Commentary & Analysis
Fuller Range of Substrates Helps Fuel Growth of Digital Printing
by Arun Chowdry,
By WhatTheyThink Staff
Published: March 8, 2004
by Arun Chowdry, Chief Technical Officer and Vice President, NexPress Solutions LLC March 8, 2004 -- Without the right substrate, digital color print pieces aren't worth much more than the paper they're printed on. In fact, digital image quality is directly linked to the suitability of a substrate. In the past, the limited selection of acceptable print stocks was an obstacle to the wide acceptance of digital printing. Papers for digital printing were often expensive stocks developed specifically for this technology. In recent years, though, print engine and paper technology have advanced to make high-quality digital printing a reality on a variety of digital print engines. But understanding what makes a paper appropriate for digital printing is a vital part of delivering a successful print job. In general, the best papers for digital output are calendered and coated papers because of their uniform surface. They offer the best conditions for producing a color-intensive, glossy printed image. Paper often reacts to variations in humidity and temperature. In the electrophotographic process used on today's digital presses, colorant is applied to the paper using an electrical charge, which is then fixed with heat and pressure. Understanding these factors can help print businesses make the most of their digital printing equipment. Electrostatic charge Electrostatic charge plays an important role in substrate performance. The better the formation of a substrate, the lower the variations in electrostatic conductivity when transferring the colorant to the printing stock. A good-formation substrate is essential for even and complete colorant transfer. For papers with a poor formation (e.g. uncoated paper), conductivity variations need to be compensated. The latest digital presses compensate for these variations so differences in printing stocks become relatively insignificant. This enables printing on traditional inexpensive offset papers. Even colorant application For textured papers, there is a risk of not enough colorant reaching the hollows of the surface, resulting in an uneven print image. Vendors offer different technologies for addressing this , enabling a press to adapt itself to the topography of the printing stock-- ensuring an even print quality throughout the piece. The NexPress 2100, for example, uses a compliant blanket cylinder. Printing heavy papers Because colorant is fixed by heat and pressure, high paper weights result in some digital presses not being able to print at full production speed. Again, different approaches are used to accommodate heavier stocks at or near rated speeds. Paper paths, imaging technology, fusing processes and temperatures, and turn radii, and other characteristics of a press generally dictate production speeds. The extreme flexural strength of the materials also leads to paper-travel problems. These factors also enable papers to be printed independently of the grain direction, providing a high level of finishing flexibility. Rapid substrate change Frequent paper stock changes within a job are common with digital color printing. Many newer digital pressesareconfigured with multiple paper drawers to allow several stocks to be used on a single job. Press operators can specify the stock in a given drawer so the press automatically adjusts to that substrate and, in most cases, paper can be changed during the printing process without having to stop the press. Substrate Flexibility Substrate flexibility is imperative to the success of digital color printing. Digital presses need to be resilient to the "moods" of the printing stock and prints, with consistent quality and speeds, and equipped to handle a diverse range of substrate parameters. Demanding substrate requirements only lead to greater obstacles in acceptance of this technology. Most major press manufacturers have programs for engineering, developing, testing and qualifying all manner of substrates and work closely with paper vendors and mills to ensure papers will deliver the desired results on their presses. Furthermore, some certify or qualify a broad range of substrates from different paper companies for use in their machines. In the qualification process, the papers are subjected to rigorous tests to determine the best settings for the press. After testing all the relevant substrate characteristics, the printing stock is qualified and adopted into the list of substrates qualified. On some presses this qualification information is stored as scripts in the front end of the press, facilitating set up when a qualified stock is used. Trend towards special substrates Digital print users have known for a long time that the combination of variable data printing and certain synthetic substrates can be used to help win over new customer groups. However, synthetic substrates are extremely sensitive to temperature. The variable fixing temperature of the latest high-end presses means users can still expect to be able to process a broad spectrum of synthetic substrates and adhesive labels. As significant advances in digital color technology offer high-quality imaging, increased speed and greater flexibility in producing new, value-added applications, digital paper becomes increasingly important to print providers. Substrate quality and the range of paper stocks available to digital press users need to accommodate this technology advances and the resulting increased demand to help drive continued growth in the industry. Arun Chowdry received a B. Tech degree in electronics from the Indian Institute of Technology, Kharagpur, and a Ph.D. in electrical engineering from The Johns Hopkins University in 1974. From 1974 to 1998, he was at Eastman Kodak, working on various aspects of electrophotography and its applications. He has been at NexPress Solutions LLC, a joint venture between Eastman Kodak and Heidelberg, since its formation in 1998. He is currently Chief Technical Officer and Vice President, NexPress.