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.

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| INDEX | OCTE 3353 | ITEC 3350 | ITEC 3351 | ITEC 3352 |
| ITEC 4372 | ITEC 4373 | ITEC 4374 | ITEC 4375 |
| OCED 5360 | OCED 6316 |