Printing – Printing members – Plate mountings
Reexamination Certificate
2000-07-07
2002-10-01
Evanisko, Leslie J. (Department: 2854)
Printing
Printing members
Plate mountings
C101S415100, C101S378000
Reexamination Certificate
active
06457410
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to printing apparatus. It relates more particularly to a calamping mechanism for securing a printing plate to a plate cylinder.
2. Description of the Related Art
In offset lithography, an image is present on a printing plate as a pattern of ink-accepting and ink-repellant surface areas. In a typical sheet-fed offset press, the imaged plate is mounted to a plate cylinder where it is inked and then brought into contact with the compliant surface of a blanket cylinder. The blanket cylinder, in turn, applies the image to paper sheets which are brought into contact with the blanket cylinder by an impression cylinder.
It should be understood at the outset that nowadays a “printing plate” may actually be a thin, flexible plastic film or sheet.
Plates can be imaged on-press or, more traditionally on an off-press platesetter. A digitally operated platesetter includes an imaging cylinder to which the plate is initially mounted and which carries the plate past the head of the imaging device. That device transfers the image to the plate. The imaged plate is then removed from the platesetter and transferred to the plate cylinder of the printing press. When mounting an imaged plate to a plate cylinder for a press mount or when mounting a plate to an imaging cylinder for imaging, it is essential that the leading and trailing edges of the plate be secured firmly to the cylinder and that the plate be wrapped tightly around the cylinder. This ensures that there will be no relative movement between the plate and the cylinder when the cylinder is rotated.
A plate cylinder typically includes two plate clamps mounted to the cylinder that extend along its surface. To load a plate onto the cylinder, the leading edge of the plate is secured to the cylinder by one clamp and the plate is wrapped around the surface of the cylinder. The trailing edge of the plate is then secured to the cylinder with the other clamp.
Some printing processes require that two superimposed plates or sheets be mounted independently to the cylinder. For example, in color proofing apparatus, a receptor plate or sheet is secured to the cylinder by a first pair of clamps and successive donor plates or sheets representing color separations are secured to the cylinder over the receptor sheet by a second pair of clamps. After the donor sheet representing each color component has been imaged, that sheet is unclamped from the cylinder and replaced by the donor sheet corresponding to the next color component which must be wrapped around the cylinder and clamped. This process must be repeated three or four times for three or four color printing.
Various devices including hydraulic clamps, vacuum clamps, mechanical and electromechanical clamps have been developed over the years for holding a plate or sheet to a plate cylinder. However, for the most part, these prior devices have tended to be relatively complex. Also, in requiring heavy metal plates as clamps, they take up a relatively large amount of space on the plate cylinder creating a substantial “void” segment on the cylinder which is the space on the cylinder occupied by the clamps and the space immediately between them. Furthermore, the ancillary mechanisms for operating such clamps, such as air/hydraulic cylinders and lines thereto, take up additional space in the printing apparatus.
The above problems are exacerbated in the case of color proofing apparatus where four such clamps are required to secure the donor and receptor sheets to the cylinder.
The prior clamping devices are also disadvantaged in that the clamping mechanisms are fixed to the plate cylinders such that the mechanisms can only secure to the cylinder a plate or sheet having a specific length. Since the plates are often precut to fit the specific plate cylinder of the printing press, this inability to accommodate different length plates substantially increases the cost of operating and running printing and proofing apparatus.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a speedy and efficient clamping mechanism for clamping a printing plate to a plate cylinder or other support surface.
Another object of the invention is to provide a clamping device for clamping a plate or sheet to a cylinder which takes up a minimum amount of space on the cylinder.
A further object of the invention is to provide a clamping device which allows the associated cylinder to accept plates or sheets of different lengths.
Yet another object is to provide a clamping mechanism which minimizes void space on the cylinder.
Still another object of the invention is to provide a clamping mechanism which does not require bulky ancillary pumps or other apparatus to actuate the plate clamps.
An additional object of the invention is to provide a clamping mechanism of this type which is relatively simple and takes up a minimum amount of space in the imaging, proofing or printing apparatus in which it is installed.
Other objects will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the following detailed description, and the scope of the invention will be indicated in the claims.
Briefly, our clamping mechanism comprises at least one pair of similar clamping devices for clamping the leading and trailing edges of a plate or sheet preferably wrapped around a rotary cylinder. As alluded to above, in those applications requiring that a second plate or sheet be secured to the cylinder independently of the first, a second similar pair of clamping devices may be required. We will describe the invention as applied to a plate cylinder in an otherwise more or less conventional printing or proofing apparatus. It should be understood however that the invention is useful in other applications in which a thin plate or sheet has to be releasably clamped to a flat or curved surface of one kind or another.
In accordance with the invention, each clamping device is a long thin blade clamp and the cylinder (in whole or in part) and each blade clamp are made of ferromagnetic materials and at least one of the cylinder and clamps is magnetized so that the blade clamp is magnetically attracted to the cylinder with sufficient force to securely clamp the leading or trailing edge of the underlying plate or sheet to the cylinder.
In one embodiment of the invention, each blade clamp is laid down on and retrieved from the cylinder by a separate dispensing mechanism. More particularly, each blade clamp is wound up on a spool which may shuttle along a track extending parallel to the cylinder. The spool is movable along the track between a home position located just beyond one end of the cylinder and an extended position located just beyond the opposite end of the cylinder. The leading end of the blade clamp extending from the spool is terminated by a hook. When the spool is moved from its home position and passes by the adjacent confronting end of the cylinder, the hook engages that end of the cylinder. Resultantly, as the spool travels along the track, the blade clamp is paid out from the spool much like the steel tape of a tape measure, such that the blade clamp is laid down onto the surface of the cylinder (or more specifically onto the printing plate thereon) parallel to the cylinder axis.
When the spool reaches its extended position beyond the opposite end of the cylinder, the trailing end of the blade clamp is released from the spool so that the blade clamp is free to rotate with the cylinder, while the now empty spool remains at its extended position until it is time to unclamp the plate from the cylinder.
In order to unclamp the plate from the cylinder, the rotation of the cylinder is stopped and the cylinder is oriented to position the blade clamp on the cylinder so that its trailing end is aligned with the spool still reposing at its extended position on the track. Then, the spool is moved ba
Evanisko Leslie J.
Presstek Inc.
Testa Hurwitz & Thibeault LLP
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