Electrophotographic Copying Arrangement

Abstract

A first charging device is used for transferring a powder image from a transporting cylinder to a copy sheet by charging the copy sheet which causes an electrostatic force attracting the copy sheet to the transporting cylinder. A second charging device charges the copy sheet with a charge of opposite polarity for substantially neutralizing the first charge so that the copy sheet easily separates from the transporting cylinder.

Description

BACKGROUND OF THE INVENTION

Electrophotographic copiers are known in which a cylinder having a peripheral layer of a photoconductive material is first electrostatically charged and then exposed by a projected image so that a latent image is formed. The latent image is developed by application of a powder which is attracted by the electrostatic force of the latent image. A powder image is thus formed which is transferred to a copy sheet which is electrostatically charged by a corona charging device.

Electrophotographic copiers of this type require the charging of the copy sheet from its rear face substantially at the voltage and with the polarity of the charged latent image of the photoconductive layer. Electrostatic forces are thus created which cause attraction between the photoconductive layer and the copy sheet so that the copy sheet tends to adhere to the photoconductive layer of the cylinder.

In accordance with the prior art, blowers are provided for producing a high-pressure air stream by which the copy sheet is separated from the cylinder surface. It has also been proposed to strip the copy sheet from the cylinder surface by stripping devices or gripping devices. The constructions of the prior art serving this purpose are rather expensive, take up much space, and are not absolutely reliable.

SUMMARY OF THE INVENTION

It is one object of the invention to improve the known electrophotographic copiers by electrically separating the copy sheet or other copy carrier from the surface transporting the powder image.

Another object of the invention is to charge the copy carrier with a charge having opposite polarity than the charge by which the powder image is transferred to the copy carrier so that the charges neutralize each other and no electrostatic attraction occurs between the copy carrier and the surface transporting the powder image.

Another object of the invention is to provide a transfer charging device and a compensating charging device for charging the copy carrier with substantially equal charges of opposite polarity so that the copy carrier is not attracted by an electrostatic force.

With these objects in view, an embodiment of the invention comprises transporting means, such as a cylinder having a movable surface; means for forming on the surface a latent image, for example by projecting an image onto a charged area of the surface; developing means for applying a powder to the latent image to form a powder image; a transfer charging device including a first corona charging device for charging a copy carrier or copy sheet with a first charge having a first polarity so that the powder image is transferred to the charged copy carrier; and a compensating charging device, such as a second corona charging device, for charging the charged copy carrier with a second charge having an opposite polarity.

The second charge is substantially equal to the first charge or slightly smaller, so that the first charge is substantially neutralized. Consequently, no electrostatic attractive force develops between the copy carrier and the surface of the transporting means, and the copy carrier does not adhere to the surface.

The copy carrier is first transported past the transfer charging device and then past the compensating charging device, each of which produces an ion stream. The charges produced on the copy carrier by the ion streams while the copy carrier passes the transfer charging device and the compensating charging device, respectively, are substantially equal to effect neutralizaton of the opposite charges.

The compensation charge must be correctly measured in order to prevent that the compensating charge substantially exceeds the transfer charge and again causes electrostatic attraction of the copy carrier.

The desired compensation of the charges can be obtained in accordance with one embodiment of the invention by providing the transfer charging device and the compensating charging device with throttling means for throttling the stream of ions from the charging devices to the copy carrier. In accordance with the invention, the ion stream of the compensating charging device is throttled to a greater degree than the ion stream of the transfer charging device, and also of the charging device which produces the charged area on which the latent image is formed.

A control grid of known construction may be used for throttling corona charging devices.

The control grids of the corona charging device which produces the charged area on which the latent image is formed, and the control grid of the transfer charging device by which the powder image is transferred to the copy carrier, are advantageously connected with a voltage source providing +600 to +800 v., while the control grid of the compensating corona charging device is connected to ground or to zero voltage.

In a preferred embodiment of the invention, the grounded housing of each corona charging device is provided with a slot facing the layer which is to be charged. The slots through which the ion stream of the compensating corona charging device passes is narrower than the slot through which the ion stream of the transfer corona charging device passes so that the compensating charge does not exceed the charge of opposite polarity required for the transfer of the powder image.

It is also practical, to dispose the compensating charging device at a greater distance from the copy carrier than the transfer charging device so that the same voltage applied to both charging devices, although of opposite polarity, produces a compensating charge which is smaller than the transfer charge.

