Research On High Speed Digital Jet Printing Quality Based On CFD

The use of computer software to control the digital printing nozzle to spray ink directly onto the fabric to form patterns has advantages of high precision,low pollution,low energy consumption,and short period,which has brought enormous impact to traditional screen printing.It is the technique combining nozzle horizontal reciprocating spray printing with conduction band longitudinal stepping that is adopted by current digital printing machines,leading to the printing at a low speed of generally 400m2/h,which is about one-tenth of the speed of circular screen printers,thus the market competitiveness of digital printing is weakened.In order to improve the speed of digital printing,a new printing mode has appeared,that is,nozzles along the width direction are fixed and spraying,while the fabric passing through continuously in high speed.But because of the high-speed movement of fabric,airflow is formed between the fabric and nozzles,which leads to floating ink droplets and grainy printing defects.For purpose of solving the above problem so as to ensure the quality of printing,this paper takes a high speed digital printing machine as the research object,carries out numerical simulation with the aid of CFX,which is one of the computational fluid dynamics(CFD)software,analyses the formation mechanism of printing defects,and puts forward the method of improving the quality of jet printing.The main research contents are as follows:Chapter 1,the background and significance of the study are elaborated,an overview of the development status of digital ink jet printing is presented,CFD technique and its applications are introduced,and on this basis the research contents of this paper are proposed.Chapter 2,this paper introduces the structure and working principle of a high speed digital printing machine,and establishes the three-dimensional model of jet printing space by applying SolidWorks.After importing the model into the professional software ICEMCFD for mesh generation,the CFX software is employed to numerically simulate the movement of fabric,the spraying of nozzle,as well as the combined motion of fabric and nozzle.Respectively,the flow field status of jet printing space under the above mentioned three conditions and their formation mechanisms are obtained.By using the high speed video technology to get the flow field images of jet printing space in working state,comparison analyses are conducted to verify the correctness of the CFD numerical simulation.On this basis,the simultaneous injection by multiple nozzles is numerically simulated,indicating that single point injection can be used approximately to simulate the actual working conditions,which reduces the difficulty of numerical simulation,and lays the foundation for the printing quality improvement program.Chapter 3,with the help of CFX,this paper numerically simulates the flow field of jet printing space of the fabric working at different speeds,and tests the spray printing in corresponding conditions,identifying the corresponding relationship among the speed of the fabric,the vortex of the flow field,and the grain defects,namely,the higher the speed is,the smaller the vortex and the defect are.According to the kinetic theory of granular flow,this paper calculates the displacement of the ink droplet along the moving direction of the fabric,finding that when the speed of the fabric increases,the displacement is also enlarged.Taken together,the most reasonable speed of the fabric is determined.In order to ensure high printing quality in different conditions,an air supply device is set on the prototype to regulate airflow according to the speed of the fabric.By this method,theoretical analysis and printing test are conducted at the speed of 0.5m/s,and sizes of airflow corresponding to different speeds of the fabric are established.The results show that the printing quality is improved obviously with the aid of air source.Chapter 4,digital printing works in damp environment.Only if the internal components of the nozzles of the machine are kept dry,can they output a stable voltage and excites pulse waveform,and as a result,high quality prints are ensured.According to the knowledge of fluid dynamics,this paper designs a print head drying system.By applying CFX,this paper analyzes the velocity and pressure in the pipe.Then by using the flow sensor,the flow in each branch pipe is measured,and it is found that the measurement results are basically the same.The comparison of the humidity value of the drying system before and after work proves the significant effect of the drying system.Chapter 5,summarized the full text,and raised further research direction.