Liquid Composite Molding (LCM) is a process group suitable for serial production of components made of fiber reinforced polymers (FRP). This is especially true for LCM process variants providing out-of-plane impregnation (perpendicular to the textile layers), such as Wet Compression Molding, which is e. g. used for automated production in automotive industry (Figure 1). For an efficient process design, detailed material knowledge is required, whereby the permeability of textile reinforcements is of particular importance. The permeability describes the conductance of porous materials for liquid flow and is direction dependent for textiles. The out‑of‑plane permeability is especially relevant for impregnation in out‑of‑plane direction. In a German research foundation (DFG) funded project, Institut für Verbundwerkstoffe (IVW) has developed a novel, particularly robust measuring system for determining out-of-plane permeability. In order to understand the relevance of this development, the specific characteristics of the out-of-plane permeability measurement have to be considered. To measure the out-of-plane permeability, a flow under controlled conditions is generated in the textile. For reasons of simplicity, current measuring systems are predominantly based on saturated measurement, i. e. a measurement liquid continuously flows through the sample. However, this does not correspond to real LCM process conditions, in which textile material is only impregnated during production. Therefore, a corresponding, unsaturated measurement provides more realistic values, but requires monitoring of the flow front - a non-trivial challenge since direct observation of the flow within the textile is not possible. The novel developed measurement system (Fig. 1) solves this challenge by using ultrasonic technology. Another difficulty arises from the complex porous structure of the textile. On the one hand, the flow channels between the fiber ranges in micrometers on the other hand, there can be gaps within millimeters between the rovings. Furthermore, the structure is characterized by a high statistical variance which leads to inhomogeneous local areas. In order to achieve a high robustness against inhomogeneous local areas, the measurements are performed in such a way that a quasi one-dimensional flow in thickness direction is achieved over a planar inlet (about 150 cm²). A sophisticated edge sealing system prevents measurement errors caused by sample leakage. A further challenge is the deformability of the textile stack. This leads to so-called hydrodynamic compacting, i.e. the compression of the textile stack due to injection pressure. Since this influence cannot be prevented and - focusing on reproducing realistic process effects - is not intended. Hence, the hydrodynamic compaction is monitored by a combination of displacement sensors and ultrasonic measurement technology, which ensures a monitoring to the level of single layer displacement.
After performing the unsaturated measurement, the saturated permeability can be determined with the measuring system on exactly the same sample. This way possible deviations between a saturated and unsaturated measurement can be defined (Fig. 2 shows exemplary results). This information can be important for liquid impregnation processes in which saturated and unsaturated flows occur equally.
In this project basic scientific knowledge of the impregnation behavior of reinforcement textiles will be gained, which can be used to design a more efficient resin injection process.
The projects „Measurement and modeling of unsaturated out-of-plane permeability of engineering textiles” and “Measurement and modeling of unsaturated out-of-plane permeability of engineering textiles” are supported by Deutsche Forschungsgemeinschaft (DFG) (Funding reference Mi 647/31-1 and Mi 647/31-2). We would also like to thank ETH-Zurich for the good cooperation in this project.
M.Sc. Björn Willenbacher
Institut für Verbundwerkstoffe GmbH
Tel.: +49 631 31607 420