Method and an apparatus for individual transport of offset printing plates

A method for individual transfer of offset printing plates from a magazine in which the plates are supported at their lower edges and stacked with their respective non-coated sides facing upwards, to a receiving table. The receiving table receives each plate with the non-coated side of the plate facing downwards. The uppermost plate of the stack is gripped by a gripping device engaging the non-coated side of the plate in such a manner that the plate is retained by the gripping means during the transfer to the receiving table. The gripping device is caused to engage the plate adjacent its upper edge, and during the movement away from the stack, the gripping device is initially moved along a path within or along a circle which has its center in the lower edge of the plate, and which has a radius corresponding to the distance between the lower edge of the plate and the engagement point of the gripping means with the plate when the plate is being gripped, until the plate retained by the gripping device is essentially in line contact at its lower edge with the plate situated immediately therebelow in the stack.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for individual transfer of offset 
printing plates from a magazine in which they are supported on their 
respective edges and stacked each with its non-coated side facing upwards, 
to a receiving table receiving a plate with its non-coated side facing 
downwards, and where the uppermost plate of the stack is gripped by means 
of a gripping means engaging the non-coated side of the plate in such a 
manner that said plate is retained by the gripping means during the 
transfer to the receiving table. 
2. Background Art 
EP-PS No. 0 037 064 discloses an apparatus for individual transport of 
printing plates. The printing plates are stacked in an inclined magazine 
with each with its non-coated side facing outwards and with intermediary 
protecting sheets arranged adjacent plates. The printing plates are 
transferred one by one from the magazine to a horizontally arranged 
conveyor advancing each plate to an exposure station. The apparatus 
comprises a pivotally arranged lifting cylinder, the displaceable piston 
rod of which is provided with suction means for gripping the uppermost 
plate in the stack and retaining that plate during the transfer to the 
conveyor. When a plate is to be gripped, the lifting cylinder is turned 
downwards into a position in which the piston rod thereof is arranged 
substantially perpendicular to the stack of plates. Subsequently, the 
piston rod is moved forwards until the suction means abut the surface of 
the uppermost plate. Then, the suction means are subjected to a vacuum in 
such a manner that they grip the plate, whereafter the piston rod is 
retracted in the cylinder until the plate has been slightly lifted from 
the plate therebelow. Then, pressurized air is blown between the plate 
lifted and the plate now on top of the stack, in such a manner that the 
protecting sheet is removed. Finally, the lifting cylinder is turned to 
its vertical position, with the effect that the plate levels with the 
conveyor path. When vacuum is no longer fed to the suction cups, the 
conveyor can advance the plates to the exposure station. The protecting 
sheets between the plates prevent the uppermost plate from adhering to the 
plate therebelow when said uppermost plate is being lifted from the stack. 
In addition, the protecting sheets protect, of course, the light-sensitive 
sides of the plates from being scratched when two plates are displaced 
relative to one another. The method and the apparatus of this prior art 
publication suffer, however, from the drawback of being rather 
complicated, partly due to the multitude of movements necessitating a 
multitude of mechanisms and partly due to the use of protecting sheets 
between the plates in the stack. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a method of the above type which, 
in a simple manner, insures that only the uppermost plate of the stack is 
gripped and transferred onto a receiving table, without the 
light-sensitive side of the plate being damaged, even when no protecting 
sheets are used between the plates in the stack. 
The method according to the invention is characterized in that the gripping 
means is caused to engage the plate adjacent its upper edge, and, during 
the movement away from the stack, the gripping means is initially moved 
along a path within or along a circle with the centre in the lower edge 
having the plate and of a radius corresponding to the distance between the 
lower edge of the plate and the engagement point of the gripping means 
with the plate when the plate is being gripped, until the plate retained 
by the gripping means is essentially in line contact at its lower edge 
with the plate situated immediately therebelow in the stack. As a result, 
the lower edge of the uppermost plate is pressed against its support and 
the plate therebelow during the first step of the transfer with, the 
effect that it is subjected to a torque in a direction away from the 
stack. Consequently, the non-coated side of the plate is slightly concave, 
and, starting from the engagement point of the gripping means at the upper 
edge of the plate, the plate is rolled off the stack without being 
displaced relative to the plate therebelow until it is in line contact 
with the plate therebelow at its lower edge. In addition to preventing a 
damaging of the light-sensitive side of the plate facing downwards in the 
stack, the above described rolling off procedure ensures that the 
uppermost plate is completely released from the plate therebelow, due to 
the gradual elimination of the adhesion between the two plates. 
