Conventional or wet-offset printing requires two different fluids—ink and water—to transfer an image from plate to paper. Wet-offset printing uses an aluminum plate with a surface that is specially grained to attract water. An image is formed on the plate in a polymer coating that attracts ink and resists water. During printing, the background of the plate is wetted with water, then ink is applied. The background area is wetted by the water and becomes ink-resistant. The polymer image area resists the water and so remains attractive to ink. Note that working with two fluids—each with very different properties—can be a complex process, especially in today’s high-speed offset presses.
To stabilize this two-fluid process, a range of chemicals has been added to the water used in offset printing. Light acids, gums, and alcohols are all added to the water to make it react more consistently with offset inks and make the process more manageable.
Waterless printing eliminates the need for water or other chemicals in the printing process by using a special printing plate that has only two surface properties: ink-resistant and ink-attractive. By working with only one fluid—ink—as opposed to two fundamentally different fluids—ink and water—the printing process is greatly simplified and some very unique benefits can be realized. Waterless runs on virtually any offset press but requires a special waterless plate, waterless ink, and a means of cooling to control temperature on press.
Compared to wet-offset plates, which must be able to selectively attract and resist two different fluids, waterless plates are simpler: The surface of the plate is made up of ink-repellent silicone and the image area is made up of an ink-loving polymer. A finished plate has only two surface properties: an ink repellent in the non-image area and ink-loving image area. Because there is no need to maintain a precise balance of two dissimilar fluids, waterless is a simple physical process rather than a complex chemical one.
Waterless inks are very similar to conventional offset inks and are available in the full range of process and PMS colors. Waterless inks are formulated to have a slightly higher viscosity than conventional inks. Since viscosity is the key physical property that causes the ink to be repelled or attracted to the waterless plate, waterless ink viscosity must be predictable and stable over a range of operating temperatures. Waterless ink formulations are available from virtually all ink suppliers.
The third key component in the waterless printing system is a means of temperature control on press. Today’s high-speed offset presses all have temperature-control systems to keep the press temperature stable during extended press runs. Waterless typically requires that press temperature be maintained within a range of roughly 15° Fahrenheit to ensure that ink viscosity is optimum for best printing results.