Offset Printing

Offset lithography is the most popular commercial printing because printers using it produce results quickly and cheaply. Other processes work better in specific situations, while lithography performs best for the most jobs. Lithography products good results because plates carry sharp images. Its plates are easy to make allow to run at high speed, so this process is fast and low price.

  • knowing how offset presses work means you can plan jobs to take advantages of different machines and avoid expensive problems.
  • Familiar with presses makes you judge good service and be on the schedule.
  • Printers very in the types, sizes and abilities of the presses in their shops.
  • Customer who understand the limitations and advantages of various presses make good choice between printers to economy and quality

 

Feeding and register units

The feeding and register units of a sheetfed press take 1 sheet of paper at a time and position it ready for printing. Good register requires that each sheet enter the printing units from precisely the same position. the feeding and register requites that each sheet enter the printing units from precisely the same position. The feeding and  register units of small, inexpensive presses may very the position of each sheet by a much as 1/16″. Largely, costly presses may have register to tolerances close to 1/1000. Perfect registration is difficult for a machine running 3 sheets per second.
Paper small presses goes in shortest dimensions 1st. For instance, on an 8/5″ x 11″ sheet, the 8.5 size is the gripper edge. Large presses take paper the wide way. For example, a 35″ press take a 23″ X 35″ sheet, with the 35″ side providing the gripper edge. Gripper hold tightly to about 3/8″ of paper, forming a strip that cannot receive ink.
With large presses, the griper edge is usually no problem because paper is almost always trimmed or cut up after printing. However, small presses usually run stock cut exactly to 8.5″ x 11″ or A4, 11″ x 17″ or A3.

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Size of Paper

Press Types, Sizes and Features Format or Paper

Offset presses are either sheetfed or web. Paper feeding into a web press come off a roll and is cut into sheets after printing. Paper feeding in to a sheetfed has already been cut.

Press size is identified according to the widest sheet or roll of paper the machine and handle. A 40-inch press can run a sheet or roll up to 40 inches wide. Small sheetfed press print sheets 18 inches or less. That size includes most presses at quick and in-plant shops whose maximum is 11″ x 17 or A3, and it is good for stationary, fliers, newsletters and forms.
Small presses run solids ink on uncoated paper 90% of the time, but many are capable of much more. Large sheetfed presses can run stock from 12″ x 18″ up to 55″ x 78″ even though few printers have machines larges than 40″. However, most commercial printers consider 19″ x 25″ and 25″ x 38″ sheet sizes versatile enough to handle almost any job.Large presses prove highly efficient for many jobs because they gang images.
The circumference of the impression cylinder and a web press is its cutoff and determines the maximum length of the images ares. A web press with a 23-inch cutoff and running a roll 35″ wide would print images imposed similarly to a sheetfed press running 23″ x 35″ stock. On the most web presses, cutoff is not adjustable. Knowing cutoffs for specific presses helps plan for efficient use of paper.

CMYK (Four-color process printing)

CMYK are images on paper consist of patterns of Cyan, Magenta, yellow and black dots: CMYK for printing.

Colorants and toners create different colors by blocking, or subtracting, different portions of the white light reflected by the paper that they are deposited on.

Removing light results in the colors Cyan, Magenta and yellow, called the subtractive primaries and abbreviated CMY.

The black colorant is abbreviated with the letter K, Resulting in the CMYK combination.

 

Changing RGB to CMYK

Black Substitution is the normal combination of CMYK colorants do not simulate the grey values well enough when an original has a shadow area that appears dark grey or black.

Substituting some black to CMY adjusts tones.

Also means more stable color on press and faster drying for press sheets.

It happens during the conversion from RGB to CMYK and in volves either under color removal (UCR) or grey component replacement (GCR).