Liquid composite molding of tackified fiber reinforcement: Performing and void removal

Both preforming and void removal have been studied in order to study the liquid composite molding process. Tackified textile fiber preforms are used widely in resin transfer molding (RTM) to produce aerospace-grade composite parts. To optimize the preforming process, a thorough understanding of many important material and process issues, such as fabric formability, preform dimension control, preform permeability, fiber wetting, and tackifier compatibility with the matrix resin, is essential. These issues are investigated based on a commercial tackifier (PT 500 from 3M). The results show that tackifier concentration, tackifier powder size and the application conditions all play important roles in governing the moldability of tackified fiber preforms. When applying the PT 500 tackifier onto the fabric, the location of the tackifier should be outside the fiber tows in order to achieve better preform compaction and interlayer adhesion. This can be achieved by applying and preforming the tackifier and the tackified fabrics at low temperatures (e.g. less than 82°C). The powder size between 106--250 mum seems to be a good choice because it provides good preforming characteristics and is easier to handle. It is also found that the AS4-6k, 5 harness fabric has much less springback than the IM7-12k, 5 harness fabric during preforming because its sizing functions like a tackifier.; The location of the tackifier in the preform affects the preform permeability. If most of the tackifier stays inside the fiber tows, preform permeability may increase greatly. This can be achieved by quickly heating the tackified preform to a higher temperature (e.g. 160°C) before mold filling. If the tackifier in the preform remains ungelled before heating, the high temperature would reduce its viscosity significantly and the capillary force may pull the tackifier into the fiber tows. Tackifier inside the fiber tows, however, tends to cause in more severe void formation problem.; It is found that a fiber preform with tackifier tends to have more severe void formation problem during mold filling than that without any tackifier. Voids are detrimental to both the surface quality and the mechanical properties of the molded composites. To minimize the void problem, a thorough understanding of void formation and removal in the molding process is need. The void removal mechanism has been studied both by observing the phenomena by flow visualization in real fiber reinforcement and systematic study in microfluidic channels. Photolithography technique is used in fabricating rectangular flow channels in micron size. Both the bubble mobility and the snap-off of bubbles are studied. It is found that the mobility of the bubble is can be correlated by the Young-Laplace equation. The time required for bubble snap off is governed by the tube capillary number based on the gas velocity by an order of -0.64.