INKETC: The Ink-Water Seesaw

In offset printing, oil-based inks are almost completely repelled by water on the plates, forming a slight emulsion. The image area of a flat lithographic plate is receptive to ink, and the non-image area is receptive to water or fountain solution. The image area has low surface energy, so water moves away easily. In the non-image area, surface energy is high, so water creates a continuous film.

Ink and water are both fed to the plate, and the only thing separating them is that the image area loves oil and the non-image area loves water.

“This balanced ink-water mixture forms a stable emulsion that is necessary for the offset printing process,” says Jim Galloway of INX International Ink's R&D center. “Anything beyond that creates pressroom print problems.”

Ink/water balance is the largest headache for offset printers. “There's always a small amount of fountain solution that emulsifies in the ink due to shearing forces and chemical affinity—for example, an ink loaded with polyesters or alkyds may draw more water than a hydrocarbon resin,” says Lisa Fine, president of consultancy Flexo Tech.

Galloway notes that if an offset ink doesn't emulsify with the fountain solution, the ink won't transfer evenly to the printing plate. “Getting the ink from the fountain to the substrate greatly depends on the split in the rollers and the adhesion properties of the rollers, plates and blankets during offset lithographic printing,” he says. “The amount of ink and water film thicknesses produced is very important to achieve the proper split emulsification film transfer.”

When ink contains too much water, it becomes over-emulsified—and you might as well shut down the press. “Generally speaking, the ink chemistry has a role as to how much affinity the ink has for the fountain solution and how stable the emulsion is,” says Don Sierzega, product manager, publication heatset for Sun Chemical.

Fountain solutions also play a key role. They lower surface tension and increase the plate's receptivity to water. For the most part, acid fountain solutions are ideal for heatset and sheetfed; neutral solutions for newspaper printing.

It's not just the ink and fountain solutions, however. Plates, substrates, temperature, humidity and pigment strength are among the many factors that can derail a press run.

“Higher press speeds increase shear forces,” says Fine. “Temperature plays a role, as a very hot day lowers the viscosity of the ink and pulls water in more easily. Too much water creates snowflaking, leaving voids and unclear images on the substrate. If there is no emulsification, the ink will mottle and the ink transfer will be poorer.”

Temperature and humidity both affect the level of fountain solution. “The only way for presses to cool down is by the evaporation of the fountain solution,” says Galloway. “Some presses (typically heatset) have water-cooled rollers to aid in this process.…Uncontrolled humidity in the pressroom will also cause varying evaporation rates.”

“Conditions can go out of control,” notes Sierzega, “whether it is temperature, process control, feedback in the dampening system, or the fountain solution not setting to the image, which leads to pushing more water. If conditions go awry the ink-water balance gets altered.”

Interestingly, higher-strength pigmented inks pose more of an ink-water balancing challenge. Read more online at graphicartsmonthly.com.