Computer-To-Plate Technology Increases Platemaking Speed and Quality
In the initial days of computer-generated typesetting, galleys (long, single-column sheets of typeset material) arrived at the design studio having been hand-set by the typesetter. A paste-up artist waxed the back of the galleys, then cut them apart and positioned the type on the paste-up boards, along with amberlith or rubylith windows (or red, rectangular plastic patches) as placeholders for the images.
At the offset print shop, the print supplier would then use a huge press camera to create negative images of the art boards and halftone negatives of the photographs on clear film. He would then strip these negatives together on a sheet of yellow plastic "goldenrod."
Using a contact frame that held photographic plate material in close contact with the composite negatives, the printer would then burn a plate, using a bright light source to expose the plate emulsion through the negative. The printer then hung one plate per color (C, M, Y, K, and any spot colors) on each of the press inking units, and the process of offset lithography could proceed.
Computer page layout software became available in the late 1980s, and it was possible to either produce composite positive images (type and photos), which could be processed like the prior generation of paste-up boards, or composite negatives, which could be burned directly to printing plates.
In the 1990s and beyond, the trend has been to omit the film altogether and produce the printing plate directly from computer-generated page-layout information, bypassing the film step entirely. This is called CTP (computer-to-plate) technology.
CTP technology has benefited the printing industry in the following ways:
- In a CTP workflow, removing one generation of image reproduction (imaging plates directly from the digital information rather than producing film and then burning plates from the film) increases the sharpness of the type and image detail.
- Computer to plate technology eliminates defects in the plates caused by scratches in the emulsion and dust on the film, as well as defects from variations in film exposure and processing.
- CTP eliminates the cost and management of film and its related processing chemistry, saving money and benefiting the environment.
- Printing plates can be imaged more quickly, accurately, and consistently (from edge to edge across the plate): from approximately 60 to 300 plates per hour depending on the size and resolution of the plates. This is a huge productivity gain over film-based workflows. Printing plates can even be burned in an unattended mode, overnight.
Unfortunately, there are also a few disadvantages.
- Everything in the workflow must be digital, including all type and photos on all individual pages, plus the imposition information itself (positioning of all job pages on a plate, and hence on a press sheet). For instance, if a magazine accepts advertisements provided as film (very unusual these days), the film must be "copy-dot" scanned so it will be part of the all-digital workflow, and so the entire press form (eight pages of the document, for instance) can be imaged at once with all pages in place.
- If a CTP plate is damaged, the entire fully-imposed form (8 pages) must be produced again.
At present, it is noteworthy that the vast majority of prepress plate production is done via computer-to-plate technology (and the CTP workflow is even becoming more common in small quick-print shops).
Ultraviolet (UV) Inks Widen Printing Substrate Options
Conventional printing inks dry through absorption into the paper substrate or evaporation into the atmosphere. In contrast, UV inks are cured (or hardened) through exposure to UV light.
This opens up a lot of options for printing.
- UV inks cure instantly, so there is no need to wait for ink on one side of a press sheet to dry before printing the other side. This dramatically speeds up the printing process. It also eliminates problems such as the "offsetting" of images (that have not yet dried completely) from the front of a press sheet to the back of an adjacent press sheet in the paper stack.
- The instant curing of UV inks with UV light rays provides outstanding ink holdout. The ink particles sit up on the surface of the paper rather than being absorbed into the paper fibers. This yields rich, vibrant colors and crisp gloss coatings.
- Using UV inks allows print providers to print jobs on unusual substrates, (including non-porous surfaces) such as plastics, vinyl (for static clings), foil, synthetic paper, styrene, and canvas.
- UV inks eliminate VOCs (volatile organic compounds). The oxidation that accounts for ink drying with conventional inks releases toxins into the air. This does not happen with UV inks, since they do not rely on oxidation to cure.
1. The cost of a UV press as well as its plates, blankets, inks, coatings, and chemistry is significantly higher than the cost of conventional equipment and chemistry. That said, it is possible to get a UV press up to color more quickly, and the savings on makeready (i.e., less wasted paper and other materials) can add up quickly.
(However, even factoring in this savings, UV printing can be more expensive than traditional offset lithography, due to higher materials costs and electrical costs for the mercury vapor lamps. To get an accurate sense of comparative pricing, the best thing you can do is find printers that do both traditional offset and UV work and then request a price comparison.)
2. The learning curve for using UV inks is steep: including running the presses, choosing the right ink for the substrate on which you are printing, and then choosing the proper press blanket
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]