An experimental investigation into ink transfer using a roller squeegee in high-speed screen printing

The paper describes a series of experiments on precision screen-printing with a roller squeegee. The work showed that this could only be achieved using a screen having a high mesh count, notably using a 150-34 screen, since a lower thread count offers insufficient resistance to flow and leads to image flooding. A detailed investigation into process parameters showed that at up to 60 per cent coverage the performance of a roller squeegee in either sliding and rolling mode is identical. Above this coverage and for smaller snap-off gaps, the roller squeegee constrained to slide led to a higher ink coverage. It is suggested that the main reason for this is the viscosity reduction at the screen surface on account of the ink characteristics and the high local shear rates. This dominates the higher hydrodynamic pressure associated with the rotary motion. However, the rolling action leads to superior tonal gain characteristics. It was observed that, when the squeegee was locked, the screen adhered to the substrate, reducing the snap-off speed and increasing the contact duration. The combined effect led to an increase in ink deposit. When the snap-off gap was increased sufficiently to prevent the screen from adhering to the substrate, the rotating squeegee produced considerably higher ink deposit than the locked squeegee. Presently, this is believed to be due to the increase in hydrodynamic pressure within the squeegee nip junction which now becomes a dominating mechanism.