According to the invention, the plate may at the upper end be subjected to 
a torque in a direction away from the stack when the gripping means is 
caused to engage the plate. In this manner, the uppermost plate is bent 
away from the stack at its upper edge, with the result that the plate is 
lifted from the stack before the transfer thereof is initiated. As a 
result, it is additionally ensured that the plate therebelow does not 
adhere to the uppermost plate and follow the movement thereof away from 
the stack. 
The invention relates furthermore to an apparatus for carrying out the 
method according to the invention. The apparatus includes a plate magazine 
to accomodate a stack of plates with their non-coated sides facing 
outwards and for supporting these plates at their lower edges. The 
apparatus further includes a gripping means for gripping the uppermost 
plate in the stack by engaging the non-coated side thereof, where the 
gripping means is associated with a moving mechanism removing the gripped 
plate from the magazine and transferring it to a receiving table such that 
the plate is placed thereon with its non-coated side facing downwards. 
The apparatus according to the invention is characterized in that the 
gripping means is adapted to grip the plate adjacent its upper edge, and 
that the moving mechanism is adapted during the removal of the plate from 
the stack to initially move the gripping means along a path within or 
along a circle with the centre in the lower edge of the plate and having a 
radius corresponding to the distance between the lower edge of the plate 
and the engagement point of the gripping means with the plate when the 
plate is being gripped, until the plate retained by the gripping means is 
essentially in line contact at its lower edge with the plate situated 
immediately therebelow in the stack. Because the uppermost plate of the 
stack is not displaced while being in surface contact with the plate 
therebelow, the light-sensitive side of the uppermost plate facing the 
plate therebelow is furthermore prevented from being damaged during the 
transport of the uppermost plate away from the plate therebelow. At the 
same time, the rolling off procedure ensures a gradual elimination of the 
adhesive forces between the two plates, such elimination starting at the 
upper edge of the uppermost plate, whereby the risk of more than one plate 
being removed from the stack is eliminated. 
According to the invention, the moving mechanism may comprise a pivotal arm 
which at its lower end is rotatably journalled relative to a frame about a 
horizontal axis of rotation positioned below the lower edge of the stack 
of plates in the plate magazine, and which at its upper end is provided 
with a gripping means arm rotatably journalled about an axis parallel to 
the axis of rotation of the pivotal arm. The gripping means may be 
arranged on the gripping means arm at a distance from the axis of rotation 
thereof. The moving mechanism further includes a guiding means guiding the 
movement of the gripping means arm in the area adjacent the stack of 
plates in the magazine. The resulting embodiment of the invention turned 
out to be particularly advantageous in practice, especially when the 
gripping means arm according to a further embodiment of the invention is 
placed in front of the pivotal arm when seen in the moving direction from 
the magazine to the receiving table. In this manner, an embodiment of the 
invention is obtained where it is possible merely by a suitable choice of 
the gripping means arm to ensure that the lower edge of the plate is 
continuously pressed against its support and subjected to a torque 
suitable for ensuring the intended rolling off in the desired direction. 
At the same time, the guide means guiding the movement of the gripping 
means arm in the area adjacent the stack of plates is particularly simple, 
because the guide means need only guide the gripping means arm into the 
gripping position thereof immediately before the pivotal arm reaches the 
corresponding gripping position during its returning movement. 
Furthermore, the pivotal arm may, according to the invention, be shaped 
substantially as an inverted L comprising a body member placed on one side 
of the magazine and the receiving table, as well as a transverse member, 
which at the upper end of the body member, extends towards the opposite 
side of the magazine and the receiving table, respectively. The transverse 
member may be arranged at a distance from the axis of rotation of the 
pivotal arm exceeding the distance between said axis of rotation and the 
upper edge of the plate magazine with plates arranged therein. The 
resulting embodiment turned out to be particularly suitable and 
space-saving in practice. 
