1. Hold the plate tightly in position.

2. Hold the plate while the dampening rollers are contacting it and wetting the nonimage area.

3. Hold the plate while the inking rollers are contacting it and applying ink to the image area.

4. Help transfer inked image to the blanket.

The plate cylinder consists of a metal body ground to close tolerances in diameter. On sheetfed presses it is usually not a complete cylinder, but has a depressed gap (about 20% of circumference) to accommodate plate damping bars. The gap also permits the inking system to recover before the next sheet is printed and compensates for individual sheet feeding. Note, Heidelberg has patented a “gapless” plate cylinder and has been able to enforce unlicensed use in the U.S.

On each end of the plate cylinder is a bearer which is a hardened metal ring attached to the cylinder body or journal. On many presses the plate cylinder bearer is in contact with the blanket cylinder bearer during printing. The diameter of the bearer is the effective diameter of the cylinder and is the same as the pitch diameter of the gear attached to the journal. The plate cylinder is driven by this gear, which is in turn driven by a similar gear on the blanket cylinder.

The cylinder gears may be spur or helical. A spur gear has teeth cut straight across, while helical gears (generally on newer presses) have teeth cut at an angle. A spur gear generally has a backlash gear, a this second gear bolted to it to reduce "play" (free or unimpeded movement). Presses which print with bearers out of contact always have helical gears. The plate cylinder can usually be rotated independently of its gear.

Adjusting the plate cylinder manipulates the printed image position. However, moving the plate cylinder to adjust the image position wastes time and paper. Standard print and plate clamp margins should be established, instead. The undercut of the plate cylinder is the difference between the bearer and cylinder radii. (Note the duplicators in our laboratory do not have bearers, so the effective undercut is not defined in the same way. For an example of bearer and undercut see the Heidelberg KORDs.

The undercut provides space for packing between the plate and cylinder. This provides proper printing height and adjusts cylinder circumference to control print length. A plate clamp is a device designed to grip the edge of the plate and pull it tight against the cylinder body. There are a variety of designs but all have provisions to move the plate laterally and circumferentially.