Instructional Package 505/6-13

THE SHEET CONTROL SYSTEM

Sheet control systems control

The positioning of each sheet of

paper
The movement of each sheet of paper
through the press from feeder to
delivery

Mechanisms in sheet control

Feeding sheet into the registering

system
Inserting sheet into the grippers of the
printing unit
Carrying the sheet through the
printing unit(s) in perfect register.
Delivering the sheet into the delivery

The feeding cycle

Paper is piled into the feeder

Sheets are separated and fed into the
register system
Sheet is front- and side-guided
Sheet is taken by--or inserted into--
impression cylinder grippers
Grippers hold the sheet in register
during printing
Sheet is delivered by an endless chain
on which gripper bars are mounted:
both impression and delivery grippers
grasp sheet at one time.
The sheet is jogged into position in
the delivery.

What is register?

Register refers to the positioning the

sheet in relation to the image on the
 blanket.
Front- and side-guiding of the sheet
creates a fixed distance from the head
and side-guide-side of the sheet to
the image.
The dimensional-instability of paper
necessitates that presses can only
register two sides of each sheet: the
head and side-guide-side.

Multicolor presses

The sheet must be transferred from

printing unit to printing unit in exact
register
Transfer cylinders carry the sheet
from one unit to the next.

Importance of gripper tension

Gripper tension must be set properly

to avoid loss of register, doubling of
the image, and damage to the sheet.

Functions of a feeder on a sheet-fed press

Keeps the sheets in alignment with

the registering mechanism.
Separates the top sheet from the pile.
Lifts the top sheet off the pile.
Forwards the sheet into the
forwarding device.
Controls the height of the pile to
insure uninterrupted feeding.

The successive-sheet feeder (AB Dick and

Chief)

The top sheet is separated and lifted

at the front or gripper side by air
blast and vacuum suckers
The sheet is forwarded onto a
registerboard which travels the same
rate of speed as the press.
Succeeding sheets are lifted and
forwarded once the tail-end of the
 preceding sheet has cleared the
feeder.

Stream feeders (most large presses)

The top sheet is separated and lifted

from the back edge by air blast and
vacuum suckers.
The front edge, floating on a layer of
air, is forwarded onto the
registerboard.
Each succeeding sheet is lifted at the
back edge and advanced as soon as
the preceding sheet has traveled a
short distance down the registerboard.
This forms a stream of underlapped
sheets.
The rate of speed of the feeder is less
than press speed.
The underlapped sheets must travel
only a short distance before reaching
the registering unit once the
preceding sheet is drawn into the
printing unit.
The stream feeder makes possible
high press speeds while minimizing
sheet control errors due to the
reduced speed of the underlapped
sheets on the registerboard (as
compared to the successive-sheet
feeder).

Stream feeder components and functions

Feedboard and/or skid--platform to

hold and lift paper to feeder height.
Hoisting mechanism--to lift
feedboard and/or skid

A photograph of a Stream Sheet Feeder on

a modern press can be viewed by clicking
here. Another view is available here.

Stream feeder components and functions

 Rear air-blast nozzles--separate top-
most sheets and provide air to
cushion and suspend the top sheet.

Stream feeder components and functions

Vacuum pick-up suckers--one set lifts

the top sheet from the stack (blue)
and another moves the lifted sheet
forward into the registerboard (red).
Pressure foot and pile height
governor (green)--controls pile height
and back edge of sheet.

A photograph of the vacuum pickup suckers

on a modern press can be viewed by
clicking here. A closeup is available here.

Stream feeder components and functions

Sheet separators--make sure that only

one sheet is lifted off the pile.
Stream feeder components and functions

Sheet steadiers--hold the back and

side edges of sheets in position

Successive sheet feeder components and

functions

Feedboard and/or skid--platform to

hold and lift paper to feeder height.
Hoisting mechanism--to lift
feedboard and/or skid
Front air-blast nozzles--separate stop-
most sheets and provide air cushion.
Vacuum pick-up and forwarding
suckers--lift top sheet at front edge
and advance it into the registerboard.
Pile height governor--controls the
height of the pile in the feeder.
 Sheet steadiers--hold the back and
side edges of sheets in position.

Conveyor tapes

Four or more cloth tapes that carry

the sheet from the feeder down the
registerboard.
Tapes should be evenly spaced to
support the sheet on the
registerboard.

