As we discussed earlier about web fed printing presses, we will discuss about the design of web fed printing presses.
Web-fed printing presses differ less from sheet-fed presses
in the configuration of their printing and inking units
than in all the machine elements actually transporting
the substrate. In principle it is much easier to transport
a web through the various press units than a sheet. The
continuous web to a great extent guides itself, because
the start of the web is pulled and the web is held under
tension in a stabilizing, defined way. Therefore web
presses are more suitable for higher printing speeds than
sheet-fed presses (sheet-fed printing up to approximately
4 m/s,web printing up to approximately 15 m/s).
Web presses are production systems that are able to produce
finished printed products, that is, finishing is an integral
part of web offset printing.
Web presses are relatively inflexible with regard to
format variation compared to sheet-fed presses, only
the web width is variable. The format is defined in the
circumferential direction of the printing plate and in
the direction of web travel by the cut-off length. The
advantage is here that, depending on the printing
process, there is no printing gap (or at least almost
none). In flexographic and gravure printing the cylinder
circumference can, within certain limits, be adapted
to the cut-off length, whereas on web offset presses
the cylinder diameters are invariable.
Fig 1 Print line of a web-fed offset printing press with the individual units (IFRA/KBA)
Figure 1 shows a schematic representation of a
web-fed offset press and its various components.Only a
general description is given below.
Transport Elements of the Web Press
Web transport through the web press begins with the
reel of paper or any other substrate on a reel; this is
clamped in reel arms. To avoid production stoppages,
the reels are almost exclusively changed in a fully-automated
process by splicers. A separate infeed unit, also
called pre-tensioning unit, is located after the reel stand
for keeping web tension constant.
Fig 2 Printing unit of a web offset press
(vertical blanket-to-blanket unit) with horizontal
web travel and possibility of a web overrun (IFRA)
The printing unit itself serves also as a transporting
element, because the web is transported under pressure
between two cylinders. In the blanket-to-blanket unit
of a web offset press these are two blanket cylinders
(blanket-to-blanket mode), in gravure or flexographic
printing (letterpress) they are a cylinder with a soft covering
and a steel cylinder. Multicolor prints are produced
by printing units arranged one after the other, which print the color separations onto the web. The
web behavior during transport varies according to the
cylinder surfaces, which requires a special dimensioning
of the diameter of the steel cylinder and/or adaptations
of the blanket cylinder packing.
Various finishing equipment is connected to the
printing unit, such as the numbering unit, gluing station
(for partial coating), dryers, chill roll assembly,
slitters, turner bars, and perforators, as well as the formers
and the folding unit, which all have some effect
on web travel. To compensate for this the web travel
is stabilized by driven draw rollers. In many applications
(e.g., newspapers,magazines, catalogs) the folder
with its cylinder section acts as a tensioning mechanism
at the start of the web. Draw rollers acting on
both sides are fitted at the entry of the folder, but it is
the tucker blade cylinder of the jaw folder that exerts
the actual tension. It is equipped with height-adjustable,
curved plates or with circular shaped segments
that correspond to the cylinder radius. The
press does not need to be stopped for readjusting the
tension, as it is
Read more »