What is web offset ink?

In a Nutshell

Web offset ink is a specially formulated type of ink designed for high-speed printing on web offset presses. These presses print on a continuous roll (a "web") of paper, rather than on individual sheets. The ink's key characteristic is its ability to dry almost instantly through a combination of heat (from dryers) and absorption into the paper.

Key Characteristics of Web Offset Ink

To function effectively on a fast-moving web press, this ink has several distinct properties:

1. Low Viscosity (Thinness): It is much thinner than sheetfed offset ink. This allows it to be pumped efficiently from the ink reservoir to the rollers and then to the printing plates.

2. Rapid Drying: This is the most critical feature. Web presses run at extremely high speeds (e.g., 3,000 feet per minute). The ink must "set" and dry within a fraction of a second as the paper web passes through a high-temperature drying oven (called a dryer) immediately after printing.

3. Heat-Set vs. Cold-Set: Web inks are primarily categorized by their drying method:

   • Heat-Set Web Ink: The most common type. It contains resins that are solid at room temperature but dissolve in the ink oils when heated. As the printed web passes through the dryer, the heat evaporates the volatile solvents (mineral oils), causing the resins to "set" and bind the pigment to the paper. The web then passes through chill rollers to cool and solidify the ink instantly.

     • Used for: High-quality, glossy magazines, catalogs, advertising inserts, and brochures on coated paper.

   • Cold-Set Web Ink (or News Ink): This ink dries primarily through absorption into the paper, without the need for heat. It is much thinner and has a lower viscosity.

     • Used for: Newspapers, phone books, and other products printed on very porous, uncoated paper stock.

4. Pigmentation: Designed for high opacity and color strength to produce vibrant, consistent images at high speed.

The Web Offset Printing Process (Where the Ink is Used)

Understanding the process highlights why the ink's properties are so important:

1. The Roll: A massive roll of paper (the "web") is loaded into the press.

2. Printing Units: The web snakes through a series of printing units (for Cyan, Magenta, Yellow, and Black - CMYK).

3. Offset Principle: In each unit, the ink is applied to a plate (which has the image), which then transfers it to a rubber "blanket" cylinder, which finally offsets it onto the paper. This is the "offset" in the name.

4. The Dryer: Immediately after being printed, the wet web enters a long, hot-air dryer.

5. Chill Rolls: The hot web then passes over chilled rollers, which cool the ink and paper, solidifying the ink instantly.

6. Finishing: The continuous printed web is then folded, cut, and trimmed into its final product (like a signature for a magazine) in-line at the end of the press.

Summary

Web offset ink is a fast-drying, low-viscosity ink engineered for the massive scale and high velocity of web offset printing. Its ability to transition from a liquid to a solid state almost instantly via a heating and cooling process is what makes the mass production of publications like magazines and catalogs economically feasible.