Impression Roller

A resilient cylinder or roller found on a gravure press forming a nip with the gravure cylinder through which the substrate passes. (This nip is referred to as the impression zone.) The impression roller, typically located above the gravure, or image-carrying, cylinder forms a hard yet resilient backing for the substrate. Its rubbery surface facilitates squeezing the substrate into the individual inked cells of the gravure cylinder where ink is transferred by capillary action.

The impression roller is a steel tube covered by an elastic polymer, such as natural rubber, or synthetic rubbers such as neoprene, Buna N, and polyurethanes (the coating having a thickness varying from H to I inch). It is driven by frictional forces generated by contact with the motor-driven gravure cylinder. The impression roller is usually smaller than the gravure cylinder, and consequently runs at a higher RPM than the gravure cylinder. The pressure at the nip also causes a deformation of the resilient surface of the impression roller, which slows down its rotation at the nip. Consequently, rotation of the impression roller is not uniform, and the velocity differential also puts additional strain on the web. The pressure of the impression roller against the gravure cylinder can be adjusted, and printers commonly adjust it to the least amount of pressure needed to generate the desired print quality, as increased pressure causes structural damage to the press bearings, the gravure cylinder, and the roller covering. The increased frictional forces also produce more heat, which can adversely affect print quality. Another impression roller setting factor is wrap, or how much contact the web of substrate has with the impression roller, described in terms of the angle formed by the web before and after the impression zone. A wrap of 120º is typical, but varies according to the substrate and the diameter of the roller; a 4:6-inch diameter roller will typically have a wrap of greater than 90º; a larger diameter roller and less wrap is desirable for heavier stock. One type of impression roller uses a system called Electrostatic Assist. ESA imparts opposite charges to the impression roller covering and the ink, the ink being attracted toward the substrate (behind which is the oppositely-charged impression roller). (See ["Electrostatic Assist [ESA]"].)

An important consideration in the use of the impression roller is heat buildup in the rubber covering, which can cause expansion, softening, and deterioration of the covering, all of which adversely affect print quality and web runnability. Expansion of the rubber due to heat can also increase the pressure in the nip enough to cause distortions in the gravure cylinder itself, or affect the cells and the ink therein, causing premature drying, drying-in, or a loss of detail in the highlights of the image. Some impression rollers utilize a method of cooling such as running cold air or water through the core, from a separate roller (which produces condensation problems, however), or utilizing a separate heat-exchange unit.

A major problem with impression rollers (especially wide ones) is their tendency to deflect. Deflection is a tendency for a cylinder or roller—especially one with a large width—to bow when pressurized or under pressure itself. Deflection commonly manifests itself in non-uniform ink transfer to the substrate. Some methods used to compensate for deflection include a three-roll system, in which a small-diameter impression roller is sandwiched between the gravure cylinder and a hard, larger-diameter back-up roller that applies pressure from the other side of the impression roller. This system is commonly used with small-diameter impression rollers. A drawback of it is the excess heat generation caused by an additional nip. Heat generation on a three-roll system can be at least twice as much as is generated by the impression nip itself. A centrally-supported, two-roll system reinforces the center of the core of the impression roller, transferring the site of the pressure from the edges of the impression roller to the center. A third means of deflection prevention is a cambered two-roll system, which uses a curved rubber surface on the impression roller to compensate for deflection of the gravure cylinder. However, this system needs to be tailored for each specific size of gravure cylinder, which may or may not be easy. Other systems being developed and utilized include a NIPCO-Roll system and an Albert Frankenthal K-Type Roller—both of which use a hydrostatic, internally-pressurized impression roller—and a Cerutti Flexible Impression Roll and Motter CDR Impression Roll.

As with rollers and cylinders in other printing press configurations, gravure impression rollers require various specifications for best results. Roller hardness is measured in durometer, and the covering should typically have a hardness between 60 and 95 durometer (for most commercial substrates). Consistency of hardness across the width and around the circumference of the roller is also important. The Gravure Association of America recommends for light, smooth subtrates (such as foil, cellophane, and some coated papers) a soft impression roller with a durometer of 60, or perhaps as high as 80 for the smoothest substrates. GAA recommends for calendered, laminated paper or coated paperboard a moderately hard roller possessing a durometer of 80:90. For rough paper (such as kraft or most paperboard), GAA recommends an impression roller with a durometer of greater than 90.

The total indicated runout (or out-of-roundness) of an impression roller should not exceed 0.003 inches. Surface roughness should also be kept to a minimum. The rubber covering should also not be adversely affected by fumes generated by solvent-based inks.

The setting of the impression roller pressure, as was mentioned earlier, is an important consideration. A typical means of determining the impression pressure is to measure the width of the nip area (called the flat). This checks several important aspects: ensuring that the impression roller and the gravure cylinder are exactly parallel, determining if there has been any expansion and/or softening of the rubber coating on the impression roller, and checking (somewhat crudely, but fairly effectively) the impression pressure. One means of accomplishing this is to apply grease and Carbon Black to the impression roller, insert bits of paper between the impression roller and gravure cylinder, and engage the impression. The size of the nip can then be determind by measuring the size of the black grease stain on the papers. A more convenient and less messy means of measuring the flat is to apply a strip of transparent adhesive tape to the carbon side of a sheet of carbon paper. Several of these tape/carbon combinations are placed strategically along the impression nip (while the web is still in the machine) and the impression is engaged, then released. A dark stripe of carbon will be apparent through the tape where the increased pressure was applied. The different sandwiches can be measured and compared.

The impression roller and incorrect settings thereof are responsible for several common gravure printing problems and defects. One potentially dangerous problem is the buildup of static electricity, caused by the rapidly moving web. This buildup (which can exceed 25,000 volts) can cause fires or dangerous electric shocks. Static is also the cause of whiskering, which manifests itself as hairlike tendrils extending off a printed image into non-image regions of the substrate. The static buildup pulls the ink from cells in undesirable directions as the substrate leaves the impression nip. Moistening the web is one way of eliminating static electric buildup.

(See Gravure.)

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