It is important that conductivity and pH levels of dampening solution be optimum and consistent. Printing Industries of America recommends measuring conductivity and pH with every fresh batch of dampening solution and every four hours when the press is running. Readings should be taken in the water pan at each printing unit and in the recirculator. Water pan readings can provide an early warning about potential print problems.
Conductivity is the measure of a material’s ability to conduct electricity. Pure water, which approaches a conductivity of 0 micromhos, is a poor conductor of electricity. The conductivity of water is directly proportional to the amount of undissolved solids (potassium, sodium, etc.) in it. As more solids are present, water becomes more conductive. Thus, conductivity can be used as an approximate measure of water quality. Materials that are mostly ion-free (alcohol is a good example) are poor electrical conductors and lower the conductivity of dampening solutions.
Unusual changes in the conductivity of dampening solution may be caused by impurities from any source (water, paper filler, paper fibers, ink particles, etc.) and justify re-checking the conductivity of the water and the dampening solution concentrate before assuming that the dampening solution was improperly mixed. It is normal for conductivity to increase during a pressrun since materials from ink and paper will contaminate the dampening solution. A rule of thumb is that if conductivity increases by more than 500 micromhos, you should replace the dampening solution. If you let conductivity climb that high, you are inviting printing problems, such as scumming.
It is wise to measure pH when you measure conductivity. pH is a measure of the concentration of hydrogen ions in water. A pH of 7.0 is neutral; less than 7.0 indicates an acidic solution and higher than 7.0 indicates an alkaline solution. Acid dampening solutions, typically used in sheetfed offset and heatset web offset printing, should generally have a pH of 4.0–5.5.
Maintaining the pH specified by your dampening solution manufacturer will ensure that non-image areas of your plates are kept clean as the gum arabic used in most solutions is redeposited on the non-image areas. This is vitally important, particularly during longer runs as the initial coating of gum arabic applied in platemaking is worn away.
If the pH is too low (below 3.5), plates may blind, inks may emulsify in the dampening solution, rollers may strip, and ink drying times may be excessive. In addition, a low pH can cause paper filler, such as calcium carbonate, to leech out and accumulate on blankets, interfering with ink transfer. If the pH is too high (above 5.5), plates may scum since gum arabic will not effectively desensitize plates at that level. In that case, excess water required to keep the plates clean is likely to result in ink emulsification.
All acid dampening solutions are buffered to some extent so that, as the amount of fountain solution concentrate increases, the pH initially drops and then levels off. This is unlike conductivity, which continues to increase in a straight line. Thus, conductivity is much better than pH for determining the amount of concentrate contained in your solution, while pH is better for monitoring that the dampening solution is working properly on the plate.
Because pH can change during the pressrun, pH should be re-checked anytime there is a problem with tinting (ink emulsified in the dampening solution), plate blinding (the image on the plate does not take ink), scumming (ink adheres to nonimage areas on the plate), roller stripping (rollers do not hold ink), or when the ink is not drying properly on the paper. If you find that the pH has risen by more than a half step—4.5 to 5.5, for instance—then it is time to use new solution.
If you don’t currently have a conductivity and/or pH meter, you can purchase one that will take both readings for $500–$1,000. Taking and monitoring pH and conductivity readings of your fountain solution every four hours of press operation will help you avoid many print problems and is part of the standard operating practices (SOPs) of a well-run pressroom.
Printing Industries of America has a variety of resources—books and workshops—for companies seeking to improve their monitoring and control of the printing process. We also offer laboratory testing to evaluate the consistency of your pressroom materials or to uncover the reasons behind print problems your company may have encountered.