In my previous post,
Stripes, Solids and Banding, I talked about many of the factors that can cause cross-process density shifts resulting in visual banding. The more you know about the problem, the better your odds of fixing it. The key is to look very closely to understand the root causes of what you are seeing.
If you see any signs of cross-process density shift, get a baseline by running a diagnostic test pattern on a high holdout inkjet coated sheet. In addition to running each process color in various percentage values, make sure to run a detailed pattern of mixed strokes, solids as well as various percentage patterns and registration marks that run the entire length of the web.
Set up the test run on its own fresh core as you will need to run 2000- linear feet of paper to effectively pinpoint the cause. Be sure to run the pattern under the same conditions as when you first saw the effect, (same paper, profile and machine settings) while closely monitoring how the web feeds into the machine.
Below are clues to look for and possible causes for each situation. In most cases you won’t be able to fix these problems yourself, but diagnosing the root cause will help you to get the right support from your OEM more quickly. So, put your cheaters on and take a close look.
If you see a visual density shift across one or more
single process colors this is likely due to an inconsistency in ink volume jetting from one print head to another.
If you only see a visual density shift in
mixed process colors, there are a couple of possible causes:
- It could be an inconsistency ink volume jetting from one print head to another or,
- The process direction timing is off for that head. The head is jetting either too soon or late which is affecting the printed dither pattern which varies the density of the ink combination forming the color.
If density shifts inconsistently
across the web, it could be an issue with paper transport or paper coating:
- If the sheet distance to the print heads vary across the web, drop flight time will vary effecting the size of the drop when it makes contact with the paper. The paper web may be baggy and feeding at varying distances under heads. Try looking down web during printing to see if the paper is showing a “wave” effect
- To determine if paper coating is the culprit, check to see if paper coating is inconsistent across xweb. Check the printed paper down-web to see if the pattern changes or goes in and out.
- If you see a density difference only on the far ends of web, you may have uneven cross process web tension causing the edges of the web to curl closer to the print heads.
Keep in mind that it is possible, although somewhat rare, to have more than one of these problems at the same time. Once you pin point what area is giving you the unwanted effect, it is easier to have a discussion with your vendors and a more productive service call.
About Mary Schilling
Mary Schilling writes about technical inkjet industry articles, provides RIP and workflow training, manages print quality analysis evaluation, ink management and color management for OEM’s and end users for pre and post machine installs. Mary Schilling consults with paper mills, fluid and inkjet machinery suppliers on how to improve color and print quality for high speed and industrial inkjet involving paper, plastics, metal, fabric and glass with UV and aqueous inkjet fluids. This experience led her to receive Innovator of the Year awards from the Flexographic Technical Association and from Xplor International for her efforts in closing the gap between inkjet printing for document, and digital corrugated packaging. She is the owner of Schilling Inkjet Consulting, Published Author and Certified ColorGate Color Trainer and Distributor.
