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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the manufacture of a sleeve-shaped
printing form for a printing machine form cylinder, and more particularly,
to a process for fabricating, from a plate-shaped blank, a printing form
sleeve having a continuous circumferential surface. Printing forms
fabricated in accordance with the present invention are especially useful
for offset printing applications.
2. Description of the Prior Art
A printing form sleeve for offset printing produced by a conventional
process is disclosed in German Patent Application P41 40 768. This sleeve
can be used in combination with a form cylinder having no grooves or
clamping segments which is already known, e.g., from German Patent DE-PS
27 00 118. For this purpose, a plate is cut to dimensions corresponding to
the circumference and width of the form cylinder and is provided, using a
plate punch, with a register device in the form of a peg-hole system. The
blank which conforms to the required dimensions must be cut to length in
an accurate manner. This is done with special plate shears. The blank is
bent or otherwise manipulated to form a tubular shape, and is clamped in a
welding device with the contiguous edges in exact alignment or
registration. The contiguous edges of the plate thus forming the beginning
and end of the printing form can then be welded together longitudinally,
preferably by means of a neodymium-YAG laser.
The prior German Patent Application P 43 11 078 further discloses a device
in which the plate which has been cut to size need only be inserted,
whereupon the plate can be bent into a sleeve shape and the plate edges
forming the beginning and end of the printing form can be positioned in
exact register for the purpose of laser welding. Disadvantageously, the
plate shears used to cut the plate-shaped blank to the desired length,
which cutting must be carried out very accurately, are very expensive.
Accordingly, it is an object of the present invention to provide an
improved printing form fabrication process in which the need for plate
shears is obviated and in which accurate cutting of the plate-shaped blank
to the required dimensions is ensured.
It is a further object of the present invention to provide a printing form
which can be manufactured in a simple and economical manner.
SUMMARY OF THE INVENTION
The aforementioned objects, as well as others which will become apparent to
those skilled in the art upon review of the teachings set forth herein,
are achieved by a process in which a sleeve-shaped printing sleeve for a
form cylinder of a printing machine is fabricated from a blank of metallic
material by cutting the blank to a width corresponding to the form
cylinder and to an intermediate length which exceeds the circumference of
the intended printing form cylinder. The blank is provided with aligning
indicia in the regions between each edge of the intermediate-length blank
and the corresponding edge to be defined by cutting. Thus, these regions
lie outside of the printing area.
The blank is bent circularly, held in exact register in a bending/welding
device, and cut to the length required for the intended form cylinder by
laser beam cutters. As a result of this cutting operation, two waste
pieces are produced. The waste pieces are removed and the now defined
plate edges forming the beginning and end of the printing form are
positioned opposite one another without overlap and welded together,
preferably by a laser.
Other objects and features of the present invention will become apparent
from the following detailed description considered in conjunction with the
accompanying drawings. It is to be understood, however, that the drawings
are designed solely for the purposes of illustration and not as a
definition of the limits of the invention, for which reference should be
made to the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
An understanding of the individual process steps of the present invention
will be facilitated by the detailed description of an illustrative
embodiment set forth herein in combination with reference to the annexed
drawings, in which:
FIG. 1 depicts the (empty) device for bending the plate-shaped blank;
FIG. 2 depicts the insertion of the plane blank in exact register into the
device for bending;
FIG. 3 illustrates the circular bending process;
FIG. 4 shows the positioning in exact register of the edges of the blank to
be connected;
FIG. 5 depicts cutting of the blank to the required length and removal of
the waste pieces; and
FIG. 6 illustrates joining of the plate edges forming the beginning and end
of the printing form without overlap and subsequent welding of the plate
edges.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
With reference now to FIGS. 1-6, there is shown in cross section a device
10 for bending and welding a plate-shaped blank, such as blank 1. As best
seen in FIG. 1, device 10 is constructed in a conventional manner and
includes elongated gripping devices 2a, 2b for maintaining the blank 1 in
a predetermined position. Illustratively, the gripping devices are in the
form of suction strips. Accordingly, each gripping device as 2a, 2b
includes a plurality of adjustable length vacuum suction members 3 which
are integrated therein so as to be distributed along the entire length at
closely spaced intervals for the purpose of holding the blank 1.
As discussed above, the blank 1 is cut from a metallic material, generally
in the shape of a strip, to form a plate having a width conforming to that
of the form cylinder for which it is intended. For a purpose which will be
explained later, however, the length of blank 1 must be greater than the
given circumference of the intended printing form. A suitable plate punch
(not shown) provides the blank 1 with edge aligning indicia in the form of
a system of spaced peg-holes located proximate the plate edges 5a, 5b to
be connected which is located outside of the provided printing area. Both
the edge aligning indicia and the edges 5a, 5b are located in a region of
the blank 1 which is outside the intended printing area of the plate. As
best seen in FIG. 2, the gripping devices 2a, 2b may be outfitted with
corresponding peg registers 4a, 4b which are insertable into the peg-holes
to center the plate-shaped blank 1 in exact registration with bending
device 10. In this regard, it will be noted that although the edge
aligning indicia of the illustrative embodiment depicted in FIGS. 1-6
comprise apertures, it will, of course, be readily appreciated by those
skilled in the art that any kind of stops, marks or other means for
ensuring that the plate edges 5a, 5b are fastened and aligned in the
welding device in an accurately positioned manner may be utilized.
