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Lithographic Plates

Ch. 17 in the Offset Lithographic Technology Book.

Laura Roberts

on 4 February 2016

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Transcript of Lithographic Plates

Basic Plate Structure
Offset plates are manufactured from any different types of materials. These include cellulose-based paper;
plastic-coated paper; acetate; aluminum; a paper base with a laminated aluminum surface; polyester; steel with a plastic surface; and copper on chromium, or aluminum.

The most common material used to make plates
is Aluminum.
(CTP) Equipment
CTP systems use an automatic processor called a platesetter to image and process plates.

These systems require the use of
platesetters and RIPs to process
information before plate
imaging can take place.

Electrostatic platesetters digitally
image plates by charging the plate
image areas so they attract dry toner.

Ink-jet platesetters image printing
plates by using ink-jet technology
to apply image areas to the
plate surface.
Conventional Lithographic Plates
Precautions on Press

When an offset plate is attached to the plate cylinder of the press, the operator should remove the gum coating from the surface of the plate before inking it. Removal is performed with a damp sponge or cotton wipe.

If the pressrun is interrupted for more than a few seconds, the plate can dry. The operator must dampen the plate again before proceeding.
Lithographic platemaking is the culmination of all prepress operations.

The plates used for offset printing vary in the way they are made and the materials in which they are composed. Most offset plates in use today are based on the principle that grease and water do not readily mix.

The image and nonimage areas of the plate are kept separate chemically.
Conventional lithographic plates are imaged with either negative or positive films, using a contact method.
Most plates manufactured today are sent CTP. Plates are categorized as surface or deep-etch, wipe-on or presensitized, additive or subtractive, and negative-or positive-working.
Lithographic Plates
Of the prepress operations, platemaking is the final stage.

Lithographic plates vary in the way they are made and in the materials from which they are composed.

The operation of offset press involves the transfer of images from one surface to another.

Lithographic plates made from thin sheets of metal, polyester, or paper serve as the image-carrying medium on an offset printing press.

Regardless of the method or materials used, the surface of a lithographic plate contains image and nonimage areas. The image and nonimage areas of a lithographic plate feel the same to the touch because they are essentially on the same level. Instead of physically higher or lower than one another.

Most offset plates in use today are based
on the principle that grease (oil) and water
do not readily mix. The image areas are
ink receptive (oleophilic) and water-repellent
(hydrophobic). The nonimage areas are
ink resistant (oleophobic) and water
receptive (hydrophilic).
Learning Objectives
When you have completed the reading and assigned activities related to this presentation, you will be able to do the following:
Explain the principles on which lithographic plates are designed and made.

Describe basic plate structure.

Name a variety of lithographic plates and explain the processes used to image and develop them.

Summarize manual and automatic processing of conventional plates.

Properly handle and store lithographic plates

Explain the various types of CTP systems.

Analyze plate problems and describe how to prevent them from happening in the future.
Surface Grain
The surface of most metal plates is grained to form a slight texture, which makes them more water receptive.

A grained surface is more
capable of carrying a film
of dampening solution than
a smooth surface is.
Surface Coatings
and Emulsions
Nearly all lithographic plates have some type of surface coating, usually an emulsion.

The type of coating used varies with the plate material and the processing method to be used.
Sizes and Shapes
Offset plates are manufactured in sizes to fit the wide range of presses.

Plate material might also
come in rolls that are cut to
size as the plates
are processed.

Smaller plates are available
in three types of plate ends.
These are straight, pin bar,
and serrated (slotted).
Surface Plates
A surface plate has a base material on which a light-sensitive coating rests.

This plate is called a surface plate because the image area is formed on the surface of the plate, instead of etched below or added above the surface.

Surface plates require exposure to high-intensity light and can be either negative or positive working.

The nonimage areas are removed through chemical processing after exposure and during processing.

The two main types of coating used on surface plates are diazo and photopolymer.

A diazo plate is a surface plate
coated with an emulsion of a
light-sensitive, organic compound.

Photopolymer plate is a surface
plate with a light-sensitive
photopolymer coating.
Photopolymers are substances that
react to actinic light by polymerizing.
and Positive-Working Plates
Another way of categorizing plates is whether they are exposed from film negatives or positives.

Negative-working plates are imaged from film negatives placed over the unexposed plates and exposed to high-intensity light. The light passing through the transparent portion (image areas) of the film hardens the coating on the surface of the plate and makes these areas insoluble in the developer. The areas on the plate not exposed to light (nonimage areas)are not hardened, and the coating dissolves when the plate is developed.