When the transfer charge of the copy carrier is compensated and neutralized in accordance with the invention, it can be separated from the cylinder by a conventional stripper. However, it has been found that the operational reliability of the arrangement is improved in an unexpected manner if a low-pressure airstream is directed towards the leading edge of the copy carrier or copy sheet which tends to adhere to the surface of the transporting cylinder. A very small pressure of about 2 mm. to 3 mm. hydraulic head is sufficient, so that it is apparent that no mechanical separation is effected by the low-pressure airstream. It is therefore assumed that the surprising effect of the low-pressure airstream is caused by an improvement of the compensation of the opposite charges by the additional movement of the ion stream caused by the low-pressure airstream.

The opposite and equal voltages applied to the transfer charging device and to the compensating charging device are preferably obtained by connecting the two charging devices, respectively, through two rectifiers to a transformer, the rectifiers being connected to block pulses of opposite polarity so that a positive voltage is applied to the transfer charging device and a negative voltage is applied to the compensating charging device. Since the two voltages are equal, the compensating charging device may be spaced a greater distance from the copy carrier than the transfer charging device, and/or the ion stream of the compensating charging device may be throttled to a higher degree than the ion stream of the transfer charging device, which may be obtained by providing the compensating charging device with a narrower slot than the transfer charging device for the passage of the ion streams.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of a specific embodiment when read in connection with the accompanying drawing which is partly a schematic sectional view, and partly a circuit diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A transporting cylinder 1 having a photoconductive layer on its cylindrical peripheral surface 1a, is rotated in the direction of the arrow A so that surface areas successively pass a corona charging device 2 having a slot 2a of predetermined width through which an ion stream is emitted toward the surface 1a of cylinder 1. Cylinder 1, and the housing of the corona charging device 2, are grounded, and the discharge electrode 2b is connected by a resistor 25 to a rectifier 21 which produces a positive voltage, as will be explained hereinafter.

An area of surface 1a is charged by the corona charging device 2 to a voltage of +600 v. to +800 v. and moves into a region of an objective 3 which forms an image of an original or master 4 illuminated by rod-shaped lamps 5 and 6 and being carried by a transparent slide 7. A rope attached to cylinder 1 and slide 7, and passing over guide rollers, causes movement of the slide 7 with the master 4 in the direction of the arrow B when cylinder 1 rotates in the direction of the arrow A. Master 4 and surface 1a move at the same speed in opposite directions so that an image of the master is formed on the surface 1a whereby a latent image of master 4 is formed on the charged area of surface 1a.

During further rotation of cylinder 1, the latent image passes developing means 8 by which an electrostatic powder is applied to the charged surface area and adheres to the latent image so that a powder image is formed. During further rotation of cylinder 1, the charged area with the powder image is brought into contact with a copy carrier or copy sheet fed by a feeding means 9 from the top of a stack of copy sheets 10 in the direction of the arrow C.

While the copy carrier moves together with surface 1a, in contact with the charged area and with the powder image, it passes a corona charging device 11 whose discharge electrode 11b is connected by resistor 26 to rectifier 21 which supplies a positive voltage between 600 v. and 800 v. so that a stream of ions passes through a slot in the housing of corona charging device 11 toward the copy sheet and charges the rear face of the same with +600 v. Due to this charge, the powder of the powder image is attracted and is transferred from surface 1a to the front face of the copy sheet.

The charged area of surface 1a passes a source of light 12 which effects discharge of the positive charge of the charged surface area whereupon surface 1a passes a rotary cleaning device 13 which removes all powder still adhering to surface 1a so that surface 1a can again be charged by corona charging device 2, and another copying operation started.

Due to the electrostatic positive charge applied by corona charging device 11 to the copy sheet, the same is attracted to the surface 1a of transporting cylinder 1 and tends to adhere to the same.

In accordance with the present invention, a further corona charging device 14 is disposed following corona charging device 11 in the direction of movement of surface 1a. The housing of corona charging device 14 is grounded, and its discharge electrode 14b connected by a resistor 27 to a rectifier 22 which produces a negative voltage. The corona charging device 14 has a slot 14a through which an ion stream passes so that the copy sheet receives a negative charge for neutralizing the positive charge produced by the corona charging device 11. The charge produced by corona charging device 14 compensates the charge produced by the corona charging device 11 for the purpose of transferring the powder image, and therefore charging device 11 will be referred to hereinafter as transfer charging device, and charging device 14 will be referred to as compensating charging device.