FIG. 2 shows the insertion of the planar blank 1 into the illustrative
bending device 10 in exact register. As will be readily appreciated by
those skilled in the art, when the pegs 4a, 4b of the register device are
fitted into the peg-hole system of the blank 1, the blank 1 is held fixed
in the device in exact register by operation of the adjustable vacuum
suction members 3. Of course, in the case of plate-shaped blanks,
fastening the blank in exact register can also be achieved without
punching in a peg-hole system beforehand. The blank can, for example, be
welded in the shape of a sleeve in accordance with subsequent steps of the
inventive process by inserting the blank into the welding device with a
sufficient projecting or excess length, positioning the two holding strips
at a closer distance than that corresponding to the theoretically correct
length of the printing form, and then tightening or taking in the plate to
the correct length.
Since pretreated aluminum plates are the most common type of printing form
used in offset printing, vacuum suction members 3 are used in the
embodiment example. If, on the other hand, the plate material is a ferrous
metal or metal alloy, it is also possible to use an electromagnetic
gripping device to fix the blank in the welding device. As yet another
alternative, a mechanical clamping device may be used to maintain the
printing form in exact register.
Once a blank as blank 1 is fixedly positioned in exact register within
bending device 10, the blank is bent in a conventional manner by moving
the gripping devices along respective paths. FIG. 3 depicts the blank 1 in
an intermediate position during bending. Continued movement of the
gripping devices and hence, bending of the blank 1, results in the exact
alignment of plate edges 5a, 5b shown in FIG. 4. In accordance with the
present invention, a portion of each end of blank 1 is severed (FIG. 5) in
order to obtain a plate having a length precisely matched to the
circumference of the corresponding form cylinder to which it will be
secured. For this purpose, at least one cutting device is utilized.
Preferably, two cutting devices, each movable along a predetermined path
to produce the desired complementary edge contours, are employed in order
to minimize the amount of time required to perform the cutting operation.
Illustratively, the respective ends of blank 1 may be cut using first and
second laser cutting devices 6a, 6b which may, in a conventional manner,
emit a suitably precise cutting beam as they are moved in, for example,
along parallel paths over the blank 1 at a predetermined distance from
edges 5a, 5b. Laser cutting of flat sheets or plates, may be utilized to
obtain both simple contours and complicated shapes having sharp or acute
corners, is known from the prior art. In conformity to its light
properties, the laser beam obeys the laws of optics. Thus, it can be
controlled by mirrors and lenses and changed in cross section in virtually
any desired manner. The chief advantage of the laser as a cutting device
consists in the low degree of thermal damage inflicted on the cut edges,
the high cutting speeds with uniform quality, and the possibility of an
automated process.
After the cutting process, one or two waste pieces 7a, 7b resulting from
the cutting process are removed from the welding device. Since the pegs
4a, 4b of the register device are centered in the peg-hole system of the
blank 1 and this peg-hole system is located in these waste pieces 7a, 7b,
the latter may be removed simply by swinging out or otherwise retracting
the peg registers 4a, 4b (FIG. 5). As shown in FIG. 6, the plate edges 8a,
8b forming the beginning and end of the printing form which has been cut
to the required length may then be joined without overlap and welded
together. Preferably, a precisely focussed laser beam 9 is used for
welding. Thus, the application of heat is restricted to a narrow zone and
the laser beam can penetrate deeply into the material. Thermal loading and
warping of the welded material is negligible in comparison to other
thermal methods.
The process of the present invention provides numerous advantages over
prior art techniques for fabricating cylindrically shaped printing forms.
In contrast to conventional techniques, workpieces which have been
punched, but not cut to the required length, are placed in the welding
device in exact register and cut to the required dimensions by means of
lasers only after being clamped into the welding device. Since the blank
is cut to the required dimensions by lasers in the welding device,
separate plate shears may be entirely dispensed with. The register punches
provided in the plate for holding the plate blank 1 in exact register
become superfluous once the plate is fixed by the holding strips 2a, 2b
and can be discarded along with the waste pieces 7a, 7b after the plate is
cut to the required length by the laser. This results in a closed sleeve
surface so that no printing media can collect in the punched out portions
in a troublesome manner.
A further advantage afforded by the present invention is that laser-cut
plate edges 8a, 8b may be made to conform to one another in an optimum
manner due to the affinity of the laser cutting and laser welding
processes, resulting in an optimum weld.
The aligning indicia employed by known devices, e.g. marks on the surface
of the form cylinder which are brought into alignment with marks on the
thin printing form sleeve, can be used for arranging the printing form
sleeve on the form cylinder. The sleeve is slipped onto the form cylinder
by expanding it with compressed air. When the compressed air is shut off,
the printing form clings to the form cylinder in a positive engagement.
The coating and exposure of the printing form can be carried out by
photochemical means outside the printing machine before the plate-shaped
blank is welded or, alternatively, after the printing form sleeve is
slipped on by coating and exposing the printing form on the form cylinder.
Thus, while there have been shown and described and pointed out fundamental
novel features of the invention as applied to preferred embodiments
thereof, it will be understood that various omissions and substitutions
and changes in the form and details of the disclosed invention may be made
by those skilled in the art without departing from the spirit of the
invention. It is the intention, therefore, to be limited only as indicated
by the scope of the claims appended hereto.
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