| The Graphic Arts Technical Foundation presents
this audiovisual on Cylinder Packing for Changing Image Length.
This information is vitally important to the maintaining
of quality press reproduction. |
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| AV 54, Offset Press Cylinder Packing, described
several reasons for changing the diameters of the plate and blanket
cylinder bodies. The first was to compensate for the varying
thicknesses of plates and blankets; |
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| ...and the second was to accurately establish
the plate-to-blanket transfer-squeeze pressure. |
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| Another reason is to change the relationship
between the diameters of the cylinders by shifting the packing
from one cylinder to another. This changes the length of the
printed image on the sheet. This is necessary to make work fit
in multicolor printing because image sizes may change with each
color printed. With presses that have bearers, we shift packing
from the blanket to the plate to get a shorter image. On presses
without bearers, all that is needed is to add or remove packing
from under the blanket and readjust the plate cylinder for proper
printing pressure. |
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| Normal printing on an offset press results in
an image that is slightly longer around the cylinder than the
image that was put on the flat plate. This extra length is considered
normal on most classes of work. |
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| One of the reasons for the image printing longer
on the sheet than it is on the plate is that, in bending the
plate around the cylinder, the image on the outer surface of
the plate lengthens due to curvature. When a piece of metal is
bowed, the convex, or outside, surface is stretched. The image
elongates with it. |
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| When designing presses, manufacturers have taken
this into consideration. Some of them build the plate cylinder
large to print an image that is the same size as the paper as
on the flat plate. |
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| Others grind the impression cylinder small to
accomplish the same result. |
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| If we keep this in mind, then we can say, in
simple terms, that the larger the plate cylinder, in relation
to the impression cylinder, the shorter the print. Or we can
say the smaller the impression cylinder in relation to the plate
cylinder, the shorter the print. When we say "short,"
we mean "relatively" short. |
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| Stretching of the plate due to excess tension
at the clamps causes additional elongation, especially when using
thin plates. There are also other variables that must be carefully
controlled if we want to prevent excessive elongation of the
printed image. |
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| One such variable is the mechanical stretch the
paper can receive going through the press. One cause of this
can be the ironing effect of too much pressure between the blanket
and the impression cylinders. |
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| Another cause of mechanical stretch can be too
tacky an ink when printing large solids. The pull necessary to
peel the sheet off the blanket stretches the paper and often
causes embossing of the solids, as well as curl. |
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| Some papers have the ability to return to size
after being stretched mechanically. Others develop a permanent
elongation. Papers of the latter type, that are multiple-sheeted
by the mill, often show different amounts of elongation in successive
sheets. This causes sheet-to-sheet misregister. |
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| Another variable is stretch of paper due to moisture
pickup in the press. This can be held to a minimum by moisture
control in the atmosphere, keeping the amount of dampening solution
on the plate to a minimum, and proper relative humidity balance
between paper and pressroom atmosphere. |
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| Paper that is out of moisture balance with the
pressroom atmosphere will stretch or shrink, mostly across the
grain, depending upon whether it picks up or loses moisture.
By having the grain run crosswise on the press, most stretch
or shrinkage will occur in the around-the-cylinder direction.
So, with long-grain paper, all normal stretch is in the around-the-cylinder
direction, and we can compensate for it. |
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| How do we do this? If the paper has stretched,
making a longer image during the first printing, we transfer
packing from the plate cylinder to the blanket cylinder for the
second printing. The more packing that is transferred to the
blanket cylinder from the plate cylinder, the longer the print
will be from gripper edge to the back edge of the sheet. Conversely,
the more packing transferred to the plate cylinder from the blanket
cylinder, the shorter the print. |
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| The resulting change in cylinder diameters causes
a slight amount of slippage or creep. Geared in, the cylinders
rotate simultaneously. The larger cylinder surface travels further
than the smaller one with which it is in contact. This can only
be accomplished by a slight amount of slippage between the two
bodies. But, as long as this is held within limits, no slur or
pate wear occurs. |
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| The printed image on a large sheet can be elongated
one-eighth to three-sixteenths of an inch without any bad effects.
It is only because of this leeway that good multicolor register
be obtained on various types of paper when moisture cannot be
well controlled. |
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| Since stretch of paper occurs more often than
shrinkage, it is important to have all possible leeway for lengthening
the image after the first color is printed. To do this, pack
the plate cylinder for the first color high, and the blanket
cylinder low. This results in a relatively short print. We can
then compensate for a great deal of stretch in printing the succeeding
colors. |
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| It is also important to start the run with a
low packed blanket cylinder because the tendency to slur increases
as the blanket cylinder diameter increases in relation to the
impression cylinder. If the first color is started properly,
the blanket will not have to be packed beyond the point where
slurring begins. |
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| Experience has shown that, if the first color
is printed somewhere near the short limit, we can take advantage
of our full permissible leeway. We can avoid getting into the
danger zone of overpacking the blanket in trying to print abnormally
long on succeeding colors. |
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| On modern presses, it is almost impossible to
reach the low limit for printing short. The impression cylinder
will not adjust that far. A blanket of normal thickness can be
applied to the blanket cylinder without any packing at all, and
the press, it is will print with such a low unpacked blanket,
will print without slur. |
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| Some presses will not print this way because
there is not enough adjustment of the impression cylinder to
reach such an underpacked blanket when printing thin paper. |
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| Let us take a practical example. For the first
color we find that, on a certain press, we can pack the blanket
to .005" below the bearers and obtain normal printing. |
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| We will have to overpack the plate to .007"
above the bearers to produce a .002" transfer-squeeze pressure.
