Experimental and computational studies of fused deposition modelling for the fabrication of microstructure patterns
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Fused Deposition Modelling (FDM) is an additive manufacturing process used in 3D printers for the fabrication of complex 3D objects by layer deposition of molten thermoplastic filaments. The FDM technology has the potential to produce customised micro/nano structured patterned surfaces with applications in medical, microfluidic, bio surfaces, etc. FDM is widely used due to its simplicity, low costs and high throughput. However, high printing accuracy and micro-manufacturing are still challenging due to the nature of the process. Even though progresses on numerical simulations of FDM have been made, there is a lack of comprehensive research on swelling and solidification of the filament and microstructures by considering the temperature dependant properties of polymers. This project aims to improve the accuracy of printed parts by controlling the polymer swelling phenomena as well as developing a cost-effective manufacturing technique, in terms of predicting the evolution of the extrusion process for the fabrication of microstructures using commercial desktop 3D printers which utilise the FDM technology. To achieve this, the temperature dependant rheological and thermal properties of polylactic acid, within the boundary of printing conditions are first obtained and analysed numerically. The mechanisms and the effects of operation and design parameters on the dimensional accuracy of the extruded filaments are then investigated by numerical simulations based on the Finite Element Method developed in COMSOL Multiphysics software. Moreover, the free surface of the polymer is determined by applying force balance and energy equations on the interface to obtain the filament swell and phase change behaviours. Finally, the model is validated using theoretical results from the literature and experimental measurements and further used to investigate the evolution of the microstructures on the filament surfaces during the extrusion process. From this study, it is identified that both printing process parameters, especially printing speed, and polymer properties have a significant impact on the formation of extrudate swell and the shape of the microstructures. The deformation of the polymer increases with the rise in temperature as the viscosity is highly affected by temperature changes. Through enhancing the solidification rate and injection rate, the accuracy of filaments and microstructures can be improved. It is discovered that the swelling phenomena can be reduced by as much as 21% through cooling the filament with water. Also, in terms of the micromanufacturing with FDM, several microstructures with different shapes including rectangles, triangles and semi-circles are simulated. The hydraulic swell value of unity is possible to achieve through adjusting the printing speed for each geometrical shape. Swell value of 1 is obtained at to 45,50 and 70 mm/s printing speed for rectangles, semi-circles and triangles respectively which can be used as a reference point for printing microstructures. This study helps to predict the shape of the filament and its microstructures using temperature dependant polymer data through simulations. This eliminates the need for time-consuming experimentations for the determination of surface topography of the filaments. To further improve this study, the effect of layer deposition on the surface topography needs to be considered.