Devices used to control the sheet on the

registerboard.

Metal, plastic, or glass balls (bottom

left)--turn in any direction and are
placed resting on the sheet throughout
its travel down the registerboard. The
balls must rest on top of a moving
tape.
Rotary brushes (top right)--rest on the
sheet throughout its travel down the
feed ramp.
Flat brushes (bottom right)--hold the
back edge of the sheet firmly against
the front stops after the sheet has
stopped moving forward. Prevents
"bounce."
Rubber-tired wheels (top right)--press
the sheet against the moving tapes.
They, like the flat brushes, are used to
hold the sheet firmly against the front
stops. They must be positioned
immediately off the tail of the top
sheet after it has been stopped by
the front stops.
A photograph of the register board of a
modern press can be viewed by clicking
here.

Setting the caliper (double sheet detector)

Prevent additional sheets from

entering the press--prevents unprinted
"slip" sheets as well as damage to the
blanket and press.
May be electrical or mechanical
(Chief 17)
The number of sheets that should
pass under the caliper depends on the
location of the device on the
registerboard.
Adjust the extra-sheet clearance so
that the proper number of sheets can
pass through unobstructed, but an
extra sheet "trips" the mechanism.

Setting forwarding roller pressure

Pressure is set so that a slight drag is

felt when a sample of the stock to be
run is drawn under the rollers using a
sideways movement.
Unequal pressure will cock the sheets
as they move down the registerboard.

Early- and late-sheet detectors

May be mechanical,
electromechanical, or photoelectric.
Turn off the pressure between the
impression cylinder and blanket
cylinder to prevent double-
impressions when a sheet
inadvertently misfeeds.
 On some presses, these sheet
detectors may also stop the feeder
and registerboard from operating.

Sequence of sheet register in the three-point

register system

Register is positioning the image in

the same place on every sheet in
relation to the head and side-guide
sides.
Three-point register refers to two
front stops plus one side guide.
The sheet is forwarded by the tapes in
conjunction with the ramp devices.
The sheet's movement is stopped by
the front stops at the bottom of the
registerboard (1).
Ramp devices, like the flat brush or
rubber-tired roller keep the sheet
against the front stops and prevent
"bounce."
The sheet is pushed or pulled into
position by the side guide (2).
The front stops swing away from the
sheet's path and the sheet is inserted
into the impression cylinder grippers
(3).
The impression cylinder grippers
carry the sheet through the printing
nip (4).
The front stops swing back down
once the sheet has moved out of the
way.

Precautions to be taken when adjusting the

three-point register system

To properly place front stops, fold a

sheet of the paper to be printed in
fourths. There will be three folds.
Place front stops approximately at
the outer two of the three folds.
Front stops are to be positioned
between the impression cylinder
grippers. Otherwise the two devices
will collide and cause expensive
damage.
If a press has more than two front
stops, all but two should be moved
out of the way (usually moved in
toward the impression cylinder) so
that only two actually guide the
 sheet. Using more than two front
stops will likely cause misregister.
Moving the front stops in (toward the
impression cylinder) or out (away
from the impression cylinder) will
change gripper bite (the amount of
paper inside the grippers). Too little
gripper bite causes misregister while
too much gripper bite causes small
tears in the sheet.
You can square a slightly-crooked
image by moving one front stop in or
out while leaving the other stationery.
This causes the sheet to enter the
press slightly crooked--which can
straighten a crooked image. It is
important to return the front stops to
their proper (square) positions after
the job is finished.
There should be space under the front
stops so that the arriving sheet can
slide under them. Usually two-sheets
clearance is recommended.
The timing of the front stops is
absolutely critical--if they swing
away too early, the sheet will lose
registration. If they swing away too
late, the sheet will be torn.

The swing-feed insertion device

Swing grippers transfer the registered

sheet from the front stops to the
impression cylinder grippers.
The swing grippers accelerate the
sheet from a dead stop (1) to full
press speed in a fraction of a second
(1-3).
At the point of transfer (3), both sets
of grippers (swing and impression
cylinder) will be closed on the sheet
at once. The timing here (4) is
critical--if both sets are closed too
long, the sheet will be torn. If one set
opens too early, registration will be
lost.

