The printing unit places a water solution (fountain) and ink on the offset plate, transfers the image to the blanket cylinder, then to the paper. It then delivers the paper to the delivery unit. The printing unit must be adjusted so that the proper amount of ink and fountain solution are deposited on the printing plate so that the image is transferred accurately, evenly, and consistently to the printing paper. Every offset printing unit is made up of the following three parts:
- The cylinder system (plate, blanket and impression)
- The dampening system
- The inking system
Each serves an important function in the total image transfer system. The only deviation in this process is with the “waterless” system (Toray) which has been explained elsewhere in this publication.
Many multicolored sheetfed presses are equipped with a Perfecting cylinder to turn the paper over so that the back side can be printed. For example, a four-color press may have the perfector cylinder between the second and third unit. After printing two colors on the top of the sheet, the cylinder can be configured to run the paper over and the third and forth unit will print 2 colors on the back side.
If the printer does not want to print “two up” (two colors each side), he will configure the unit so that the paper is not turned around thus printing 4 colors on the top.
Many 7 color presses have the perfector unit between the 2nd and 3rd unit. This enables the printer to print 2 colors on the inside of the sheet and 5 on the outside. Greeting cards are often printed this way. Sheet length is very important in perfecting. Most of these perfecting units will tolerance only a 1/16 of an inch variation. If a sheet is off more than that the grippers will miss picking up the tail causing a jam in the perfector. Thus sheet cut length is very important.
When printing light weight papers such as a 50, 60 or 80 lb text, the transfer cylinder of a press will be fairly small. When running heavier weight papers, however, such as a Bristol or a board, a larger transfer cylinder is usually preferred. The reason press manufactures will make the transfer cylinder larger for a press that runs mostly board is because of the stiffness of the board. When transferring between printing units the heavier weight board will not wrap around the small transfer cylinder resulting in “sheet slap.” If the transfer cylinder is larger in diameter, the board does not warp near as much and sheet slap is eliminated. Heidelberg, for example, makes a “CD” (carton and diameter) Speedmaster. This press uses a special double dimension hard chrome plated impression cylinder and a singe triple dimension transfer drum to ensure smooth transport for all board.