During a run of five or six colors, on a single color press,
our paper stretches on each color. But we have a latitude of
.009" of packing to take out from under the plate and put
under the blanket for the succeeding colors before we get into
serious trouble. This should take care of almost any amount of
paper stretch with the exception of very abnormal conditions. |
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| Suppose we had started this job with normal packing.
If we had started the first color with the plate even with the
bearers and the blanket .002" over the bearers, our latitude
would be small indeed. A shift of packing of even .004"
from plate to blanket would result in a blanket overpacked .006"
We might be over the limit for packing a blanket above the bearer
surface, and would be lucky if we avoided slur and worn plates
on our last color. |
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| Whenever slurred dots have their tails toward
the back of the sheet, the blanket is packed too high, or there
is excess impression cylinder pressure, or both. The squeeze
of the blanket at the nip and the bulge which results, added
to the bulge of the paper at the nip, may contribute to slur.
By keeping pressure to a minimum, especially impression cylinder
pressure, we can avoid it. |
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| Streaks are very often due to the same causes.
A blanket that is packed too high will travel faster through
the nip than the plate or the paper. Strains and stresses are
built up within the blanket due to resistance. When the stresses
build up to a point where they overcome the resistance, there
is a slip that causes a streak. These streaks will sometimes
be so regular that they are mistaken for true gear streaks. This
is especially true on coated paper. Streaks of this type can
occur between plate and blanket, or between blanket and paper.
|
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| Ink acts as a lubricant on the surface of the
blanket, reducing the slip resistance in heavily inked areas.
In such areas, there will be a greater tendency to slur than
to streak. |
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| Too high a blanket, or excess pressure between
plate and blanket, is not readily noticeable unless a tendency
to streak appears. This does not mean, however, that there is
no trouble here. If the slippage that is being employed to compensate
for paper stretch is not kept within its allowable limits, plate
wear will result. |
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| The image on surface plates, in particular, will
have a tendency to break down. Their lives will be materially
shortened. Most of our troubles occur on late colors in multicolor
work. Halftone does will first start to enlarge and thicken the
tones. Reverse lettering will tend to fill in. Then plates will
show wear. |
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| It bears repeating that excess pressure also
aggravates the condition. When adding or removing packing from
the blanket cylinder, adjustment must be made to the impression
cylinder. A minimum amount of pressure is best for quality reproduction.
|
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| When the packing gauge indicates that the cylinders
are packed correctly to the manufacturer's specifications, and
there are still portions of the image that print light, resort
to local packing. Do not, at any time, put a whole packing sheet
under a blanket--or increase impression cylinder pressure--to
take care of small, low areas. Allowing a low area to go unpatched
will require excess impression cylinder pressure, and that is
where the damage is done. |
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| The register rule is an invaluable tool for the
analysis and correction of register problems that occur when
sheets go through the press more than once on a sheetfed press.
|
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| It is used to measure and compare the distance
between register marks on the plate and those on the printed
sheet. The comparison of these measurements will indicate whether
the first color is printing long or short on the sheet from front
to back as well as along the lead and back edges. These measurements
will also show if there is an uneven stretch or distortion in
the sheet length from front to back. |
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| If the sheet has stretched or shrunk and
can be corrected by shifting the cylinder packing, the register
rule will help determine the amount of packing that should be
shifted. On a properly adjusted press, image enlargement is directly
proportional to the ratio of the diameters. Using this as a starting
point, we can calculate enlargement from plate to blanket. As
the formula states, the original cylinder diameter multiplied
by 3.1416 divided by the new cylinder diameter multiplied by
3.1416 is equal to the original image length divided by the enlarged
image length. |
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| Every time we remove .001" of packing from
the plate and put it under the blanket, we change the ratio.
The plate cylinder diameter is decreased by .002".
The blanket cylinder diameter is increased by .002".
This is a total change of .004". In one revolution, then,
we have an enlargement of .004 x 3.1416 or .0126". |
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| However, the image does not extend all the way
around the cylinder. We must multiply this figure by the percentage
of the circumference covered. If the image extends only 2/3 of
the way around the cylinder, we have 2/3 of .0126 or approximately
.008" enlargement. Several factors have a bearing on whether
all the elongation gained on the blanket will be carried over
to the paper. Some of these are: |
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| (1) The diameter of the impression cylinder,
in relation to that of the plate cylinder with the plate and
its packing; |
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| (2) Impression cylinder pressure; and |
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| (3) The ability of the paper to return to original
size after the impression. |
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| To summarize the problem of cylinder packing
on the offset press, we must observe the following: (1) Find
the exact cylinder undercut; |
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| (2) Pack plate and blanket to proper printing
height, using the minimum number of packing sheets; |
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| (3) Keep impression cylinder pressure to a minimum;
and |
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| (4) When printing the first color on a multicolor
job, shift packing from the blanket to the plate cylinder to
print as short as possible. This allows plenty of leeway to compensate
for paper stretch. |
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