Development of a direct process for the production of long glass fiber reinforced phenolic resins

Parts made from phenolic molding compounds reinforced with glass fibers have a good heat resistance, a high dimensional accuracy, and an excellent chemical resistance. However, these parts typically have a low elongation at break and a low impact toughness. Conventional glass fiber-reinforced phenolic molding compounds have an average fiber length which is below the critical fiber length for this material combination. To increase the fiber length in the molded parts, a new injection molding process was developed, which allows the direct feeding of long glass fibers into the plasticizing unit of the injection molding machine. For opening and dispersing long fiber bundles, a thermoset-specific screw mixing element was developed and compared to the conventional conveying screw geometry. The process stability was quantified by analyzing the plasticizing work and injection work. Despite the increased fiber shortening when using the mixing element, the dynamical mechanical properties were improved compared to the standard conveying screw geometry, which is attributed to the better fiber-matrix homogenization.