Guidelines for Printing on Supercalendered Paper
Leading industry printers, equipment and consumables manufacturers joined SC Council members on special SC Council Technical Advisory Boards to create the landmark Guidelines for Printing on Supercalendered Paper.
The Guidelines contain the collective recommendations of the Technical Advisory Board for obtaining optimum results with both pre-press and on-press stages of printing with SC paper using both heatset web offset and gravure.
Heatset Web Offset
These general prepress guidelines for heatset offset web presses represent an important starting point for getting excellent printed results with SC papers. These suggested prepress benchmarks can help position jobs for successful production runs on SC papers before they enter your pressroom.
Quality film separations are the first important step toward achieving outstanding printed results on SC papers.
Four-color film separations with a Total Area Coverage (TAC) of 260% to 280% (film densities) are recommended for SC papers. This lower level of film densities helps keep ink and water requirements at levels requiring lower oven temperatures resulting in maximum print and paper gloss. Lower TAC also minimizes web fluting.
Using higher film TAC that exceeds the recommended range can -- depending on paper properties and press temperatures -- result in a condition called "dry back", a chemical reaction that takes place over three to five days. Printing affected by "dry back" appears noticeably dull and lifeless, and is in stark contrast to the vivid, glossy product originally delivered by the press.
Dot gain is also a factor. To compensate for the additional dot gain commonly seen in running SC papers, it is suggested that four-color separations be sharpened by 3% to 5%.
Negative plating film, which picks up less dirt, is preferred by most printers with emulsion side down, right-reading.
Computer-to-plate systems provide opportunities to easily adjust prepress specifications. Recognizing that each press has its own individual operating characteristics, we suggest consulting with your suppliers to develop the CTP specifications that will deliver the best performance on SC papers.
On the majority of heatset offset presses in use today, the best results on SC papers are seen with screens of 120 to 133 lines per inch. Higher screens, such as 150 lines, generally are not recommended.
Square and oval dot screens minimize dot gain in mid-tones and generally have been found to produce the best results on most heatset offset web presses.
Printers using stochastic screens report excellent printed results on SC papers. The technology appears to be a very promising area for further development in the use of SC papers on heatset offset presses. We suggest contacting your suppliers to obtain information about stochastic screens for SC papers on your presses.
Control of water pickup is especially important for achieving outstanding results with SC papers. Using a plate that minimizes water pickup improves print quality and runability of SC papers.
By carrying water below the surface, fine-grain, deep-etched plates offer optimum water control. Excessive water on the surface reduces optimal print quality and can promote linting and blanket build-up.
Best results with SC papers are obtained with quick-release buffed blankets which provide better release and register.
Blanket Wash and Lubrication Spray
Blanket wash should not exceed 5% in water content. Check with your supplier for the range that works best for SC papers on your presses.
Many printers report good results on SC papers with the same process inks they use for standard lightweight coated papers on their presses. With easily managed ink tack adjustments, they achieve excellent production with SC papers on presses that handle other types of paper as well.
Optimum ink tacks will vary depending on the SC paper you're running and your press performance characteristics. We recommend working with your SC paper, ink and press technical representatives to determine the ink tacks that work best to produce good results with SC papers on your equipment.
In general, ink tacks are approximately 8.0 for first down blacks, followed by decreasing tacks down to 6.5 for the last color down. The usual ink sequences are black, cyan, magenta and yellow.
The industry's leading ink manufacturers also offer inks formulated specifically for use with SC papers. With their special viscosity and yield value, SC inks minimize dot gain for higher dot fidelity. In some cases, the flow characteristics of SC inks differ from those of mono-tack and coated inks.
Minimizing the amount of water in the ink-and-water balance without scumming, produces several benefits. It allows lower oven temperatures, better register control, limits paper surface roughening, produces higher print gloss, improves ink mileage and reduces lint problems.
Fountain solutions should be kept to a pH range between 3.8 and 4.5 for SC papers. Check with your fountain solution supplier for the conductivity range recommended for use with SC papers.
Maintain the infeed tension at the minimum level needed to hold register, while avoiding doubling as well as slurring. Too tight a folder nip can override back to the infeed and cause web breaks.
