Impact of stitching on permeability and mechanical properties of preforms manufactured by dry fiber placement

Dry fiber placement (DFP) is a technology that enables outstandingly high mechanical performance of composite parts, as preform structures with load-related fiber orientations can be created with minimum of fiber crimp and undulation. Yet, the poor impregnation behavior of DFP preforms during liquid composite molding processing methods remains a main challenge. Encouraging an out-of-plane impregnation, for example, by using a vacuum assisted or compression resin transfer molding process, reduces the flow length within the preform and is a first step towards reducing cycle times. However, further improvements are required in order to apply DFP preforming methods in serial production. In a previous work, the influence of various process parameters as well as post-treatment steps on out-of-plane permeability have been investigated with the conclusion, that tufting/stitching led to the highest increase in out-of-plane permeability (more than two decades). For further understanding, this study deals with the influence of a stitching step on the out-of-plane permeability of DFP preforms as well as the mechanical properties of the finished laminate depending on a variety of stitching parameters such as thread tension during stitching, denier of the used thread, stitch distance, seam distance, seam displacement, and treated DFP layup. It is shown that the biggest impact on out-of-plane permeability results from the achieved stitching density (as a product of stitch distance and seam distance) and denier while thread tension and seam displacement show negligible effects. For the mechanical properties, it can be stated that the presence of stitching channels in thickness directions leads to a minor decrease of tensile properties while bending properties are more significantly reduced by about 30%. POLYM. COMPOS., 40:1631-1642, 2019. (c) 2018 Society of Plastics Engineers