Wetting and levelling of toner during fusing of electrophotographic prints

Sammanfattning: This licentiate thesis comprises two studies dealing withthe fusing step in electrophotographic printing. The firststudy addresses the development of a method for monitoringwetting/spreading of single toner particles in-situ on a heatedsubstrate. The method employs imaging from above and thus makesit possible to use toner particles of realistic size (around 10µm in this case). The results of this first study clearlyshow that high energy substrates not only exhibit far greatertoner particle spreading, and work of adhesion generated by it,but also a strong dependence on the toner type. For the Xeikontoners used the cyan and black spread far more than themagenta, with yellow clearly the lowest. Conversely, low energysubstrates exhibit limited spreading and lack of sensitivity totoner type. The results permitted the proposal of a simplegeneralised law to describe thespreading kinetics. Thisempirical relation was able to fit all experimental resultswith the bare minimum of input information from the equilibriumstate. The method provides a readily visualisable and flexiblemeans for more systematic design of toner-substrateinteractions in electrophotography. A broad range of differenttoner particles can be rapidly screened in contact with modeland real substrates, and the spreading then correlated toelectrophotographic runnability, print quality and durabilityissues. The second study is based on evaluation of print qualityfrom trials on a Xeikon digital press with varying fusingconditions. The study illustrated the utility of white-lightinterferometric profilometry in quantifying the thickness andsurface roughness spectrum of toner layers transferred andfused on two different paper substrates (coated and uncoated).These measurements generate increased insight into themechanisms giving rise to toner print quality. Significantdifferences in surface roughness behaviour were observedbetween different toner colour combinations as a function oftemperature settings and both paper type and fusing units(radiant and heated roll). The correlation between gloss andprint surface roughness at micron length scales extends tolonger length scales with increasing toner film thickness andon use of the heated roll glossing unit. These observationsfacilitate a more systematic approach to designing fusing unitsto avoid typical problems such as gloss variations inelectrophotographic prints.