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, says Leon Minnie, product manager: production systems at Konica Minolta South Africa.
Printability relates to the image quality and overall appearance of the printed piece; whilst, 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.
Below are the characteristics of paper for digital printing:
* Runnability – a leading cause of paper jams is out-of-plane deformation (such as curl or cockle – an uneven wavy surface), a problem that is exacerbated at the higher toner levels and fuser temperatures used in full colour printing. Compared with many offset press requirements, sheet properties for digital printing must be more stringently controlled in terms of stiffness, moisture level, edge quality and dimensional integrity in order to meet the jam-free requirements of complex high-speed paper paths.
* Stiffness – stiffness is the ability of a sheet to resist an applied bending force. It has a significant effect on runnability. It is closely related to formation, thickness and moisture level.
* Calliper (thickness) – automatic feed systems, high capacity stackers and inline finishing equipment can only function effectively if paper calliper is sufficiently uniform, with some systems employing real-time inline thickness measurement to detect and compensate for variation.
* Grain direction – the grain direction, or the direction in which most fibres lie in a sheet, can vary between the width and length of a sheet. With digital presses, feeding sheets with the grain in the wrong direction can cause paper jams if the stiffness is not in the functional range.
* Formation – formation is the arrangement of fibres and other components in the substrate and refers to the orientation and distribution of fibres, fillers, pores and voids. How paper performs in digital printing has been very closely linked to its formation.
* Surface properties and print quality – the surface characteristics of paper must also be matched to the specific ink or toner, as well as to the press technology. Surface characteristics important to toner printing are uniformity, adhesion, strength and smoothness.
* Surface strength – in the toner fusing stage, paper surface strength must be adequate to prevent the delamination of coatings, or fibre-picking with uncoated papers.
*Smoothness – the smoothness of the paper surface is often described in marketing terms as its “finish”, and a wide range of finishes are available, from cast-coated gloss with an almost mirror finish, to low-gloss matte surfaces, to rough-textured surfaces such as linen. Special embossed finishes with specific patterns can add interest, but these substrates are notoriously difficult to print on in most dry toner systems.
* Dimensional stability – dimensional stability refers to the change in shape or dimension of a sheet and also can refer to the change in planarity. In a digital press, papers are subjected to heat, which can cause expansion, contraction, curl, cockle and, in some cases, an accelerated creep.
* Moisture – of all digital paper properties, the moisture level and moisture history are arguably the most critical and are often the only rigid paper specifications. Moisture affects resistivity, which in turn affects the magnitude of the dielectric force in toner transfer and the resulting image quality.
* Paper conditioning – dimensional stability on press requires sufficient paper conditioning time, this means allowing paper to condition and find equilibrium within the relative humidity and temperature in the press room or storage area.
* Recycled papers in digital printing – there are many challenges in manufacturing high quality printing papers using recycled fibres. This is a growing segment of the digital paper market, especially for transactional and business applications for which a company’s environmental policy may dictate choices.
“Stickies” and other contaminants in recycled papers are a particular hazard for electrophotographic printing because the non-uniformities interfere with toner transfer. Defects may be exaggerated by a surrounding charge field and even small contaminants can result in deletion spots that are easily visible in highly toned areas.
New technologies are developing to increase the efficiency of recycling and management of contamination levels. It is anticipated that improved quality in uncoated recycled grades will increase the use of recycled papers in digital printing.