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Offset
press blankets are made of supporting
fabric and a rubber composite. Various
rubber materials are used for the transfer
of the image from the plate to the blanket.
Blankets come in compressible to hard
finishes along with smooth to rough surfaces.
The majority of printers today use compressible
type blankets. These blankets give good
compression which eliminate many smashed
blankets. Slightly rough blankets are
also preferred as they have less contact
with the paper surface thus reducing blanket
contamination.
Offset
compressible blankets are constructed
from two, three or four plies of a strong
woven fabric fastened together with thin
layers of "rubber" adhesive to form a
laminate termed the carcass. The carcass
then receives many coats of a suitably
compounded polymer, applied to one side
to produce the face layer which is approximately
0.020" thick Total thickness, i.e. face
and carcass together, is between 0.030"
and 0.075", depending upon the number
of fabric piles employed. The number of
fabric layers equals the number of plys.
Thus a 4 ply blanket would contain 4 fabric
layers. To make the blanket "compressible"
a thin layer of air cells (for compression)
is layered between the fabric and rubber
face. Conventional blankets are made up
of a rubber face and the fabric carcass.
A compressible blanket is made up of a
rubber face, fabric, compressible layer
and a carcass.
The
conventional blanket will bulge at the
printing nip (displace not compress),
especially when over packed. The conventional
blanket is good when blanket packing is
critical, however, squeeze pressure must
be precise. The conventional blanket is
recommended where high print pressures
are required. The compressible blanket,
on the other hand, will form only a very
slight bulge when overpacked.
The
diagram above shows the difference in
compression between the compressible and
the conventional blanket
Unlike
the conventional blanket the compressible
blanket has many more features such as:
- Smash
resistance which translates into longer
blanket life
- Compensates
for variation in Blanket thickness and
substrate
- Wider
latitude for packing
- Reduction
in plate wear
- Reduction
of slurring
- Minimizes
mechanical press problems
Blankets
are manufactured in three types of surfaces:
- Cast
- Surface finish produced by curing
paper and talc
- Ground
(buffed) - Mechanical process of finishing
the surface of a blanket
- Texturized
- Surface finish produced by chemical
means
Below
are magnifications (100X) of various blanket
surfaces (Cast, Buffed and Textured)
Blankets
will generally last 3 to 6 months depending
on the type of press (web/sheetfed) and
types of jobs run. Conventional blankets
are hard and give a high amount of resistance
when the blanket and plate cylinder are
squeezed together, as shown in the drawing
on the next page. Compressible blankets,
as their name implies, are compressible
and give under the squeeze.
Properties
required in an offset blanket:
The
fabric piles must be strong and stretch
very little. There should be, however,
some degree of stretch to allow the blanket
to fit tightly on to the cylinder, since
a slack blanket can cause doubling and
slurring. On the other hand, excessive
tensioning through high stretch will cause
uneven thickness and result in low areas
in the print. The thickness must be uniform
between closely defined limits.
With
an compressible blanket, slight bulges
do form, but to a much lesser degree than
the conventional blanket. Compressible
blankets are used to print uncoated and
lighter coated papers. When a blankets
wrap occurs, due to a web break or paper
sticking to the blanket, the compressible
blanket has the best chance for surface
recovery. The conventional blanket, due
to its very hard surface, will not retract
like the compressible surface.
The
conventional blanket bulges at impact
point. Rubber displaces rather than compresses
and will bulge when subjected to pressure.
Because of this displacement, the surface
speed of the conventional balnket is slightly
different than the surface speed of the
plate. One possible result is slurring.
Conventional blankents can be used when
highly coated papers.
The
face of the blanket should be
- free
from pinholes, and blemishes which could
affect print quality non-abrasive to
reduce plate wear
- resilient
- of
uniform surface hardness and hard enough
to be capable of reproducing a facimile
of the printing image
- very
smooth, having a matt surface with no
low spots or raised areas
- resistant
to ink vehicle, cleaning solvent and
varnish
- ink-receptive
- resistant
to peeling, blistering, embossing, debossing,
glazing or tackiness, also abrasion
from paper or board
- capable
of giving good release (ink and paper)
Packing
is very important to the life of a press,
blanket, plate, etc. When a new blanket
is put on a press, it will first stretch.
Usually the pressmen will, after installing,
run the press under impression for several
hundred or thousand impressions then retorque
the blanket. Blankets should always be
torqued to assure proper tension. To attain
the proper squeeze between the plate and
blanket (to assure proper transfer of
the image) special paper, known as packing
material, is placed under the blanket
and plate. A special “packing micometer”
is used to determine the amount of packing
material needed. See page 3-35 for a diagram
of blanket packing. It is important that
you always recheck packing, after installing
a new blanket, several thousand impressions
after.
- Common
blanket problems include:
- Incorrectly
installed (over/under packed)
- Overtightened
- hand vs. torquing
- Poor
blanket maintenance - using harsh cleaning
materials that contain acetone, toluene,
etc.
- Out
of square blanket
- Wrong
blanket for application
- Poor
release blanket
As
there are many types and styles of blankets,
the printer must evaluate the type of
blanket he wishes to use and work with
a blanket supplier such as Day International.
Most printers are printing a wide range
of papers and can not change a blanket
for every paper printed. A printer must
work with a supplier and choose the right
blanket that gives him the best overall
performance for a wide range of papers
printed.
Using
a packing gauge
How
to pack a blanket...
Plate
cylinder is .020” undercut. The
offset plate is .012”thick. To bring
the plate to a pressure squeeze of .004”
(over bearers) we need to add 0.012”
packing material.
The
blanket cylinder is .090” undercut
and the blanket is 0.065” thick,
then 0.025” packing is required
to bring the blanket to the surface.
This
then will allow the squeeze to be 0.004”
between the plate and the blanket.
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