Although often specific to paper, picking and loose contamination can sometimes be an interrelationship involving the entire printing process.

Definition
Although often specific to paper, picking and loose contamination can sometimes be an interrelationship involving the entire printing process. When these problems become pervasive rather than the exception, certain considerations may significantly broaden the operating window. Static electricity can cause loose contaminants to cling to the paper surface. This can be a problem when pressroom relative humidity is below 35% and when paper is not acclimated. The ideal range is 40-50% Rh.

Press Attributable
Excessive back cylinder pressure stressing paper surface;

• Heavy plate moisture application breaking down paper or coated surface;
• Hardened rollers do not set properly, impeding ink transfer and thorough wash-up — Check for proper roller durometer;
• Synthetic fibers and contaminants from rags, hickey picker rollers, etc;
• Spray powder contamination;

Ink Attributable
Tack of ink exceeding the cohesive strength of paper coating is causing stressful blanket release;

• There is ink tack build on rollers from excessive downtime and long make-readies. Spray rollers with anti-skin inhibitor or similar retardant;
• The setting speed of ink/paper is too fast causing subsequent ink tack build from unit to unit. Check slope of ink tack build relative to paper;
• Dry ink particles;

Blanket Attributable
Stressful release causing pick or delamination (fast ink demands “quick release” compressibles);

• Excessive blanket packing causing extreme back-cylinder pressure;
• Blanket wash or solvent causing blanket tack or swelling on start-up;
• Use edge-sealed blankets to minimize swelling from solvent and fountain solution contamination;
• Maximum slip and clean release at the blanket/sheet impression nip is important in non-image areas as well as image areas. Fountain solution should offer good lubrication and release;

Paper Attributable
Pickoffs (Loose contamination);

• Slitter dust/cut dust;
• Coating particles;
• Pickouts (Evidence of “crater” void at point of origin);
• Poor coating adhesion;
• Wood vessel segment fibers;
• Basestock contaminants;

Pressroom Environment
Monitor relative humidity and temperature. The ambient temperature and humidity of the pressroom may be too high or too low, affecting ink tack and paper strength.

• Ideal climate control is 45% (±5%) Rh and 72°(±5°)F.
• Allow paper to acclimate to pressroom.
• Paper acclimation time is relative to environmental extremes.
• Properly conditioned paper runs with a broader operating window on press.

pH
pH is the measure of the hydrogen ion content in water. Todays fountain solutions are buffered at 4.0 and do not change if you add more solution. As a press runs the pH will change depending on the compatibility of the ink with the fountain solution. An increase in pH means that the fountain solution is attacking the salts in the ink. If the rise continues unabated, then there will be problems with the plates because the solution is no longer buffered. Most plates are designed to run with fountain solution in a pH range of 3.8 to 4.2. When the pH rises above 4.2, the gum arabic will not adhere properly to the plate and becomes sensitive to ink causing scumming. Thusly, it is important to record pH reading to monitor the relationship of the fountain solution with ink. For quality printing, it is important to maintain the optimum pH for dampening systems.

 Conductivity

Conductivity refers to the measure of a material's ability to conduct electricity. As ionic materials are dissolved or go into solution in water, they form ions and the water becomes more conductive. Conductivity increases by about three percent for every degree the temperature rises. Most conductivity meters have built-in temperature compensation. Conductivity doesn't indicate fountain solutions' lithographic properties. Any material that dissolves in water changes conductivity readings. Clays and minerals in paper coatings affect conductivity, as will some soluble salts from ink pigment. Calcium carbonate, which is used in alkaline paper making, can cause problems on press. Calcium ions can react with ions of certain fountain solution chemicals, thus changing the nature of the solution itself and causing the conductivity to change. Calcium also reacts with other materials, forming compounds that can be deposited onto plates, rollers or blankets. In turn, these deposits precipitate glazing, leading to toning in the non-image areas, plate blinding and other problems.

Paper companies use calcium carbonate in the base-sheet as a filler and in the coating as a pigment. Most competitive sheets also use calcium carbonate as both filler and pigment. Calcium carbonate provides brightness and a more blue-white shade than clay. Calcium carbonate is used in neutral or alkaline paper making, which results in a more permanent sheet than acid paper making. Permanence is essential for reduced yellowing and brittleness of paper as it ages. This is especially important in the manufacturing of books. Acid sheets contain little, if any, calcium carbonate. This is due to the generation of foam when calcium dioxide is released from calcium carbonate in an acid environment. Therefore, acidic sheets typically contain more clay.