Moreover, according to the invention, the guide means may comprise a guide 
pin placed on the gripping means arm as well as a guide surface 
cooperating with the guide pin and placed in the frame. In this manner, a 
simple guiding of the movement of the gripping means arm and consequently 
of the gripping means is obtained, both when the guiding means is guiding 
the gripping means into its gripping position and when it is ensuring the 
specified path of the gripping means once the gripping means has gripped 
the plate or serves both purposes. 
In addition, according to the invention, the gripping means may be formed 
by a plurality of suction cups aligned on a transverse support which is 
connected to the gripping means arm. As a result, offset printing plates 
made of all known material combinations and of varying widths can be 
gripped and retained by one and the same gripping means. 
Moreover, according to the invention, the suction cups may be 
bellows-shaped such that their length is reduced when they are supplied 
with vacuum while abutting the plate so as to grip said plate, and a 
tilting edge may be present immediately below the suction cups. The 
tilting edge is placed immediately behind the engaging surface of the 
suction cup when seen in the moving direction. In this manner, the 
uppermost plate is slightly bent about the tilting edge when vacuum is 
supplied to the suction cups, with the result that the upper portion of 
the uppermost plate is released from the plate therebelow before the 
pivotal arm starts to transfer the plate. 
Finally, according to the invention, the support may be tiltably connected 
to the gripping means arm, the tilting movement only being allowed in the 
same direction as the rotating direction of the pivotal arm, by means of a 
stop which determines the engaging position of the suction cups when the 
plate is being gripped. Accordingly, the plate gripped is prevented from 
being subjected to such a torque during the transfer that would provide 
the plate with an S-shaped bending curve when seen from the side, i.e. a 
bending curve having an inflexion.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 is a diagrammatic side view of a machine for producing offset 
plates, i.e. for the exposure and developing thereof in such a manner that 
they are ready for printing use. The machine comprises a material unit 1 
including an apparatus according to the invention. The apparatus comprises 
a magazine 3 which inclines relative to vertical and contains a stack 2 of 
plates. The apparatus comprises, furthermore, a roller conveyor 4 and a 
pivotal arm 17 with a gripping means transferring the uppermost plate of 
the stack onto the roller conveyor 4 which advances the plate to an 
exposure unit 5. When the plate has been positioned on an exposure table 
6, the exposure unit 5 exposes the light-sensitive side of the plate. When 
seen in the moving direction of the plate through the machine, the 
exposure unit 5 is followed by a buffer unit 7 comprising a conveyor 8, 
which stores the exposed plate until the plate is advanced to a developing 
unit 9. The developing unit 9 has a developing section 10, a washing 
section 11, a fixing section 13, a rinsing section 12 and a drying section 
14. Having passed these stations, the ready-made plate leaves the machine 
and is transferred to a delivery shelf 15. 
FIG. 2 is a diagrammatic, more detailed view of a first embodiment of an 
apparatus 16 according to the invention. Below the magazine 3 and the 
conveyor 4, the pivotal arm 17 of the apparatus 16 is at its lower end 
rotatably journalled about a horizontal axis 18 relative to a frame 19. 
The plate magazine 3 is inclined relative to vertical, and, when seen from 
the side, the magazine 3 is arranged on one side of a vertical plane 
through the axis 18 of rotation of the pivotal arm. The horizontal roller 
conveyor 4 is substantially placed on the other side of the above 
mentioned vertical plane. 
The pivotal arm 17 can be moved by means of a chain drive 47, between a 
gripping position shown to the left of FIG. 2, and a delivery position 
shown to the right of the Figure. The chain drive 47 comprises a chain 48 
which is secured at its ends to the pivotal arm 17, a resilient member 
(not shown) and coupled to one end of the chain, a driving sprocket 49, 
which is driven by means of a motor (not shown) and which is rotatably 
journalled in a frame, as well as two idler sprockets 50, 51 which are 
also rotatably journalled in the frame. 
