The role of paper and process technologies for mechanisms and image quality in digital electrophotography

摘要

Image quality and the mechanisms involved in digital dry toner electrophotography are influenced by the interactions between the printing machine, toner and paper in the last two steps where the paper is involved, i.e., in transferring the developed image toner to the paper, and in fusing the image to be fixed permanently on the paper surface. This study discusses the role of paper in these two steps in different technologies and its effect on the printing mechanisms and image quality. The control of optical, electrostatic and mass and heat transfer phenomena in the printing process are affected by the unevenness of the properties of paper due to its heterogeneous structure and its sensitivity to humidity conditions and printing process parameters. In this research, a set of experiments was conducted to understand the electrostatic behaviour of paper in toner transfer and thermal behaviour in toner fusing. The results show that not only image quality is affected by the variability of paper properties, but also the mechanisms of toner transfer and fusing. Accordingly, the research suggests that the paper should be included as part of the printing mechanism, performance (printability and runnability), and image quality. Consequently, if there is a change in paper properties due to a change in ambient conditions or the use of another grade for a specific application, the process parameters can be adjusted to compensate for these changes in order to meet the requirements for image quality. It was found that the variability in image quality in terms of colours (the requirement for different toner layers), grey scale (halftone structure) and the location of the image in the xy-plane is affected when rendered through the electrophotographic process. The fast mechanical speed in printing machine direction drives the toner transfer and fusing mechanisms differently from the cross machine direction. As a result, a certain image element such as a line will have different quality in these two printing directions, or if the line is placed in the length or width direction of the page. The conclusion was that the electrophotographic process should be designed to reduce or even to neglect the effect of paper when printing a high-quality colour image in a high-speed process. This can be achieved by eliminating the contact with paper from the image side in both transfer and fusing by adopting the technologies of toner jumping transfer and non-contact flash fusing. These technologies have special requirements for chemical and physical toner properties, such as modification for equal absorbance of the flash radiation by CMYK colours, a suitable melting viscosity and surface energy, and a small and narrow toner particle size and shape distribution to unify the charge-to-mass ratio of the toner, which is important for transfer quality and efficiency. To ensure high print quality for different applications, some of the transfer and fusing parameters need to be automatically adjusted according to substrate specific properties and levels of image coverage.