Numerical and experimental investigation of long glass fiber breakage in hot runner systems

Long glass fiber-reinforced polypropylene composites are widely used in industry because of their low cost and high performance. However the strength of the parts depends on the average fiber length, and the fibers are subjected to breakage during the whole process, in the plasticating phase as well as during the flow through the nozzle and in the cavity of the mold. The aim of this paper is to determine how hot runner systems (HRS) influence the fiber breakage. The type of HRS (open or tip nozzle) and its main geometrical characteristics (e.g. the manifold length and the diameter of the nozzle) could strongly induce a breakage of the fibers during the injection molding process. Experimental tests were conducted with a modular HRS. A polypropylene reinforced with long glass fibers was injected through different configuration of the HRS, maintaining the same process parameters. The flow in the modular HRS channels was also numerically simulated and the fiber breakage was predicted by a coupled fracture model. Eventually, the numerical simulation, calibrated by the experimental results, was used to optimize the geometry of the HRS, thus determining the configuration that retains the optimal fiber length throughout the injection molding process.