A methodology for determining preform compaction in bladder-assisted resin transfer molding with elastomeric bladders for tubular composite parts

A common approach for the manufacturing of hollow composite parts based on textile reinforcement materials is the utilization of bladder-assisted resin transfer molding (BARTM). Here, the process-induced compaction of the preform is a decisive factor in the injection stage as it significantly influences filling times and part qualities. However, the use of expandable elastomeric bladders impedes the determination of local compaction pressures and thicknesses of compliant preforms during BARTM. This paper therefore presents an efficient methodology for evaluating the compaction state of tubular fabrics during preform compaction and subsequent resin injection by considering the membrane stiffness of an elastomeric bladder as well as the compressibility of the textile preform. First, different process models are developed to describe preform compaction based on single-point and full preform compaction data. The acquisition of exemplary model data for bladder expansion and preform compaction is accomplished through experimental methods. A specifically developed test rig comprising optical measurement techniques is used to directly characterize the radial expansion behavior of tubular silicone rubber bladders. The compaction behavior of single- and multi-layered braided preforms is evaluated by means of dry compression experiments. The resulting measurement data is used to create an integral model-based process window for combined bladder expansion and preform compaction. Lastly, a prediction of relevant local compaction pressures and preform thicknesses is conducted for an exemplary BARTM process.