How to deal with plate filling-in in offset printing?

A long-run color job goes on press. The first few hundred sheets look perfectly normal, and the press operator has just begun to relax, when the sheets on the sample table start to change: the dots in the shadow areas fill in and lose their gradation; small reversed-out characters are filled solid, with ragged stroke edges; scattered dirt specks and ink spots appear across the blank areas of the sheet. The whole printed sheet looks “gray and dull” — the solids are no longer solid and the gloss drops away. The reject rate climbs from a few parts per thousand into double digits.

The first reaction on the floor is almost always the same: “There is too much ink.” So the ink feed is cut back — and the result is worse than before. The solid ink density falls, but the filling-in does not go away. The plate is wiped down and the press is restarted; it runs well for a few dozen sheets, and then the filling-in returns exactly as before.

This is a scene all too familiar in any offset pressroom. Once the transfer of offset printing ink falls out of balance, the real culprit behind filling-in is rarely to be found in the ink fountain.

Filling-in (also called plugging) refers to the loss of the water-receptive, ink-repellent character of the non-image areas of the printing plate, which are then occupied by ink, or to the recesses between dots becoming filled up. The consequence is ink appearing where no ink should be on the printed sheet, and neighboring dots merging into one another.

To understand it, we must go back to the underlying logic of offset printing: the ink–water balance. Offset printing can form an image only because of the difference between the image areas, which are oleophilic (ink-receptive) and hydrophobic (water-repellent), and the non-image areas, which are hydrophilic and oleophobic. The non-image areas are not naturally ink-repellent; they are protected by an extremely thin film of fountain solution that holds the offset printing ink off. This water film must not be absent (the moment the film breaks, the ink immediately “creeps” onto the blank areas), and it must not be too thick (with excessive water, the ink becomes over-emulsified, its viscosity drops and the dots spread). Imbalance in either direction ends in filling-in.

In day-to-day production, filling-in is frequently confused with several related faults, so it is worth drawing the distinctions clearly:

One point must be stressed: filling-in is a symptom, not a cause. Behind it lie at least six distinct causal chains, and the problem is only truly solved when the right chain has been identified.

Experienced operators do not guess; they look — at the form, the position, the timing and the accompanying symptoms. The checklist below can be used directly for comparison:

Position and timing are the two most valuable clues. A fault that recurs at a fixed position indicates a “static” element such as a cylinder, the blanket or a dampening roller; one that grows worse as the run accumulates can almost always be traced to paper dust build-up, ink skinning or the decay of the hydrophilic layer on the plate.

3.1 The plate itself

Poor hydrophilic treatment of the non-image areas or an excessively shallow grain; incomplete developing or gumming leaving residual photosensitive coating; incorrect baking temperature or time; plate stored for too long, oxidized or exposed to light, with its hydrophilicity degraded. Problems of this kind usually show up the moment the press starts.

3.2 Fountain solution — the culprit most easily overlooked

A low pH value (excessively acidic) corrodes the plate surface and destroys the hydrophilic layer; a high pH value causes the ink to over-emulsify and the dots to spread. Excessively high conductivity (hard water, ion accumulation), an incorrect proportion of isopropyl alcohol or of the alcohol substitute, and an over-high temperature in the water tank cause the same problems. Both too much and too little water lead to filling-in, which is exactly where on-site adjustments most easily go the wrong way.

3.3 Offset printing ink

Excessively high tack in the offset printing ink pulls fibers from the paper and causes piling; excessively low viscosity, or ink that is too soft, spreads and produces dot gain. In addition: over-emulsification (high water uptake), drying too fast and skinning on the plate, coarse pigment particles with poor dispersion, and overdosing of additives such as tack reducer, drier or varnish, or a mismatch with the paper and fountain solution.

3.4 Paper and paper dust — the highest-frequency cause on site

Low paper surface strength with dusting and linting: paper dust mixed with offset printing ink accumulates on the blanket and the plate and literally “fills” the dots out. Abnormal paper pH, moisture content and ink absorbency add to the trouble, while excessive spray powder becomes a new source of contamination.

3.5 Pressure and mechanical condition

Excessive pressure between the plate and blanket cylinders squeezes and spreads the dots until they fill in solid; incorrect pressure on the form rollers or dampening rollers; a blanket that is aged, slack or dented by local compression; worn bearings and gears causing uneven pressure; dirty, hardened dampening roller sleeves giving an uneven water feed.

3.6 Environment and operation

Uncontrolled pressroom temperature and humidity (high temperature thins the ink and accelerates drying); incomplete wash-up leaving dried ink behind; long stops with no plate wiping and gumming; lint-shedding wiping cloths and the wrong cleaning agent.

A summary widely quoted in pressrooms: “Out of ten cases of filling-in, seven can be charged to the account of paper dust and water.”

The key to solving the problem is a one-to-one correspondence with the cause, with quantitative criteria and a means of verification for every action taken. The table below is arranged in the order of “highest probability first, smallest change first”:

Note: the values given above are commonly used industry rules of thumb; the standards of the equipment and material suppliers shall prevail in any specific case.

Turning the following procedure into a card kept at the press is more useful than memorizing ten folk remedies:

• Examine the form: is it local or across the whole sheet? At a fixed position or wandering? Worsening gradually, or present from the very first sheet?
  

• Examine the accompanying symptoms: are there paper-dust white spots, ragged dots, streaks and bars, or set-off on the reverse side?
  

• Troubleshoot in sequence: water → ink → paper → pressure → plate → machine. This sequence is ranked by “probability of failure × cost of adjustment” — the highest probability and the cheapest change come first.
  

• Change only one variable at a time: after each change, run 30–50 sheets and compare samples. Change two things at once and you will never know which one did the work.

Verification cannot rely on the eye alone: measure the dot gain and solid ink density with a densitometer, a dot gain meter or a magnifying glass, and confirm the improvement with numbers.

Should the matter simply be closed once the filling-in has been dealt with? No. Printing companies that truly save money turn this occurrence into a contingency plan for the next one:

Establish a standard ink–water balance curve for this particular machine, together with a per-shift log (water setting, pH, conductivity, temperature and humidity).

Calibrate periodically: verify cylinder pressures quarterly, and replace the blanket and dampening roller sleeves at the end of their service life.

Carry out material compatibility verification before make-ready: test the compatibility of the offset printing ink with the paper and the fountain solution, rather than waiting until the job is on press to discover a problem.

Turn the troubleshooting SOP into a card kept at the press and build it into operator training and shift handover checks.