Injection molding of LGF-reinforced thermoplastics: numerical and experimental investigations of fibers breakage

In recent years polymer composites are gaining an extraordinary success: the ability to provide high mechanical properties and at the same time low density makes them suitable for a wide range of applications. The employment of these materials at a cost that is becoming more competitive has made them attractive not only for specific sectors, such as aeronautics or automotive for which they were originally developed, but also for mass production of consumer goods. In this context, thermoplastic composite materials are becoming particularly interesting, because they combine the well known physical-mechanical characteristics with low environmental impact, and with all the benefits derived from being suitable for injection molding processing. Indeed, this technology allows the production of large size components, with very complex geometries in low cycle times: it is therefore possible to mold articles with high performance at low cost. The polymer matrix composites most commonly used are the long glass fiber reinforced thermoplastics. This type of reinforcement aim at increasing the improvement of mechanical properties, especially the tensile strength and elastic modulus, given by the presence of the dispersed phase within the matrix; as the load is supported by the fibers, the longer the fibers, the greater the increase in the property itself. In previous studies it was highlighted that the transformation process lead to the breakage of the glass fibers, reducing the benefits given by the presence of the reinforcement. The purpose of this thesis is to analyze how the main characteristics of the process affect the rupture of the fibers, in order to define some general rules that can be used to optimize the process. In this way it is possible to manufacture components with the best obtainable mechanical performances. Using a numerical and experimental approach, different topics have been tackled to achieve this goal: I. It was first analyzed the experimental procedure commonly used for measuring the length of reinforcement fibers, examining its characteristics and critical points. Based on this analysis, an innovative procedure was implemented by using image analysis software, which can overcome the limitations of traditional methodology, and is particularly effective for measuring long fibers. II. Through the use of the Design of Experiments (DOE) techniques, the influence of the main process parameters on the fibers length reduction during injection molding was analyzed. The study was based on an extensive experimental campaign. III. In the same way, using DOE techniques, the influence of the Hot Runner System geometry on fibers degradation was analyzed. The experimental study, conducted in collaboration with a company that is leader in this industry, was carried out using a custom-built modular hot runner system. IV. The flow of the reinforced polymer through the hot runner system has been simulated using advanced finite element software. The numerical approach was designed to identify an analytical correlation model between fluid dynamic variables calculated by numerical simulations and experimental results obtained during the previous tests. V. A procedure to optimize the geometry of a channel, in order to minimize fibers breakage during the flow of the reinforced polymer, has been developed by using a multi-objective optimization software. The optimization procedure, which provides the simultaneous implementation of different applications, was used for an industrial application of a water assisted injection molding of an automotive component. VI. Eventually, the degradation of the fibers in composite materials with high long glass fibers weight fractions was analyzed. The experimental tests lead to new, original results, which were analyzed to propose new hypotheses in support. Specific tools were developed and designed for the verification of these hypotheses. The work presented in thesis was carried out at Te.Si., a laboratory of DIMEG - University of Padua, Italy, from January 2007 to December 2009, under the supervision of prof. Paolo F. Bariani and of ing. Giovanni Lucchetta.