Abstract:
The invention relates to a method and a device for powdering printed paper sheets. According to the invention, the performance of the air-pressure generator which generates the air stream for the powder-air mixture can be adjusted in line with the machine speed or the speed of transportation of the paper sheets.

Description:
BACKGROUND 
     The invention pertains to a method for powdering printed sheets, wherein the powder is removed from a supply container and mixed with air in a mixing device, the powder is distributed by an air stream and applied onto the sheet by a nozzle unit that, if so required, can be adapted to the width of the sheet, the number of outlet openings or the outlet cross section, in particular, of said nozzle unit is changed correspondingly, and the air stream is generated by an air pressure generator. The invention also pertains to a device with a supply container for the powder, a mixing device, in which the powder is mixed with the air stream, an air pressure generator for generating the air stream, and a nozzle unit, from which the air stream containing the powder is discharged and blown onto the sheet, wherein the nozzle unit contains several outlet openings or an outlet cross section that allows the nozzle unit, if so required, to be adapted to the width of the sheet. 
     It is known, for example, that paper sheets printed by a printing machine are stacked by means of a sheet stacking device. Before the stacking process, the sheets are powdered such that the still moist printing color does not smear during the stacking process. A powdering device, in which powder is mixed with an air stream, is used for powdering the sheets, and the air stream containing the powder is blown onto the surface of the sheet. A nozzle unit that, for example, is mounted on a nozzle beam that extends over the entire width of the sheet transverse to the transport direction is used for this purpose. In order to maintain the powder loss as well as the soiling of the sheet stacking device and the printing machine at a minimum, the active region of the nozzle unit is adapted to the width of the sheet. This is realized by switching on or off individual nozzles or by adapting the outlet cross section to the width of the sheet. For example, if nozzles are switched off, i.e., if the outlet openings of these nozzles are closed, the pressure drop between the blower that generates the air stream and the outlet openings changes. The compressed air made available by the blower now must be discharged through a smaller number of nozzles and a respectively smaller outlet cross section, whereby the flow speed as well as the pressure gradient change so that the air emerging from the nozzles is discharged with a higher speed. Due to the lower pressure drop that occurs if a smaller quantity of outlet nozzles remains open, less powder is stirred up in the mixing device that is constructed in the form of a turbulence chamber, so that less powder is concentrated and, thus, distributed in the air. The specific powder volume which is distributed for a narrow sheet consequently is lower than that for a wider sheet. In addition, the quantity of powder required is highly dependent on the transport speed of the printed paper sheets. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and/or a device by means of which the specific powder volume admixed in the air stream and, consequently, the distributed powder volume can be adapted to the instantaneous requirements of the sheet in a superior fashion. In a method of the initially mentioned type, this is attained according to the invention due to the fact that the performance of the air pressure generator can be changed, in particular, continuously during the operation. 
     According to one aspect, the performance of the air pressure generator is adapted to the machine speed or the transport speed of the paper sheets. The method according to the invention essentially provides the advantage that the distributed quantity of powder can be exactly adapted to the requirements of the paper sheet. Consequently, compromises with respect to a slow operating speed and a normal operating speed are eliminated. When starting the operation or reducing the speed, the powder quantity is automatically reduced. 
     According to other aspects of the invention, the performance of the air pressure generator can also be adapted to other ambient conditions, if so required. These conditions pertain, for example, to the printing material, the size of the printed surface, the printed image, the type of powder, the type of paper, the relative humidity and the temperature. Suitable sensors are provided for this purpose. These sensors forward the measured data, in particular, control currents, to a control unit of the air pressure generator. 
     According to another refinement, the power of the air pressure generator is adapted to the number of open outlet openings or to the outlet cross section. Due to this measure, wide sheets that, for example, are powdered by  12  nozzles are provided with the air quantity required for this purpose, and a lesser quantity of air is supplied for narrower sheets (e.g., with a half-format) that, for example, are powdered by six nozzles. Consequently, the air volume discharged per nozzle remains the same. This also ensures that the quantity of powder per nozzle remains the same. This means that the performance of the air pressure generator is reduced if the number of outlet openings or the outlet cross section is reduced. In this case, the performance of the air pressure generator can be linearly adapted. 
