Wet Compression Molding of Biocomposites for a Transportation Industry Application

The transportation and automotive industries are slowly integrating biocomposite materials into products where the economics make sense; this typically means a short manufacturing cycle time, not using expensive prepreg, and with little waste generated from the process. In a previous investigation into the use of biocomposites for electric bus seats and backs, three different material systems (hemp, flax, and pure cellulosic fibers, each paired with a high-bio-content epoxy) and two manufacturing processes (wet layup followed by compression molding, vacuum-assisted resin transfer molding) were investigated, but neither process proved to be viable. In this paper, a relatively obscure process called Wet Compression Molding (WCM) is considered for economical production of the biocomposite bus seats using the same three material systems. Darcy’s law predictions of full impregnation time for a nominally 3.5 mm thick part using experimentally determined permeability values are all less than 2 s. Furthermore, prepreg is not used, and net-shape parts without excess resin show potential. Important design details of the WCM mold set, used in the manufacturing of flat test panels from each material system, that are generally not discussed in the literature include a high-pressure O-ring seal, and semi-permeable membranes covering injection pins and vacuum vents (evacuates trapped air) to prevent resin ingress. Biocomposite laminate specimens are fabricated using the mold set in a thermal press and a vacuum pump. Part characterization includes fiber volume fraction estimates and measurements of thickness, density, flexural modulus, and outer fiber maximum stress at failure. Due to its rapid impregnation with just enough resin, WCM should be considered for the economical manufacture of parts similar in shape and size to electric bus seats and backs.