Numerical Simulation Of Motion Characteristics Of Non-spherical Fibre Particles In Fused Deposition Modeling Process Using CFD-DEM

Fused Deposition Modeling(FDM)is a typical 3D printing technology which utilizes the hot melt and cohesiveness of thermoplastic materials to extrude the semi-fluid hot melt material to form a production layer by layer with the aid of computer technique.In order to improve the strength,stiffness and heat resistance of the printed parts,discontinuous carbon fibres are usually introduced to reinforce thermoplastic materials.However,due to the irregular shape of the carbon fibre particles,bridging structures may be formed in the print nozzle and the printing progress is hindered.In order to solve this problem,it is critical to understand the movement and distribution of non-spherical fiber particles in the print head to achieve in-depth and substantial printer structure and process parameter optimization.In this thesis,the coupled Computational Fluid Dynamics-Discrete Element Method(CFD-DEM)model is used to conduct in-depth research on the motion and force characteristics of non-spherical fibre particles.The numerical results can provide a theoretical basis for FDM experimental research and design.Firstly,the Particle Resolved Direct Numerical Simulation(PR-DNS)method is used to explore the force and heat transfer characteristics of non-spherical particles in uniform and shear fluids,respectively.The distribution characteristics of the drag coefficient(Cd),lift coefficient(Cl),moment coefficient(Cm)and average Nusselt number(Nu)of non-spherical particles under different working conditions are obtained through the sub-particle scale analysis.The effects of aspect ratio,Reynolds number,particle spin number and shear rate on Cd,Cl,Cm and Nu are investigated.The changing trends of various coefficients with the particle incident angle in different cases are comprehensively analysed and compared.It is confirmed that the influence of particle rotation and shear rate should be considered when calculating the momentum and heat transfer between non-spherical particles and fluid.Furthermore,a combined CFD-DEM model is developed to numerically investigate the FDM process of discontinuous carbon fibre reinforced polymer composites.The effects of fibre length,fibre initial volume fraction and properties of thermoplastic fluid on fibre force and motion are discussed.It is shown that nozzle clogging is more likely to take place with the increase of fibre length and initial fibre volume fraction.Low viscosity of the polymer matrix can be effective for eliminating the clogging issue of relatively short fibres.However,fibre clogging seems unavoidable when the fibre is very long.Finally,a potential solution to the nozzle clogging using a cone sleeve insert located above the shrinking region is proposed and the feasibility of the solution is confirmed based on numerical simulation.The influence of the key characteristic parameters in the scheme on the movement characteristics of the fibre particles in the print head and the formation of bridging structure is studied,and the optimal parameter group to avoid fibre clogging is confirmed.