 Background

Paper companies have used calcium carbonate in all paper produced in the last 30 years. It has been used in coated papers, in general, since the 1930s. There are significant benefits to the use of calcium carbonate in the alkaline paper making process, some of which include:
An environmentally responsible process;
•    Increased opacity and brightness;
•    Faster ink set for quicker turns;
•    Cost-effective manufacturing process.

Most of the North American uncoated wood-free sheet capacity uses the alkaline or neutral paper making process with calcium carbonate as a filler and pigment. Some printers have experienced scumming problems when printing alkaline, uncoated paper and have determined that calcium carbonate is often the issue. Sizing is difficult to obtain on an uncoated sheet when alkaline paper-making chemistry is used. This may cause increased fountain solution absorption, which can contribute to scumming problems.

Fountain Solution

There are different types of calcium carbonates. Some may contain free lime, which may be more prone to leaching into the fountain solution. Highly acidic or overly aggressive fountain solutions can further impact this tendency, especially on uncoated papers. Although the calcium compounds are essentially insoluble in acid-based fountain solutions, calcium carbonate can adversely react to the acid and form calcium ions, which when combined with other ions such as citrate, can leach out onto the rollers as a hard, white glaze. This leaching can cause the pH and conductivity of the fountain solution to increase. This condition coupled with non-image blanket build can reduce the effectiveness of the fountain solution and contribute to a scumming or toning problem. If this condition is suspected, the contaminated fountain solution should be analyzed to determine whether calcium carbonate or free lime has been absorbed. The comparison of the calcium ion concentration in fresh fountain solution and fountain solution that has been in contact with alkaline paper will indicate whether calcium carbonate or free lime has been leached from the paper.

Ink Setting or Drying Rates

Calcium carbonate may help improve ink setting by pulling oils or solvents out of the ink more rapidly than clay. The pore volume of the coating (the amount of small openings in the coating) is one factor that determines the rate of oil or solvent removal. If a large number of small pores are present, more oil or solvent can be removed, and the ink will set faster. The pore volume is determined by pigment particles’ size, shape of the clay, and calcium carbonate used in the coating. Any potential chemical effect of calcium carbonate should be addressed by the ink and coating suppliers, as some inks and coatings are more sensitive to chemistry changes than others. If the calcium carbonate contains free lime, problems may occur either directly with the ink or through fountain solution chemistry changes that may affect the ink.

Dynamics on Press

Calcium is also a component of ink formulations, with higher levels typically present in magenta. Depending on hardness, calcium is also present in untreated tap water. Chemical incompatibilities (in some isolated and extreme instances with respect to water, fountain solutions, ink, and paper) can cause break down and build-up which, in turn, create printing problems. It is important to note that calcium carbonate contamination severe enough to overwhelm the printing system is usually the product of cumulative build-up resulting from the absence of preventative maintenance. Though more typically experienced in high volume printing with uncoated paper where calcium carbonate is used as a relatively unsealed base-stock filler, some symptoms of calcium may include:

Progressively poor ink transfer usually seen as dot sharpening;
•    Ink roller stripping;
•    Fountain solution progressively becoming more alkaline if not buffered for alkalinity;
•    High conductivity gain of fountain solution;
•    Excessive foaming of fountain solution only when contamination is present. Compare the performance of fresh solution with contaminated solution;
•    Build-up of calcium glaze on the ink rollers. This typically appears as a white haze which is not easily removed with conventional roller wash;
•    Build-up or piling in the non-image area of the blanket;
•    Progressive toning or scumming as a result of increased alkalinity, poor water receptivity, poor ink transfer, and accelerated plate wear.

Press Consideration and Analysis

Monitor fountain solution pH and conductivity before, during, and after each job.
•    Hard tap water contains calcium which can adversely react to fountain solution causing “calcium soaps.” Tap water can also vary in conductivity from day to day. Therefore, monitor tap water conductivity daily, and consider a water treatment system which is specifically designed for the lithographic printing process.
•    As a weekly maintenance consideration or when contamination is suspected, ink rollers should be deoxidized in addition to a regular roller wash. Consider any of the following:
•    Vinegar and hot water;
•    Avoid using solutions with detergents or surfactants that may be counterproductive to the printing system. Follow up with a regular roller wash to ensure proper ink receptivity.
•    If magenta or red pigmented inks seem to be the source of most problem, consult with ink supplier.
•    Test fountain solution for the presence of high levels of calcium carbonate by comparing fresh and suspect samples.
•    Check roller durometer and settings for proper specifications to ensure good ink transfer.
•    Potential for paper or paper coating breakdown initiating the leach of calcium ions.