Blanketing Offset Printing
by Dennis Cheeseman
The offset blanket is a modern wonder. To be effective, the blanket must:- Be a shock absorber
- Be ink- and water-receptive without mixing the two or swelling
- Transfer ink both as solids and dots without streaking
- Be an impression cylinder when printing blanket-to-blanket.
The blanket must perform these functions while being soaked from the back and face with solvents and water—all without coming apart at high speeds. It must work with various papers, inks and plates, often at incorrect packing heights, and with ink-and-paper lint built up over long periods of use.
Though not as long-lasting as letterpress blankets, some offset blankets reach 16 million impressions before being changed. However, most blankets are replaced earlier due to damage from web breaks.
Several different blanket surfaces are available, including ground (buffed), spread (coated) and caste. All can be used on newspaper presses, but never mix them on units feeding into the same folder. It goes without saying that units feeding into other units in color work should not only have blankets from the same manufacturer but also the same type of surface.
Most newspaper blankets have a compressible layer built into the carcass. Closed cell layers offer the advantage of retaining compressibility and “smash resistance” over a longer period of time.
Although compressible blankets offer many advantages, the main one is the elimination of the “bulge” in printing zones common in conventional blankets (see diagram). This allows using relatively high packing levels for good solids without causing undue dot gain. An excellent compressible layer is necessary for a good blanket, but just as important are the fabrics which comprise the bulk of the blanket. Stable fabrics mean a stable blanket that doesn’t move on the cylinder, maintains its width around the cylinder, and doesn’t shrink at the gap.
Inevitably, a blanket will lose its thickness. Part of this gauge loss is due to the tightening of the blanket around the cylinder. Also, during printing, air is forced out of the fabrics and, in some cases, the compressible layer. All blankets lose some thickness immediately, called setting in, then continue to lose thickness throughout their life. The initial loss is not only unavoidable but also necessary, since during this period the blanket absorbs unevenness in the packing, the press and the blanket itself.
The second stage of thickness loss is an area that blanket manufacturers try to control. Stable fabrics and compressible layers control this to a large degree. Blankets that lose 1.5-to-3 percent of their original thickness are generally acceptable.
Blankets should not only be dimensionally stable but also pliable enough to bend around the cylinder and into the gap. When a blanket is too stiff, its thickness increases at the gap, increasing the “bump” at the gap and the load on the bearers.
A frequently asked question is how much torque a blanket requires. As long as the blanket lies flat on the cylinder and does not move, it is tight enough to print. Many more problems are caused by over-tensioning than by under-tensioning. A printer can see and hear the effects of a loose blanket; an over-tightened blanket only shows the bad effects later. In other words, use no more torque than necessary. Every press manufacturer uses different tensioning methods and specifies the required torque.
Careful attention must also be given to the height of the blanket over the bearers. For this reason the thickness of the blanket, plate and packing materials should be measured, as these products all have variances which could lead to an over- or under-packed blanket.
Do not take the gauge stamped on the blanket as an absolute. In addition, the use of a suitable blanket micrometer is important—machinists’ micrometers may not be suitable.
Indications of too much packing include premature plate wear, plate cracking, contamination of succeeding inks, paper-transport problems, blanket damage and excessive dot gain.
Too little often leads to poor solids, excessive ink and paper piling, and paper-transport problems.
Along with never mixing blankets from different manufacturers on units feeding into the same folder, it is best to replace both blankets on a cylinder at the same time, even when only one is damaged.
Dennis Cheeseman is director of customer services for US Ink. Phone, (201) 935-8666; fax, (201) 933-2291.
Flexo Vendors Incorporate
by Hoshin Seki
At the fall Newspaper & Publishers Flexo Users Group annual meeting in Monterey, Calif., the Flexo Vendors Alliance announced it had officially incorporated as a not-for-profit organization.Started just three years ago, FVA represents vendors supplying newspapers with flexo presses, plates, anilox rolls, inks, newsprint, doctor blades, blankets and other supplies. Even though many members are in direct competition with each other—KBA-Motter Corp. and MAN Roland, Inc., for instance—they share the common goal of supporting and propagating the flexo process.
FVA operates a World Wide Web site (www.flexovendors.com), where users can find the complete flexo print specification guide for newspaper printing, a list of flexo vendors, and a message board to discuss production and technical issues.
Seki is president of KE Solutions. E-mail, Hoshinseki@aol.com; phone, (914) 664-2414.