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FLEXO Magazine : October 2010
Technologies & Techniques In the modern prepress room, the need for the intermediary step of creating films is increasingly rare. Digital platemaking technology allows the image (now computerized with desktop publishing software) to be output directly to the printing plate. Flexo versus Offset. For the folding carton industry, lithography (more commonly litho or offset) has long been the domain of the highest possible print quality. Improvements in computer and press technology, however, are leveling the playing field, so that purchasing decisions can be made more and more on other factors, such as run length, setup times and the cost of makeready materials such as plates, blankets and ink. Lithography is based on the principal that basically, oil and water don’t mix. There is no raised surface on the printing plate; that is, it is planographic, consisting of imaging sur- faces that repel water, and non-printing surfaces that attract it. The printing plate is attached to a cylinder that is covered in water before the ink is applied. When the ink is delivered to the plate, it adheres only to the image area. As the plate cyl- inder turns, the ink is transferred to a blanket cylinder which does the actual printing to the substrate. Flexography, commonly known as flexo, has been making incremental in-roads in the folding carton industry for years. As flexo carton printing technology improves, this is expected to increase over time. (See FLEXO August 2009, page 62; or November 2008, page 32.) Flexo uses a rubber or photopolymer printing plate with raised surfaces that carry the ink. The ink is transferred to an anilox roll, which, carries a fixed volume of ink on its knurled surface. This roll then transfers the ink to the raised image areas on the flexo plate. Improvements in flexo technology are challenging the print quality of lithography and other print processes. Flexo press- es are always web-fed, but may have in-line rotary or flat bed die cutting units at the delivery end. This means it is possible to start with a roll of unprinted paperboard at the feed end of the flexo press, and finish with printed, die cut and scored folding carton blanks at the delivery, ready for windowing, gluing or other finishing (depending on press capabilities, it is possible for those processes to occur in-line before scoring/ die cutting, or even after). Cutting and Creasing Once a sheet or web of paperboard has been printed, the shape of the container created in the design phase is used to manufacture the cutting die (either flatbed or rotary). Think of the die as an elaborate cookie cutter. Back in the early days of folding carton manufacture, printing was accomplished primarily on flatbed presses. The sheet was hand-fed into the press, wrapped around a rotating cylinder and printed as it passed over a reciprocating letterpress die moving back and forth beneath it (Figure 7). By removing the ink rollers and replacing the printing die with a cutting and creasing die, this same configuration could be used to cut and score the dried, printed sheet. Today, in the modern reciprocal die cutter, the die moves up and down, not back and forth. The pile of printed sheets is fed from the right, registered on the feed board and pulled into the cutting station. The steel rule die is secured to a recipro- cating platen which descends with precise timing and many hundred tons of pressure onto the sheet. Figure 6. An example of CMYK separations. 46 FLeXO oCtober 2010 www.flexography.org FLX_Oct10_mech.indd 46 10/15/10 12:32 AM
Sustainable Fall 2010