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

provide high mechanical properties and in the same time low density makes them suitable for a wide range of applications. The change to adopting 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, particularly interesting become thermoplastic composite materials, because they combine the cited physical-mechanical characteristics with low environmental impact, and with all the benefits derived from being processed by injection molding. Indeed, this technology allows to product components of large size, 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 try to increase the improvement of mechanical properties, especially the tension strength and elastic modulus, given by the presence of the dispersed phase within the matrix; as the load is supported by the fibers, the greater is fibers length, the greater is 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’s possible to product components with the best mechanical performances obtainable. Using a numerical and experimental approach, to achieve this goal have been treated different topics: 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 which is particularly effective for measuring long fibers. II. Through the use of Design of Experiments techniques, was analyzed the influence of the main process parameters on the fibers length reduction during injection molding. The study provided the implementation of an extensive experimental campaign. III. In the same way, using DOE techniques, was analyzed the influence on fibers degradation of the Hot Runner System geometry. The study, conducted in collaboration with a company leader in this trade, 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 breakage of fibers during the flow of the reinforced polymer, has been developed by using multi-objective optimization software. The optimization procedure, which provides the simultaneous implementation of different software, was used for an industrial application of a water assisted injection molding of an automotive component. VI. It was finally analyzed the degradation of the fibers in composite materials with high long glass fibers weight fractions. The experimental tests lead to new, unpublished results, which were analyzed to identify possible justifications. Specific tools was developed and designed for the verification of these assumptions, which will be verifying in future DIMEG research works. 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.