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Printing Plates

Printing processes such as offset lithography use printing plates to transfer an image to paper or other substrates. The plates may be made of metal, plastic, rubber, paper, and other materials. The image is put on the printing plates using photomechanical, photochemical, or laser engraving processes. The image may be positive or negative. Typically, printing plates are attached to a cylinder in the press. Ink is applied to the plate's image area and transferred directly to the paper or to an intermediary cylinder and then to the paper. The printing plates used depend on the type of press, the printing method, and quantity of the print run. A plate is prepared for each color used, or four plates in the case of 4-color (CMYK) process printing. In general, metal plates are more expensive but last longer and have greater accuracy. Paper plates are usually more suitable for shorter runs without close or touching colors.



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The function of the original stone printing surface is now served by thin aluminum plates, although other materials, such as stainless steel and plastic, can also be used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber rollers carry ink and water to the plate surface. The ink is transferred first to the blanket cylinder and then to the paper.

Lithographic plates are the least expensive printing surfaces available today, and this fact has contributed greatly to the success of the process. Aluminum plate materials have a thin surface coating of light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an intense source of blue and ultraviolet light. Images are transferred to the surface by exposing the plate through a film positive or negative. Some materials can be exposed directly, as in a graphic-arts camera or by a computer-controlled laser beam, thereby eliminating the expense of film and speeding up the plate making process.

Modern offset lithographic presses range in size from small sheet-fed duplicators-used for small, single-color jobs such as brochures and newsletters-to massive web presses capable of printing millions of copies of magazines, catalogs, mailing pieces, and packaging materials in full color. No other process has such a broad range of applications.

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Solutions to Common Printing Problems
Problem: Buildup on inking rollers or plate

Possible Cause


Too much alcohol in fount solution.
Fount additive not fully compatible with alcohol.
Incorrect addition of the fount additive to the recirculation tanks causing precipitation of the gum from the additive.

Remedy

Reduce alcohol level.
Change fount additive.
Add in sequence water, fount additive, and then alcohol mixing thoroughly after each addition.
Problem: Feedback on dampening rollers

Possible Cause


pH too acid. the ink is not taking up enough water in the image area of the plate
Fount solution too cold, leading to increased ink viscosity and reduction in the water uptake of the ink.
pH too high, the ink emulsifies too much ( interfacial tension between water and ink too small).

Remedy

Decrease fount additive level or change the additive. do not reduce the level below 2%) by volume
Raise the temperature in the recirculation tank(s).
Reduce the pH
Problem: Pilling on the blanket

Possible Cause


Surface tension of the water too high.
Ink over emulsifying due to high pH.
Level of dissolved solids too high causing over emulsification.
Temperature of fount too high
Setting too fast because the ink is not sufficiently emulsified caused by
    - low water temp
    - low pH

Remedy

Change fount additive or increase the alcohol level
Reduce the pH
Clean out recirculation tanks and make up a fresh fount solution
Reduce the temperature of the fount
Increase water temperature
More Problems and Solutions