GB1568751A - Sheet huide - Google Patents

Description

(54) SHEET GUIDE (71) We, SOLNA OFFSETAB, a Swedish Body Corporate, of Knistavagen 21, S-191 47 Sollentuna, SWEDEN, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a sheet guide and particularly to a sheet guide for printing in an offset press.

In offset printing the printing surface consists of a thin plate which is chemically treated so that it attracts water on the nonprinting surfaces and ink on the printing surfaces. The plate is fastened onto a plate cylinder, and ink and damping water are con tmuously applied to the plate during printing. The image is transferred from the printing plate to a rubber blanket tensioned on the blanket cylinder and from there the actual printing is done on the paper.

The printing paper is transported from the stack of paper into the press and is gripped by the counter-pressure cylinder which presses the paper against the blanket cylinder with the image. Printing against a counterpressure cylinder is most common in sheet offset presses. The paper lies on the outside of the counter-pressure cylinder and is pressed while curved against the rubber blanket.

Instead of merely printing one side of a paper, both sides can be printed simultaneously, with two blanket cylinders acting against one another. This is especially common in web offset presses but also occurs in sheet presses.

For printing in several colors, multicolor presses are usually constructed according to two different principles, either as so-called unit presses or with the use of a common counter-pressure cylinder. In unit presses the sheet is picked up from the feeder pile by the suction head, air is blown under the sheet and it is fed forward to conveyor belts which transport it to the register table where it is registered from the front and from the side as it remains momentarily completely stationary. The sheet is then accelerated by some form of mechanism, whose construction varies considerably in different presses, to the same velocity as the counter-pressure cylinder at the moment when the sheet is drawn in by its grippers into printing engagement.

From the counter-pressure cylinder the paper is given to an intermediate cylinder, and it is held momentarily by the grippers on the counter-pressure cylinder as well as the grippers in the intermediate cylinder.

Depending on the design of the press, there are one or three intermediate cylinders between each printing unit. Especially in presses with several intermediate cylinders between the units, the paper will pass a large number of gripper changes as it is fed through the press.

During the actual changing of the grippers, the grippers on one cylinder will press the paper against its gripper pads, while the grippers on the other cylinder will take the paper between the first grippers and press it over against its own gripper pads. The operation of the grippers is controlled by mechanical cams which are very exactly designed and made with small tolerances. Normally, the paper moves about 3 mm while being held by both sets of grippers.

In sheet transfer it can also be desirable to turn the sheet with a so-called sheet turner.

The sheet is gripped at its rear edge instead of at its front edge by the grippers of the sheet turner.

In multicolor presses with a common counter-pressure cylinder, the sheet is moved to and from the press in the same manner as in unit presses, but during the printing the sheet is held against a single counter-pressure cylinder for the first as well as the second color. The sheet is then usually taken over by a gripper trolley, which is moved by chains over to the next printing unit, where the third and fourth colors are applied, and then possibly by chains again to another unit for the fifth and sixth colors.

Reducing appreciably the number of changes of grippers reduces the risk of deformation of the paper during the change of grippers, but on the other hand there are problems in registering the trolley which moves the sheet from one printing unit to another.

All of these problems are eliminated in web offset presses, but there are problems with keeping the paper tension constant in the paper web, and in printing in several successive colors there are strict requirements on the evenness of the web tension.

After printing, the sheets or web is/are moved out of the press for drying and completion.

Problems can occur in printing due to the fact that freshly printed sheets, when they hang freely without support, can flap or slide against parts of the press and smear.

The present invention relates to a guide means for completely or partially freely movable sheets of paper, which prevents them from coming into contact with structural parts of the press or flapping out of control.

The invention will be described in detail as a sheet guide for an offset press with reference to the accompanying drawings, of which Fig. 1 is a schematic representation of a sheet press for offset printing, Fig. 2 shows an ordinary changing of grippers, and Fig. 3 shows a sheet guide according to the invention.

