1. Field of the Invention
This invention concerns a sheet-fed press in which the paper feed is stabilized. More specifically, the invention concerns stabilizing the movement of the sheet of paper in a sheet-fed press. The sheet-fed press according to this invention has first and second press cylinders. The first press cylinder is defined as an intermediate cylinder or a delivery cylinder whose curved surface serves to guide the sheet through the space between the curved surface and a sheet guide unit. The second press cylinder is defined as an impression cylinder or the like which is positioned next to the first press cylinder via a reception area.
2. Description of the Related Art
Multiple-color sheet-fed presses which employ a series of printers each of which prints a different color ink are well known in the prior art. As can be seen in FIG. 6, the basic structural elements of such presses are feeder unit A, which consists of feeder device 39; printer unit B, which has four printers, 132a, 132b, 132c and 132d, arrayed in tandem to print cyan, magenta, yellow and black; and delivery unit C, here paper delivery unit 04.
In multiple-color sheet-fed presses with this configuration, a sucker unit with an inlet for sheets 11, which are piled on table 141 of the feed unit 39, separates a single sheet and transports it on conveyor 120. Swing gripper 121a delivers the sheet to intermediate cylinder 121b of printer 132a. The sheet is fed between blanket cylinder 22a and impression cylinder 23a, and the first color is printed.
Once the first color has been printed, the sheet is fed out between the blanket cylinder 22a and impression cylinder 23a and taken up by intermediate cylinder 27a of the second printer 132b. From the intermediate cylinder 27a, the sheet is delivered to impression cylinder 23b. The next process, the printing of the second color, is executed by blanket cylinder 22b and impression cylinder 23b. 
The subsequent colors are printed one after the other. When sheet 11 is fed out between blanket cylinder 22d and impression cylinder 23d, which perform the final-stage printing, it is pulled onto delivery cylinder 35 of delivery unit C. From delivery cylinder 35, the now completely printed sheet 11 is taken onto chain conveyor 124 and transported to delivery unit 04, where it is added to the stack on table 40 of the unit 04.
Generally, the sheets 11 which are printed in a sheet-fed press are of a thickness which ranges from 0.04 m/m to 0.8 m/m. At times, high-rigidity sheets of metal plate or synthetic resin might also be printed. As the sheet is fed from printer 132a to printer 132b to print the various colors, various mishaps may occur. A thin sheet of paper will generally have low rigidity, and its rear portion will tend to flap. A thicker sheet of paper or sheet metal will have high rigidity, and its reaction force (stability) against the centrifugal force of rotation and its own curvature will cause its rear portion to separate from impression cylinder 23, and collide with the sheet guide unit 1xe2x80x2 below the cylinder resulting a paper rebounding.
When the paper flaps or rebounds in this way, the print may be smudged or the paper folded or torn. This phenomenon is a significant cause of a reduction in print quality. Two typical methods employed to counteract this problem are to use a skeleton cylinder or a drum cylinder for the intermediate cylinder 27. This allows the most appropriate scheme to be used for the rigidity of whatever sheet is being printed.
The example shown in FIG. 7(A) is a skeleton-type intermediate cylinder 27, which is used primarily when printing thicker sheets of paper. One of these skeleton cylinders 27 is placed on each side of each printer 132. Each skeleton cylinder consists of a pair of rotors (arms) 271 which rotate on axis 270. Each arm 271 has a series of pawls 29 on its shaft 272 (see FIG. 8(A)) running from the end of arm 271 to the end of arm 271 on the opposite side of the shaft. The distinguishing feature of the skeleton cylinder 27 is that the area of the cylinder which comes in contact with impression cylinder 23 when the paper passes between them is extremely small. The sheet 100 which is being rotated forward is allowed to bend beyond point P where it comes into contact with pawls 29. In other words, the point of contact P becomes the point of action. By lengthening the distance from this point to the end of sheet 100, we reduce the reactive force exerted by the sheet in its attempt to return to its original shape.
As a result, we reduce the amount of rebounding at the end of the sheet which strikes sheet guide unit 1xe2x80x2, the curved guide which conforms to the hypothetical circumference of the lower portion of skeleton-type intermediate cylinder 27. This scheme minimizes tears and folds; but on the other hand, because this sort of skeleton cylinder 27 provides a larger region in which the end of sheet 100 is free, a thin sheet will have more opportunity to flap.
The example shown in FIG. 7(B) is drum cylinder-type intermediate cylinder 27xe2x80x2, which is used primarily for thinner sheets of paper. This sort of drum cylinder 27xe2x80x2 has a number of pawls 29 in two places along the circumference of a roller which rotates on axis 270.
