A study of the off-contact screen printing process. I. Model of the printing process and some results derived from experiments

A physical model of the screen printing process is described. Interrupted printing tests provided guiding information on the flow pattern in the paste during the deposition process. The paste flow region ahead of the squeegee was divided into three regions: the pressurization region (region I), the downward screen cross-flow region (region II), and the paste accumulation region (region III). During printing, the squeegee tip becomes deformed and its angle of attack decreases. Region I extends from the leading edge of the deformed squeegee tip to the beginning of the deformation region. As a result of pressure buildup in region I ahead of the squeegee, there is a flow of paste under the squeegee. The paste deposition process depends mainly on the flow process in region I. Analysis of experiments indicates that the squeegees used deform so that their fronts can be approximated by a wedge. The angles of hard squeegees decrease by approximately 20° from the undeformed angle of 45°. The corresponding angle decrease for soft squeegees is on the order of 30-40°. The vertical forces acting on the squeegees were estimated to be between 60 and 47 lbf. The average speed of the paste under the squeegee, caused by the developed hydrodynamic pressure, was found to be between 0.1 and 0.2 of the squeegee speed