Experimental investigation of stamp forming of unconsolidated commingled E-glass/polypropylene fabrics

Stamp forming of two unconsolidated commingled E-glass fiber/polypropylene fabric composites with nominal weights of 743 and 1485 g/m² has been studied for simple mold geometry. For this manufacturing process, unconsolidated commingled fabrics are transferred directly from an oven to a press where they are stamped using a matched-dies metal mold to achieve simultaneous consolidation and conformation to the mold shape. In this study, the influence of the stamping parameters such as the stamping pressure, stamping temperature, mold temperature, loading rate and holding time are determined on the flexural properties, void content and void distribution. Results obtained for the stamp forming process of the unconsolidated fabrics were compared with results obtained by compression molding of the unconsolidated fabrics and by stamping pre-consolidated fabrics. For the fabric with the higher nominal weight, the flexural properties were found to be lower than the optimal properties, while for the fabric with the lower nominal weight the flexural properties were equivalent to the optimal properties determined by compression molding. Good correspondence was found between the variation of the flexural properties and the variation of the void content. This allows the mechanical properties to be approximated by only measuring the void content. Finally, the minimum temperature at which the stamping pressure has to be applied in order to successfully process the unconsolidated fabrics is determined.