Graphic Design & Printing Technology

Graphic Design & Printing Technology

powder grain that serve as spacers ensuring oxidative drying by the inclusion
of air between the sheets. The grain laying between the sheet provide air cush-
ion between the induvidual sheets.

Offset Blanket

Offset blankets are formed from vulcanised rubber bonded on a fiber base.
Blankets should have the ability to accept ink from the plate and to transfer it
to the paper with satisfactory ink density.

Structure of a blanket

The bottom layer or the back side of the blanket
is formed from rubber or synthetic materials
which are specially woven to maintain the
quality and strength of the blanket. They are
called carcass. The backing on a blanket
includes atleast one layer of soft resilient
material, such as form, rubber, cork, or loose
fabric fibres that gives blanket its compressible
character. To make the blanket compressible a
thin layer of air cells is layered between the fabric
and rubber face.

Depending on the number of fabric layers on a
blanket it may be classified as single ply, 2 ply, 3 ply, 4 ply, etc. The number of
fabric layers equals the number of plys. Thus a 4 ply blanket would contain
four fabric layers.

Two basic types of offset blankets are conventional and compressible blankets.

Conventional blankets tend to bulge at the impact point. They are made without
a layer of form.

Compressible blankets are designed to compress and instantly rebound to its
original shape and thickness. They are the most widely used blankets because
they prevent minor blanket smashes from effecting the print quality.

Under blankets:- They are underlay blankets, made of form rubber, cork or
loose fabric that can be used under the blanket to improve its performance.
Benefits of using underblankets are reduced downtime, sharper definition of
fine images, and reduced roller and cylinder pressure that lead to reduced plate
wear.

Hardness of blanket is measured in terms of shore hardness. (eg. 80oA or
80 shore A) The device used for measuring the shore hardness of a rubber
blanket is shoredurometer.