The increasing importance of liquid composite molding (LCM) techniques in the composites industry requires the development of adequate methods for characterizing the permeability of reinforcing textiles. This enables the comparison of flow simulations with real impregnation experiments as well as the prediction of the impregnation behavior for industrial applications. For flat fiber reinforcements numerous approaches for one- or multidimensional RTM-based permeability measurements are known. However, permeability characterization of tubular braidings used for infusion-based bladder inflation molding (BIM/RTM) requires the development of an alternative concept.
In this work, a novel approach for measuring the one-dimensional unsaturated in-plane permeability of tubular braided textiles is introduced. The approach involves a robust mechanical setup, an industrial camera system and a fully automated testing procedure with specifically developed digital image processing algorithms. This enables the automatic detection of the advancing fluid flow front during testing at a high level of repeatability. As the impregnation behavior of the braided sleevings predominantly shows unidirectional characteristics, the permeability is evaluated by using the one-dimensional form of Darcy’s equation for steady state flow in a porous medium.
The infusion-based bladder inflation molding process is influenced by various process parameters. The compaction pressure during impregnation, which primarily depends on the ratio between injection pressure and internal bladder pressure, is one of the most crucial parameters because it severely affects both laminate quality and fiber volume content. In this paper, the results of measurements investigating the influence of the compaction pressure as well as the interdependence of bladder and injection pressure on the in-plane permeability of a biaxial braided carbon sleeving are presented. The analysis shows that an increasing compaction pressure significantly reduces the textile permeability, while a minimum compaction of the preform should be maintained to avoid unwanted effects such as fiber washout and overfilling of the preform.