The contact area between lithographic plate and blankets needs to be under sufficient pressure to transfer the ink appropriately. There are two ways to adjust this distance: cylinder placement and blanket thickness. 

Proper cylinder placement, or iron-to-iron distance, is commonly tested with a metal strip which varies in width by 0.001 inch from one end to the other. Various presses require different distances and gauges. If the thin end of the gauge fits between the cylinders, but the thick end does not, the iron-to-iron distance is in the correct range between these two values. 

Blanket packing can also be adjusted in order to change the effective diameter of the blanket cylinder. This is the poorer choice because the pressure created will build up differently over the contact area as the blankets are over-compresssed. This is not how the press was designed to operate and will result in sheet overfeed, loss of registration, shorter blanket life, and image distortion.

The best transfer of ink from plate to blanket occurs when the circumferences of the plate and blanket roll are equal.

When the printing contact pressure is too light, more ink is required to fill the gap. In this case, ink is wasted and paper tends to shed lint into the ink. Print show through to the back side of the paper is increased, as well as ink set-off onto other sheets.

When contact pressure is overly heavy, excessive wear occurs on all parts in contact including plates and press gears. The printed image will be distorted by excessive dot gain.

Author: Frank Borlon
Source: Newspapers and Technology, Oct. 1999, p. 26.
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