Development of optimized rib geometries for the application of expanded long-fiber reinforced thermoplastics
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Date
2024-07-04
Authors
Τσιλιάκου, Ευδοκία
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Abstract
Long fiber reinforced thermoplastic (LFT) materials are extensively used in automotive applications as a substitute for metal materials, because of their higher strength and impact properties. One of the main challenges in the automotive industry is the development of lightweight components reducing energy consumption and emissions during production. To achieve these goals, improvement of certain composite material groups and optimization of the production process must be undertaken. Among those materials, polypropylene reinforced with long glass fibers (LGF- PP) is often used due to its improved tensile strength, impact resistance, and thermal stability. Furthermore, utilizing LFTs like LGF- PP for high-speed production of complex parts is commonly done by injection molding process. During this process, the orientation of the fibers is affected, and the fiber length decreases considerably. This results in low stiffness and mechanical performance of LFTs. To minimize the reduction of fiber length, a low-shear process must be used like foam injection molding (FIM). FIM is a high potential process which improves fiber length and allows higher design freedom and dimensional accuracy. Foaming leads to the development of cell nuclei into the core of the part reducing the density of the component. This indicates minimizing material usage, and plausible energy saving throughout this method. To eliminate the need for constructing thick-walled areas, parts are subdivided into ribbed sections. Additionally, this results in better stiffness and stability, contributing to lightweight industrial applications.
The current thesis investigates the development and optimization of rib geometries in polypropylene (PP) reinforced with long glass fibers (LGF) for lightweight, in high-performance applications such as in the automotive industry. The main focus is the comparison of conventional injection molding with FIM process for LFTs and investigating the advantages of FIM. The ProFoam process is tested, which is a physical FIM process in which nitrogen, as blowing agent, is introduced during melting in the plasticizing unit. Various parameters in the injection molding process - in particular injection speed, melt temperature- and rib thickness, rib height, and rib orientation are analyzed in a full factorial experimental design. The fiber orientation and length are investigated as well as the foam structure through microscopic analysis. The mechanical properties are also determined on each case by using 4- point bending tests. Through those results the influences of the process parameters and the rib geometry are presented for both methods.
The results for the foamed parts are obtained by setting 10 % weight reduction during FIM process. It is found that cell formation is significantly affected by melt temperature and injection speed. Opti-mum cell diameter and cell distribution are observed at higher melt temperatures and injection speeds. Moreover, fiber orientation is mainly influenced by the angle between the rib and the melt flow direction, which in combination with the fiber length will influence the mechanical properties. The weight-specific failure load of the foam parts is comparable and even higher than the compact parts. Overall, the results provide an insight into the impact of foaming on the mechanical characteristics and rib geometry, as well as useful information in the development of high-performance thermoplastic materials.
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Keywords
Long liber reinforced thermoplastic (LFT), Thermoplastic foam injection molding, Rib geometry, Glass fibers, Foam structure