Precautions with swing-feed grippers

Timing is critical in maintaining

register.
Grippers must be checked
periodically for dust and dirt than
may interfere with their functioning.
 Gripper bite must be adjusted
carefully and checked frequently.

Sheet bridges

Some presses have these devices (not

shown) that prevent sheets from
accidentally falling down under the
feedboard once the front stops have
swung away.

Feed roll insertion device (AB Dick 360)

The pre-registered sheet (1) is

pinched between upper and lower
feed rollers that, in turn, drive it
against stops in the impression
cylinder gap (2).
The feed rolls pinch in time with the
press' rotation.
The sheet is pinched between free-
moving rollers (in this case on the
top) and a driven roller (in the
example, the roller is a cam on the
bottom).
The sheet is accelerated to a speed
slightly faster than press speed.
The sheet is rammed into a stop in the
impression cylinder gap at higher-
than-press speed (2). This causes a
slight buckle (there is a buckle
control on the AB Dick 360 to
control the timing of the feed roll
insertion device).
Once the sheet has buckled, the
impression cylinder grippers close (3)
and the feed rollers disengage the
sheet (4).
The impression cylinder grippers
carry the sheet through the press.

Bowing

Refers to accurately aligning a sheet

that has a concave or convex lead
edge.
Can only be done on presses using
the feed roll insertion device.
Consists of stops known as gauge
pins in the impression cylinder that
are flat on one side and ground to
various depths on the other.
 The left image illustrates a sheet of
square-cut paper against the flat sides
of the gauge pins.
The center shows how the various
depths of the opposite (non-flat) sides
of the gauge pins can be used to align
a concave sheet.
The right illustration shows how a
convex sheet can be aligned.

The rotary drum insertion device.

A derivative of the swing-gripper

system except a cylinder is used
instead of a swing-arm.

The direct insertion (tumbler grippers)

insertion system (Chief 17)

There is only one set of grippers--the

set mounted in the impression
cylinder.
The registered sheet is parked at the
front stops.
The impression cylinder revolves
slightly under the stopped sheet.
At the appropriate time, the
impression cylinder grippers tumble
around and grasp the sheet from
below.
 The front stops move out of the way
and the impression cylinder grippers
carry the sheet through the printing
nip.

Push side guide

Before the sheet is inserted into the

impression cylinder grippers, it must
be side-guided.
Push side guides are generally found
on small presses (like the Ryobi
3302H-A), board presses, and metal
decorating presses.
Push guides push the sheet that has
already been stopped at the front
guides to a preset location.
Ramp devices must hold the sheet
against the front stops--without
impeding its side-to-side motion--
while the side guide pushes the sheet
into position.
If a push guide is used on a large
paper-printing press, a corrugating
bar must be used to stiffen the sheet
before the push guide is engaged.

Pull side guides

Most common on large paper-printing

presses
Friction fingers or rollers pull the
sheet against a stationery side-guide
plate.
The system must allow for slippage
once the sheet contacts the plate.
Otherwise, the sheet will be damaged.
Grippers

Low-lift grippers are used on presses

with separate insertion devices (AB
Dick 360). They only open about 1/8
inch or less.
Tumbler grippers are used on presses
with the direct insertion system
(Chief 17). They may tumble 360
degrees during their motion.
Grippers generally have a finger-like
construction.
Gripping tips are generally roughened
for better traction on paper.
Some grippers are spring-loaded to
provide even pressure and reduce the
need for constant adjustment.

Location of grippers

Swing and drum insertion devices.

Direct insertion devices.
Delivery grippers
Transfer cylinders on multicolor
presses.
Impression cylinders.

Cautions to be observed when setting

grippers.

Gripper pressure must be tight due to

the great pull exerted on the sheet by
the printing nip (ink tackiness and
blanket stickiness).
 The grippers must firmly hold the
sheet without marking it (too high
pressure causes tell-tale dents in the
paper along the lead edge).
If there is too little pressure, the sheet
will pull out of the nip. This causes
distorted images, registration loss,
problems with transfer to other
grippers, or the sheet may even stick
to the blanket cylinder!
A too-tight gripper (not spring-
loaded) will loosen the others.
The entire opening and closing cycle
of the grippers must be smooth and
free from any jerky or springy
motion.
Uneven gripper pressure results in
wrinkled sheets.

