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A detailed
description of web offset press operation is beyond
the scope of this publication or training. However,
the growth of web printing for books, newspapers,
magazines, business forms, etc. has made the process
a major part of the printing industry. The following
information will give you a brief introduction and
understanding of the workings of a web offset press.
Web printing
refers to the printing on a continuous roll of paper
(or some other substrate) rather than printing on
individual sheets as a sheetfed press does. There
are two basic types of web presses with two sub
types. These include:
- Heatset
web - heat dries the ink
- Blanket
to blanket - prints both sides of the web
at the same time.
- Coldset
web - ink dries by air and absorption
- Blanket
to impression - only prints one side at a
time
The web
press is similar to the sheetfed press in that it
contains an infeed section, printing units and delivery
section. However, because the web prints rolls the
various sections are different from those used on
a sheetfed press.
There
are differences between the various web presses
not only in web width but speed, configuration,
etc. Web press widths can vary from 11" up to 56".
Speeds can vary from 300 feet/minute to 3,000 feet/minute.There
are basically two types of web presses:
High
speed commercial web
- Used
mostly for high volume printing such as publication,
newspaper, book, advertising, etc.
- Speed:
900 - 3000 ft/min
- Heatset
- ink dries by heat
- Combination
sheeters and folders
- Prints
both sides of the web at the same time.
- "Half-web"
meaning web widths in the 25" range
- "Full-web"
meaning web widths in the range between 33" -
56"
Small
(narrow) Web
- Used
mostly for lower volume printing such as business
forms, small publications, etc.
- Speed:
300 - 900 ft/min
- Coldset
- ink dries by air and absorption (into the paper)
- Roll
to roll or roll to sheet or roll to fan-fold
- Able
to perforate, punch, etc.
- Width
ranges from 11" to 25"
All web
presses have some commonaities such as an infeed,
printing units and delivery.
Infeed:
Paper is delivered to the press by the infeed section.
The infeed section typically contains a roll stand
to hold the paper rolls; a splicer which automatically
splices the end of one web to the beginning of another
(except on small business forms presses), a web-steering
device which controls the sidelay (side-to-side
position) of the infeeding web; and a tensioner
that maintains the proper tension on the web as
it enters the press.
Printing
Units:
Web presses are made up of several printing units
usually numbering from 3 up to or more than 9. Like
sheetfed, each printing unit contains an inking
system, a dampening system, a plate cylinder, a
blanket cylinder, and an impression cylinder.
Delivery:
The delivery
section can consist of a variety of devices, from
a simple sheeter, which cuts the moving web into
sheets of the required size, to a combination sheeter
and folder, which can both fold the web into final
signatures (group of pages) and trim the signatures
to size. Small business forms presses can print
from a roll into another roll for future processing.
While
there are similarities between the various web presses,
there are also many differences.
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