Sandwich composites with fiber reinforced plastic skin and foamed plastic core are widely used in industries in land transportation, automotive, aerospace, and marine structures for its lightweight and high in-plane and flexural stiffness. Besides traditional 3-layer sandwich composites, multilayer sandwich composites (5-layer or more) have been drawing increasing attention from both academic institutes and industry sectors because they offer additional benefits, including more choices for structure design and even higher specific strength and toughness.
To form the sandwich composites with specific layered structure to deliver the property benefits, some manufacturing technics were researched and published. The literature review of this thesis work shows that most of the technics are batch molding process, e.g. vacuum molding, which are time and labor intensive thus not efficient for mass production. A manufacturing method, namely double-belt laminating, was attempted in forming sandwich composites continuously, however, it cannot be applied to manufacture multilayer sandwich composites with foamed plastic core due to the heating method used which may destroy the plastic core.
Obviously, in order to take full advantages of multilayer sandwich composites, a continuous processing method for the efficient manufacturing of the composites is urgently needed. In view of this, the present thesis work aims to research and develop a laminating process for continuously joining multilayers of
fiber reinforced thermoplastic belt and foamed thermoplastic belt simultaneously to form the sandwich composite belt that has required layer structure and may be employed in manufacturing a product with complex structure using 3D printing technics.
The thesis work includes design, fabrication and parameter optimization of a roll laminator for continuously joining thermoplastic multilayers to form the sandwich composites with foamed plastics as core. The roll laminator consists a heating chamber with five heating channels for continuously heating the skin and core layers non-isothermally. The reason to use non-isothermal heating is, on one hand, to provide sufficient heat at the skin-core interface for bonding via fusion; on the other hand, to prevent the foamed plastic core from absorbing too much heat to collapse. After the multilayers pass through the heating channels, pressure is applied on the preheated layers, followed by cooling and solidification, resulting in a multilayer composite belt, of which the skins and cores are cohesively joined by fusion bonding. A LabVIEW-based operation system is also developed for process control and data collection.
To understand the governing mechanisms behind the roll laminating process, a 2D dynamic thermo- mechanical coupled model is developed based on the ABAQUS/Explicit analyzer. The coupled model can provide the temperature and pressure distribution pattern in the skin and core layers during laminating process, which is critical to establishing the relationships between the processing parameters and bonding quality and core structure integrity. To validate the coupled model, temperature and pressure distribution pattern of the laminating process were measured experimentally. Good agreement between the experimental results and the predicted ones from modelling was achieved.
Based on the well-established interfacial bonding mechanisms of polymers under thermal treatment, a theoretical bonding model, considering both intimate contact mechanism and healing mechanism, is also developed in this work. The bonding model was validated by flatwise tensile test results. Use of the temperature and pressure distribution obtained from the thermo-mechanical coupled model, the bonding model can predict skin-core interfacial bonding strength of multilayer composites manufactured under different processing conditions.
Material properties involved in this study, including melting temperature, viscosity, weld time, surface roughness, tensile strength, were characterized experimentally.
Based on the thermo-mechanical coupled model and the interfacial bonding model, a numerical correlation between material properties, process parameters, skin-core bonding strength and structure integrity of the foamed plastic core was proposed. The influence of three critical processing parameters (heating temperature, compression pressure and laminating velocity) on the bonding strength was investigated. Both experimental and predicted results show that the bonding strength increases with increasing temperature, pressure and decreasing laminating velocity. However, overheating and excessive pressure should be averted to avoid foam core collapse. The microstructure of skin-core interface under different conditions is examined by Scanning Electron Microscope (SEM). Finally, a processing window for sufficient bonding strength and well-protected core structure is defined. The thesis work shows that the models developed in this work can be used as a powerful tool to obtain optimized processing parameters in manufacturing of multilayer polymer composites with foamed plastics as core and can also be further extended in the manufacturing of composites with different materials.
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