Deformation of the flexographic printing plate is an important factor in determining the quality of the printed image. A numerical model of the individual dots has been developed and used to examine the deformation of the plate under a range of printing conditions and image characteristics. Two mechanisms have been identified for the deformation of the image on the plate: expansion of the dot surface and dot barrelling. These results have been combined with those from an experimental study to apportion the dot gain due to ink spreading and physical deformation of the dot. The results have shown the low-coverage dots at high line rulings to be particularly affected by the effect of variation in the impression pressure. This has significant implications for the ability of the process to reproduce high-resolution images that combine both highlight and shadow regions successfully and consistently. Ink spreading has been identified as the major cause of dot gain, except at low coverages, where the deformation of the dots makes a significant contribution.
A combined experimental and numerical investigation into the behaviour at the nip formed by the junction of a hard and soft elastomer covered roller is described. Experimental measurement of pressure at the film and elastomer-substrate interface are presented along with the speed differential due to microslip and thermal transients due to elastomer hysteresis. A unified finite element model of the nip is also presented where Newtonian behaviour is adopted to reflect the use of a gear oil as the lubricant in the experimental work. The system response was found to be dependent on engagement, contact width, surface speed and elastomer properties in particular. It was found that this contact type cannot be described accurately with a Hertzian model which has been adopted in all previous studies.
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.
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