By selecting the width of slot 14a of the compensating charging device 14 to be smaller than the width of slot 11a of the transfer charging device 11, and also than the width of slot 2a of charging device 2, the ion streams have a corresponding width in the direction of movement of surface 1a and produce corresponding charges so that by selection of the free width of slots 2a and 11a, the desired charge of between 600 and 800 v. is obtained. The ion stream of compensating charging device 14 emitted by discharge electrode 14b is limited by the narrower opening 14a to a greater degree than the ion stream of the preceding charging devices 2 and 11. Furthermore, compensating charging device 14 is disposed at a greater distance from the copy sheet than transfer charging device 11, and also at a greater distance than charging device 2 is spaced from surface 1a.

Due to the limiting of the ion stream of the compensating charging device, the compensating charging device 14 cannot charge the copy sheet to a negative voltage having a similar value as the positive charge produced by the transfer charging device 14. Only compensation and neutralization of the positive charge required for the transfer of the powder image is effected by the compensating charging device 14 due to its narrower slot 14a and the greater spacing from the copy sheet.

The neutralized copy sheet is no longer electrostatically attracted to the cylinder surface 1a, and drops off the same so that it can be supplied to a heating device 15 by which the transferred powder image on the copy sheet is permanently fixed to the same.

The separation of the copy sheet from cylinder surface 1a is further improved by a low-pressure stream of air blown out of a nozzle 16 whose discharge opening 16a extends over the entire axial length of cylinder 1. Nozzle 16 is disposed closely adjacent surface 1a, and the airstream is blown in tangential direction of cylinder surface 1a toward the leading edge of the copy sheet which moves in the direction C. The pressure of the airstream is only 2 mm. to 3 mm. hydraulic head, so that it is not the mechanical power of the airstream by which a copy sheet is separated from the cylinder surface. Rather, the effective separating action of the low-pressure airstream is due to an improvement of the compensation of the transfer charge of the copy sheet by the additional movement of the ion stream of the compensating charging device 14 produced by the low-pressure airstream.

A transformer 17 is connected to a voltage source 18 and 19 of an alternating voltage of 220 v. and 50 Hz. The secondary winding of transformer 17 produces a voltage between 5000 and 6000 v. and has one grounded terminal, and a terminal connected by line 20 to rectifier 21 which blocks the negative pulses of the alternating current and to a rectifier 22 which blocks the positive pulses of the alternating current so that rectifier 21 supplies a positive voltage to charging devices 2 and 11, while rectifier 22 supplies a negative voltage to charging device 14. Equal voltages of opposite polarity are supplied to the transfer charging device 11 and to the compensating charging device 14.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of electrophotographic copying arrangements differing from the types described above.

While the invention has been illustrated and described as embodied in an electrostatic copier having a compensating charging device for neutralizing a copy sheet charged by a transfer charging device for transferring a powder image to the copy sheet, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that from the standpoint of prior art fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

Claims

I claim:

1. Electrophotographic copying arrangement comprising, in combination, transporting means having a movable surface; means for forming on said surface a latent image; developing means for applying a powder to said latent image to form a powder image on said surface; means for feeding a chargeable copy carrier to said surface in the region of said powder image; a transfer charging device emitting a first ion stream toward said copy carrier for charging said copy carrier with a first charge having a first polarity so that said powder image is transferred to said charged copy carrier and an electrostatic force attracts said charged copy carrier to said surface; and a compensating charging device emitting a second ion stream toward said copy carrier for charging said charged copy carrier during passage of said compensating charging device by said copy carrier with a second charge having a second polarity opposite to said first polarity and being substantially equal to said first charge produced by said first ion stream during passage of said transfer charging device by said copy carrier for at least substantially neutralizing said first charge so that said copy carrier does not adhere to said surface of said transporting means.

2. Copying arrangement as claimed in claim 1 wherein said transfer charging device includes first throttling means for throttling said first ion stream; wherein said compensating charging device includes second throttling means for throttling said second ion stream to a greater degree than said first ion stream is throttled by said first throttling means.

3. Copying arrangement as claimed in claim 2 wherein said means for forming a latent image include a third charging device emitting a third ion stream directed at said surface for producing a charged area thereon, and including third throttling means for throttling said third ion stream to a lesser degree than said second throttling means throttles said second ion stream, and means for projecting an image onto said charged area.

