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FLEXO Magazine : October 2010
Technologies & Techniques flexographic printing/converting operations, printing is the first step, as well as any other decorative processes. A Word About Grain. The slurry that enters the “ wet end” of the paper making machine is about 99 percent water and 1 percent wood fiber. Through the giant length of the machine (Figure 3), as water is pressed and dried from the wood pulp, the individual solid fibers are aligned in the direction the slur- ry is traveling as it makes its way toward the “dry end,” where the resulting paperboard contains about 5 percent moisture. This alignment is called grain, and in the carton shop, folding a score on a box that runs with the grain is much easier than folding it across the grain. Back at the sheeter, a mill roll 28in. wide is being pulled off an unwind stand and cut into sheets 28in. X 40in. The grain direction always runs in the same direction in which the board was made, which in the sheeter is called the cutoff, in this case the 40in. length of the sheet. For consistency, grain direction is always expressed as the second dimension of a sheet of paperboard. A good packaging designer will use grain to his or her advantage to make a box that sets up eas- ily and closes securely. The way a box is laid out on a sheet can mean the difference between a tuck flap closure that locks securely or one that won’t lay flat and wants to pop open when it shouldn’t. Printing Printing presses cannot print continuous tones: smooth graduations from light to dark tones of the same color. The way printers have worked around this problem is to create an optical illusion called a halftone. Look at any photograph in a newspaper through a magnifying glass and you will see that it is composed of thousands of dots. The halftone converts a continuous tone into a pattern of solid dots. The dots are either very small for highlighted areas or very large (relatively speaking) for shadows. (In stochastic screens, the dots are all the same size, but randomly distributed, producing a simi- lar effect.) Viewed at a distance, the eye cannot distinguish between these tiny printed motes, and fills in the missing information to “reform” the image as a continuous tone. Color images are created in the same way. A color pho- tograph of continuous tone is separated and screened into the four process colors: cyan, magenta, yellow and black (CMYK). By combining various combinations of CMYK dots of differing sizes, many more colors may be simulated. These are called manufactured colors. See an example of how the four process colors combine to form the final image in Figure 6. Most, but not all colors may be reproduced with process inks. The expanded-gamut printing adds two or three more colors to the original four—either red, green and blue; orange and green; or custom colors—increasing the spectrum that may be produced. Some colors, such as certain shades of gray, red and violet do not manufacture perfectly from a combination of the four CMYK inks. In this case, these colors may be matched by a single spot color ink, providing there are enough units on the press. The number of halftones that are printed in a linear inch is called line screen. Its value is expressed as lines per inch (lpi). Newspapers, which are printed on porous, lower quality paper may require as little as a 70 line screen to reproduce a reason- ably sharp image, but high quality images printed on packag- ing will typically be printed between 133 and 150lpi. Glossy, color magazine print may go as high as 300lpi or more. Prepress. How does original artwork wind up on the fin- ished folding carton? In the old days (the distant 1980s), just about every printer using offset lithography, made films from four-color negatives which were then exposed onto printing plates, which in turn were inserted into the press, one printing unit for each color. If the customer required a color that could not be manufactured from the basic four-color process (cyan, magenta, yellow, black) and the printer only had a printing press with four stations, this spot color would require a sec- ond pass once the first four colors were laid down. Figure 4. Diagram of a folding carton in sheet (left) and tube form. Figure 5. Multiple cartons strategically laid out on the web to reduce waste. 44 FLeXO october 2010 www.flexography.org LIFE CAN BE GOOD AGAIN NO print defects were experienced NO unnecessary press stops occurred NO blade edge breaks Printing Doctor Blades Your blade life can be 10 times longer with SWEDCUT Distributed exclusively by CANADA – USA – MEXICO FLXON Incorporated +1.704.844.2434 www.flxon.com Exclusive Distribution in CANADA – USA – MEXICO FLXON Incorporated +1.704.844.2434 www.flxon.com Performance Through Quality FLXON_3LeadingPrinters_kd.indd 1 7/15/10 9:45 AM FLX_Oct10_mech.indd 44 10/15/10 12:32 AM
Sustainable Fall 2010