The principal advantage of offset over other types of printing production is the economy of preparatory operations. The use of paper prints and reproduction proofs instead of engravings and large metal forms affords substantial savings of preparatory time and materials in favor of lithography.
There is another advantage in the lithographic process which until recently has been largely ignored. It is possible to print both sides of a sheet of paper simultaneously. This procedure utilizes the blanket cylinders as simultaneous impression and printing cylinders. The use of this principle with roll stock instead of cut sheets easily provides a substantial measure of economy over conventional lithographic procedures.
In addition, the web press delivers printed and folded signatures ready for gathering. As a measure of comparison, a sheet-fed press delivers approximately 5,000 sheets printed one side per hour. A web press can deliver 2o,000 signatures per hour, printed both sides, folded, and packed ready for gathering.
WEB PRESS COMPONENTS
The following components are usually minimum requirements for a web-offset installation:
1. Automatic in feed roll unit
2. Printing unit (one or more to print multicolor work)
3. Dryer unit 4- Chill unit
5. Folding unit
6. Packing unit
AUTOMATIC IN PEED ROLL UNIT
This component stores rolls of paper and pastes a new roll to the end of the roll being printed without interruption of production. As the operating roll approaches exhaustion, a new roll is brought into close position to the traveling web. The operator starts rotation of the new roll until its speed is identical to the expiring roll. The operator at the proper time actuates a paster arm and a cut-off knife which simultaneously attach the new roll to the traveling web and sever the old core. No interruption to the printing occurs during this changing cycle. On machines not equipped with automatic pasters, it is necessary to stop the press, cut off the old roll, remove the old core, lock the new core in position, paste the new roll to the tail of the old web, and start production again.
THE PRINTING UNIT
The function of this unit is to transfer an image from the plate cylinders to the blanket cylinders and to opposite sides of the web of paper simultaneously. Inks are usually special heat-set formulas. The fountain solution is circulated continuously through a filter tank to maintain cleanliness in the dampening system.
Since there is no need for gripper and sheet-control devices as with sheet-fed work, the entire circumference of the cylinder, except for about % inch, is used to produce a printed image. The dampening method is identical to sheet-fed work.
The travel of the sheet is 800 feet per minute or more. At this speed ink could not normally set sufficiently hard to process through the folder without special drying action. The web is passed into a gas-heated drying chamber about 8 feet long or longer as soon as it leaves the printing unit. The temperature of this chamber is over 300° F. and the volatile solvents are burned or evaporated almost instantaneously and carried off in high-velocity vents to the outside of the building.
The dryer must provide adequate gas control cutoffs to permit stopping of the web for necessary adjustments and changes to the operation of the press. Drying may also be accomplished by passing the printed web over large steam-heated drums.
THE CHILL SYSTEM
The sheet as it leaves the dryer has a temperature of over 300° F. Although the solvents have been removed, the ink is in a plastic condition and would smear during the folding operation. The function of the chill rollers is to return the sheet to (1) normal temperature condition of 60° to 80° F., (2) set the binders in the ink, and (3) reduce any static condition of the web.
This also adheres the ink on the sheet. Chilling is accomplished by passing the sheet around two or more large polished cylinders through which refrigerated water of the desired temperature is continually flowing. From the chill rollers the web travels into the folders.
The folder converts a web of paper into printed signatures and pamphlets for trimming or gathering into multiple-signature books and pamphlets. Webs which are not to be converted into signatures or which require additional operations in flat-sheet form, which cannot be accomplished on the press, can bypass the folder and be sheeted onto a pile delivery similar to sheet-fed presses.
There are several types of folding actions which can be performed mechanically on the entire web or on portions of a web which have been slit into ribbons. The principal folder types are as follows: (1) former, (2) jaw or parallel, and (3) chopper or angle. A single press installation may contain any or all of the above folding actions depending on the type of product desired.
In installations which use this folding device, usually the first folding action of the web, a large triangular-shaped metal V is used, with width of the web forming the top of the V and, as the web moves toward the apex, it is formed into a double ribbon with the point of the V forming the first fold. The web is usually folded in half, lengthwise of the ribbon.
This folding operation may be in sequence with the former or it may receive one or more individual ribbons from a slitting operation of a wider web.
The action of a jaw folder requires two cylinders. One cylinder will have cam-activated jaws or open grooves into which a tucker blade of the second cylinder forces the paper web to produce a parallel fold.
This jaw cylinder, twice as large as the printing cylinder, contains a double row of impaling pins which permit control of the signature which is being cut from the web, as well as the following signature which is still attached to the web. This action provides continuous web control at the time of the separation of the signature from the web stream.
CHOPPER OR ANGLE FOLD
The signature is free of the roll after passing through the jaw folder unit and can be passed into additional parallel folding units or can be folded across the web travel direction. The chopper fold is one made at right angles to the previous parallel fold in the jaw folder unit.
As the signature leaves the jaw folder, it is carried by pulleys along a smooth table into position above a pair of smooth folding rollers. At the correct instant, a blade above the signature drops and forces the signature into the turning fold rollers and thus a quarter or angle fold is completed.
THE PACKING UNIT
The signatures, as they are delivered from the folding unit, are not folded tightly and are bulky at the fold point.
These are not suitable for saddle or side gathering machines to process for sewing or stitching into bound books. The signatures must be pressed tightly to provide a smoothly folded signature for other binding operations. This is usually a separate operation in the bindery.
On the web press, the folded signatures are dropped onto traveling tapes and carried to a vertical delivery position and deposited on the table of the packer in a vertical position with the folded edge down. Successive signatures are delivered and deposited in line and carried along the packer bed by tapes until a desired pack size is accumulated. A clamping chain is placed in a groove at the far end of the packer table and the signatures to be packed are moved over the chain against an end board. A second board is placed at the front of the pack and hydraulic clamps compress the pack into a rigid tight bundle. The chain is then fixed around the bundle of signatures and the hydraulic clamps released. The bundle now contains fully folded signatures ready for subsequent binding operations.