Fig. 1 shows schematically an offset sheet press with four print units 1,2, 3 and 4. From the stack 5, the paper 6 moves over the register table 7 to the first printing unit 1, where it is pressed between the counter-pressure cylinder 8 and the blanket cylinder 9. The plate cylinder 10 is inked from an inking unit (not shown) and is dampened by a damping unit (not shown).

The paper is then fed after printing with the first color by the intermediate rollers 11, 12 and 13 to the next printing unit 2, is printed with the second color between the counter-pressure cylinder 8a and the blanket cylinder 9a, is moved to printing unit 3 by intermediate cylinders 1 lea, 12a and 13a, and so on.

After printing in the last printing unit 4 the completely printed paper is laid on the stack 14.

Fig. 2 shows schematically the transfer of a sheet 6 from one roller to another, for example from intermediate roller 13 to counterpressure cylinder 8a. The sheet is held by the grippers 15 of the intermeidate roller 13 and is drawn into the grippers 16 of the counterpressure cylinder 8a. For a very short distance, about 3 mm, the paper is held by both grippers 15 and 16. Grippers 15 then open and the paper is pulled on by the counterpressure cylinder 8a.

The rippers are controlled by mechanical cams (not shown) in a control sequence which must be very exact and synchronized with the cylinders.

During a portion of the sheet transfer sequence, a large portion of the rear section of the paper, as can be seen from Figs. 1 and 2, will float freely and can flap and cause smearing, paper damage etc. To prevent this and achieve a contact-less and thereby smear-free guiding of the sheet, a sheet guide according to the invention is used. Its construction is shown in Fig. 3. The roller 13 shown there has grasped a sheet of paper 6 with the gripper 15 and pulls it with it. The sheet guide means 16 is mounted above the roller. It comprises two perforated distributor pipes 17 and 18 for blowing compressed air into a distribution chamber 19.

The air is pressed out from the distribution chamber through a slot 20 at high velocty, and is guided in the direction of motion of the sheet along a guide plate 21. Thus the air will "run along the surface", i.e. follow the guide plate, thereby creating a negative pressure which lifts up the passing sheet towards but spaced from the sheet guide. The sheet will also be spaced from the roller or backing.

This is the so-called Coanda effect which applies to gases and flowing fluids. It is an effect which has been applied previously to fluidistors, airplane aerodynamics, etc.

The boundary layer of air protects the sheet from contact and smearing. The air then flows along the guide plate and is mostly diverted by a diverting plate 22 and exits through slot 23 between the guide plate 21 and the diverting plate 22.

Suitably, the length of the sheet guide means is made so great that it extends across the entire cylinder, i.e. as large as the maximum paper width.

The mounting of the sheet guide in the press frame can be movable so that the guide can be moved parallel with the peripheral surface of the cylinder, since the flapping of a sheet can occur at varying distances from the grippers.

The present sheet guide has been described in conjunction with an offset press, but it can of course be used in sheet presses for printing books or the like, and at various locations in the press, such as at delivery and feeding, and in book-binding machines, etc.

By supplying heated air to the sheet guide, it is also possible to accelerate the drying, and different additives can be used in the air for reactions with the printing ink and/or paper to produce desired effects.

Claims (3)

WHAT WE CLAIM IS:

1. Means for guiding sheets in movement along a path, preferably in printing presses, characterized by pipes for supplying air, a distribution chamber for the air supplied, an elongate slot perpendicular to the direction of motion of the sheets, an elongate guide plate defining one side of said slot and extending at a distance from the sheet path to guide the air blown out from said slot across the guide plate, and a diverting plate parallel with and spaced from the edge of the guide plate opposite from said slot for diverting the excess air, said guide means being disposed in the vicinity of the sheet path, so that air is blown out between the guide plate and the sheet which is thereby sucked up to the guide means but without touching the same.

2. Means according to Claim 1, characterized in that it extends essentially across the entire width of the sheet path.

3. Means for guiding sheets in movement along a path, preferably in printing presses, substantially as hereinbefore described with reference to and as shown on the accompanying drawings- A . . r-