Abstract
Hot-press molding of glass-fiber-reinforced polypropylene (GFPP) laminates was monitored using embedded fiber Bragg gratings (FBGs) in unidirectional laminates. These sensors allowed the monitoring of process-induced strains. Because the cooling phase is critical in thermoplastics manufacturing, affecting crystallinity and ultimately residual strain, two metallic molds with different heat diffusion capacities were used (steel and aluminum). The optical sensors proved to efficiently characterize some material properties; for example, strain variations could be related to physical changes of the laminate, revealing key transition points such as the onset of melt or solidification. After the GFPP plates were released from the mold, residual strains were estimated. The longitudinal behavior is controlled by the behavior of the glass fibers. The transverse residual strain of the laminate made with the aluminum mold is higher than that of the laminate made with the steel mold. The thermal expansion behavior was investigated during a post-process heating procedure. A particular behavior was observed at the first heating ramp for the laminate made with the steel mold, indicating that the relatively poor thermal diffusion had a significant influence on the crystalline microstructure. The thermal expansion coefficient is higher for the aluminum mold made laminate. A DSC (Differential Scanning Calorimetry) of both materials showed a similar degree of crystallinity after processing and a similar increase after the heat treatment. This suggested that the difference of thermal expansion properties is related to the initial strain state of the laminates.