Paper characteristics for digital printing

New technological developments in electrophotographic printing are placing stringent demands on paper performance. With higher run speeds and higher image quality expectations, paper manufacturers are challenged to produce papers with the appropriate characteristics at acceptable price points. Digital substrates must be able to handle higher levels of toner while maintaining sharp line edge acuity and accurate dot placement, as end-users are demanding snappy colours, defect-free areas of solid colour and sharp text with high edge sharpness.

Paper performance for printing may be broken down into three functional areas: runnability, printability and fitness for use. Runnability is generally understood to encompass the performance of papers throughout the printing cycle, in other words, that the sheets run smoothly through the print engine without jamming. Printability relates to the image quality and overall appearance of the printed piece; while fitness for use, or usability, of the final printed piece is assessed in terms of grade-related properties such as colour, texture, basis weight, the ability to be finished and distributed in the required manner and the ability of the image to meet permanence requirements for the specific use.

In order to discuss the technical requirements of digital papers, it is useful to consider the steps that paper goes through during the electrophotographic marking event. The two steps that are critically related to paper properties are toner transfer and fusing. Inside an electrophotographic printer, the image is written using a laser or other light-based system to a photosensitive drum or belt known as the photoreceptor. Charged toner is attracted to the image areas of photoreceptor, which are charged differently than the background (or non-image) areas. The dielectric force that drives the toner transfer arises from a charge placed on the paper before it reaches the transfer "gap". The strength and uniformity of this force determines the efficiency of toner transfer.