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The
printing unit places a water solution
(fountain) and ink on the offset plate,
transfers the image to the blanket cylinder,
then to the paper. It then delivers
the paper to the delivery unit. The
printing unit must be adjusted so that
the proper amount of ink and fountain
solution are deposited on the printing
plate so that the image is transferred
accurately, evenly, and consistently
to the printing paper. Every offset
printing unit is made up of the following
three parts:
- The
cylinder system (plate, blanket and
impression)
- The
dampening system
- The
inking system
Each serves an important function in
the total image transfer system. The
only deviation in this process is with
the "waterless" system (Toray) which
has been explained elsewhere in this
publication.
Many
multicolored sheetfed presses are equipped
with a Perfecting cylinder to turn the
paper over so that the back side can
be printed. For example, a four-color
press may have the perfector cylinder
between the second and third unit. After
printing two colors on the top of the
sheet, the cylinder can be configured
to run the paper over and the third
and forth unit will print 2 colors on
the back side.
If
the printer does not want to print "two
up" (two colors each side), he will
configure the unit so that the paper
is not turned around thus printing 4
colors on the top.
Many
7 color presses have the perfector unit
between the 2nd and 3rd unit. This enables
the printer to print 2 colors on the
inside of the sheet and 5 on the outside.
Greeting cards are often printed this
way. Sheet length is very important
in perfecting. Most of these perfecting
units will tolerance only a 1/16 of
an inch variation. If a sheet is off
more than that the grippers will miss
picking up the tail causing a jam in
the perfector. Thus sheet cut length
is very important.
When
printing light weight papers such as
a 50, 60 or 80 lb text, the transfer
cylinder of a press will be fairly small.
When running heavier weight papers,
however, such as a Bristol or a board,
a larger transfer cylinder is usually
preferred. The reason press manufactures
will make the transfer cylinder larger
for a press that runs mostly board is
because of the stiffness of the board.
When transferring between printing units
the heavier weight board will not wrap
around the small transfer cylinder resulting
in "sheet slap." If the transfer cylinder
is larger in diameter, the board does
not warp near as much and sheet slap
is eliminated. Heidelberg, for example,
makes a "CD" (carton and diameter) Speedmaster.
This press uses a special double dimension
hard chrome plated impression cylinder
and a singe triple dimension transfer
drum to ensure smooth transport for
all board.
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