Studies Of The Processing Mechanics For Controllable Microscale Morphology Of Direct-ink-writing 3D Printing

Direct-ink-writing(DIW)3D printing technology can print a variety of rheological materials with different functions at low cost and high precision,including conductive pastes,elastomers,hydrogels,etc.Among these rheological materials,self-leveling inks(such as conductive inks,hydrogels,PDMS,etc.)have excellent printability and versatility,showing broad potential for electronic applications.However,due to the complex rheological behavior of inks during extrusion,how to precisely control the microscale morphology after printing has always been a difficult challenge.During the printing process,the ink extruded from the nozzle deposits a pattern on the substrate with the movement of the nozzle.In general,printing parameters are obtained based on experience or trial and error,so the process of optimizing printing parameters is time-consuming and laborious.In addition,the optimized parameters may not be applicable to other equipment and inks.It turns out that a comprehensive understanding of the mechanics of ink extrusion is key to guiding high-quality 3D printing.In order to solve the above problems,the following research content is carried out in this paper:1)Based on the in-depth understanding of the thermodynamic and rheological properties of self-leveling inks,the effects of surface tension,viscosity and other properties on the morphology and size of ink lines were discussed.The effects of the air pressure and the inner diameter of the nozzle on the extrusion form of the ink were investigated,and it was pointed out that printing should be carried out when the ink flowed steadily and continuously.The width of the ink line before and after curing was measured,indicating that the width of the ink line would not change significantly after curing.2)According to the thermodynamic characteristics of self-leveling ink,the formula for calculating the width of the ink line is derived by using arc to approximate the contour of the upper surface of the ink line.By investigating the influence of the setting of each DIW printing parameter on the extrusion morphology of the ink and the morphology of the print line,two dimensionless parameters V and H were constructed.With these two parameters,6 different morphologies of the ink line(or printing behavior modes)can be adjusted,i.e.meandering,accumulation,thinning,smearing,chain and discontinuous.Through experimental combined theoretical methods,starting from the parameter settings of V=1 and H=1,the parameter boundaries between different printing behavior modes are discussed.Finally,based on the conditions of each printing behavior mode,a parametric phase diagram controlled by two dimensionless parameters is constructed.With this phase diagram,ink lines with a specific topography can be printed quickly.3)The process of forming the ink layer stacked with multiple ink lines was discussed,and it was pointed out that the ink layer would form a very flat surface due to the leveling characteristics of the ink before curing,but after curing,the surface of the ink layer would appear in rows of wavy ripples.Through experimental observation,the key parameters affecting the ripple morphology were identified,and a geometric model describing the surface contour of the ink layer was constructed.This model can be used to guide the printing of high flatness ink layers.Finally,the possible causes of ripples on the surface of the ink layer after curing are discussed,and it is pointed out that the phenomenon may be similar to the coffee ring effect.4)Using the smearing mode,the width of the ink line can be further reduced when using the same nozzle,and the limit of V_f on the nozzle movement speed can be broken.In the printing process,the printing parameters are continuously changed,and the printing behavior mode can be continuously converted,so that the surface of the same ink line appears with different line morphologies.Based on the DIW process model that controls line topography,two fully functional arrays are printed,the sensor array and the resistance array.Based on the rheological properties of materials and the forming mechanism of DIW3D printing,this paper obtains the microscopic morphology control equation of self-leveling ink printing lines by establishing a process mechanics model,which realizes the accurate prediction of different morphological printing process parameters,simplifies the steps of process parameter optimization,and provides a reference for further establishing the parameter system and control method of high-precision DIW printing.