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Offset-Press Fundamentals

Ch. 20 from the Offset Lithography Technology Book

Laura Roberts

on 6 March 2017

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Transcript of Offset-Press Fundamentals

Offset-Press Fundamentals
Learning Objectives
When you have completed the reading and assigned activities related to this presentation, you will be able to do the following:

Distinguish between an offset press and an offset duplicator.

Explain the function of a waterless offset press.

Describe the features common to offset presses and duplicators.

Give examples of the control of an offset press and an offset duplicator.

Define the six major operating units of offset presses and duplicators.

Identify the roller and cylinders locations on offset presses and duplicators.

Summarize blanket properties and proper installation and care.

Discuss the capabilities of a direct imaging press.

Explain the concept of VDP.
Conventional-Press Classifications
Conventional offset presses are tied directly to the prepress function. Plates are imaged - either conventionally or digitally - away from the press and then carried to it.

In contrast, digital offset presses use direct imaging (digital). That is, blank plates are mounted onto the press and then imaged by digital-driven lasers.

Offset printing presses are categorized by the maximum sheets they handle, the number of colors that can be applied in a single pass, whether one or both sides can be printed in a single pass, how paper feeds into the press, and other special features.
Duplicators and Presses
Small sheetfed offset presses printing up to a maximum sheet size of about 11"x17" or slightly larger are generally referred to as offset duplicators. Those that print larger than this are referred to as offset presses.
Sheetfed Presses
Offset presses and duplicators designed to print a single sheet of paper at a time are referred to as sheetfed presses. These presses pick up individual sheets of paper from the feed table.
Webfed Presses
Presses that feed from a roll of paper are referred to as webfed presses or web presses. These presses are used for work requiring long runs, such as magazines, newspapers, and other similar publications.
Multicolor Presses
Many offset presses and duplicators have only one printing unit, and therefore, can print only one color at a time. Presses that print more than one color during a single pass are referred to as multicolor presses.
Perfecting Presses
Most offset presses print on only one side of the paper at a time. presses that print on both sides of the paper at the same time are known as perfecting presses or perfectors.
Townsend Color Head Units
The Townsend Color Head unit is a unique self-contained printing unit allowing second-color printing on most offset duplicators. The unit permits two colors to be printed in one pass through the press.
Waterless Offset Presses
Waterless offset presses do not use dampening solution to keep the plate's nonimage areas free of ink. These presses function because the plate's nonimage areas consist of a layer of silicon that repels ink.
The Feeding Unit
The feeding unit of a press is responsible for sending the substrate into the press properly. With sheetfed presses, the feeding unit is responsible for picking a single sheet from the top of the pile of paper that has been loaded into the press. With webfed presses, the feeding unit sends a long ribbon of paper into the press.

Continuous feeding means that long ribbons of paper or individual sheets are fed into the press constantly. Continuous feeding systems used on sheetfed presses are commonly referred to as stream feeders. When substrate is fed into a sheetfed press with a successive feeding system, it is fed one sheet at a time.
Webfed-Press Feeding Units
Webfed offset presses use large rolls of paper that unwind as the length of paper makes its way through the series of rollers in
the press.

The paper winding through the press is referred to as a ribbon or web.
Sheetfed-Press Feeding Units
A continuous feeder on a sheetfed press ( a stream feeder) sends the sheets to the press on a conveyor platform called the feedboard, which uses a set of rollers, balls, or brushes to move the sheets forward.

The double-sheet detectors are used to detect when more than one sheet of paper is being fed to avoid paper jamming and loss of time.
Sheet Transfer
A vacuum blower pump inside the press forces
air through plastic tubes near the front of the paper stack
to separate the top sheets and improve feeding.

A set of suction feed drops down against the front edge of the top sheet of paper on the pile and lifts it to the edge of the feedboard.

Sheet separators are springy, steel fingers extending over the lead edge of the pile of paper in the feeder. When the suction feet grip the top sheet on the pile an pull it upward and away from under the sheet separators, the sheet separators prevent the next sheet from being picked up at the same time.
Pile Feeders
A pile feeder is a mechanism used to lift individual sheets from the pile and feed them into the press.

The sheets of paper are stacked on a platform called a pile table or paper table. The pile of paper must be straight and neat when placed on the table.
Fanning out allows for easier separation and feeding by introducing a blanket of air between the sheets.

Jogging refers to squaring and aligning all the sheets just before loading them onto the feed table.

A pile height regulator controls the speed of elevation of the pile table or piling bars, allowing the table or bars to run higher or lower, depending on the thickness of paper being used.
The Register Board
and Insertion Unit
As the substrate enters the register board, it goes under the double-sheet detector, which is used to detect when multiple sheets of paper are being fed at the same time. The register board accepts a sheet from the feeder unit and inserts it in register, ready for printing.

Register is the proper positioning of the sheet with respect to the image on the blanket.
The Three-Point Guide System
With the three-point guide system, the sheet is forwarded to the front guides by conveyor tapes.

While it is held against the front guides, a side guiding mechanism pushes or pulls the sheet into proper side alignment.
Once the sheet is preregistered into the proper
alignment it is fed into the grippers on the
impression cylinder.

