How to distinguish barring from white-ring dirt spots
Barring is an elongated light or dark mark; a white-ring dirt spot is a small dot surrounded by a ring of unprinted white. Barring mostly comes from the doctor blade, anilox roller, or gears; white-ring dirt spots come from a particle of dirt on the plate or blanket. Look at the shape first, then check three consecutive sheets; only clean the plate if the position stays the same.
First, Look at the Shape
Streaks and white-ring spots are often lumped together as the same kind of “dirt.” Their shapes are not the same.
A streak is an elongated mark, darker or lighter than its surroundings, running in one direction. A white-ring spot is a small dot: a bit of ink in the center and a ring of paper around it, because the plate has both picked up ink and blocked the surrounding ink.
In the image above, the left is a streak, and the right is a white-ring spot. A streak has length; a spot has a ring.
Next, See Whether It Stays in Place
| Streaks | White-ring spots | |
|---|---|---|
| Shape | A dark/light mark | A small dot plus a white ring |
| Common sources | Doctor blade, anilox roller, gear, or plate cylinder bounce | Ink skin, paper lint, or dust on the plate or blanket |
| Three-sheet check | Often the entire streak is present, in the same direction | Repeats in the same position—stuck to the plate or blanket |
| What not to do | Wipe just one proof sheet and continue printing | Treat it as a doctor blade line and grind the blade |
Pull three consecutive sheets. If a streak appears on every sheet in the same direction, check that roller or blade; do not look for a nonexistent speck on the plate. If a white ring lands in the same place on all three, stop the press and clean the plate or wash the blanket. If it moves to a different place on each sheet, it is dust that landed on the substrate; washing the plate will not remove it.
How to Diagnose on Press
- First identify the shape: length means a streak; a white ring means a spot.
- Look at three in a row and note its position relative to the artwork.
- For a streak, measure the spacing. If the spacing equals the circumference of a roller, check that roller first.
- For a white-ring spot, stop first at the spot with the ring, remove the particle, and then restart the press. Do not loosen the threshold and let this kind of ring remain.
Passing color does not mean these two types of defects are absent. Solid-area color difference looks at the entire color patch, while streaks and white rings are local. Detection thresholds must also be set separately: use direction and spacing for streaks, and diameter for spots.
Read Next
- What Is Print Quality Inspection puts spots and streaks back into the full inspection set.
- How Tight Should a Delta E Tolerance Be on Press explains why passing color may still require stopping the press.
