Offset printing separates itself from other methods by one basic physical fact: ink and water do not mix. The same water that makes the process practical is also the hardest part of the run to keep under control. When the operator manages water well, the press runs smoothly and print quality is far easier to hold. This article covers the four areas that decide the result: pH value, concentration, dosage, and a set of practical precautions worth checking on every shift.
pH control for the fountain solution
On a conventional positive-working PS plate, the fountain solution is normally held between pH 5 and 6. A reading far outside that band creates specific faults on press.
When the pH is too low:
1) The image areas are attacked, dots shrink, and severe cases lead to image wear, known in the trade as plate blinding.
2) The hydrophilic layer on the non-image area breaks down, and those areas start to blind as well.
3) Ink drying slows and can stop altogether.
4) Ink emulsification speeds up and ink transfer becomes irregular.
5) The ink film loses gloss and looks flat.
When the pH is too high:
1) The non-image area loses its water receptivity, ink creeps outward, and scumming appears.
2) The damaged hydrophilic layer is hard to restore, so the background picks up ink.
3) Image contrast drops and colors look washed out.
4) On positive-working plates the image layer may dissolve, leaving broken strokes and missing detail.
Adjustment rules used on the floor:
1) Heavy, oily inks need a slightly lower pH, otherwise the plate scums.
2) Dark inks run at lower pH than light inks.
3) Thick ink films need lower pH than thin films.
4) Solid forms run lower than screen forms, and coarse screens lower than fine screens.
5) In hot, dry conditions, lower the pH; in cool, humid conditions, raise it.
6) Poor quality paper calls for a slightly lower pH.
7) Match the pH to paper chemistry: acidic paper needs a higher pH, alkaline paper a lower one.
Three ways to measure the value:
1) A pH meter, or acidity meter, reads the potential difference of the solution. Calibrate the instrument, rinse the electrode with distilled water, then immerse it and read the display.
2) Indicator dyes such as litmus or methyl orange change color according to the pH of a measured sample.
3) Test paper is dipped for about a minute and compared with the color chart.
The meter gives the most accurate and fastest reading and is not affected by the color of the solution. Indicators are cheap and quick but carry more error. Test paper is convenient, though less precise. Most plants still rely on test paper for routine checks.
Concentration control
Concentration means how much fountain concentrate, powder or liquid, is present in the mix. Operators often watch pH and forget concentration. Many concentrates contain buffers, so pH barely moves even when the concentration changes a lot. Controlling pH alone is therefore not enough.
Set concentration from these factors:
1) Paper. Open, porous stock sheds lint and coating dust onto the blanket, rubbing the plate and causing scumming, so the mix should be stronger. Smooth, dense stock, coated art paper and cast-coated board need a weaker mix.
2) Ink. The usual order is magenta over black, black over cyan, cyan over yellow; dark over light; low viscosity over high viscosity.
3) Drier. Adding drier speeds ink setting but coarsens pigment, raises viscosity and increases the ink receptivity of the non-image area, which invites scumming. Add concentrate whenever drier is used.
4) Form type. Solid lines and heavy text carry more ink and need a stronger solution. All-screen forms need a weaker one. Mixed forms with solids, screens and lines require a compromise based on ink demand.
5) Shop temperature. Warmer air thins the ink and releases more free fatty acid, so the plate scums more easily. Raise concentration as temperature rises.
6) Press speed. Higher speed means more friction between rollers, more heat, thinner ink, weaker color, and a tendency to compensate with more ink, which again scums the plate. Increase concentration with speed.
7) Other conditions. Heavy impression pressure and hard blankets or packing also call for a stronger mix.
Concentration is tracked by conductivity, which rises in a roughly linear way as concentration climbs. Mix with soft water, hold pH at 5 to 6, and keep conductivity inside the range given by the supplier. Heidelberg alcohol solutions, for example, are usually held between 800 and 1200 microsiemens per metre.
Dosage control
Fountain solution use is surprisingly large next to ink. Depending on the job, a press may consume several times, sometimes dozens of times, the volume of ink. The solution leaves the system in four ways:
1) Emulsification into the ink. Some emulsification is unavoidable and even necessary; no offset ink runs completely water free. More ink laid down means more water carried away.
2) Absorption by paper. Paper is porous and water receptive. Looser sheet and lower sizing mean higher absorption.
3) Evaporation. Plate, blanket, ink rollers, dampening rollers, sheet surface and open fountain tray all give up moisture, faster at high speed, high temperature and low humidity.
4) Squeeze off. At the nip, solution on the plate edges is pushed to the ends and tail, and excess is thrown off.
Because all these paths end in evaporation and vary from job to job, water dosage is harder to set than ink dosage. The guiding rule is simple: use the least water that keeps the non-image area clean, so water and ink stay in balance.
Factors to weigh:
1) Plate. Solid forms demand the most water, text less, screen forms least, roughly in proportion to ink demand.
2) Ink. The critical water uptake of an offset ink sits around 20 to 25 percent. Beyond that point, pigment particles become free, viscosity, tack and transfer fall, and the ink turns into an oil-in-water emulsion. Low viscosity, free-flowing inks need less water; heavy ink feed needs more.
3) Paper. Damp stock needs less water, dry stock more. Low surface strength stock sheds lint onto the blanket, so opening the water slightly reduces piling and cuts blanket washing.
4) Temperature and humidity. A hot shop and a hot machine speed evaporation, so add water. Low relative humidity also calls for more water.
5) Other. High speed and heavy impression pressure both raise the demand.
Practical precautions
1) Keep the formula stable. Frequent changes make the behavior impossible to learn.
2) Measure concentrate with a dedicated measure, not by eye.
3) Avoid skin contact with chemicals, and use a meter or test paper for pH rather than taste.
4) Mix with the softest water available. Calcium in hard water reacts with citrate in the concentrate and forms calcium citrate deposits that block the micropores on roller surfaces, causing ink stripping.
5) Hold pH steady through the run.
6) When a color change needs a big shift in concentration, adjust the mix while 500 to 700 sheets of the current job remain.
7) Watch water visually. A reflective sheen on the plate means too much water. The target is no sheen and no scumming.
8) After a stop of a few minutes the plate dries, so increase water before restarting.
9) Keep the water line clear when refilling the tank, and avoid stirring up flocculent sediment that blocks pipes.
10) Before switching to a new solution, clean the circulation system with a dedicated cleaner to remove biofilm and slow new growth.
11) Clean dampening rollers, the fountain tray and ink residue on roller covers regularly so water transfer stays even.
12) Drain the fountain tray during long shutdowns to prevent corrosion.
13) Choose concentrates with a good microbiological inhibitor to extend cleaning intervals.
Water control is not a single setting. pH, concentration and dosage move together with paper, ink, form, temperature and speed. Operators who read those signals and adjust early keep the press running clean, and the sheets that leave the delivery show it.