The feature which distinguishes drum cylinder 27xe2x80x2 is that the amount of its surface area which comes in contact with impression cylinder 23 as sheet 100 is fed between them is maximized. Because the portion of sheet 100 which is beyond pawls 29 is guided along the circumference of the drum cylinder (27xe2x80x2), this scheme makes it very difficult for the end of the sheet to flap, so it minimizes doubling, tearing and other defects resulting from the end of the sheet wrinkling or flapping. However, when this sort of drum cylinder 27xe2x80x2 is used to convey thicker varieties of paper, the fact that there is very little area where the end of the sheet is free will result in significant rebounding.
In recent years, as print quality has improved, there has been a tendency to use the skeleton cylinders even for thinner papers. To keep thin sheets from flapping, a sheet guide unit 1 is provided which has a sheet guide surface 1d following the contour of the lower portion of intermediate cylinder 27 (or 27xe2x80x2) and delivery unit 35 (hereafter referred to as the intermediate cylinder). In order to address the problems in this sort of sheet-fed press, a sheet guide unit is provided in which specifically pressurized air is blown through a number of vents in the sheet guide unit into the space between intermediate cylinder 27 and surface 1d of the sheet guide unit. This air is blown along the bottom of sheet 11 as it passes through the space along sheet guide surface 1d. Because of the Bernoulli effect, the air blown through the vents causes the sheet 11 to be suspended.
One such sheet guide unit is described in Japanese Patent Publication (Kokai) Hei 10-109404. We shall explain the relevant technology with reference to FIG. 8. The sheet guide unit, which runs along the circumference of skeleton-type intermediate cylinder 27 or delivery cylinder 35, both of which are studded with pawls 29, consists of air ducts 06. On the upper surface of the air ducts 06 are numerous air vents 4a and 4b. The vents 4a and 4b face in opposite directions and are located on either side of the center of the intermediate cylinder 27 or of delivery cylinder 35. The vents distribute the air toward the outer edges of the intermediate cylinder 27. The vents 4a and 4b produce two streams of air which originate at the vents and continue to move in the directions determined by the vents. These air streams keep the sheet of paper suspended at a specified height, thus stabilizing the travel of the sheet.
In the prior art technique, then, air is blown through a space between sheet guide surface 1d and the intermediate cylinder underneath sheet 11. The sheet is caught on pawls 29 of skeleton-type intermediate cylinder 27, the type of cylinder used for thicker papers. The air is blown into the space from ducts 06 below the guide surface through the air vents 4a and 4b. The Bernoulli effect which results from the differential flow rate above and below the sheet causes the sheet 11 being conveyed around the circumference of the intermediate cylinder 27 to be pulled toward surface 1d of the sheet guide unit and to be suspended slightly above that surface as it is conveyed until it is delivered to the subsequent impression cylinder 23.
However, in this prior art technique, when the sheet exits the guide space and is released from the pawls of the skeleton cylinder, there is nothing to hold it. And particularly if the sheet is thin, the Bernoulli effect due to the flow velocity of the air stream will not be sufficient to stabilize the end of the sheet.
In addition, with this prior art technique, in the reception area for the sheet between the intermediate cylinder and the impression cylinder, in other words, at the point where the intermediate cylinder and impression cylinder come in contact with each other (and at this point in stages 2, 3 and 4), the rotation of the two cylinders creates a vortex (a rotary airflow dragged by the rotation of cylinders) in the direction that the cylinders are rotating. In particular, the turbulent boundary layer 37 shown in FIG. 1 develops above impression cylinder 23, whose lower surface lacks a sheet guide unit.
When the vortices act on the end of sheet 11 which is about to be transferred or has been transferred to impression cylinder 23 from intermediate cylinder 27, the end of the sheet will not be able to remain stabilized. Sheet 11 will, then, behave improperly, either moving around or flapping up and down. If the intermediate cylinder 27 is a skeleton cylinder, and a thinner paper is used, when the sheet 11 is transferred from skeleton cylinder 27 to the pawls of impression cylinder 23 and the rotational phase progresses, the gap between cylinders 23 and 27 will be even larger. When sheet 11 is released by pawls 29 (see FIG. 3) of the skeleton cylinder, it is very likely to move around or flap, as described above, since it is then in an unrestrained state.
With a drum-type intermediate cylinder, the end of the sheet is held between the intermediate cylinder and the impression cylinder, so it cannot move around or flap as described above. Because the sheet is clasped between two cylinders, however, a thicker and more rigid sheet will be more likely to tear or have printing defects.
The tendency of sheet 11 to be adversely affected by air vortices will vary according to its thickness. Solutions offered in the prior art, including the invention disclosed in the Japanese Patent Publication 10-109404, have not provided any means to insure that the action of sheet 11 be controlled properly, as discussed above. If, as has become common in recent years, the same printer were used to print on both thicker and thinner papers, it would be necessary to change from skeleton to drum cylinder each time a different thickness of paper is used. Practically speaking, this is simply not possible.
In view of the problems discussed above, the objective of this invention is to provide a sheet-fed press which will prevent air vortices in the reception area between the intermediate and impression cylinders from causing the end of the sheet to move around or flap; which would allow sheets of thinner grades of paper to be conveyed in a stable fashion; and which would prevent sheets of thinner grades of paper from moving around or flapping when a skeleton cylinder is used as the intermediate cylinder, so that the paper can be conveyed in a stable and continuous fashion.
