Effect of screw geometry on long glass fiber breakage during injection molding

The aim of this work is to compare the effects of two different 40 mm screws on fiber breakage during plastication. A polypropylene, reinforced with 30% long glass fibers, was tested. A standard three zone screw and a special profile screw were compared. The special screw has low compression ratio, a dynamic mixer and it is tapered over the whole length. The pitch is variable, increasing from the feeding zone to the screw front; the combination of these features gives higher productivity with better melt homogeneity and a very consistent dosing time end cushion. The plastication was performed at two different process conditions and the screw pull-out technique was used. The fibers length was measured at different distances from the hopper after the matrix pyrolysis. The experiments show that the different geometry of the screws, within the selected conditions, did not affect the fiber breakage. Residence time and aspect ratio of pellet length to channel width and length were also considered. A procedure was developed and validated in order to allow an accurate comparative analysis of the fiber breakage progression during plastication.