In the plate magazine 3, the plates in the stack 2 are supported by a 
supporting member 21 at their lower edge 20. The plates are arranged with 
their light-sensitive sides facing the base 22 of the plate magazine. The 
supporting member 21 can be upwardly displaced relative to the position 
which is shown in FIG. 2, which position allows the largest possible 
height of plates capable of being stacked in the magazine 3. Accordingly, 
the upper edge 23 of the plates can be placed at the same location in the 
magazine irrespective of their size. Correspondingly, adjustable side 
guiding means 54a, 54b are arranged in the magazine in FIG. 4, these 
guiding means positioning the plates in response to their width. The plate 
magazine is pivotally connected to the frame 19 about a horizontal axis 25 
of rotation through an arm 24 in such a manner that the magazine can be 
moved from the inclined position indicated by solid lines in FIG. 2, to 
the horizontal position which is indicated by dotted lines, and allowing a 
feeding of plates. In connection with the magazine 3, it should 
furthermore be noted that the plates are arranged in such a way that their 
upper edges 23 are flush with the upper edge 26 of the base 22 of the 
magazine 3, which appears clearly from FIG. 3. 
The pivotal arm 17 is shaped substantially like an inverted L with a body 
member 27 arranged on one side of the magazine 3 and the roller conveyor 4 
as well as a transverse member 28 which extends towards and past the 
opposite side of the magazine 3 and the roller conveyor when seen from 
above as in FIG. 4. For the sake of good order, it should be mentioned 
that the body member of FIGS. 2 and 3 is indicated by dotted lines because 
it is not visible in these Figures due to its position (see FIG. 4) on the 
opposite side of the magazine and the conveyor, respectively. The pivotal 
arm 17 comprises, opposite the body member 27 and adjacent its lower end, 
a short arm 55 connected to the body member 27 by means of a lower 
transverse beam 29. The short arm and the lower end of the body member 27 
are pivotally connected to the frame 19 so as to form the axis 18 of 
rotation of the pivotal arm 17. The upper transverse member 28 is placed 
at a distance from the axis 18 of rotation of the pivotal arm 17 which is 
greater than the distance between the axis 18 of rotation and the upper 
edge 23 of the stack 2 of plates in the magazine 3 and the upper edge 26 
of said magazine 3, respectively, with the result that the upper 
transverse member 28 can pass the edges. 
A bearing bracket 30a, 30b is arranged at each end of the upper transverse 
member 28. A gripping means or suction cup arm 31 is rotatably journalled 
on the bearing bracket about the axis 52 of rotation, this axis 52 of 
rotation being parallel to the axis 18 of rotation of the pivotal arm. The 
suction cup arm is made of bent sheet material and comprises an upper 
transverse wall 32 with an opening 56, as well as, at each end, an end 
wall 33a, 33b perpendicular to the transverse wall 32. The pivotal 
connection with the bearing brackets 30a, 30b is provided in the latter 
end walls. Opposite the above described pivotal connections with the 
bearing brackets 30a 30b, the suction cup arm 31 comprises a transverse 
supporting wall 57 provided with a transverse row of suction cups 34 which 
can be supplied with vacuum through a tube 58. Immediately below the row 
of suction cups, the supporting wall 57 continues into a tilting part 35 
which extends towards the gripping surface 37 of the suction cups and ends 
in a tilting edge 36 immediately before the gripping surface 37 in the 
non-activated state of the suction cups, i.e. when the suction cups are 
not subjected to vacuum. As clearly illustrated in FIGS. 3 and 4, the 
suction cups 34 are bellows-shaped such that when loaded they will try to 
reduce their length to such an extend that their gripping surface 37 is 
moved to the other side of the tilting edge 36. A roller 38 is mounted on 
one end wall 33a of the suction cup arm 31, this roller being adapted to 
roll on a guide surface 39 of a guide rail 40 arranged on the inner side 
of the frame 19 in an area adjacent the upper end of the plate magazine. 
The guide surface 39 serves to guide (see FIGS. 2, 3 and 4) the suction 
cup arm 31 into a position in which the gripping surface 37 of the suction 
cups is substantially level with the uppermost plate 41 in the stack of 
plates, whereby the uppermost plate 41 may be gripped and transferred onto 
the roller conveyor 4. 
A guide rail 43 is also secured to the inner side of the frame 19 in the 
area adjacent the edge 42 of the roller conveyor 4 farthest from the axis 
18 of rotation of the pivotal arm 17. The guide rail 43 serves to guide 
the movement of the suction cup arm 31 immediately before a plate is 
placed on the roller conveyor 4, and when the pivotal arm is returned 
towards the plate magazine 3 after the plate has been released so as to 
grip another plate. 
