Stripes can be a wonderful design element, but when we see stripes when printing solids, we might call that “banding.” In inkjet terminology, it is more correctly referred to as “cross-process density shift” which is a bit of a mouthful. When this type of print quality issue exists, they are easiest to spot when printing darker colors on higher hold out papers. The wider the web width you are printing on, the more likely it is that you will see a density shift at some point.
Jetting accurate sized drops cross hundreds of tiny print heads is a feat in itself. It requires an extremely complex calculation sometimes called a uniformity process. Each print head must be identified by its machine position, and measured in real time as it jets a color swath. Depending on the variance factor assigned by the OEM, the machine will attempt to average adjustments to reduce the optical density difference for the entire row of heads. On most devices this is conducted when changing out a roll of paper during the registration and alignment process.
The slightest variance in drop size jetting onto the page from one head can become easily visible when you are stitching 100’s of print heads across the sheet and expecting the same visual optical density from all of them. When the drops from one print head are a different size from the others, visually you will pick up a shading or density difference as banding.
Print head alignment is also critical since all of these small drops must line up down the web (process direction timing) and across the web (cross-process direction). Cross-process direction alignment is rarely an issue on high speed inkjet as print heads are microscopically aligned in a fixed position to sit accurately in bars across the web. This is to ensure that heads are stitched accurately side to side and that proper jetting angles are achieved.
However, process direction timing is sometimes is a factor. Although the heads are mounted in a fixed position within a fixed bar, there is a timing program which tells the print head when to fire. This is set when the machine is initially set up and should be verified during the OEM maintenance process. If one or more heads of a process color jets either too quickly or too late, the dither pattern of combined colors will become misaligned creating a visual darker appearance for that print area.
But jetting is only one part of the equation, keeping the paper stable is just as important as jetting accuracy. Even though we are talking micron measurements, if the paper is closer to some print heads than others, the drop flight time from the print head to the paper will vary, causing an area to visually look lighter or darker. A variation in distance can also cause the drop size and shape to change. The farther the paper is away from the paper, the longer the flight time and larger the drop size and variance.
The paper itself can be a factor as well, particularly the consistency of the treated or coated surface. If the coating weight to one side of the web to the other varies, it can cause a difference in dot spread. Coating differences substantial enough to cause banding are rare and differences are more likely to be seen from one side of the paper to the other (2 sidedness) rather than across the web.
There are many components that have to work perfectly together to deliver flawless print quality. Some “flaws” are nearly unavoidable with certain color, coverage and paper combinations. However, problems can be minimized if you can pinpoint the root cause or causes.