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Feb 1, 1996 12:00 AM
Okey, okay, it's true. They're not as exciting as, say, a brand-new eight-color press with all the fancy features. And, they certainly aren't one of the larger expenditures a printing firm must concern itself with.
But, printing blankets are one of the most important factors influencing quality print output. They're an important investment, and may be the ultimate determination of customers' acceptance or rejection of finished products.
In essence, your firm's reputation may be riding on that little sheet of rubber.
There are a number of questions printers must consider to ensure a smooth ride. Which blanket type is best for them--conventional or compressible (most likely the latter)? What factors, such as uniform thickness, surface texture, squareness and ink receptivity, are important in blanket selection?
Printers also must understand the guidelines for preparing new blankets for press, including cutting, punching and packing rules. In addition, how can you prevent smashes, embosses and other difficulties? Lastly, how do you properly maintain blankets?
Blankets--synthetic rubber-covered textile composites--have exhibited slow, steady technological progress over the years. One relatively new change is the cylindrical blanket available for the Heidelberg Harris Sunday press, reports Jon Kirtsey, sales manager for Reeves International (Spartanburg, SC).
"This blanket is gapless, which reduces paper consumption," he says. "A conventional flat blanket, when wrapped around a unit and secured by some kind of locking mechanism, typically has a 1/16 or 18-inch gap. Since this cylindrical blanket has no gap, shops don't see that paper waste." Reeves and Day International are the only authorized manufacturers of this type blanket.
"Blanket stability also is improving--in the past year or two, we've seen an increased weave in the carcass, more threads per inch, which offers more stability," notes Lloyd P. Dejidas, director of graphic services for the Graphic Arts Technical Foundation (Pittsburgh). "Also, all manufacturers strive for a blanket that's packed once and stays put, and they're getting pretty close to it. We're testing blankets now that perhaps we're only repacking once or twice at the most, so we see much more stability in that blanket when it comes to plate-to-blanket pressure."
Two fundamental types of blankets exist--conventional (rarely used today) and compressible. "While the former are still available, when using them everything in the process must be almost perfect," relates Kirtsey. "Packing must be exactly right, for instance. Conventional products lack the latitude of compressible blankets."
"Compressible blankets feature an additional layer that functions similar to a shock absorber, allowing a blanket to rebound after it has taken a hit," explains Dave Kennedy, vice president of sales and marketing for D.Y.C. Supply Co. (Fort Lee, NJ).
According to the experts, compressible types also offer greater packing latitude, or the ability to give high-quality print over a broad range of thickness of total pack. While conventional blankets once offered better print quality, with less dot gain and better dot shape, developments in compressible blankets have evened out those types of differences.
Of course, blankets differ in other ways as well. Various surfaces are available, from ground and buffed to cast and texturized. A buffed surface, for instance, is said to translate into more uniform printing pressure on press, which, in turn, means a more uniform print. Blankets, too, feature different hardnesses. Reportedly, the harder the blanket, the better the release. They also vary in terms of uniform thickness, squareness, resistance to tackiness and glazing, etc.
But, which type is right for your shop? It depends on your equipment (whether sheet-fed or web), substrate, type of job and other factors.
"We don't recommend using conventional blankets on webs," offers Wayne Rentz, manager of sales and technical services, Pacific Rim, for Day International (Dayton, OH). "One advantage of compressible blankets is that they absorb vibrations caused by higher speeds. A conventional blanket won't do that. Instead, it transfers essentially everything, so your running speed will be reduced. Still, I am aware of some customers who employ conventional blankets on web presses."
"We would first determine whether or not the shop used coated or uncoated stock, ink type (ultraviolet or conventional) and press size," explains Herb Mycroft, technical director of the David M. Co. (Longwood, FL). "The bigger the press and the faster shops run it, the more demands are made on the blanket."
"Calendered papers and heatset printing, for example, require a different type of blanket than does newsprint," adds Rentz. "Newspapers require a very absorbent paper, and the blanket must carry a lot of ink. Therefore, the blanket doesn't require as smooth a finish for newspapers as it would for heatset. After all, in heatset, ink lays atop the paper, and too rough a surface could result in distortion."
What about blanket hardness? According to Day International experts, hardness is a relative term. What one vendor describes as hard may only be medium hardness to another. This blanket characteristic also varies with use. Frequent printing of large solids, for instance, softens a blanket.
"Hard blankets tend to have better release," Mycroft says. "But, if you print on a rough substrate, soft blankets do a better job; they get more ink into the pits of the material and offer better ink lay."
"Some printers use one type of blanket for all their work, selecting one style of medium hardness," notes Mark Terrel, marketing services administrator for Day International. "That means sacrificing a certain amount of quality, rather than changing blankets for various jobs. But, some shops that handle a variety of work may use several types of blankets, changing between jobs."
