The paper presents a numerical model of image transfer in screen-printing that is supported by experimental data. The model focuses on a roller squeegee system. It combines thin lm hydrodynamic behaviour with roller squeegee deformation and ow through a porous screen. The work within this investigation has, for the rst time, enabled an estimate of deposit thickness at different halftone coverage. For low coverage, ink transfer is governed by hydrodynamic behaviour in the nip contact, and deposited lm thickness is represented by an ink spread model. For larger open areas, dependent on squeegee load, the lm is removed from the top of the screen and therefore the deposit is likely to be controlled by the screen thickness. The work con rms that a roller squeegee system leads to higher nip pumping capacity in comparison with a sliding squeegee of nominally the same shape. The roller system is therefore appropriate when heavy ink deposits are required. The velocity gradients through the lm when ink ows through the screen are reduced as the open area increases. The consequent reduction in shear rate leads to a recovery in viscosity within the nip contact.
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 insuf cient resistance to ow and leads to image ooding. 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 suf ciently 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.
The paper focuses on the solution of a numerical model to explore the sliding and non-Newtonian fluid behaviour in soft elastohydrodynamic nip contacts. The solution required the coupling of the fluid and elastomer regimes, with the non-Newtonian fluid properties being described using a power law relationship. The analysis showed that the fluid characteristics as defined by the power law relationship led to large differences in the film thickness and flow rate with a movement of the peak pressure within the nip contact. The viscosity coefficient, power law index and sliding ratio were shown to affect the nip performance in a non-linear manner in terms of flow rate and film thickness. This was found to be controlled principally by the level of viscosity defined by the power law equation. The use of a speed differential to control nip pumping capacity was also explored and this was found to be most sensitive at lower entrainment speeds.
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