When the uppermost plate 41 in the stack 2 of plates is to be transferred 
onto the roller conveyor 4, the pivotal arm 17 is moved out of a resting 
position, such as the vertical position of FIGS. 2 and 3, and towards the 
stack 2 of plates. As a result, the roller 38 on the suction cup arm 31 is 
moved into contact with the guide surface 39 of the guide rail 40, with 
the effect that the suction cups 34 are moved into their gripping position 
when the pivotal arm 17 reaches its gripping position, which is shown to 
the left in FIGS. 2 and 3. When vacuum is supplied to the suction cups 34 
in the above described gripping position, the cups grip the uppermost 
plate 41 in the stack 2 adjacent the upper edge of the plate. The 
bellows-shaped form of the suction cups and the tilting edge 36 
immediately below the row of suction cups 34 imply that the upper end of 
the plate is lifted from the plate therebelow. Subsequently, the pivotal 
arm 17 is moved to the right towards the roller conveyor 4. Due to the 
weight and dimensions of the suction cup arm and due to the positioning of 
the various parts of the apparatus, the suction cups are moved along a 
path during the above described movement. The path is placed within or 
along a circle C with the centre in the supporting point of the gripped 
plate 41 against the supporting plate 21 and having a radius corresponding 
to the distance between the supporting point and the engagement point 45 
of the suction cups 34 with the plate when the plate is gripped in the 
magazine 3. The suctions cups follow the above-described path, until the 
gripped plate 41 is essentially in line contact with the plate therebelow 
at its lower edge. As a result, the gripped plate 41 is gradually rolled 
off the plate therebelow while forming a curve when seen from the side. 
While the plates are in surface contact, no relative displacement occurs 
therebetween, with the result that the risk of damaging the 
light-sensitive surface facing inwards of the gripped plate has been 
eliminated. In this connection, it should be noted that it is important 
that the contact surface of the plate towards the plate therebelow is not 
broken during the entire separating procedure, whereby no relative 
displacement occurs of the gripped plate relative to the plate therebelow. 
During the continued movement of the pivotal arm 17 towards the roller 
conveyor 4, the lower edge of the gripped plate 41 is at a time lifted off 
the supporting member 21 and slides along the upper surface of the plate 
therebelow. At the same time, the side of the gripped plate facing upwards 
in the magazine slides on the upper front edge 46 of the roller conveyor 
so as to end up facing downwards on the roller conveyor. 
Before the pivotal arm 17 reaches the delivery position indicated to the 
right in FIG. 2, the roller 38 of the suction cup arm 31 abuts the guide 
surface 44 of the guide rail 43, with the result that the suction cup arm 
31 is guided into the delivery position of the plate. In the delivery 
position, the supply of vacuum is interrupted to the suction cups, with 
the effect that the plate is released and can be advanced by means of the 
roller conveyor onto the exposure table 6 of the exposure unit 5, the 
plate thereby passing through the opening 56 in the transverse wall 32 of 
the suction cup arm 31. When the pivotal arm is returned from the delivery 
position to its vertical position, the movement of the suction cup arm 31 
is guided by the contact of the roller 38 with the guide surface 44. In 
response to the rigidity and height of the plate relative to the weight of 
the suction cup arm, the movement of the suction cup arm 31 is, of course, 
also guided by the contact of the roller 38 with the guide surface 39 of 
the guide rail 40 during the removal of the uppermost plate from the 
stack. 
FIGS. 5 and 6 are a longitudinal sectional view and a fragmentary plan 
view, respectively, in the direction 6--6 of FIG. 5 of an alternative 
embodiment of the invention corresponding to the embodiment indicated in 
FIG. 1. The embodiment of FIGS. 5 and 6 corresponds in all respects to the 
embodiment of FIGS. 2 to 4, apart from the details shown in particular in 
FIGS. 5 and 6, the reason why only the shown details are described below. 
