Reference Book
roll. The paper from the depleted roll is cut off and the roll is brought to a stop.
The new roll begins to feed the press at normal running speed.Zero speed paster:- It is a device in the infeed section of a web press that
splices a fresh roll to an expiring roll by stoping the expiring roll briefly but
not stoping the press.
Web break detectors:-A web break detector is a device that automatically detects
the site of a web break and immediately shutdown the press. The detector also
prevents any free paper from whipping back and wrapping around the cylinders.
Five main causes of web break are:
a) wet spot on web,
b) edge cuts,
c) felt hair, calender and fiber cuts,
d) tension control malfunctions and
e) humidity problems.
Image alignment and register:- There is a number of printing adjustments that
can be made to place the printed images correctly on the paper web and achieve
register in backup with other colours. The plate cylinder can be angled, moved
laterally or be packed. The entire printing unit can be advanced or retarded
and check the back to back register with the opposite plate image. The operator
can check the image quality and register on one or both sides of the web while
the press is running.
Ink drying systems:-Heat-set inks are most commonly used inks in web offset
printing. The composition of heat-set inks requires the use of heat, for drying
and chilling or cold setting to bond the pigments to the paper. Web-offset
presses using heat-set inks must be fitted with an exhaust system to eliminate
solvent vapours from the driers.
Non-heat set inks do not require the use of heat from the driers. They are used
on highly absorbent uncoated papers. Radiation curing inks,UV curing inks
and thermal curing inks are also used in web-offset presses.