Reference Book
Printing Unit
The printing unit places a water solution and ink on the plate, transfers the
image on to the paper and delivers the paper to the delivery unit. The printing
plate must be adjusted so that the proper amount of ink and water solution is
deposited on the printing plate so that the image is transferred accurately, evenly
and consistantly to the printing paper. The printing unit of an offset press
consists of cylinder system, the dampening system and the inking system.
Cylinder system:- The lithographic press consists of a group of cylinders that
transfer images from the printing plate to the substrate. The components of a
cylinder system are the plate cylinder, the blanket cylinder and the impression
cylinder.
Cylinder structure:- The components of a cylinder includes the body, bearers,
bearings, clamps and a gear that drives the cylinder.
Cylinder body:- is made up of steel or cast iron. Normally it is coated with
chromium to prevent rusting. It should have perfectly even surface. Body of
a press cylinder has two parts namely cylinder gap and cylinder bearer. The
cylinders have some gap on its surface where the clamping mechanism is
attached. It is called the cylinder gap or depression gap. The narrow metal
band at both ends of press cylinder used to allow packing and easy alignment
is called the bearer. The height of the bearer is always higher than the cylinder
body to accommodate plate. The difference between the radius of the cylinders
bearers and the radius of the cylinders themselves is called undercut. The
cylinders have a series of clamps in the depression gap to hold the plate or
blanket.
Gutter:- is the small gap between the bearer and the cylinder body. It prevents
foreign bodies like grease and other lubricants coming into contact with plate
and blanket surface.
Drive gears:- are gears that make the cylinders rotate.
Cylinders
Plate cylinder:It holds the printing plate on the press, receives water on the
non image area from the dampening system and ink on the image area from
the inking system and transfers the image to the intermediate blanket on the
blanket cylinder. This cylinder brings the inked plate image into contact with
the blanket cylinder.
Blanket cylinder:- It is located between the plate and the impression cylinders.
This cylinder holds the rubber blanket, a synthetic rubber mat that accepts the