Choice Coats

Not all two-side stocks are coated alike. Most have better printability and consistency on one side or another, which on press, results in unevenness in color and gloss. This is why one side of a sheet is typically preferred when a job requires high-quality graphics and color. But if both sides require the same high-quality graphics and color, significant press adjustments are required, creating press downtime, inefficient use of labor and wasted press sheets. And in the end, usually one side still presents better quality over the other. Why the difference?Why does coating each side of a sheet with two coatings make a difference, as opposed to double-coating the front side and single coating the back? For starters, the thickness of the coating may differ from one side to another. While some paper mills attempt to achieve even thickness on each side, it's a difficult task. To simulate a double-coating, a single coating is applied at greater thickness than the two individual layers on the other side. In contrast, when the coated, two-sided stock (C2S) receives coating from two coaters in a single pass, the mill is better able to achieve consistent coat weight across the sheet. But if an excess amount of coating is applied in one pass (more than six or seven lbs. of coat weight), it's difficult to control the thickness across the full length of the machine. A true double coating is the only guarantee for overall quality assurance.And printers?On press, two problems can arise for printers working with double-sided coat sheets. For instance, if the coatings have two different thicknesses, printers will experience different ink holdout on each side of the sheet. The single- and double-coated sides of the sheets are essentially two very different surfaces; ink can vary dramatically. Smoothness is another issue with which a printer must reckon. Since the single-coated side tends to be slightly rougher than the double-coated side, the objective is to fill in fiber inconsistencies and create a smooth surface. Starting out with a fairly rough fiber-base sheet, as most Mills do, the coating is used to further fine-tune the quality of the sheet. When mills pull down only one coating, they are attempting to duplicate a two-step operation with only one application. The same example can be applied to painting a house. First, you apply a primer coat. If you skip the primer, the finish coat tends to soak into the wall. But if you are willing to apply enough of a finish, you can achieve a surface that looks fairly uniform, but not perfect. However, painting your house with the appropriate amount of primer and top coat will always result in a better end product. Bound by historyThe reason more mills don't apply two-sided double-coating is because of history. Years ago, the paper and board markets demanded one-side coated stock when specifications were not as critical. At best, customers were demanding superiority over the uncoated sheet. Over the years, however, the need for high-quality C2S papers has risen even if it's not always possible to add coaters to existing machinery after the fact. Building a paper machine is a substantial financial and time investment that requires an enormous amount of floor space. If a mill wants to add a coater, it requires a dryer on one end of the paper web, even though paper machines are typically followed by a winder unit that slices the web down into smaller rolls. So, if a mill decides to add a second coater, it has to relocate the winder, as well as invest in new equipment. Because the machines are fixed in concrete, making changes often requires a significant infrastructure adjustment.Does coating really make a difference?The question asks whether C2S makes a difference for printers and publishers. The answer is that it depends on the application. If a printing job requires complex graphics on one side, but simple text or graphics on the reverse side, the answer may be no. But in most cases, if quality requirements are equal on both sides, C2S is ideal. In fact, even if graphical requirements are not the same on both sides, a true, coated two-side sheet still saves time and money.First, a two-sided coat results in significantly better runnability because press operators don't have to make adjustments when printing on one side to the other. Secondly, two-sided coated stock improves overall productivity because without press stoppages, jobs can go straight to finishing faster, saving labor, makeready and waste sheets. And thirdly, using C2S improves quality by eliminating the human factor from the operation since press operators are called to get involved less in stops and switches.Questions to askBefore using a C2S, it's important to understand not only the paper, but also the printer's capabilities. Ask about the range of C2S papers a mill offers. If your shop is printing a wide range of applications, you want a supplier that offers true double-coated paper in all point ranges. If you are a combination printer, you may also want to ask if they offer both heatset and nonheatset grades. Don't forget technical service. Although many suppliers will reimburse printers for problems wit paper stocks, they may not take the extra step to diagnose the problem to help prevent it in the future. Real technical service is more than paying off a claim; it involves helping printers solve problems and getting presses back up and running.Finally, C2S sheets may all look the same unprinted, but it doesn't mean they print alike. Printers using true coated two-sided paper can reap considerable time savings and quality benefits while improving ink holdout and print consistency. They can also improve productivity by keeping their presses doing what they do best—rolling out high-quality work.-Steven Parker, product development manager, and Rich Wicks, manager of technical sales and service, Tango Papers

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