Sheet transfer on a multicolor press.

Chain
Single drum
Three-drum (shown)
Multiple drum

Anti-marking devices

Air cushion prevents ink from

transferring onto transfer cylinder--
which could case blurring.
Net-like covering can be used over
transfer cylinders so the ink won't be
able to stick to a smooth shiny
cylinder.

Sheet delivery

Delivery begins when the delivery

grippers grasp the sheet from the
impression cylinder grippers.
Skeleton cylinder--first part of the
delivery system--is composed of two
sprockets on a shaft that drive
continuous chains on which grippers
are mounted (they are the same
distance apart as the impression
cylinder circumference).
Additional supporting segments keep
the sheet from whipping, rubbing, or
 waving.
The supporting segments must be
positioned so that they do not smudge
wet ink.

A photograph of the delivery of a modern

press can be viewed by clicking here.

Delivery sheet release

Idler sprocket wheels--the delivery

chains pass around the idler sprocket
wheels that can be adjusted to
provide the proper tension on the
chains.
A cam is located near the idler
sprocket wheels to force open the
delivery grippers. This causes the
sheet to fall, face-up, onto the
delivery pile. The cam can be set to
cause the sheet to fall sooner or later.

Sheet control devices

Help control the sheets so that they

form a neat stack.
Blow-down nozzles
Static neutralizer bars
Vacuum wheels/slow-down rollers
Joggers
Wedges
Sheet decurlers

Vacuum wheels/slow-down rollers

The sheet is helped down and into

position by air vacuum wheels.
Blow-down nozzles

The sheets are also helped down by

blow-down nozzles.
The blow-down nozzles also increase
the airflow surrounding the sheet and
help the ink to dry faster.

Wedges

Wedges are used to flatten the top of a

stack of paper if the paper has
become curled.
Paper becomes curled by the molding
action caused by the stickiness
(tackiness) of the ink combined with
the pressure of the cylindrical
blanket. End-of-sheet curl is often
called tail-end curl.
Register marks

Used to indicate to the press operator

if there is misregister at either the
front stops or side guide.
Various types are available: some
show head-to-tail misregister while
others indicate side-to-side
misregister.
Register marks may be burned onto
the plate during prepress or may be
added manually by the press operator.
All register marks must be positioned
on the side-guide edge of the sheet.
The side-guide edge of the sheet
varies from side to side: on most
presses, the front side of a sheet is
guided with the side guide found on
the operator's side of the press.
Then, using a motion called turning,
the sheet is replaced in the feeder and
printed on the back using the gear-
side side guide. The Chief 17 has
both operator and gear-side side
guides.

Paper conditioning

Paper subjected to changes in

temperature or humidity may
radically change size around its
perimeter.
Paper subjected to an increase in
temperature or a decrease in relative
 humidity will shrink (become tight-
edged) around the edges while the
center remains its original size.
Paper subjected to a decrease in
temperature or an increase in relative
humidity will expand (become wavy-
edged) around the edges while the
center remains its original size.
A skid of cold paper brought into a
warm pressroom will cause the
moisture in the air around it to
condense on the edges of the stack
(condensation always forms on the
cold edge of a warm thing). Then, the
moisture will be absorbed into the
paper causing wavy edges.
A skid of warm paper brought into a
cold pressroom will release its
moisture into the air around it and
become tight-edged.
Therefore, paper should always be
allowed to acclimate to the ambient
pressroom conditions before the
package is opened. If you do this,
the condensation will form on the
outside of the package where it does
no harm. Or, the moisture will not be
allowed to leave the package--it will
remain within the paper.
The greater volume of paper to be
acclimated, the longer the necessary
standing time. See chart at right.

Problems that occur when paper is not flat

Wrinkles
Misregister
Doubling
Difficulty feeding

To prevent wavy- or tight-edged-paper

Keep all paper sealed while in

storage.
Maintain constant temperature and
humidity in the pressroom 24/7/365.
Allow paper to stand according to the
Paper Conditioning Chart before
opening it.

Piling skids

Follow lifting ergonomics

 Stack empty skids as shown.
Stacks should never be higher than
five feet.

Cutting steel strapping

You've really never felt pain until a

metal strap cuts you!
When cutting a metal strap with
cutters, be sure to protect yourself
from the strap when it snaps free.