| With the rise of national fitness as a national strategy,sports consumption has gradually become a new hot spot in the 21 st century,and sneakers have become one of the most eye-catching categories in the field of shoes and clothing.The sneakers with warp-knitted vamp printing have advantages in comfort and appearance due to its special fabric structure and printing characteristics.Therefore,it has broad market and development prospective.Screen printing is the technical basis of additive printing of warp-knitted vamp,which evolved almost purely through evaluation of engineering experience and is a manufacturing process forming the complex three-dimensional pattern with a thickness of 0.8mm or more through dozens of overprinting and overprinting on the warp-knitted spacer fabrics.At the moment,it is conducted by hands.In view of the labor-intensive characteristic of warp-knitted vamp printing,improving the automation level of additive printing of vamp has been listed in the technological plan of green and intelligent manufacturing in the field of shoes and clothing.The work in this paper is a part of National Key Research and Development Program(2018YFB1308800).In order to understand the printing mechanism and develop intelligent printing equipment,theoretical analysis on paste rheology,mechanical property of squeegee,and the deformation and permeability characteristics of screen are carried out,based on which flow field simulation is done by the fluid-solid coupling method to obtain the printing mechanism.Then the automatic printing equipment is developed as well as corresponding process.The main contents and achievements of this paper are as follows:(1)The physicochemical properties and rheological properties of printing paste as well as their effects on printing process are studied.The physicochemical characters of aqueous polyurethane dispersion(PUD)with high solid content,including main component,particle size distribution,surface charge and Zeta potential,are characterized using Fourier transform infrared spectroscopy,dynamic light scattering and laser Doppler electrophoresis.The paste rheology is studied by steady and dynamic shear tests.The viscosity with shear rate,temperature and solid content as well as viscoelastic properties with strain and frequency were analyzed.It can be known that paste is pseudoplastic non-Newtonian fluid with weak gel structure and shear thinning characteristics.Based on Arrhenius formula and Carreau equation,the viscosity model of paste under the influence of multiple factors is constructed.Meanwhile,the fractional derivative Zener and Poynting-Thomson models are derived and optimized,based on which the linear viscoelastic properties of paste are analyzed by regression.Using the above constitutive equations,the theoretical boundary values of paste subjected to shear,strain and oscillation in printing were obtained.(2)A mathematical model of printing flow field considering rheological properties of paste,squeegee form and printing speed is constructed,and the distribution characteristics of flow field are studied.An analytical procedure is developed to solve the stress,strain,and displacement distribution of wedge squeegee with variable section based on elastic mechanics theory.As the plane strain problem,the mechanical characteristic of hyper elastic squeegee is conducted to obtain the deformation equation.Then the lubrication theory is applied to the mathematical modeling of additive printing,leading to non-dimensional velocity as well as the integral expression of hydrodynamic pressure that is solved by the Romberg integration algorithm.Then the extended Fourier amplitude sensitivity test is carried out to quantify the impacts of printing force,angle and velocity on hydrodynamic pressure.Finally,in order to verify the effectiveness of the mathematical model,a CFD model is developed to simulate the flow field of additive printing,taking into account Newtonian and non-Newtonian properties.And the simulation results are in good agreement with that in theoretical analysis.(3)The screen form and printing coordination conditions are analyzed and calculated.In addition,the rheological splitting of paste during screen rebound is studied.By calculation,the deformation expressions of printing pattern with screen size,snap-off distance,pattern position and printing force is obtained.Based on Poiseuille flow,the relation between the printing velocity and mesh count,thickness,fiber diameter,viscosity is obtained through kinematics compatibility conditions.Meanwhile,the relationship between rebound velocity of screen and screen size,snap-off distance and printing velocity is also obtained.Based on Hooke’s law and Newton’s law of internal friction,the mathematical equations for screen size,squeegee size,snap-off distance,mechanical parameters of screen and screen specification are proposed from the mechanical compatibility conditions of screen rebound.Using the surface wetting process and interface reconstruction technology,the rheological splitting of paste in the screen rebound is analyzed,through which it can be known that surface free energy and fiber diameter have important effects on the rheological splitting.(4)Fluid-structure coupled model of paste transfer in printing is established,using the rigid body dynamics,multiphase flow,porous media and dynamic grid technologies,based on which the variation in flow field and transfer mechanism of paste are studied.The distribution of flow field is analyzed as well as the variation of pressure between permeable and impervious areas.Moreover,the relationship between transfer amount of paste and pressure is analyzed,which is verified by single factor experiment.The transfer mechanism of paste is summarized: the wedge flow field constructed from squeegee and screen meets the hydrodynamic lubrication,forming a hydrodynamic pump,and the steep increase in pressure near the squeegee tip inject the paste into the aperture of screen.Under the effect of printing force,the screen is in contact with the warp knitted vamp,forming a liquid bridge.As the squeegee moves on,the screen“peels off” dynamically from the warp knitted vamp at some distance behind the squeegee,and the contact line moves up.Meanwhile,the liquid bridge is stretched up to rupture.Since the surface free energy of the warp knitted vamp is greater than that of the screen,most of paste is transferred to the warp knitted vamp.(5)The process of additive printing is analyzed to identify some special mechanisms.On the basis of transfer mechanism,additive printing equipment is researched and developed.Based on the fuzzy mathematics theory,the reliability of additive printing system is analyzed using failure mode and effect analysis to obtain the failure modes and risk priorities.Orthogonal experiment with seven factors and four levels is designed and performed on the additive printing equipment,then variance and range analysis are carried out.The optimizing model of process parameters is constructed by support vector machine for regression and genetic algorithm to obtain the optimum printing parameters.The high-quality printing products produced by the additive printing equipment can meet the requirements of tensile strength,flexometer and hydrolysis testes.This paper develops additive printing equipment and design the corresponding process to produce high-quality printing products based on the in-depth analysis of transfer mechanism of paste.It has important theoretical significance and application value for understanding the mechanism of additive printing for warp knitted vamp,improving the automatic level of the industry,promoting competitiveness of products and achieving the green intelligent manufacturing. |
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