One of the most important reasons for measuring and controlling surface smoothness is for print quality. For the contacting-type printing processes, the ink film will transfer to a paper surface upon physical contact. When the voids in the paper surface are deep enough prevent such contact, ink will not transfer to the low spots, and non-uniform ink transfer causes poor print quality. When the ink film is adjusted to achieve satisfactory print density on the rough areas of a web, the same ink film may be too heavy to achieve optimum print quality on the smoother portions, perhaps causing mottle and other problems.
There are many reasons why the manufacturers of photocopy machines have target ranges for Sheffield roughness. A xerographic machine needs optimum paper surface properties for reliable sheet feeding, image transfer, and image fix. The fix level decreases as the Sheffield roughness increases, as it affects toner adhesion. Print density loss is observed as roughness increases. There also can be image problems with papers that are too smooth. Toner particles can be flattened and appear as larger dots, thus increasing the perception of the background. Rougher papers produce less background. With regards to paper handling, smoother papers are less stiff for a given basis weight. Smoother papers increase “electrostatic tacking” in the image transfer process. The coefficient of friction decreases with increasing roughness, a factor that is important in sheet transfer operations. The Sheffield roughness properties are carefully specified for the
electrostatic copier printing process.
Similar to the photocopy machines, inkjet printers have paper handling requirements. The method in which a single sheet is transferred from the supply stack generally relies upon the friction differences in paper-to-rubber versus paper-to-paper in a stack. In recent years, there has been development work on optimizing 2-sided surface roughness for ink jet printers. The printing surface was manufactured to be smooth for image quality and the back side was rough in order to facilitate paper feeding and also to avoid excessive contact with a freshly-printed surface as printed sheets are successively stacked in the printer tray.
Related posts include information on the relationship between paper roughness (smoothness) and the following items:
- Papermaking Process
- Printing processes
- Parker Print Surf Test
- Sheffield Test