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Lithographic Printing

( Originally Published 1963 )



MECHANISM

The lithographic press consists of many component parts. It is a precision machine which, during manufacture, requires very precise tolerance—in many instances less than 0.0005-inch variation.

BROAD CLASSIFICATIONS OF TYPES OF PRESSES

There are two general types of machines manufactured: (1) sheet-fed presses and (2) roll-fed presses.

The distinction is simple. The sheet-fed press prints cut sheets which are placed on skids at the mill. The roll-fed presses utilize a roll of paper as it is produced in the paper-manufacturing machine. Either of the above types of presses could be further classified as (I) single-color presses with only one printing unit and (2) multicolor presses with more than one printing unit which provides continuous printing of a single sheet or roll without a reloading requirement.

SHEET-FED PRESSES

Sheet-fed presses contain the following groups of operating components which have been separated into two functions: (1) sheet-moving components and (2) non-sheet-moving components.

Sheet handling:

1. Feeder and transfer system

2. Guide and register system

3. Impression system

4. Delivery system

I. THE FEEDER AND TRANSFER SYSTEM

This system must have a means to hold and lift a pile or skid of paper so that the top sheets are on a level with the feed table; separate the top sheet from the pile; advance the sheets in sequence to a guide and register mechanism; and place each sheet into the grippers of the impression cylinder in a predetermined exact position for printing registered sheets of very close tolerance with each subsequent sheet.

The corners of the top sheet of the pile are broken by rotary combers so that vacuum suckers which telescope can grasp and lift the sheet near the rear corners. As the sheet is combed and lifted, a compressed air tube moves into position below the sheet, releasing a blast of air, which lifts it free of the sheet below. The lower sheet is held in position by thin flexible steel fingers. As this sheet is lifted free from the pile by a layer of forced air, a second set of suckers at the forward end of the feeder lifts the sheet and forwards it onto the feedtable where it is moved to the feedboard by endless tapes. The tapes on the feedboard carry the sheets to the guide and register system.

2. THE GUIDE AND REGISTER SYSTEM

As the sheet moves down the board, it encounters front guides which control the front or gripper printing position of the sheet. At the time that the sheet is in firm position in the front guides, the side guide moves the sheet into its correct side position. The sheet is now ready for the impression and is transferred to the grippers of the impression cylinder. (The sheet is under constant gripper control once it has been positioned at the guides until the sheet is in the delivery pile.)

3. THE IMPRESSION SYSTEM

The sheet rolls onto the impression cylinder held in constant position by the cylinder grippers, and passes between the blanket (which contains the image to be printed) and the impression cylinder, at which time the image is transferred to the printed sheet. This is the third and final splitting of the original ink film picked up from the ink fountain and passed to the form ink rollers. After the sheet is printed, it is transferred from the impression cylinder directly to the delivery system.

4. THE DELIVERY SYSTEM

The delivery system consists of a continuous chain at the side extremities of the press containing several sets of delivery gripper bars which travel between the transfer cylinder and the pile or skid of printed sheets. The sheet is picked up from the impression cylinder by a transfer cylinder and then gripped by one of the bars of the delivery chain. The chain gripper system is timed to pick the sheet from the transfer cylinder at the exact instant it is released. The sheet is carried through the press in delivery position over a skid. As the sheet is moved into correct position, the delivery gripper is released, and the sheet falls into the reciprocating jogger plates. At the top of the delivery pile on the sides and back are reciprocating jogging plates which continually jog the sheets into an even pile as they are released from the delivery grippers and fall onto the load of finished work. The sheets are delivered in the pile face up. The skid of printed work is removed from the press by the use of handlift platform trucks.

Non-sheet-handling components:

1. Ink system

2. Dampener system

3. The plate cylinder

4. The blanket cylinder

These systems are concerned only with the image on the plate cylinder. The ink system supplies ink in proper quantity and consistency. The dampener system maintains the nonimage areas of the plate in a condition which will not accept ink.



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