KEEP YOUR C1S PRINTING TROUBLE FREE
Editorial Contact: Lisa K. Winegar A.B. Isacson Associates, Inc. 225 Park Avenue South New York, NY 10003 Tel: 212-529-4500 Fax: 212-529-4442 E-mail: lwinegar@abipr.com By Bryan J. Ortman, Ph.D. What is the most important quality a printer looks for in a paper? Simply put - runnability. The question is then, what does it take to achieve optimum runnability with a C1S/C2S sheet? The performance of a C1S sheet is largely dependent on the configuration of the sheet itself. What happens to a web in the mill is directly related to what will happen on your press. Smoothing Things Out Quality problems on press are often directly related to a lack of paper smoothness. Mottle, poor ink holdout, and poor ink coverage are typical problems with rough C1S sheets. Obtaining smoothness is a mechanical process. The more times a mill can iron the sheet, the smoother it becomes. Papermakers' terms for the ironing process include calendaring, wet stacking and dry stacking, but what matters to a printer is that more ironing equates to better runnability. How can you know whether your sheet has that extra bit of ironing ? Ask if the sheet goes through a dry stack after the wet stack. In a typical paper making process, after the web is run through a hot press to remove excess water, a wet stack adds moisture back into the paper in order to iron it. But if you stop there, your sheet is not going to be as smooth as you want it to be. Your mill should add the extra step of running the paper through a dry stack, which not only adds smoothness but also allows the mill to target certain problem areas to achieve more uniformity. And of course, uniformity translates into a more forgiving sheet on your press. The use of an extended nip press also helps in the manufacture of smooth paper. Most mill press sections have two or three nips through which the paper web is run. An extended nip distributes the weight more evenly against the smoothing roller and imparts a greater degree of smoothness to the sheet prior to the drying and ironing stages. As a final stage, the paper goes through a finishing process. This can be done in a way that maximizes smoothness. The mill can use either a gloss calendar roll or a hot/soft calendar roll. The hot/soft roll is preferable because it finishes the paper at a higher temperature, which results in a smoother surface. Like ironing, the higher the temperature, the more wrinkles are eliminated. Don't Forget Your Coat Sometimes coated board will have scratches and streaks caused by particles trapped under the coating blade. Talk about problems on press! This is one you want to avoid. The best coating operation helps minimize these problems. There are two coating methods used in producing C1S stock: air-knife and blade-knife - and they are quite different when it comes to delivering a blemish-free sheet. Traditional processes use an air knife coater that follows the surface of the paper. It's a nice idea, but the end product is only as smooth as the substrate. If the base sheet has bumps this process will simply follow the highs and lows - giving you hills and valleys to print on. Blade knife coating is what you want to look for. It applies an even coating no matter what the underlying surface smoothness might be. Coatings can be made even smoother by using a double blade coating with a bent blade, rather than a straight one. The coating formulation itself is another area that can make a huge difference to the printing process. This is the actual recipe a papermaker uses for your printing surface. One of the most important qualities to ask about is coating viscosity. A high solids coating formulation makes the sheet smooth and pliable. Go for Clean A great frustration on press is sheet debris that leads to hickeys. A clean mill - you guessed it - contributes to a clean sheet. And the cleaner the sheet, the longer you can run the press without having to clean up. If C1S/C2S is an important part of your mix, you might consider a mill tour. For your paper to perform well, the mill needs good internal housekeeping. For some mills various contaminants, such as dust or lint, fall into the machine and collect on the sheet as the machine is running. These contaminants adhere to the press rollers during print production and require printers to either shut down and clean up the press, or reduce the tack on the ink. To a certain extent, the degree to which a mill produces a clean sheet also depends on its coating formulations. Coatings with strong binders reduce the amount of debris. Dimensional Stability A good C1S/C2S sheet should perform consistently. But that's not possible if the sheet is warping, has welts, or shrinks. A sheet can loose its dimensional stability due to improper fiber furnish, incorrect drying, inconsistent moisture, or any combination of these. Good fiber furnish is a matter of balance between long and short fibers. Long fibers create a stable, yet rough sheet. Short fibers create a much smoother surface, but do not add much strength. Printers should consider a C1S stock that combines a mix of both fibers. When the right balance is achieved, the resulting sheet is durable, smooth and stable. Non-uniform moisture is another reason for poor dimensional stability. This is often caused by insufficient control over a mill's internal processes. A mill with a high level of automation, has a better chance of reducing this variability. Though many mills employ scanning equipment, some use more automation throughout the entire production process. This can only add to the consistency of the final product. Light and Bright Brightness of the sheet and light-fastness, or the degree to which sheets will yellow over time, also make a difference to the success of your project. Light-fastness is affected by the formulations used to create the coating. If the mill uses dyes or optical brightners, the coating fades over time. Conversely, when pigments are used, the sheet better represents a true white paper. Customers concerned about the staying power of color should stick with sheets that use the pigment method. There are two main pigments used in the papermaking industry - clay and calcium carbonate. Until recently, most C1S stocks used clay, which resulted in a brightness of about 82. Tango paper was the first C1S sheet to introduce calcium carbonate brighteners in 1997, which elevated its brightness to 88. Today, several mills have followed suit, resulting in similar brightness levels among various C1S stocks. Choosing the Right C1S Stock for You The brief time spent choosing the right stock will be worthwhile. Not only are printers able to produce higher quality jobs, but a growing number are using these stocks to replace higher cost papers. Although you may not be taking a trip to a paper mill any time soon, asking a few questions will help you determine the quality of a sheet. Here is a quick checklist of things to look for. Does the mill: · Have a high level of plant automation? · Take extra steps to ensure quality control (such as visual inspection systems and electronic scanners)? · Take extra steps in the calendaring and ironing processes? · Use blade rather than knife coating (specifically double blade coating with the use of a bent blade)? · Use pigments rather than optical brighteners? · Combine higher percentage of long fibers with extra refining and calendaring steps? · Use hot/soft calendar finishing? In the end, the biggest benefit for printers using a high quality C1S sheet is simply that they can avoid problems when printing every day jobs. This means better color, more consistency and less press downtime - run after run. Terms: Extended nip - a mill feature that distributes the weight of a sheet more evenly against the smoothing roller. Dry stack - a mill process that is added after the wet stack to create greater sheet smoothness. Double blade knife coating - a coating method that fills in the hills and valleys of the base substrate. About the author: Dr. Bryan Ortman is the Group Leader, Coating Development for Tango Paper, Westvaco. About Tango Paper: As an exceptionally clean sheet, Tango C1S eliminates repeated blanket and plate wash-ups related to hickeys. The paper's blemish-free, consistent coating also provides an even foundation for ink dots that result in razor-sharp printed images. Tango is manufactured in Evadale, Texas, and is a product of Westvaco Corporation. Swatch books can be ordered by calling (800) 597-2437. # # # #