Abstract:
The invention is a method and a device for blocking a stack of stacked objects. A device for blocking in accordance with the invention includes two opposed side walls which engage the stack, a pressing device which has at least two pressing elements opposite one another which contact the stack, between which elements of the stack are compressed while contacting the side walls, and charging electrodes which charge the stack, and wherein the charging electrodes are located in at least one of the pressing elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a device as well as a method for blocking a stack of stacked objects, for example stacked newspapers, magazines, or the like. 
     2. Description of the Prior Art 
     A device of this kind is known (WO-96/33118) and has a lifting table by which a stack of stacked objects lying thereon can be pressed against two flaps serving as counterpressure plates. During the compression of the stack, the stack is blocked by charging electrodes located laterally with respect to the stack. Since the paper stack has a higher dielectric constant than air, the electrical field concentrates in the X stack. In addition to this concentration, the field is concentrated in the superfluous air inclusions in the stack, so that the field force effect in the stack and in the charge accumulated at the surface compresses the air in the stack. As a result, the adhesion of the objects is decreased, so that the stack is mechanically and firmly blocked and consequently is secured. During the compression of the stack, charging electrodes located laterally next to the stack, depending on the height of the stack, can be switched on. 
     Similar devices are known for blocking stacks (DE-44 34 946 A1 and DE-44 41 431 A1) that have a transport table, with lateral charging electrodes located on the transport table, said electrodes extending perpendicularly to the transport direction of the stack and extending perpendicularly to the transport table for the entire height of the stack. Opposite the transport table is another grounded charging electrode that contacts the stack from above. This charging electrode can be provided with one or more rollers that are pressed from above against the stack. 
     Finally, a device is known for aligning, pressing, and blocking packets of loosely stacked printed products (G 295 06 231 U1). This device consists of a packet conveyor with a front packet stop that can be raised and lowered, two packet aligning plates located laterally with respect to the packet conveyor and capable of being brought together or moved apart, and a pressing device above that can be raised and lowered and can be placed between the packet alignment plates on top of the packet to be pressed. It has a charging assembly for electrostatic blocking of the printed products, whose charging electrodes are connected to the packet alignment plates located laterally with respect to the packets and to the pressing device, so that the packets are charged with a voltage on their lateral wall areas and their tops. 
     SUMMARY OF THE INVENTION 
     The object of the invention is a device for blocking a stack of stacked objects, stacked newspapers, magazines, or the like for example, by which the stack to be blocked can be charged very effectively. 
     This object is achieved in a device and a method. 
     The device according to the invention has a pressing device with two pressing elements opposite one another, between which a stack can be pressed. The charging electrodes for charging this stack are integrated into the pressing elements opposite one another, so that the latter are pressed against the stack when compressing the stack. In addition, charging electrodes can also be provided in the side walls to which a high positive or negative voltage is applied, in any case with a different polarity than on the upper charging electrodes. 
     The device according to the invention is very simple in design, because the charging electrodes, which are known from the prior art and as a rule are located laterally next to the stack to be blocked, can be completely eliminated, since they are integrated into the pressing elements. 
     The pressing elements opposite one another thus form, in the manner of a plate capacitor, an electrical field that runs essentially parallel to the pressing direction and passes perpendicularly through the objects to be stacked, for example newspapers, magazines, or the like, so that the desired displacement charges accumulate on their surfaces and build up a capacitive displacement field that opposes the electrical field applied externally. Since the directions of the electrical field applied externally and of the displacement field induced in the stacked objects run parallel to one another, during pressing a capacitive displacement current flows so that the charging of the stack is extremely effective and by which high adhesion forces are produced. The displacement current is produced by the air being forced out of the stack. 
     Although it is known (WO 96/33118) to ground the supporting table that serves as a pressing element and the flaps located opposite the supporting table or to apply the same potential, so that here as well, charging electrodes are provided above and below the stack for blocking the latter. Since the supporting table and flaps are at the same potential however, and additional charging electrodes are located laterally, a field pattern is produced that passes diagonally through the stacked objects. 
     Preferably, the stack is charged by the device according to the invention during pressing at the same time, since as a result of the compression of the stack, its dielectric constant is increased during pressing, so that a correspondingly high electrical field forms between the charging electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described as an example with reference to the drawing in greater detail. 
     FIG. 1 shows a first embodiment of the device according to the invention, in a schematic perspective view; 
     FIG. 2 shows a second embodiment of the device, likewise in a schematic view. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The device according to the invention for blocking a stack of stacked objects, for example stacked newspapers, magazines, or the like, is designed as a vertical shaft  1  with two opposite vertical side walls  2 ,  3 . The side walls are fastened by their lower ends using outwardly bent ribs  4 ,  5  to a working device, especially a stacker. 
