TL;DR
Ink density variation is most often caused by an inconsistent anilox roll, ink viscosity drift, or doctor blade wear. Check anilox condition, ink viscosity, and doctor blade loading before adjusting press settings.
What you might see
- density variation in MD or CD on printed output
- density dropping over the course of a run
- print mottle or streaking in solid coverage areas
- ink viscosity reading drifting up during production
Likely causes
Anilox roll cells plugged with dried ink or filler, reducing ink volume delivery
Ink viscosity rising due to solvent evaporation or water evaporation in water-based inks
Doctor blade worn or loaded, not metering ink consistently
Plate impression pressure inconsistent across the print width
Required tools
- Densitometer
- Zahn or DIN viscosity cup
- 10x loupe for anilox inspection
- Anilox cleaner (approved grade)
- Doctor blade replacement
- Nitrile gloves
Safety first
- Printing inks and solvents are flammable and may contain hazardous chemicals. Use nitrile gloves and maintain adequate ventilation at all times.
- Doctor blades are sharp. Use cut-resistant gloves during doctor blade removal and installation.
Procedure
- 1
Pull a density read across the print width at the current production point. Identify whether variation is MD or CD.[1]
- 2
Measure ink viscosity with a Zahn or DIN cup. Compare to the grade recipe viscosity. Adjust ink viscosity to spec by adding approved solvent or water.
Warning: Ink solvents are flammable. Work with adequate ventilation and away from open flame or ignition sources. Use nitrile gloves. - 3
Inspect the anilox roll under magnification (10x loupe). Filled or damaged cells indicate the roll needs cleaning or replacement.[1]
- 4
Clean the anilox roll with the approved anilox cleaner per the press maintenance SOP. Chemical cleaning under ultrasonic may be required for heavily plugged cells.
- 5
Check the doctor blade loading pressure. Inconsistent pressure across the blade width causes variable ink metering.
- 6
Inspect the doctor blade edge for wear or chipping. Replace the blade if wear is visible.
- 7
After all corrections, run a short print run and pull density reads at 5-minute intervals to confirm stability.
Sources
Bobst Bobst MASTER M5 / DM5 Flexo Press general technical documentation, Bobst
Flexographic press ink density control and anilox maintenance, general flexo printing practice (general)
More guides for Bobst Bobst MASTER M5 / DM5
How to fix doctor blade streaking on a Bobst MASTER M5 Flexo Press
Doctor blade streaking produces ink lines or streaks in solid print areas. Replace the blade, verify blade material matches the ink system, and check anilox roll cleanliness.
How to fix print registration error on a Bobst MASTER M5 Flexo Press
Registration error between print colors is caused by web tension variation, plate mount error, or driven roller slip. Check web tension, verify plate position, and inspect all drive rollers before re-registering.
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