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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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 Offset Lithography
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

Backing Away (Hang Away)

Print progressively lighter in color due to ink failing to flow and replace the ink that is removed by the fountain roller.

Backing Away

Problem Solution
Ink is short and buttery. Improve flow of ink with an addition of good litho varnish.
Ink is thixotropic. Ink gels when sitting without agitation. Work ink in fountain frequently to keep in fluid.

Blocking

Undesired adhesion sheets in a load causing then to stick together.

Blocking

Problem Solution
Not enough spray powder being carried. Increase volume of spray powder
being carried. select a bigger size of spray powder.
 
Sheets being piled too high in delivery. Remove and allow to dry in small lifts.
 
Inks not setting fast enough on stock. Consult our representative.
 
Excess ink required to obtaion desired strength. Increase the strength of ink to allow less ink to be carried.
 
Ink does not dry completly through and dried only on surface, creating
adhesion between two printed surface.
Consult our representative.
 
Wax content on ink is too high Consult our representative.
More Problems and Solutions