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The Big Picture
From conception to presentation, an image (or an entire text) goes through at least six stages--image design, image generation, image assembly, image carrier preparation, image transfer and finishing. The second-to-last stage, image transfer, is the printing process--transferring the image to paper or other material.
Offset Lithography Printer Setup
The most common contemporary printing technology is called offset lithography. These presses can be sheet-fed, meaning that they can print on individual sheets of paper or other material; they can also be web-fed, meaning long webs of paper or other material are fed through the machine on wheels. For each colour used, a separate printing tower or unit is activated. Presses can have as many as 12 towers: six print on one side of the paper/material, and six print on the other side. (A perfector flips the sheets.) The four main and most commonly used colours, however, are black, cyan, magenta and yellow.
Offset Lithography Process
Each tower has three cylinders with surface areas a bit larger than the area of the paper/material being printed upon. These cylinders rotate at the same speed so to press against each other. Lithographic plates with inked images (created by lasers) are clamped to the plate cylinders of each tower. When the printing press is turned on, the cylinders begin to roll against each other. The plate cylinder then presses against the blanket cylinder, so-named because a soft, thin sheet is wrapped around it to pick up half of the ink from the plate cylinder. The blanket cylinder then rolls onto the paper clamped onto the impression cylinder. Half of the ink from the blanket cylinder goes onto the paper (or other material receiving the image). As the machine continues to run, the cylinders continue to roll: more ink is applied to the plate cylinder, transferred to the blanket cylinder, and pressed from there onto the paper on impression cylinder.
Grippers in Sheet-Fed Presses
The individual sheets are held by grippers that alternatively clamp down and let go as the paper (or other material) travels through the press. Each impression cylinder has grippers to hold the leading edge of the sheet as the ink is pressed into it by the blanket cylinder. Other sets of grippers (not on a cylinder) transfer the sheets from one impression cylinder to the next. A set of grippers is always holding the sheet, and another set is always ready to receive the sheet when the previous grippers let go.
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