FIGS. 5 and 6 illustrate the pivotal arm 67 in the gripping position. The 
pivotal arm 67 comprises a body member 77 and an upper transverse member 
78. A bearing bracket 80a, 80b is secured at the upper end of the 
transverse member. A suction cup arm 81 is pivotally connected to the 
bearing brackets 80a, 80b about an axis 102 of rotation. The suction cup 
arm 81 comprises two end members 103a, 103b providing the bearing 
presenting the axis 102 of rotation. The suction cup arm 81 comprises, 
furthermore, a transverse connecting rod 82 and a guide plate 70 
interconnecting the two end members. The guide plate 70 guides the plate 
in the delivery position (not shown) of the pivotal arm 67 when the plate 
is advanced by the roller conveyor 4 onto the exposure table 6 (see FIG. 
1) of the exposure unit 5. 
A suction cup beam 71 of a Z-shaped cross-section and with two side members 
73a, 73b is rotatably connected with the end members 103a, 103b of the 
suction cup arm 81 opposite the axis 102 of rotation between the suction 
cup arm 81 and the pivotal arm 67, this rotatable connection defining an 
axis 72 of rotation. The body 107 of the suction cup beam 71 is provided 
with a transverse row of suction cups 34 corresponding to the suction cups 
referred to in FIGS. 2 and 4, and which can be supplied with vacuum 
through the tube 58. One leg 74 of the suction cup beam 71 extends 
immediately below the row of suction cups 34 in a direction towards the 
gripping surfaces 37 of the suction cups. A tilting part 85 with a tilting 
edge 86 is adjustably arranged on the leg 74. In the non-activated state 
of the suction cups 34 and when the pivotal arm 67 is moved towards the 
magazine so as to grip the uppermost plate therein, the tilting edge 86 is 
placed immediately behind the gripping surface 37 of the suction cups. The 
lengths of the suction cups 34 is reduced, as previously described, when 
the suction cups are supplied with vacuum. 
A stop pin 75 extends outwardly of one side member 73a of the suction cup 
beam 71. The stop pin is adapted counter-clockwise (see FIG. 5) to limit 
the turning of the suction cup beam 71 to the position of FIG. 5, by 
providing an abutment against the edge surface 76 of the adjacent end 
member 103a of the suction cup arm 81. The suction cup beam 71 is allowed 
to turn freely clockwise about the axis 72 of rotation (see FIG. 5). The 
pivotal mounting of the suction cup beam 71, and consequently of the 
suction cups, prevents the gripped plate from being subjected to a torque 
during the transfer which would otherwise have provided the plate with an 
S-shaped bending curve, i.e. a curve having an inflexion, and which would 
have deformed the plate unecessarily. 
Finally, a roller 88 is rotatably journalled on the outer side of the end 
member 103a of the suction cup arm 81. The roller 88 is adapted to roll on 
an upper guide surface 90 of a lower guide rail 89 and a lower guide 
surface 91 of an upper guide rail 92, these guide rails being placed on 
the inner side of the frame 19 in an area adjacent the upper end of the 
plate magazine. The upper guide surface 90 of the lower guide rail 89 is 
adapted to limit the convex bending of the gripped plate when the plate is 
being removed from the stack of plates. The guide surface 90 is 
furthermore adapted to guide the suction cup arm 81 into the position 
corresponding to the gripping position of the suction cups 34 when the 
pivotal arm 67 is moved from the delivery position to the gripping 
position of FIG. 5. The lower guide surface 91 of the upper guide rail 92 
is adapted to provide the gripped plate with the desired bending 
independent of the rigidity thereof. 
The invention may be modified in many ways without thereby deviating from 
the principles thereof. Thus, the movement of the suction cup arm may, for 
instance, during the entire transferring movement of the pivotal arm and 
during the entire returning movement thereof, be limited by a lower and an 
upper guide as indicated by means of the guides 89 and 92 in the area 
adjacent the gripping position of the pivotal arm. Furthermore, the 
angular turning of the suction cup beam may be forcibly guided during the 
movement of the pivotal arm so as to achieve the desired curving of the 
plate. The forced guiding can, for instance like the guiding of the 
suction cup arm 81, be provided by means of a guide pin and guide plates. 
Correspondingly, the suction cup beam can advantageously be guided by 
means of other mechanisms beyond a pivotal arm and a suction cup arm, such 
as by means of a chain, guides and guideways for providing the desired 
moving path for the suction cup beam.