Discuss the optimum tension control settings with your SC paper and press manufacturer's technical representatives to assure the best runability performance with SC papers.
Keep paper web exit temperatures below 270o F. Staying at or below that heat level will retain the folding properties of SC papers, and will also preserve the gloss and overall print quality of the finished product. The lower temperature also helps to lower energy costs. Excessive drying heat can roughen the surface of SC papers, dulling the appearance of the printed product.
For best performance on double-web jobs with a configuration requiring a narrow and a wide-width roll, it is recommended that the narrow roll run as the bottom web, centered in press, and printing in the second set of units. This procedure minimizes web tension, stresses and eliminates air turns on the narrow rolls.
SC paper rolls should be kept in their mill wrappers until shortly before their use in the pressroom to minimize excessive moisture absorption that could affect on press performance.
Since each gravure press has its own operating fingerprint, the following prepress guidelines offer a general perspective on points to consider when readying jobs for production on SC papers.
Cell shape tends to influence ink release on the very smooth surface of SC papers. Engraving a geometric cylinder cell shape that makes it easy to evacuate all ink from the cell is extremely important. A smooth cell shape maintains consistent ink release onto the paper at high press speeds. Electrostatic assist (ESA) also contributes significantly to the thorough ink evacuation of cells on press.
The use of new, advanced digital engraving systems can provide more flexibility and better control of results than film-based processes. The systems offer opportunities to develop operating algorithms that can be used to engrave cylinders with cell structures that optimize results based on specific SC papers, presses and customer job requirements. These algorithms containing precise specifications, like press fingerprints, subtle gradation and sharpness values, can be stored and later downloaded for use in producing recurring jobs such as magazines and catalogs.
Software is now available that can also help analyze and reduce Total Ink Coverage (TAC) for individual gravure presses with customized cell engraving specifications. With the growing use of PDF files by customers, engraving systems that can automatically optimize incoming job files for gravure production on SC papers can play an important role in assuring consistent high quality and productivity.
Ink and Engraving
Printers running SC on gravure presses report good results using both coated formulation inks and uncoated formulation inks. The bottom line is that printers should work closely with customers to create sets of internal standards to determine which inks and engraving specifications will deliver the printed results they prefer on SC papers.
Normal viscosity levels for SCA generally are in the 14 second to 18 second range, using a Shell no. 2 eflux cup. Staying within this viscosity range will, in most circumstances, usually allow printing of the light tones without plugging the solids.
SC papers are manufactured with internal strength that allows them to perform extremely well on state-of-the-art gravure presses, designed to operate at speeds up to 3,000 feet per minute. Maintaining those high press speeds for maximum productivity is not unusual for SC papers, even at the lighter 28- to 30-pound weights. Moreover, the consistent production reliance of SC webs on high-speed gravure presses is well documented.
Maintaining the right durometer readings for the hardness of the rubber impression rollers is very important to achieving optimum results on SC papers. Lower tension adjustments are also important to on-press performance with SC papers. Ink and cylinder temperatures should also be checked. House-cleaning the end of the pressure roller connectors, and consistent preventive maintenance for the ESA systems, is essential for good operating conditions. Because the ESA improves the printability of both SC and coated papers on a gravure press, it is important to keep the system clean and in peak operating condition.
In producing a job, it is recommended that SC and other grades of paper run their rolls by position to maximize performance. Roll position is defined by identifying the trim position on the master reel at the paper mill. Roll labels note the positions of each roll taken from the master reel.The heavier weights of the SC paper rolls now being manufactured by the mills for the wider gravure presses is an important consideration in establishing roll handling procedures. A 132-inch roll weighs over 10,000 pounds, and a 137-inch, 50-inch diameter roll of SC+ weighs nearly 12,000 pounds. Before handling rolls of that size, it is essential to check and adjust clamp truck pressures to carry those greater roll weights while avoiding roll-handling damage.
Some producers of SC paper manufacture specific grades for use on offset or gravure presses. Other producers provide a duplex grade suitable for either printing process.
It is important to note that when a grade is specified for gravure "only", it will not be suitable for production on an offset press since it will lack the needed chemistry to withstand the water and heat rigors of the high-speed offset printing process.
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