4. Copying arrangement as claimed in claim 1 wherein said transfer charging device includes a first throttling slot for throttling said first ion stream; wherein said compensating charging device includes a second throttling slot narrower than said first throttling slot for throttling said second ion stream to a greater degree than said first ion stream is throttled by said first throttling slot.

5. Copying arrangement as claimed in claim 1 wherein said transfer charging device includes first throttling means for throttling said first ion stream; wherein said compensating charging device includes second throttling means for throttling said second ion stream to a greater degree than said first ion stream is throttled by said first throttling means; and comprising means for applying equal voltages of opposite polarity to said transfer charging device and said compensating charging device, respectively.

6. Electrophotographic copying arrangement comprising, in combination, transporting means having a movable surface; means for forming on said surface a latent image; developing means for applying a powder to said latent image to form a powder image on said surface; means for feeding a chargeable copy carrier to said surface in the region of said powder image; a transfer charging device disposed at a first distance from said copy carrier for charging said copy carrier with a first charge having a first polarity so that said powder image is transferred to said charged copy carrier and an electrostatic force attracts said charged copy carrier to said surface; and a compensating charging device disposed at a second distance from said copy carrier for charging said charged copy carrier with a second charge having a second polarity opposite to said first polarity for at least substantially neutralizing said first charge so that said copy carrier does not adhere to said surface of said transporting means. Said second distance being greater than said first distance whereby the same voltage applied to said charging devices produces a lesser second charge.

7. Copying arrangement as claimed in claim 6 comprising a transformer adapted to be connected to a source of alternating voltage; first and second rectifiers connected with said transformer to block negative pulses and positive pulses, respectively, said first and second rectifiers being, respectively, connected with said transfer charging device and with said compensating charging device so that said transfer charging device produces a positive first charge and said compensating charging device produces a negative second charge of said copy carrier.

8. Electrophotographic copying arrangement comprising, in combination, transporting means including a rotary cylinder having a peripheral surface moving in one direction; means for forming on said surface a latent image; developing means for applying a powder to said latent image to form a powder image on said surface; means for feeding chargeable copy carrier to said surface; a transfer charging device for charging said copy carrier with a first charge having a first polarity, said feeding means feeding said copy carrier to said surface in the region of said transfer charging device so that said powder image is transferred to said charged copy carrier and an electrostatic force attracts said charged copy carrier to said surface; a compensating charging device disposed following said transfer charging device in said one direction for charging said charged copy carrier with a second charge having a second polarity opposite to said first polarity for at least substantially neutralizing said first charge so that said copy carrier does not adhere to said surface of said transporting means; and blower means for blowing a low pressure stream of air toward the leading edge of said copy carrier and past said compensating charging device.

9. Electrophotographic copying arrangement comprising, in combination, transporting means having a movable surface; means for forming on said surface a latent image; developing means for applying a powder to said latent image to form a powder image on said surface; means for feeding a chargeable copy carrier to said surface; a transfer charging device for charging said copy carrier with a first charge having a first polarity, said feeding means feeding said copy carrier to said surface in the region of said transfer charging device so that said powder image is transferred to said charged copy carrier and an electrostatic force attracts said charged copy carrier to said surface; and a compensating charging device for charging said charged copy carrier with a second charge having a second polarity opposite to said first polarity for at least substantially neutralizing said first charge so that said copy carrier does not adhere to said surface of said transporting means; and blower means for blowing a low pressure stream of air toward the copy carrier on said surface for separating the same from said surface.

10. Electrophotographic copy arrangement comprising, in combination, transporting means having a movable surface; means for forming on said surface a latent image; developing means for applying a powder to said latent image to form a powder image on said surface; means for feeding a chargeable copy carrier to said surface in the region of said powder image; a transfer charging device for charging said copy carrier with a first charge having a first polarity so that said powder image is transferred to said charged copy carrier and an electrostatic force attracts said charged copy carrier to said surface; a compensating charging device emitting an ion stream toward said copy carrier for charging said charged copy carrier with a second charge having a second polarity opposite to said first polarity for at least substantially neutralizing said first charge so that said copy carrier does not adhere to said surface of said transporting means; and blower means for blowing a low pressure stream of air past said compensating charging device toward said fed copy carrier and into said ion stream so as to displace said ion stream for separating said copy carrier from said surface, the pressure of said stream of air being insufficient to produce mechanical separation of said copy carrier from said surface.