     The bottom of shaft  1  is formed by a lifting table  7  that can be moved vertically up and down (arrow  6 ), said table consisting of a horizontal lifting plate  8  and a lifting cylinder  9 , at whose upper end lifting plate  8  is fastened. 
     On the vertical side walls  2  and  3 , flaps  13 ,  14  are fastened with articulation to each of the upper horizontal lengthwise edges  7 , 12  so that flaps  13 ,  14  can be folded inward around the lengthwise edges  11 , 12  between a horizontal position in which they project into shaft  1  and a vertical position illustrated in phantom in which they are folded out as straight-line extensions of side walls  2 ,  3 . In the horizontal position, the flaps  13 ,  14  can be secured by a locking device (not shown) so that they can withstand a considerable pressure from below. In flaps  13  and  14 , charging electrodes designed as electrode plates  15 ,  16  are provided, each of which is electrically insulated from the other flap areas of flaps  13 ,  14 . Preferably, the charging electrode plates  15  and  16  are designed so that when flaps  13 ,  14  are folded in, they project downward slightly at flaps  13 ,  14 . 
     The two charging electrode plates  15 ,  16  are connected to a high-voltage source and both can preferably be connected to the same potential, The lifting plate  8 , which is opposite the two flaps  13 ,  14  in shaft  1 , is designed as an electrically conducting metal plate and is preferably connected to ground or connected to a voltage which has a polarity opposite that of the voltage applied to charging electrode plates  15 ,  16 . 
     FIG. 2 shows a second embodiment of the invention, with the same reference numerals representing identical parts. The important difference from the embodiment shown in FIG. 1 lies in the use of a conveyor belt  30  which is designed to be grounded and capable of carrying objects away. The stack, represented by  21  as a whole, is introduced on this conveyor belt  30  into the device according to FIG.  2 . Then the side walls  2 , 3  are brought together laterally to align the stack. These side walls are additionally provided with electrodes  31 ,  32  and  33 ,  34  inserted into them, said electrodes being electrically insulated from the remaining side walls. 
     In another deviation from the embodiment according to FIG. 1, the pressing element  13  located at the top can be moved by means of a preferably pneumatic lifting cylinder represented schematically by  35 . A charging electrode plate  15 , likewise electrically insulated, is inserted into the upper pressing plate  13 , said plate  15  also being capable of being connected to a high voltage source. During pressing in the vertical direction as indicated by directional arrow  6 , stack  21  is compressed, with a capacitive displacement current being produced by the escape of air from the individual objects  20  forming the stack  21 . 
     The operation of the device according to the invention for blocking the stack will be explained below with reference to FIG.  1 . 
     With flaps  13 ,  14  opened, the objects to be stacked, for example newspapers, magazines, or the like, are introduced from above into the shaft  1 , whereupon they fall on lifting plate  8  and are aligned by the side walls  2 ,  3  that serve as packet aligning plates. When a predetermined quantity of objects  20  to be stacked has accumulated in shaft  1 , flaps  13 ,  14  are folded into their horizontal positions and secured. The stack  21  located between the lifting plate  8  and the flaps  13 ,  14  is compressed and squeezed by raising the lifting plate  8  between flaps  13 ,  14  and the lifting plate  8 . At the sate time, a previously determined, preferably negative high voltage is applied to the charging electrode plates  15 ,  16  so that an electrical field is formed between the charging electrode plates  15 ,  16  and the lifting plate  8  connected to ground, said field passing through the objects  20 , stacked on top of one another in layers, essentially perpendicularly to their wide sides. As a result of the sudden voltage rise dU/dt and a simultaneous compression of the stack  21 , a capacitive displacement current dI/dt flows through stack  21 , said current being produced by the sudden voltage rise, the simultaneous increase in the dielectric constant, as well as a decrease in the resistance due to the pressure contact between the charging electrode plates  15 ,  16  that act as charging electrodes, the stack  21 , and the lifting plate  8  that acts as a charging electrode. 
     As a result of the direct contact between the charging electrode plates  15 ,  16  and the stack  21 , in contrast to the known zero-contact transfer of the electrical charge known from the prior art, an improved efficiency is achieved so that the high-voltage generator used with the device according to the invention can be made smaller than the high-voltage generator used in known devices for blocking. 
     The charging electrodes can be adjusted or adapted to the stacked height of the stack  21  located in shaft  1  and composed of objects  20  to be stacked automatically in the device according to the invention, since the charging electrodes are designed on the pressing elements located opposite one another between which stack  21  to be blocked is pressed. 
     Since the charging electrode surfaces are in mechanical contact with the objects to be stacked, no contamination can adhere to them as it is known to do on the high-voltage electrodes that operate without contact conventionally and which must be cleaned at regular intervals, often within a few days.