Retaining apparatus and method for holding printing plates...

Printing – Printing members – Plate mountings

Reexamination Certificate

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Details

C101S383000, C101S415100

Reexamination Certificate

active

06267054

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to retaining printing plates on the cylindrical surface of a vacuum drum. The invention has particular application in computer-to-plate or computer-to-press systems.
BACKGROUND OF THE INVENTION
In the printing industry it is sometimes necessary to retain a plate on the cylindrical surface of a vacuum drum. For example, many computer-to-plate or computer-to-press systems image a lithographic printing plate which is held onto the outside surface of a rotating drum. In such systems the rotating drum has a cylindrical surface onto which a plate can be held. The cylindrical surface is typically perforated with an array of holes or other apertures. Air pressure in a plenum behind the cylindrical surface is reduced by a suitable vacuum pump. The resulting pressure differential between atmospheric pressure on one side of the plate and a reduced pressure on the side of the plate in contact with the cylindrical surface holds the plate against the cylindrical surface.
While such vacuum drums are well adapted to holding flexible films, they cannot always reliably hold metal printing plates. Metal printing plates have greater stiffness and thickness than films. Because of this, metal plates tend not to seal to the cylindrical surface of a vacuum drum as well as films. This reduces the pressure differential across the plates and reduces the forces which hold the plates to the drum. Leakage is especially significant in the areas near the edges of the plates.
Because metal plates are often heavier than comparable films the centrifugal forces which act on such plates when they are mounted on a rotating drum tend to be larger than those which act on films. For heavy plates mounted on a large drum which is rotating with a high angular velocity the centrifugal forces acting to pull the plates off of the rotating drum can approach the forces caused by the pressure differential across the plate which tends to hold the plate to the drum. The pressure differential across the plate is no more than one atmosphere and tends to be less in regions adjacent the edges of the plate where leakage is a problem.
Prior art approaches to the problem of holding plates to a rotating vacuum drum provide mechanical or magnetic clamps to hold both edges of a plate to a rotating drum. Providing such clamps is complicated because it should be possible to easily and quickly affix plates of different lengths to the same drum. A first clamp for holding a first edge of a plate may be at a fixed location on the surface of a drum. A second clamp for holding a second edge of the plate must be movable relative to the first clamp.
It is known to use magnetic clamps for the second clamp. A magnetic clamp consists essentially of an elongated member which contains one or more magnets. The magnets are attracted to the drum. The magnetic clamp may be simply placed over a second edge of a plate to hold the second edge of the plate to the drum. Such magnetic clamps have the advantage that they may be positioned as necessary to hold down the second edge of a plate. A disadvantage of such magnetic clamps is that the magnets which are required in the clamp are reasonably heavy. As a magnetic clamp is moved to accommodate plates of different lengths the balance of the drum will change. This can cause undesirable vibrations, especially when the drum is rotated at higher angular velocities. Furthermore, if a heavy magnetic clamp does fly off a drum during use it can cause significant damage to adjacent parts of the machinery.
It is known to hold steel plates to a drum by providing magnets inside the drum. This approach does not work with aluminum plates.
There is a need for a clamping system suitable for clamping the edges of printing plates of different sizes to a vacuum drum which avoids or reduces the above-noted disadvantages of currently available clamping systems. There is a particular need for such clamping systems that may be used on existing vacuum drums without the need to make structural modifications to the existing vacuum drums.
SUMMARY OF THE INVENTION
This invention provides a retainer for holding one end of a plate onto a cylindrical surface of a vacuum drum. The retainer has an elongated member which can be positioned on the cylindrical surface of a vacuum drum adjacent the edge of a printing plate. The member has a curved surface portion which conforms to the cylindrical surface of the drum. A lip extends along an edge of the member. The curved surface portion may be held by vacuum on the cylindrical surface with the edge of the printing plate received under the lip.
The retainer can be very light in weight so that it does not affect the balance of the drum, even if the retainer is not always at the same location on the drum. In preferred embodiments the retainer has a seal extending around a peripheral edge of the curved surface portion. The seal maximizes the force with which the retainer is held to the drum by preventing air from leaking under the edges of the retainer. A similar benefit may be obtained by facing the curved surface portion with an elastomeric material.
Another aspect of the invention provides a vacuum drum assembly wherein a retainer according to the invention holds a plate on a cylindrical surface of a vacuum drum.
A further aspect of the invention provides a method for retaining an edge portion of a plate to a vacuum drum. The method provides a retainer and a vacuum drum having a cylindrical surface with a radius of curvature. The retainer comprises an elongated member having a curved surface portion, the curved surface portion having a radius of curvature substantially equal to the radius of curvature of the cylindrical surface and, a lip extending along an edge of the member. The method includes wrapping a plate around the cylindrical surface of the vacuum drum; placing the retainer on the vacuum drum so that the lip projects over an edge portion of the plate and the curved surface portion adjoins the cylindrical surface; and, holding the retainer against the cylindrical surface by reducing a pressure within the vacuum drum so that the lip prevents the edge portion of the plate from lifting away from the cylindrical surface.
Further aspects of the invention are described below.


REFERENCES:
patent: 2999453 (1961-09-01), Roberts
patent: 3824927 (1974-07-01), Pugh et al.
patent: 4505199 (1985-03-01), Hasegawa et al.
patent: 4587900 (1986-05-01), Oshio
patent: 5410964 (1995-05-01), Koelsch
patent: 5526746 (1996-06-01), Capdeboscq
patent: 5562039 (1996-10-01), Fox et al.
patent: 5865433 (1999-02-01), Morrissette
patent: 2028722 (1980-03-01), None

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