Grippers are small metal tabs or clips that grabs a sheet of paper and feeds it through the printing press.
The Printing Unit
The printing unit is the heart of the offset press. Everything about the actual printing cycle centers on the three cylinders in the printing unit. The three cylinders are the plate cylinder, the blanket cylinder, and the impression cylinder.
The Plate Cylinder
The plate cylinder holds the printing plate and its image. The plate clamps are set so one holds the leading edge of the plate, while the other holds the tail edge of the plate firmly.
The Blanket Cylinder
The blanket cylinder is located between the plate and impression cylinders. This cylinder holds the rubber blanket that accepts the image from the plate cylinder and transfers, or offsets, it to the substrate.

Underblanket is packing material made of foam rubber, cork, or
loose fabric fibers used under the printing blankets to improve the blanket's performance.
The Impression Cylinder
The impression cylinder carries the paper through the printing unit and presses it against the inked blanket, where the image is transferred to the paper.
The Dampening Unit
The dampening unit moistens the nonimage areas of the plate with fountain solution so ink does not adhere to them.
The Inking Unit
The inking unit (also referred to as the ink train) consists of an ink fountain and several rollers that control the feeding of the ink.
The Delivery Unit
The delivery unit on most duplicators and presses consist of a chain delivery, a receding delivery table, joggers, and some type of antisetoff-spray attachment.
Dampening Systems
The fountain roller rotates in a pan containing the fountain solution. Depending on the dampening system being used, the fountain roller transfers a thin film of fountain solution either to the ductor roller or directly to the form rollers.
The intermittent dampening system uses oscillator rollers, form rollers, a ductor roller, and a fountain roller.The disbursement of the fountain solution is controlled by regulating the rotating speed of the fountain roller in the solution.

Continuous dampening systems do not use ductor rollers to transfer fountain solution from the pan. This system uses a metering roller to move the solution to a transfer roller.

Integrated dampening systems link dampening rollers and the inking rollers to the ink fountain. The fountain solution and ink are fed to the plate from the same set of form rollers.
There are two alcohol dampening systems commonly found in industry are Dahlgren and Kompac. These recirculate the fountain solution making it possible to maintain pH consistency in the fountain solution.
Inking Fountain
The ink fountain is a reservoir that holds the ink before the
rollers transfer the ink to the printing plate.

On conventional systems, a set of screws called fountain keys, regulate the supply of ink to the plate at the ink fountain.

By adjusting the keys, the press operator moves the fountain blade against or away from the fountain roller. The fountain blade is a metal plate forming a barrier between the ink fountain and the fountain roller. By turning the ink keys clockwise allows the maximum amount of ink.

Some presses have ink agitators, which is a motorized, revolving metal cone that runs along the ink fountain, constantly stirring the ink.
Night Latch
A handle, called the night latch, allows the press operator to lower the rollers against the plate and lift them out of contact with the plate.
Inking Train

The ductor roller is the first roller in the train. This roller comes in alternate contact with the fountain roller and the first oscillator or distributing roller.

Most of the rollers in the ink train are intermediate rollers. The two types of intermediate rollers are the distributing rollers, which contact two other rollers, and riders, which contact only one other roller.

Distributing rollers, not only carry ink forward in the ink train, but they also reduce the ink's viscosity at the contact points between rollers. Riders often collect debris, such as coating or filler particles. The oscillating rollers move from side to side as they rotate.
Chain Delivery
Most duplicators and all large presses are equipped with delivery grippers consisting of a series of small metal fingers attached to a bar extended between two continuous delivery chains.
The delivery unit transfers the printed sheets from the final impression cylinder to the delivery pile.

The delivery cylinder carries the printed sheets from the impression cylinder to the delivery grippers attached to the delivery chains. This cylinder is usually covered with skeleton wheels.

Skeleton wheels are adjustable disk
on the delivery cylinder that holds the sheet as it travels. This disk is a
paper-guide wheel.
Direct Imaging Presses
The grippers carry the sheet to the end of the duplicator or press and releases onto the delivery platform.
Ejector Delivery
On duplicators using an ejector delivery system, also referred to as a chute delivery system, a simple ejector mechanism is used to delivery the printed sheets to a tray.
After impression takes place, a cam on the impression cylinder causes the cylinder grippers to open and release
the sheet.

Ejector fingers then rise to lift the edge of the sheet away from the cylinder and force it out over a set of stripper fingers.

Stripper fingers separate the sheet from the impression cylinder and direct it under two ejector rollers that force the sheet into the paper-receiver tray.
A direct imaging press applies the image to plates after they have been mounted onto the plate cylinder. A laser images the plates directly from electronic files, using technology similar to the CTP technology used with platesetters.

The plates are automatically imaged in register with one another because they are already on the plate cylinder of the press. Each plate is imaged with its own laser. Direct imaging technology is also referred to as computer-to- press (CTPr).

Some digital offset lithographic presses are also capable of VDP (Variable Data Printing), which means the plate cylinder can be reimaged between impressions.

One of the many applications of VDP is direct-mail promotions in which each piece is tailored to its recipient.
Antisetoff Spray
The delivery system on some presses uses an antisetoff spray. Antisetoff spray is a powder made from fine starch particles that is sprayed on the surface of the printed sheets. The powder prevents setoff, or the transfer of wet ink from one sheet to another.
Auxiliary Press Functions
In addition to printing, many offset duplicators and presses can be fitted to perform additional operations.

Numbering, perforating, slitting, scoring, pasting, folding, tipping, and trimming attachments can be added to the duplicator or press.
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