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

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Aalto-yliopiston teknillinen korkeakoulu | Doctoral thesis (article-based)
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TKK dissertations in media technology, 3
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.
electrostatic transfer, thermal fusing, dry toner
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  • [Publication 1]: Pirkko Oittinen, Hussain AL-Rubaiey, Katja Sipi, and Katri Vikman. 2001. Research on paper-ink-process interactions in electrophotographic and ink jet printing. In: Proceedings of the 2001 International Conference on Digital Production Printing and Industrial Applications (DPP 2001). Antwerp, Belgium. 13-16 May 2001. Springfield, VA, USA. Society for Imaging Science and Technology. Pages 327-330. ISBN 0-89208-233-X.
  • [Publication 2]: Hussain AL-Rubaiey and Pirkko Oittinen. 2001. Transfer current and efficiency in toner transfer to paper. In: Proceedings of the 17th International Conference on Digital Printing Technologies (NIP 2001). Fort Lauderdale, Florida, USA. 30 September - 5 October 2001. Springfield, VA, USA. Society for Imaging Science and Technology. Pages 648-652. ISBN 0-89208-236-4.
  • [Publication 3]: Hussain AL-Rubaiey, Timo Hartus, and Pirkko Oittinen. 2002. The influence of flash fusing variables on image fixing quality. In: Proceedings of the 2002 International Congress of Imaging Science (ICIS 2002). Tokyo, Japan. 13-17 May 2002. Pages 575-576.
  • [Publication 4]: Hussain AL-Rubaiey and Pirkko Oittinen. 2003. The future potential of digital electrophotography. In: Proceedings of the 2003 International Conference on Digital Production Printing and Industrial Applications (DPP 2003). Barcelona, Spain. 18-21 May 2003. Springfield, VA, USA. Society for Imaging Science and Technology. Pages 114-115. ISBN 0-89208-246-1.
  • [Publication 5]: Hussain AL-Rubaiey, Nonkululeko Khanyeza, and Pirkko Oittinen. 2004. The effect of coating colour on toner transfer in digital printing. In: Proceedings of the PulPaper 2004 Conference, Coating (PulPaper 2004). Helsinki, Finland. 1-3 June 2004. Pages 35-40.
  • [Publication 6]: Hussain AL-Rubaiey, Pia Räsänen, and Pirkko Oittinen. 2005. Detail rendering in digital dry toner electrophotography. In: 31st International Research Conference of iarigai. Copenhagen, Denmark. 5-8 September 2004. Advances in Printing Science and Technology, volume 31, pages 13-26.
  • [Publication 7]: Hussain AL-Rubaiey and Pirkko Oittinen. 2006. Controlling fusing parameters by optical image quality in electrophotographic printing. In: 32nd International Research Conference of iarigai. Porvoo, Finland. 3-7 September 2005. Digitalisation and Print Media, volume 32, pages 123-127.
  • [Publication 8]: Hussain AL-Rubaiey and Pirkko Oittinen. 2008. Thermal behavior of paper in contact fusing technology. Journal of Imaging Science and Technology, volume 52, number 3, 030507, 10 pages.