Printing – Work supporting members – Grippers
Patent
1998-08-12
1999-10-05
Burr, Edgar
Printing
Work supporting members
Grippers
101116, 101118, B41F 2104
Patent
active
059607165
DESCRIPTION:
BRIEF SUMMARY
The invention concerns an impression cylinder for a sheet printing machine, having at least one cylinder pit in which sheet grippers are arranged.
Such impression cylinder are long known in the art, particularly in the field of indirect printing machines such as offset printing machines (see for example GB patent 1138369), in direct printing machines such as intaglio printing machines and also in screen printing machines. The known impression cylinder have always had printing saddles separated by pits, the circumferential length of the printing saddles corresponding to the size of the sheet to be printed and the pits being necessary for housing the sheet grippers.
Such impression cylinders with an external surface interrupted by pits make problems during the printing process. At each outset of pressure, that is when a printing saddle comes into contact in a collision-like manner with the cylinder printing the image, a collision excitation is created which may influence the quality of the impression. In offset printing, changes in the pressure between the blanket cylinder and the impression cylinder may even occur. In the case of slim cylinders, that is when their length is greater than their diameter, undesired vibration streaks may be created because of the insufficient rigidity or because of the collision excitation.
A particular problem occurs in rotary screen printing machines, in which an elastically flexible screen printing stencil is stretched on the circumference of a form cylinder. This screen printing stencil is stressed on the inside by a doctor blade which accumulates upstream of the printing nip the ink bead necessary for the printing operation. During the passage of a cylinder pit of the impression cylinder, the screen printing stencil is not supported anymore by the circumference of the impression cylinder and is more or less forced into the pit by the pressure of the doctor blade and may thus be damaged. In order to avoid this danger, it has been proposed in the previous European Patent Application N.sup.o 96810043.8 issued to the same applicant to mount the doctor blade displaceable inside the screen printing cylinder in such a way that the blade is lifted off and away from the screen printing stencil each time a pit of the impression cylinder passes the printing nip.
The object of the present invention is to provide an impression cylinder for a sheet printing machine such that the effects and the drawbacks created in an impression procedure by the presence of cylinder pits can be removed in a simple way.
This object is achieved by the impression cylinder according to the invention, wherein the sheet grippers, when in closed position, do not rise above the peripheral surface of the cylinder and in that each cylinder pit is provided with a cover which is movable by an adjusting mechanism in a first position in which the cover completely covers the cylinder pit with the grippers in closed position thereby completing the peripheral surface of the impression cylinder, and in a second position in which the cover opens at least partially the pit in the region of the grippers.
The impression cylinder and its adjusting mechanism are hence constructed in a way such that each cylinder pit is always completely closed when, during the printing process, the pit passes the printing nip, and sufficiently opened during a transfer of a sheet onto the impression cylinder or away from the impression cylinder so that the grippers can be opened and the transfer of the sheets can take place in the usual manner. When transfer cylinders with grippers are used for transferring the sheets onto and away from the impression cylinder, the cylinder pit is then so opened during the transfer that the grippers of the transfer cylinder can penetrate inside the pit of the impression cylinder, as is usually done.
With the invention, one obtains that, during the printing process, the impression cylinder rolls with a closed and practically continuous surface against the cylinder printing the image, which can be in particular e
REFERENCES:
patent: 1597250 (1926-08-01), Rowley
patent: 4813356 (1989-03-01), Abendroth et al.
patent: 4875069 (1989-10-01), Takada et al.
patent: 5671671 (1997-09-01), Wyssmann et al.
patent: 5690027 (1997-11-01), Takahashi et al.
Burr Edgar
De la Rue Giori S.A.
Grohusky Leslie J.
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