     According to yet another refinement, the powder is removed from the supply container by means of a fixed metering device, e.g., a drum metering device or a cell metering device. The exact quantity of powder required for powdering the respective sheet can be withdrawn by the fixed metering device. If one or more nozzles are switched off for powdering narrower sheets, the quantity metered by the fixed metering device is correspondingly reduced. According to a refinement, the pressure of the air stream is adjusted to a value between 0.1 bar and 0.5 bar, in particular, 0.3 bar. A relatively large air volume provided by the air pressure generator corresponds to this low pressure. Consequently, the pressure of the air stream can be maintained at a constant value independently of the number of open outlet nozzles. 
     According to one preferred aspect of the invention, a side channel compressor is used as the air pressure generator. This compressor requires no maintenance and is not sensitive to dust. In conventional powdering devices, rotary vane compressors are used. However, these compressors require regular and relatively frequent maintenance. In addition, the vanes must be exchanged relatively often. Another problem is that rotary vane compressors must run dry because the air must be free of oil, and, thus, wear is promoted. Rotary vane compressors of this type also generate a relatively high pressure of approximately 1 bar and only a small volume. Rotary vane compressors can only be operated within a very narrow speed range, where this speed range of 2860 l/min to 3430 l/min corresponds to a frequency of 50-60 Hz. At low rotational speeds, the pressing force of the vanes is low resulting in an insufficient seal. At higher rotational speeds, the pressing force becomes excessively high due to the centrifugal force such that the wear increases impermissibly. 
     The power of the side channel compressor is preferably controlled or regulated with the aid of a frequency converter. Due to this measure, the rotational speed of the compressor and consequently its performance can be adapted relatively easily to the respective requirements. The frequency can be adjusted, for example, between 0 and 100 Hz such that a rotational speed range between 0 and 6000 l/min is achieved. 
     According to one refinement, an injector is used as the mixing device. In this injector, the air stream is realized in the form of an enveloping air jet, into which the powder is attracted by suction. This provides the significant advantage in that the suction tube can be arranged vertically and the powder must merely drop from the metering device due to gravitational force in order to be transported into the enveloping air jet. Consequently, the transport direction of the powder does not have to be deflected and, thus, no transport medium is required since, as mentioned above, the gravitational force suffices for transporting the powder into the air stream. 
     According to the invention, the compressor can be controlled by a sensor. This sensor determines the speed of the paper web or another component that is related to the speed of the paper web, e.g., the rotational speed of a driving motor or the like. The quantity of powder being withdrawn can be controlled exactly in this fashion. 
     The sensor determines, for example, the number of open outlet openings or the outlet cross section and controls the compressor drive correspondingly. However, sensors which determine the volume flow or the pressure in the air stream could also be used. For example, the compressor can be adjusted by means of a pressure sensor in such a way that a uniform pressure always exists in the powder/air stream. However, it would also be conceivable to provide sensors for determining the quantity of the powder contained in the air stream. The abovementioned sensors may be provided alternatively or simultaneously. In addition, it would also be conceivable to provide sensors which determine the printing material, the size of the printed surface, the printed image, the powder type, the paper type, the relative humidity, and the temperature. In this case, the performance of the air pressure generator is varied depending on the sensor data. The paper sheet is always powdered with an optimum amount of powder in this fashion. 