Another objective of this invention is to provide a sheet-fed press which will allow paper of a wide range of thicknesses to be conveyed in a stable fashion without moving around or flapping, even when a skeleton cylinder is used as the intermediate cylinder.
Yet another objective of this invention is to provide a sheet-fed press which would control, according to the thickness of the sheet of paper, undesirable movement of the sheet resulting from air vortices in the reception area between the intermediate and impression cylinders.
To address these problems, the current invention is designed as follows. The sheet-fed press according to this invention has two printing cylinders, the first of which is an intermediate or delivery cylinder with a sheet guide unit under its lower surface consisting of a space through which the sheet can pass, and the second of which is an impression cylinder or alike positioned adjacent to the first cylinder via the reception area. This press is distinguished by the fact that it has an additional second air supply chamber in the rear side of the sheet guide surface which is located in the downstream segment of the flow of sheet, and by the fact that there are air vents in the downstream segment of the reception area through which air from the second air supply chamber is blown in the direction that the second cylinder is rotating.
In this case it is desirable that there should be an air guide side wall facing along the circumference of the second printing cylinder. This air guide side wall should be located at the downstream from the air vents of the second air supply chamber. The air stream blown through the air vents can flow along the air guide side wall and be directed toward the tangent of the second cylinder.
Since the air guide side wall consists of the wall of the second air chamber at the air vents side, no additional wall will be needed.
With the invention, the downstream portion of the air guide side wall gradually narrows as it approaches the second cylinder. The venturi effect which occurs on the downstream portion of air guide side wall will produce a negative pressure on the lower surface of the sheet being conveyed. Because the air stream is moving toward the tangent of the second cylinder, it creates a flow which can counteract the vortex near the surface created by the rotation of the second cylinder (i.e., it creates a flow opposite the direction of rotation of the second cylinder).
By canceling or reducing the speed of the vortex, this arrangement can prevent the sheet from breaking free or flapping. Even when a skeleton cylinder is used, the sheet can be conveyed without problems.
With the invention, it is desirable to provide a means to draw the air flowing along the path of rotation of the second cylinder on the downstream side of the air vents. The drawing means might be a hood which extends along the breadth of the air guide side wall so as to cover the rotary surface of the second cylinder downstream from the reception area.
With this invention, the air in the vicinity of the reception area will be collected and drawn into the hood. This will prevent the air from being dispersed and so prevent the adverse effect which the dispersed air would exert on the sheet. The hood allows the sheet to be transported more smoothly from the first cylinder to the second cylinder.
The quantity of air drawn into the drawing means should be greater than the quantity blown through the air vents. This will further insure that the air near the reception area will not be able to disperse.
It is effective to create a return channel for the air so that at least a portion of the air drawn in by the drawing means is recirculated to the second air supply chamber.
By creating the second air chamber, air vents, and return channel by which the air in the hood can recirculate back to the second air chamber, we provide a system by which we can use the continuously circulating air, by temporarily accelerating the air in the channel, to counteract the speed of the vortex. We then need no extraneous air; and we can reduce the energy required to accelerate the air. And because we need only a single air pump, we can reduce our equipment cost.
The press according to another embodiment of this invention comprises a second air blowing means to supply the air flow from the second air supply chamber as mentioned above which blows air along the circumference of the second printing cylinder from a point downstream from the reception area; a third air blowing means of an air jet unit to blow air toward the reception area between the two aforesaid cylinders from a point upstream from that reception area; and an air control means to control the air flows to the two air blowing means mentioned above, by selecting one of two air blowing means according to the thickness of the sheet being conveyed from the surface of the sheet guide unit, or by constricting the volume of air supplied to the air blowing means.
The press according to another embodiment further has a first air blowing means to supply an air stream to blow air into the space along the sheet guide unit and the first press cylinder so that the sheet is suspended slightly above the guide surface of the sheet guide unit as it is conveyed. The air control means to control the air flow mentioned above can constrict the volume of air supplied to the first air blowing means according to the thickness of the sheet.
With this invention, if for example a sheet of a thicker paper were being conveyed from the sheet guide unit to the reception area, it would select the third air blowing means to blow air toward the reception area between the two cylinders from a point upstream. If a sheet of thinner paper were being conveyed, it would select the second air blower, which is downstream from the reception area between the two cylinders, to blow air toward the second cylinder. Even if a skeleton cylinder is used as the intermediate cylinder, this scheme insures that sheets of a wide range of thicknesses can be conveyed in a stable fashion without buckling or flapping.
The air control means to control the air flow mentioned above may, not only control the control signals for selecting the air blowing means or constricting the volume of airflow supplied to the air blowing means, but also select a preset signal for the pressure to be exerted on the cylinders according to the thickness of the paper.