Once you've selected the blankets that are right for your needs, the next step is installing them on press. Before installation, experts from Reeves International recommend that press operators:
* Decide on a packing type: polyester, paper, etc.
* Examine the packing for correct thickness, size and squareness. The width of the packing should be calculated to finish 1/8 inch inside the maximum paper web width.
* Check blanket and packing for correct thickness, size and squareness. Thickness should be checked with a desktop micrometer.
* Combine the right gauge blanket and packing to attain the required overpack. Most blankets will lose approximately .001 inch during tensioning, and allowances should be made to compensate for this loss.
* Clean the blanket cylinder thoroughly.
Install the blanket and packing as recommended by press manufacturers, advises Reeves. A torque wrench reportedly is the best way to assure consistent and uniform tensioning.
"Based on manufacturer recommendations, printers can establish the ideal torque values for each press and blanket type," relates Kirtsey of Reeves. "First, they should tension the blanket initially to the established torque value. Then, apply the impression pressure and rotate the cylinder several times, which helps distribute the tension around the blanket and ensures it will conform smoothly to the packing surface. Retension the blanket before press startup and again after approximately 5,000 impressions."
"However, if operators overtension blankets, they can draw them down too thin and lose print," warns Rentz. "If, on the other hand, printers overpack blankets, they can distort the dot and cause web feed problems."
"Overtensioning blankets means operators won't get the full amount of life out of the blankets because they're basically squeezing all the compressibility from the blankets. The print will suffer as well," remarks Laurelle Sciola of Polyfibron Technologies (Billerica, MA).
Just because operators have mounted blankets properly doesn't mean they can sit back and relax, though. A wide array of potential hazards exists during runs, from smashing and embossing to a buildup of coating on blankets that causes them not to pick up ink. "Blankets typically don't die a natural death--they're murdered," asserts Kirtsey.
A smash can be caused by numerous factors, such as wood chips or extra pieces of paper going through the press. For example, if a sheet-fed machine picks up four or five sheets instead of one, it forces a low spot or cuts the blanket, and the press won't transfer ink in that area.
An emboss, on the other hand, refers to a swelling or raised surface on the blanket, reports Kirtsey. An emboss occurs when the vehicle in the ink or the solvent used on press is absorbed into and thickens the blanket surface. The result? Dot gain or dot slur. Prevention is easy: simply use a blanket resistant to your solvents.
But, these difficulties aren't the only problems blankets can encounter.
"A coating buildup on blankets can cause them not to pick up ink. Also, washing solvents can make the blanket tacky and cause the paper to stick and cause wrapups," points out Rentz, "Printers can't always prevent coating buildup because customers may supply the paper. The only remedy is just to clean the blankets more often."
Does any remedy exist for smashes? "It depends on their severity," he adds. "If the surface of the rubber is damaged, operators must remove the blanket. But, depending on press design, printers sometimes can roll the smash out of the print image area. Also, if the smash doesn't damage the face of the blanket, blanket rejuvenators will raise the portion of the blanket that's been smashed back up to proper print height.
"But, that's only a temporary fix and we recommend using it only until operators get the job off press, at which time they should change blankets. Those rejuvenators actually remove plasticizers, which enables them to work and swell the blanket, but then that portion of the blanket becomes tacky."
Proper maintenance is vital to get the most from your printing blankets. How often they should be washed depends on the amount of ink being used and the type and quality of stock, according to Rentz. "We prefer to wash by hand rather than utilize automatic blanket washers because operators use much less solvent during hand washing."
Water and solvents both are necessary to clean blankets properly, according to experts at Reeves. Paper dust, talc and gum arabic are dissolved or loosened by water, but solvents must be used to dilute and remove ink.
"When cleaning blankets, press operators should rub the blanket surface with a cloth or soft sponge moistened with the wash solution and immediately dry any excessive solution with a clean cloth," says Kirtsey of Reeves. "They also must take care to protect blanket edges from excessive exposure to the wash solution."
According to the manufacturer, these solvents generally aren't harmful to blankets if used in moderate amounts:
* Petroleum distillates with boiling points from 190 [degrees] F to 265 [degrees] F
* Fast-drying mineral spirits
But, avoid these solvents, since they can adversely affect the blanket surface:
* High boiling solvents
* Chlorinated solvents
* Solvents with an aromatic content higher than 15 percent.
Storage is a final important component of blanket care, and there are two recommended methods. Operators can store blankets in their original shipping tubes and stand the tubes on end, according to Reeves. Or, blankets can be removed from their tubes and laid flat, rubber to rubber or back to back, adds Kennedy of D.Y.C. Blankets, he continues, should be stored away from sunlight in a cool, dry area.
Obviously, the often overlooked offset blanket is more than a fringe accessory to the printing process. Never mind the multi-million dollar press you just invested your business life in; improperly used and cared for blankets can smother any chance for quality and profits.