     The device according to the invention contains an injector in which the air stream is used as the driving air. The driving air is conveyed around a suction line, into which surrounding air is attracted by suction and the powder is attracted by suction with this ambient air. This suction line can be directly connected to the fixed metering device such that the conveyed powder is entrained in the surrounding air. According to the invention, the fixed metering device may consist of a metering drum that is described, e.g., in patent application serial nos. DE 199 37 090.7 and DE 199 37 557.7 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Additional advantages, characteristics and details of the invention result from the subordinate claims as well as the following description, in which one particularly preferred embodiment is described in detail with reference to the single drawing figure. The characteristics illustrated in the single drawing figure and mentioned in the claims as well as in the description are significant for the invention either individually or in arbitrary combinations. 
    
    
     DETAILED DESCRIPTION 
     The single drawing figure shows a mixing device of a device for powdering printed paper sheets which is identified in entirety by the reference numeral  1 . In this mixing device  1 , powder is conveyed in the direction of the arrow  2  from a (not-shown) supply container by a (not-shown) fixed metering device. This fixed metering device is arranged directly above the mixing device  1  such that the conveyed powder drops directly into a funnel  3  from the fixed metering device. This funnel  3  flows into a suction line  4  that is arranged directly underneath the funnel  3  a short distance from the fixed metering device. 
     A line  5 , by means of which compressed air is supplied to the mixing device  1 , also flows into the mixing device  1 . This compressed air is generated by an air pressure generator  6  which is realized in the form of a compressor, in particular, a side channel compressor  7  that is controlled by a frequency changer or a frequency converter  8 . The line  5  flows into a distribution chamber  9 , in which the air is uniformly conveyed around the suction line  4 . A sleeve-shaped air channel  10  is connected to this distribution chamber  9 . The sleeve-shaped air channel  10  is formed by arranging the suction line  4  within a bore  11  and at a distance from the inner wall of the bore such that the air in the distribution chamber  9  is conveyed into an outlet  12  between the suction line  4  and the bore  11 . Since the cross section of the sleeve-shaped air channel  10  is relatively small, the air in the channel  10  is accelerated to a high speed such that a vacuum is generated in the suction line  4  and air is entrained from the suction line  4  in the direction of the outlet  12 . Due to this vacuum, however, the powder that drops into the funnel  3  is also conveyed in the direction of the outlet  12  where it is mixed with the compressed air generated by the side channel compressor  7 . 
     The drawing figure clearly indicates that the powder which is conveyed in the direction of the arrow  2  must merely drop into the funnel  3 , and from there it is attracted by suction. The transport direction of the powder is vertically downward, and the transport is preferably realized due to gravitational force. A transport line  13  that transports the powder/air mixture into an annular distributor  14  is connected to the outlet  12 . In this annular distributor, the powder/air mixture is distributed over several individual lines  15 , via which the mixture is conveyed to the individual outlet nozzles. The individual lines  15  can be closed by means of valves  16 , wherein the valves  16  are actuated, for example, by means of electromagnets  17 . 
     If one or more powdering nozzles are deactivated, e.g., by closing individual lines  15  by means of the valves  16 , the powdering device is adapted to sheets of smaller width and the conveyed powder volume is reduced by reducing the performance of the fixed metering device. In addition, the volume of the compressed air stream can also be reduced by means of the frequency converter that reduces the rotational speed of the side channel compressor  7 . Although this causes a reduction in the suction performance in the suction line  4 , exactly the required quantity of powder which was admixed by the fixed metering device is discharged through the still open powdering nozzles. In addition, the volume of the air that contains powder can be maintained. The volume of the air that contains powder and is discharged per powdering nozzle can also be maintained independently of the number of activated nozzles by adapting the blower  6 . 
     In any case, the same quantity of powder is discharged per powdering nozzle. 
     It should also be mentioned that if one or more powdering nozzles are deactivated such that the resistance in the annular distributor  14  is increased, the power of the side channel compressor  7  is reduced such that air as well as powder is not discharged toward the outside via the suction line  4  due to the increased dynamic pressure in the annular distributor and consequently the outlet  12  of the mixing device  1 . The adaptation of the compressor power ensures that a permanent vacuum is present in the suction line  4 .