Investigation On Key Forming Processes Of Inkjet-type Three-dimensional Printing(3DP) System

Owing to rapid processing,low-cost maintaining,wide selection of material and office-friendly superiority,three dimensional printing?3DP?has been one of the most developing and competitive AM processes,and additionally,it is very potential to be extensively accepted by the manufacturing enterprises.As China lagged a lot behind the western countries in terms of the research on 3DP,the western suppliers have been always monopolizing the 3DP market in China,which makes the minor business in China not able to access 3DP productions due to the high prices of the related machines and processing materials.Therefore,according to the thread of optimizing the 3DP sub-processes to improve manufacturing qualities,this dissertation presents the detailed development flow for a 3DP system,which can be shortly described as follows: firstly,the 3DP sub-processes,which sequentially include the phase of micro-droplet jetting,the interaction between the droplet and powder bed,the phase of bonding powder together and the spread of powder layers,were investigated for the purpose of obtaining optimization of the sub-process parameters which can improve the 3DP results,then,the development of 3DP machine and 3DP data-processing software were conducted based on the above parameter optimization,and finally,via the above self-developed pair of 3DP machine and software,the 3DP fabrication of alumina/borosilicate-glass composites was investigated in this study.The second chapter in this dissertation firstly proposes a novel thin-type pneumatically diaphragm-driven microdroplet-jetting unit,whose structure is so compact that the multinozzle-array printhead composed by several these jetting units can be realized,and consequently,the following experiments on the 3DP sub-processes can be carried out under such a pneumatically diaphragm-driven jetting technique.Besides,the mathematical model of microdroplet generation aiming at this thin-type jetting unit is established by the governing equations of force equilibrium,mass conservation and energy conservation,and the jetting principle can be interpreted with this mathematical model.Finally,based on the above jetting unit,the “micro-jetting experiment” is carried out,and microdroplet-regime parameters?i.e.the dimensions and velocities?,which resulted from different actuating conditions set for the jetting unit,were acquired through the method combining the image capture and digital image processing.The third chapter firstly presents a theoretical investigation on the droplet/powder impact interaction effects,and thereby makes several items of “energy hypothesis” from the perspective of energy conversion to describe the influences of process parameters on the droplet/powder impact process and further the qualities of 3DP parts;then,according to the results from the above “micro-jetting experiment”,the thin-type jetting unit set with different actuating conditions can eject the microdroplets with the corresponding regime parameters to impact onto the powder bed samples,and thus,the influences of process parameters?i.e.the microdroplet regime parameters and powder bed parameters?on the droplet/powder impact interaction effects and the qualities of 3DP parts can be experimentally investigated;finally,it is proved that these experiment results and above “energy hypothesis” have a good conformance,and through the above theoretical and experimental investigations,the optimization plans for the process parameters are finally proposed.In order to fulfill the optimization plans for the powder bed parameters,the fourth chapter proposed an experiment research on ultra-thin powder layering,and it is experimentally indicated that the “double-smoothing” layering method is much more superior to the conventional “counter-rolling” method in the aspect of spreading?and densifying?the fine powder particles into ultra-thin powder layers without obvious powder-layering defects;moreover,as an innovation,a theoretical powder-layering framework is established to describe the powder behaviors in the process of powder layering;this theoretical powder-layering framework is also adopted for interpreting the observed phenomenon during the above powder-layering experiments and further the superiorities of double-smoothing powder-layering method over the conventional powder-layering methods.Conforming to the optimization proposals for the process parameters concluded from the researches in the previous chapters,the fifth chapter presents the development details of a 3DP fast data-processing method?on the basis of SSL-format files?and a 3DP machine system composed of a three-functional-storey construction.A feasibility study is conducted in the sixth chapter for the purpose of verifying the use of urea-formaldehyde resin as the in-powder adhesive for 3DP-based fabrication of high-performance alumina/borosilicate-glass composites via the above-developed 3DP machine;the 3DP material system for alumina/borosilicate-glass composites and the related postprocessing procedures?sintering and melt infiltration?are also emphasized in the investigation;the fabricated alumina/borosilicate-glass composites exhibit the fine mechanical properties: 86.7MPa for the tensile strength,200.7MPa for the flexural strength,1503.4MPa for the compressive strength,278.7GPa for the Young's modulus,14.1GPa for the Vickers hardness,and 4.2MPa m^1/2 for the fracture toughness,and additionally,the CTE of such composite parts is very low,below 5.3×10^-6?^-1;finally,the potential of such composites for direct use in engineering applications is also proved by fabrication of rapid tools and functional parts with complex geometries based on the current 3DP-based fabrication process.