Why Coated Cardstock Mottles
Mottling on coated cardstock is a cloud-like unevenness of tone in a solid area, with no fixed direction and no equal spacing. Common causes are uneven coating, ink not laying flat, or insufficient pressure. It is not ink bars, and you should not wash plates as if it were white rings or dirt spots.
Mottling Is Not Banding
Mottling looks like a solid area that has not printed solid. Dark patches and light patches with soft edges, like clouds, with no repeating spacing to be found.
The image above has no horizontal bars and no white rings, only an area of color that will not lay down evenly. If the light and dark areas form evenly spaced horizontal bands, that is banding, not mottling.
First Distinguish Paper, Ink, and Pressure
| Symptom | More likely cause | What to do first |
|---|---|---|
| The coating on blank paper already shows mottling | Uneven coating | Change the paper or batch; do not adjust ink first |
| Only large solid areas mottle; fine type is normal | Ink is not laying down evenly, or ink volume is insufficient | Check ink viscosity and ink volume on solids |
| Mottling decreases after increasing pressure | Pressure is insufficient; ink is not pressed into the coating | Increase pressure in small steps while checking whether the plate is filling in |
| Disappears immediately after changing paper | Absorption or coating of this batch of paper | Keep a paper sample, not just a printed sheet |
Look at the coating on the unprinted edge of the paper. If the blank area is already mottled, printing on it will only make it more obvious. If the blank area is uniform and only the solid areas mottle, then adjust ink and pressure.
Washing the plate has almost no effect on mottling. There is no speck of dirt on the plate that can be wiped off; the cloudiness is throughout the entire solid area.
How to Judge Acceptance
- Do not use “it looks acceptable” as the standard. Take a signed-off solid and compare it with the current shift’s printed sheet under the same light source.
- If the customer accepts by color difference, the average color difference over the whole area may still pass. Mottling must be recorded separately: unevenness, not color cast.
- For cameras, do not use only dirt-spot diameter. Mottling is variation within an area; spot-based rules will let it pass.
- Keep paper samples and printed sheets. If the same batch of paper mottles again next time, you can check the paper directly instead of suspecting the machine again.
See Also
- Why Does Banding Occur on a Flexo Label Press to distinguish spaced bands from mottling with no spacing.
- How Tight Should a Delta E Tolerance Be on Press explains why average color difference cannot cover local unevenness.
