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Transport Elements of the Web Press




Text 2 Web-fed Printing Presses

Vocabulary

single-sheet feeder самонаклад з почерговою подачею аркушів

cylinder configuration конфігурація циліндра

plate cylinder формний циліндр

impression cylinder відбитковий циліндр

unit configuration конфігурація секції

middle drum проміжний (середній) барабан

reversing drum перевертальний барабан

sheet reversal перевертання аркуша

Rotation обертання

sheet transfer передача аркуша

transfer drum передаючий барабан

metal sheeting металеве покриття

triple-sized transfer drum трициліндровий передаючий барабан

joint impression cylinder з’єднаний відбитковий циліндр

register приводка

circulating chain циркулярний ланцюг

conveying distance довжина передачі

feed drum барабан самонакладу

doctor blade ракель

conveyor chain ланцюг конвеєра

perfecting двосторонній друк

rotating cutting blade дисковий ріжучий ніж

 

Web-fed printing presses differ less from sheet-fed presses in the configuration of their printing and inking units than in all the machine elements actually transporting the substrate. In principle it is much easier to transport a web through the various press units than a sheet. The continuous web to a great extent guides itself, because the start of the web is pulled and the web is held under tension in a stabilizing, defined way. Therefore web presses are more suitable for higher printing speeds than sheet-fed presses (sheet-fed printing up to approximately 4m/s,web printing up to approximately 15m/s).

Web presses are production systems that are able to produce finished printed products, that is, finishing is an integral part of web offset printing. Web presses are relatively inflexible with regard to format variation compared to sheet-fed presses, only

the web width is variable. The format is defined in the circumferential direction of the printing plate and in the direction of web travel by the cut-off length. The advantage is here that, depending on the printing process, there is no printing gap (or at least almost none). In flexographic and gravure printing the cylinder circumference can, within certain limits, be adapted to the cut-off length, whereas on web offset presses the cylinder diameters are invariable.

Web transport through the web press begins with the reel of paper or any other substrate on a reel; this is clamped in reel arms. To avoid production stoppages, the reels are almost exclusively changed in a fully-automated process by splicers.A separate infeed unit, also called pre-tensioning unit, is located after the reel stand for keeping web tension constant.

The printing unit itself serves also as a transporting element, because the web is transported under pressure between two cylinders. In the blanket-to-blanket unit of a web offset press these are two blanket cylinders (blanket-to-blanket mode), in gravure or flexographic printing (letterpress) they are a cylinder with a soft covering and a steel cylinder. Multicolor prints are produced by printing units arranged one after the other, which print the color separations onto the web. The web behavior during transport varies according to the cylinder surfaces, which requires a special dimensioning of the diameter of the steel cylinder and/or adaptations of the blanket cylinder packing.

Various finishing equipment is connected to the printing unit, such as the numbering unit, gluing station (for partial coating), dryers, chill roll assembly, slitters, turner bars, and perforators, as well as the formers and the folding unit, which all have some effect on web travel. To compensate for this the web travel is stabilized by driven draw rollers. In many applications (e.g., newspapers, magazines, catalogs) the folder with its cylinder section acts as a tensioning mechanism at the start of the web. Draw rollers acting on both sides are fitted at the entry of the folder, but it is the tucker blade cylinder of the jaw folder that exerts the actual tension. It is equipped with height-adjustable, curved plates or with circular shaped segments that correspond to the cylinder radius. The press does not need to be stopped for readjusting the tension, as it is equipped with a mechanism that allows adjustment during paper run. After the web or the ribbon bundle has been cross-cut in the folder, the

sheets are transported and delivered by means of pins or grippers in the cylinder section of the folder. The web can also be cut into individual sheets after printing using “sheeters”, and then stacked in a delivery like with sheet-fed presses.




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