced resin flow inside large-scale, complex-shaped mould cavities during molding, with insufficient predictability and controllability of flow behavior.
Permeability is a physical parameter that describes the flow resistance of fabric or reinforcement to resin.
It characterizes how easily resin flows through porous glass fiber fabric and is a function of porosity.
During the RTM mold-filling process, accurate depiction of resin infiltration properties within reinforcements is critical for optimizing the layout of injection gates and vents in mold design,
shortening production cycles and guaranteeing product quality. Hence, in-depth research into resin permeability to fibers during mold filling, identification of its influencing factors,
and realization of design and control over these factors will greatly boost the industrialization of the RTM process.
Influencing Factors of Fiber Permeability in RTM Process
2.1 Effect of Fiber Volume Content on Permeability
The fiber volume content inside the mold cavity exerts a significant impact on resin permeability of the reinforcement. Permeability decreases markedly as fiber volume content rises. Meanwhile, fiber volume content or porosity greatly affects the quality of molded products.
High fiber volume content (low porosity) creates tighter compaction of fiber mats, facilitating air bubble discharge in the mold cavity and improving product quality. However, elevated fiber volume content or reduced porosity leads to a sharp drop in permeability and extends mold-filling time. Hence, there exists an optimal porosity range for fiber reinforcement, within which fiber wetting performance reaches its peak.
2.2 Effect of Fiber Ply Number on Permeability
With other conditions unchanged, ply number has little influence on fiber stacking state; fiber volume fraction barely varies with ply number under constant pressure.
Ply number imposes a notable effect on plain weave fabric permeability: when fiber volume fraction is below 57%, permeability drops obviously with increasing plies; when above 57%, the influence becomes negligible. For unidirectional and orthogonal layups, ply number has minimal impact on permeability.
2.3 Effect of Fiber Fabric Structure on Permeability
For identical fiber volume content, reinforcements with different fabric structures yield distinct permeability. Research proves that glass fiber mats possess far better permeability than plain woven glass fiber cloth.
Reason: Although large voids exist between warp and weft fiber bundles of woven cloth, the overlapping contact zones of bundles are highly compact, causing a dramatic permeability reduction. Resin can hardly fully infiltrate these overlapping areas during molding, impairing mechanical properties of composite parts. By contrast, glass fiber mats consist of fine fiber bundles arranged across multiple planes without sharply differentiated permeability zones, delivering superior fiber infiltration performance.
2.4 Effect of Stitching Yarn on Permeability
Scholars have conducted quantitative numerical simulations to study how stitching yarn affects the equivalent permeability of flow channels between rectangular fiber bundles. Results reveal that even tiny stitching yarns drastically alter channel equivalent permeability. Therefore, the influence of stitching yarn cannot be ignored when establishing equivalent permeability models for stitched textile composite preforms.
2.5 Effect of Resin Viscosity on Permeability
Resin viscosity ranks as one of the critical factors governing permeability and mold-filling duration. Excessively high viscosity brings high injection pressure, prolonged filling cycles, poor fiber wetting, incomplete cavity filling, and difficult removal of entrapped air. Overly low viscosity easily triggers turbulent flow during filling, trapping massive air bubbles and degrading finished part performance.
Proper resin viscosity must be determined per experimental requirements and ambient conditions to guarantee full fiber wetting and stable product quality. Within a moderate viscosity range, resin viscosity shows no obvious correlation with permeability.
2.6 Effect of Molding Temperature on Permeability
Resin starts curing immediately upon entering the mold cavity during RTM filling. Production demands both shortened curing cycles for higher efficiency and complete fiber impregnation. The core requirement is that resin initiates gelation only after full cavity filling.
Injection temperature modulates resin viscosity, gel time and fiber surface tension. Low injection temperature increases resin viscosity and weakens fiber wetting; excessively high temperature deteriorates resin inherent service performance.
2.7 Effect of Capillary Pressure on Permeability
Capillary pressure is acknowledged as an unignorable variable in permeability testing. Flow during filling splits into two forms: macroscopic inter-bundle flow controlled by filling pressure, and microscopic intra-bundle flow dominated by capillary action.
Darcy’s law is derived from fluid flow in pre-saturated media under external pressure, yet actual RTM filling involves resin flowing over dry fiber bundles, so capillary effects must be accounted for.
2.8 Effect of Vacuum Assistance on Permeability
Vacuum-aided filling enables resin to penetrate voids more easily and improves fiber wetting. Vacuum assistance drastically reduces entrapped air bubbles in RTM molding, effectively boosting product strength, lowering void fraction and elevating overall permeability.
03 Conclusion
As permeability research advances, its inherent complexity becomes increasingly apparent, with numerous unresolved research challenges remaining. To date, analysis of permeability influencing factors mostly stays at a qualitative stage, making quantitative characterization a vital research priority.
- Single-layer fiber reinforcements have been extensively studied with abundant modeling work, yet few theoretical models describe how compression deformation alters fiber volume content and subsequent permeability;
- Permeability through the thickness of planar woven fabrics lacks thorough investigation;
- For multi-layer reinforcements, layup patterns and inter-layer fluid flow rules require deeper exploration;
- Research on permeability changes induced by stitching yarns is scarce, despite wide adoption of stitched preforms for high-performance textile composites;
- Few predictive permeability models exist for 3D textile reinforcements, which see growing application in aerospace manufacturing;
- Calculation standards for fiber content of reinforcements remain unsystematic and unstandardized.
Furthermore, establishing a national permeability database, standardizing permeability measurement methodologies, and deploying online simulation, monitoring and control of RTM filling processes based on accurate permeability data will deliver tremendous momentum to the industrial advancement of RTM technology.
