Why Do Flexo Labels Get Ink Bars?
Ink bars on flexo labels are dark and light bands that repeat across the web at roughly fixed intervals. They usually come from anilox roller, gear, or plate cylinder runout, not from a random speck of dust falling on the substrate. First measure the distance between two bars; whichever roller circumference it matches, check that roller first.
What ink streaks look like
An ink streak isn’t a random patch of light or dark. It is a series of light and dark bands running across the web, spaced about the same distance apart, and recurring throughout a roll.
The image above shows this repetition: one dark band, one light band, with consistent direction and spacing. Random dirt spots don’t line up this neatly.
What roll does the spacing point to?
| Spacing you measure | Check first | Why |
|---|---|---|
| Close to the anilox roll circumference | Anilox roll clogged cells, wear, or bearing | Each revolution prints the same ink volume difference |
| Close to the gear pitch | Gear wear, mesh too tight or too loose | Ink transfer stutters once per gear tooth |
| Close to the plate cylinder circumference | Plate cylinder runout, uneven plate mounting, tape too soft | Pressure changes once per revolution |
| No fixed spacing | Not an ink streak | Check instead for dirt spots or web defects |
Use a ruler to measure the center-to-center distance between adjacent streaks in a solid area, then compare it with the roll circumferences on the press. If it matches, stop at that roll; don’t take the entire machine apart.
If the streaks are still there after plate cleaning, you can rule out “a speck of dirt on the plate.” A hickey will disappear when washed off; an ink streak will not.
How should inspection be set up?
- Treat ink streaks as a separate class; don’t rely only on dot diameter to catch them. Thin, long streaks are often invisible to dot thresholds.
- Alarms should look at repetition, not a single faint mark on one sheet. A single mark on one sheet might just be something spotted by chance during sampling.
- Record the spacing and the speed at the time. When speed changes, gear streaks sometimes lessen or worsen.
- For color, still use solid color difference. Ink streaks are variations within the same color area; the average color difference for the whole area can still pass.
Next
- How to tell a streak from a hickey helps confirm whether you’re seeing a streak or a spot.
- What is print quality inspection explains why inspecting a single sample can miss this kind of repeating defect.
