Preparation And Process Parameter Optimization Of PLA Based Biological Composite By Fused Deposition Modeling

At present,Fused Deposition Modeling（FDM）has developed as one of the key core technologies for the design and manufacture of high-performance complex components.However,due to the limitation of the principle of layer-by-layer printing and manufacturing,it is inevitable that the parts printed by it produce macro fuse interface between layers（Z direction）,leading to the problems of weak interlayer bonding strength and poor overall mechanical performance.In order to solve the above problems,this thesis proposed the following solutions:Firstly,the PLA based biological composite filaments were prepared with Poly Lactic Acid（PLA）as the matrix,nano-hydroxyapatite（n-HA）and micron chopped carbon fiber（micron chopped CF）as the reinforcing phase.Secondly,the PLA based biological composite filaments with different composition ratios at different temperatures were used for rheological experiments to obtain the rheological data needed in the numerical calculation of fluid.Finally,the results of fluent numerical calculation and APDL transient numerical calculation were combined with the fused deposition modeling experiments to verify the best process parameters,so as to improve the distribution of reinforcement phases in the interlayer interface area and the local collapse and warpage of the sample,and further improve the interlayer bonding strength,overall mechanical performance,and appearance accuracy of the PLA matrix biological composite samples.Based on this,this thesis carried out the following work:（1）For the preparation of n-HA@PLA core-shell structure,the results showed that,after the modification of n-HA by heating in the constant temperature water bath at 60℃for 4 hours,the core-shell structure with n-HA in the core and PLA in the shell was successfully obtained.For the enrichment of CF functional groups,the results showed that,after hydrogen peroxide oxidation and microwave heating for 7min,the corrosion grooves on the surface of the micron chopped CF showed the best effect,which was beneficial to mechanical meshing with the PLA matrix.At the same time,the number and types of oxygen-containing functional groups were improved,among which the hydroxyl group or ethoxy group（C-O bond）and carboxyl group（COOH bond）were the most significant,reaching 20.52%,which increased by29.5%compared with that before microwave oxidation treatment.Then,the modified n-HA and oxidized micron chopped CF were evenly coated on the surface of PLA filaments by ultrasonic impregnation treatment to form PLA based biological composite filaments.Finally,PLA based biological composite filaments with different component ratios at different temperatures were prepared for rheological experiments,and rheological data such as non-Newtonian exponent n,viscosity coefficient K,and visco-flow activation energy E_ηwere obtained for fluid numerical calculation.（2）In the numerical calculation section of fluent,the distribution of n-HA and micron chopped CF in the interlayer interface region was very important,and the distribution was closely related to the velocity of the fluid at the nozzle exit.The flow velocity of nozzle outlet at 190℃,200℃,210℃and 220℃was respectively analyzed with different structural parameters and material composition.The orthogonal experimental parameter table with three factors and three levels was set,and then imported into Excel to calculate the mean and variance of the fluid velocity at the nozzle outlet under each set of parameters,and further calculated the Delta value through Minitab.The results showed that the influence degree of these three factors on the nozzle outlet fluid velocity was as follows:the filament feeding speed had the greatest influence,followed by the nozzle diameter,and the micron chopped CF content with relatively weak.At the same time,with the same printing parameters,the tensile samples were printed and prepared by the printer,and the tensile properties were analyzed by tensile experiments.The dispersion state of micron chopped CF and n-HA at interlayer interface was observed by SEM.The results showed that the larger the mean and variance of the fluid velocity at the nozzle outlet,the more uniform the dispersion state of the reinforced phases at the interlayer interface,and the better the tensile properties of the samples.Finally,the optimal parameters were obtained:melting temperature of 210℃,nozzle diameter of 0.5 mm,filament feeding speed of14 mm·s^-1,and micron chopped CF content of 7%.The maximum tensile yield strength of the sample printed on the basis of these parameters was 36.54 MPa.（3）In the transient numerical calculation section of APDL,the APDL transient numerical calculation was conducted to monitor the temperature of 7948 node in the first layer and 10016 node in the second layer respectively under different bottom plate temperatures.At the same time,it was experimentally analyzed by fused deposition modeling and printing.The results showed that when the bottom plate temperature was 50℃,55℃and 60℃,the local collapse occurred,and the mechanical properties were reduced.On this basis,the temperature monitoring of5404 node in the top corner region and 5485 node in the side region of the first layer was carried out by numerical calculation,and the analysis was carried out by combining the warpage measurement and tensile property experiment of the samples.The results showed that the optimum printing temperature was 45℃,the warpage degree was 0.47 mm,and the tensile yield strength reached 39.86 MPa,which was9%higher than that of the sample when the temperature of the bottom plate was50℃.There are 49 figures,14 tables and 163 references in this thesis.