Non Sheet Handling Components
( Originally Published 1963 )
1. THE INK SYSTEM
The particular ink required for a job is placed in a trough or fountain which extends the full width of the press. An agitator moving continually across the fountain keeps the ink in a workable condition. Ink which is not worked continuously has a tendency to stiffen. In this condition good ink distribution is not possible. The consistency of ink is like a very heavy paste. At the lower edge of the fountain is a thin metal blade in contact with the fountain roller. This blade is of spring steel and is held in position by numerous adjusting screws in the course of its length. The adjustment of these screws separates the blade from the roller and controls the amount of ink fed into the ink system. By careful adjustment, varying amounts of ink can be selectively fed to several areas across the width of the press. Through a series of rollers, continually spreading and transferring ink, a continuous controlled supply of ink is provided to composition form rollers, usually four in number, which contact the image on the plate. The form rollers are adjustable as to their pressure on the plate and the life of the image through abrasive action of the rollers is dependent on the careful setting of these rollers. A good system may have as many as 16 different types and kinds of composition or steel rollers to give satisfactory distribution of ink to the image on the plate.
The lithographic inking system encounters one condition which does not occur in any other printing process. This condition is the emulsification of ink by the fountain etch during the inking process.
It can be seen that each time the plate cylinder revolves, the ink rollers contact not only the image area, but also the nonimage area. With each revolution, some of the etch is picked up by the form rollers and redistributed through the entire inking system. This is why it is important to use only the amount of etch required, and no more, to avoid contamination of the ink system as much as possible. The inking system must absorb this material or evaporate it. The use of high-quality inks which tend to resist emulsification gives much better quality than inks which absorb relatively large amounts of etch. Fountain etches which contain volatile components will produce less emulsification because of evaporation of the solvent from the ink rollers and the plate.
2. THE DAMPENER SYSTEM
This system is not as elaborate as the inking system, because the fountain etch which consists of a small amount of a dilute weak acid and water or volatile component is relatively easy to distribute. The etch is placed in a tray or fountain the entire width of the press. A metal roller which takes up the etch from the fountain rotates in the etch. By a duct transfer roller, the etch is transferred to two form dampeners. These form rollers may be covered with a special cloth called molleton or with parchment paper.
Each revolution of the press causes both image and nonimage areas to pass under both form dampeners. During this cycle, the nonimage areas are given a charge of etch. The image areas are to a consider-able degree etch resistant, but under pressure some etch is also left on the image area. This tends to contaminate the ink distribution and weaken the image density. The extent that the inking system can absorb or evaporate the etch taken up determines the capability of the inking system.
In similar fashion, the form dampeners tend to pick up ink from the image areas and this is redistributed back through the fountain system. The more ink which is taken into the system, the less effective is the ability of the system to keep the nonimage areas sharp and clean.
Cleanliness.—The importance of cleanliness of inking and dampening systems cannot be overemphasized. Frequent and thorough cleaning of the fountain system is a must. Ink which has become emulsified should be discarded and replaced with undiluted ink.
Failure to maintain these systems in clean and good operating order can lead to any of the faults indicated below:
Scum: This is a tone of the color of ink being used which appears in the nonprinting areas of the sheet.
Tackiness: A condition which pulls fibers from the printed sheet and appears as defects in solid areas of color.
Blinding: Image areas have been attacked by the etch acid or other etch ingredients because the ink has become too emulsified to protect the image.
Poor density: Ink is emulsified to a point that full color of ink is impossible or there is too much etch in relation to ink on the form.
A NEW SYSTEM OF DAMPENING
In recent years a dampener system which does not contact the form in any way has been developed. This system feeds the etch to the plate through the inking system. This would seem to be impractical, but it is quite successful. One of the reasons this is so is because the system uses a substantial content of alcohol instead of water. This permits the inking system to quickly evaporate that portion which stays in the system, and the alcohol provides a rapid distribution of the etch to the nonimage areas of the plate. The image areas of the plate have less water to emulsify, and a more uniformly dense printing image is the result. With less emulsification, less tack is present and less strength of paper is required.
This system has not yet found wide acceptance in lieu of conventional dampener systems, but is in operation on both sheet-fed and web equipment.
3. THE PLATE CYLINDER
The plate cylinder is undercut sufficiently to receive the aluminum, zinc, or paper plate. As previously noted, sufficient packing is placed behind the plate to bring the image to cylinder bearer height. It is possible under required circumstances to pack the cylinder above bearer height and thus lengthen the circumference of the image and correct an error in the printed image length. The blanket cylinder would be given less packing in order to keep the required impression of o.003 inch. Such a method of adjusting print length is not a suitable substitute for exact platemaking and should be resorted to only on infrequent occasions.
As the plate cylinder turns on its axis, it comes into consecutive con-tact with dampener rollers (usually two in number) and inking rollers (from two to four rollers) which maintain a hydrophilic condition of the nonprinting areas and a hydrophobic condition of the printing image. In this manner the ink is transferred only to the image areas and kept out of the nonimage areas. This transfer of ink to the plate is the first split of ink film in the printing process. The inked image is now ready for transfer to the blanket.
4. THE BLANKET CYLINDER
After the image has passed the form ink rollers, the plate rolls into contact with the blanket on the blanket cylinder. Normal makeready would pack the blanket so that a few thousandths of an inch pressure would exist between the blanket and the plate image. This is the second transfer or split of the ink film. From the blanket, the image is transferred to the paper as previously described.