Positive-working plates are exposed from film
positives and a high-intensity light. Light
passing through the transparent portions
(nonimage areas) of the film begins
decomposing the coating. The coating in the
exposed areas of the plate become soluble
and is dissolved during processing.
Subtractive and Additive Plates
Subtractive and additive describe how the exposed plates react to the developer.

Subtractive plates require a developer to dissolve the unexposed coating so the coating can be easily removed (subtracted) from the base material. Areas with the coating removed become the nonimage- dampening solution -receptive areas. The areas of the light-sensitive coating exposed to light are hardened and become insoluble when the developer contacts them. These exposed areas are the ink-receptive image areas of the plate.

Additive plates are imaged
with a high-intensity light
and a film negative. The
light shines through the
transparent (image) areas
and hardens the image areas
on the plate. During
processing, a special
lacquer is applied (added)
to the image areas, and the
coating in the nonimage
areas is removed from
the plate.
Deep-Etch Plates
Deep-etch plates carry the coating slightly below the plate surface These plates are exposed from film positives, rather than film negatives. After exposure and processing, the ink-receptive, or unexposed, areas are chemically etched, making the image areas lower than the nonimage areas.
Multimetal Plates
Multimetal plates are made with two layers of metal (bimetal) or three layers of metal (trimetal). Multimetal plates are sturdy and useful for long pressruns because they carry the image above the plate surface.
Electrostatic Plates
Electrostatic plates are imaged by a photoelectrostatic process that uses light and electrostatic properties. The principles on which the process is based are the same as those used in most photo-copying machines and laser printers - a surface must be selectively charged.

The process begins when light is reflected from a black-and-white original. When the light is reflected from the white nonimage areas of the original light strike portions of a positively charged surface, the light causes those portions to lose their charge and become neutral.
Aqueous Plates
Aqueous plates are developed with chemistry that is primarily water based, instead of solvent based. Aqueous plate developer is the result of environmental regulations that seek to reduce the use of organic solvent-based chemistry in the United States.
Conventional Platemaking Equipment
Conventional platemaking processes are
photo-mechanical processes. In these processes, light-sensitive coatings on the plate surface physically change after exposure to a
high-intensity light source. After exposure,
the plate is processed so the nonimage portions of the emulsions are receptive to dampening solution and image areas are receptive to lithographic ink.
Most platemaking machines use alight source to expose the plate from film negatives or positives. The platemaker has a vacuum frame with a rubber blanket and a glass cover. The blanket is bordered by a rubber sealing gasket and contains an outlet fitting connected to the vacuum pump by a rubber hose. A flip-top platemaker is one example of a self-contained unit.

A platemaker's gray scale is used to determine the correct exposure time of a presensitized plate.
Digital Plates
Today's technology allows plates to be imaged directly from digital files in a process known as computer-to-plate (CTP) or direct-to-plate (DTP).

CTP plates consist of a base coated with a light-sensitive emulsion. They can be either negative or positive working.
Base Material
Digital plates can be aluminum, polyester, or paper. The main distinction among the three materials is their durability during the press run.

Aluminum plates have a grained surface to improve the retention of a thin film of dampening solution. Depending on the emulsion, aluminum plates can print between 100,000 and 2 million impressions.

Polyester plates are not grained. Instead they are treated chemically to improve their hydrophilic characteristics.

Paper plates do not carry
a light-sensitive emulsion.
Instead they are imaged
Imaging Technologies
Several types of emulsions are available on digital plates. Some emulsions are formulated to react to visible light. Others react to invisible light. Still others react to heat.

Emulsions are also different in whether the laser exposes the image areas (negative working) or nonimage areas (positive working).

Silver-halide plates is a printing plate with a light-sensitive emulsion of silver and iodine, bromine, or another halogen.

Thermal -conversion plate is a printing plate with a coating that is imaged by being exposed to heat instead of light.

Thermal-ablation plate is a printing plate
with a coating that is ablated by a laser's heat.

Waterless plate is a printing plate made with
an aluminum base and three to four coatings
and designed for waterless offset printing.
Plate-Related Problems
The printing plate is one of the most critical components governing the quality of the final printed product because it carries the image to be placed on the paper or substrate. Various problems can result if the plates are processed or used improperly.
Black spots - from plates not printing well on press.
Bridging - when developer solution reacts to connect two or more image areas on the plate.
Plugging - images on the plate fill in where they should be open and clean.
Piling - Excessive amount of developer solution allowed to build up on certain image areas of the plate.
Halation - soft, fuzzy, unwanted circles, similar to halos.
Plate blinding - visible ares of the plate that do not accept ink.
White spots - parts of image areas have pinhead-sized openings where there is no image.
Weak image - inadequate development.
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