DEVICE FOR ELECTROSTATICALLY CHARGING SHEET MATERIAL

Abstract: A charging device for charging paper and other flexible material comprising a cylindrical cavity (16) in a housing (10) having an inlet and an outlet (typically a single slit (12)) into which and from which the flexible medium (14) can pass. Centrally along the axis of the cavity is a charging conductor (28) and the conductive interior of the cavity (20) and the conductor (28) are connected to opposite sides of a source of potential (38). A perforated cylindrical guide (30) may be located within the cavity so as to define a narrow annular gap around which the flexible medium can pass. Three rollers (32, 34, 36) forming entrance and exit nips for the flexible medium (14) may be located at the entrance/exit slit.

Description: Title : Device for ' electrostatically charging sheet material . - -_._ repπ.?τpτi0N Field of invention This invention concerns a device for electrostatic charging of sheet material eg paper typically for use in photocopiers and the like. Background to the invention Hany photocopying processes involve the electrostatic charging of a sheet or roll of paper and the subsequent exposure of the charged surface to a light image of the document which is to be copied. The application of the electrostatic charge to the paper should be as -uni orm s possible to avoid di erences in sensitivity over the surface of the paper.. Hitherto a charging device has typically comprised one or more lengths of wire which are stretched betveen insulating supports and are charged to a high potential from a high tension source and adjacent to which the paper is moved. Although a non-uniform pattern of charge would result from a static exposure of a sheet of paper to the charging device a relatively uniform charge can be produced over the surface of the paper by moving the paper relative to the charging device. In addition a sufficient level of charge is obtained by using a charging device at a very much higher potential than in theory is necessary so as to ensure that the whole surface of the paper receives a charge which exceeds a minimum potential. Known devices therefore require very high potential sources and appropriate traction means for moving the paper uniformly past the _ charging device and are not part ctilarly.-suited to photocopying systems in which the power supply requirements are limited and/or in which the speed of movement of the paper past the charging device is not neces¬ sarily well controlled or uniform and in which the source of high tension does not necessarily produce a continuous charge but produces a series of pulses as in the case of a power supply incorporating a pie∑rO-electric crystal which is squeezed so as to produce a high voltage across the crystal which can then be dissipated through the ' charging device onto the paper. Objects of the present invention With this in Blind the present invention has as its object the production of a charging device which can be used to charge a paper more -uniformly and more readily than hitherto particularly when using a non-uniform, traction of paper past the charging device and/or a lowe power and/or pulsed operation charge source. The invention Accordin to the present invention a device for charging paper an like flexible material comprises a housing containing a cylindrical cavity at least the curved surface of which is conductive t an inlet an an outlet to allow the flexible meditun to be introduced into and extend around the' internal curved surface of the cylindrical cavity and to leave via the outlet, and an elongate conductor extending coax * .ally through the cavity and electrically insulated from said conductive surface and a source of electric potential with one terminal connected to the conductive surface and the other to the elongate conductor. The housing oay be of insulating material and the conductive sur- face may be a film or coating or sheet of conductive material such as metal. Alternatively the housing may be of conductive material and may O I be mounted in an insulating manner with the elongate conductor ~ - j^ ε*tι«tche-i -be--wβ« * ιt-33isu-_-at -rø-_B&ing-. a£..*αppo6iie- Wids of the.housing.. The inlet and outlet may be separate slits in the housing or may be combined into a single slit into which the flexible medium is fed and from which the flexible medium leaves the housing. The ends of the cavity may be adapted to -receive and grip the incoming paper and say be rotatable relative to the remainder of the housing so as to provide a traction drive for the flexible material around the interior of the cavity. A cylindrical guide may be located within the housing, the diameter of-the ~ guide being-just less than the-internal diameter of the housing, so as to leave a narrow gap around which the flexible medium will pass. The guide is preferably perforated or otherwise apertured to permit the passage of electric charge from the conductor to the medium. It has been found that the .presence of the perforated guide enhances the charge delivered to the paper. Preferably the perforated guide revolves within the housing. The flexible medium say be a length of paper pulled from a roll of paper or individual sheets of paper or any other flexible medium. Whatever the material it may be untreated or chemically treated as with a photo-sensitive material such as zinc oxide to form a photo- receptive surface. The invention allows more even distribution of charge to be ob¬ tained over the flexible material than has hitherto been the case. Particularly this is the case when the source of charge is the type which produces a burst of potential which then decays over a period of time until the next burst is produced as for example an H.T. source - -* - incorporating a piezo-electric crystal which is squeezed periodically to obtain high voltage bursts between its faces. The invention will now be described by way of example with refer ence to the accompanying drawings. In the drawings Fig. 1 is an end view of a charging unit embodying the invention, Fig. 2 is a cross-section tbrougi the unit of Fig. 1 on the line AA. Fig. 3 is a end view in cross-section of another embodiment, an Fig. is an end view also in cross-section of a further embodi- aent of the invention. Detailed description of drawings As shown in the drawings, a charging unit embodying the invention comprises a generally cylindrical housing 10 having a single aperture 12 through which a sheet of paper shown in dotted outline at _k can be « fed and out of which the other end of the paper can exit after having traversed the interior of the cylindrical housing which forms a cylin¬ drical cavity generally designated 16. The inside surface of the cavity 16 is coated with a conductive material as denoted by reference numeral 20 in Fig. 2 and for con¬ venience the conductive coating 20 is connected via a conductor 22 to earth (or to one termi al of a high tension source). In the embodiment shown in the drawings the housing is formed fro insulating material and two circular end caps 2k and 26 are fitted int t-fae opposite ends of the generally cylindrical tube 10 to form the cavity and each of the end caps ≥ and 26 is apertured to receive a wire electrode 28 which is stretched therebetween. A stop 0 at one O * $_ end of the wire prevents the wire from pulling through the end cap 2k and a tensioning device 32 is located at the other end of the wire 28 so that the latter can be stretched between the two end caps 2 and 26 and the tension adjusted to suit. A conductor (which may be an extension of the wire 28) con¬ nects the latter via a rectifying device 36 such as a diode to one terminal of the output of a high tension source 38 the other terminal of which is connected via a conductor kQ to earth. Although not shown in detail the source of potential 3 comprises a piezo-electric crystal with means for squeezing sane to produce a high voltage potential across its faces,which in turn is conveyed to the two terminals of the source. As is well known when the squeezing action is released, a poten¬ tial of equal and opposite polarity is produced across the faces of the crystal and it is for this reason that the diode rectifying device 3 is incorporated so as to prevent charge of one polarity from reach¬ ing the electrode wire 28. The conductive interior of the cavity formed by the lining 20 may be formed by spray painting with a metallic suspension paint onto the interior surface of the electrical insulating tubular member 10 or by securing a sheet of electrically conductive foil such as aluminium foil onto the interior surface of the cavity by means of an adhesive or the like. The aperture 12 allows the paper or other flexible medium 1 * to. be fed into and around the interior of the cavity and to exit there¬ from. Alternatively two apertures may be provided suitably angled so as to introduce the paper substantially tangentially to the surface of OMPI _ the interior of the cavity and to allow same to leave and the one aperture serves as an inlet and the other as an outlet. The circular extent of the aperture into and from which the flexible material enters and leaves the cavity is as small as possibl compatible with the requirement that the flexible material will leave the cavity as shown in Fig. 1. By ensuring that the circular extent of the aperture is as small as possible so the maximum area of the . paper is presented to the electrode wire 28 and by moving the paper around the interior of the cavity so different points on the circum- ference of the wire electrode 28 are presented to the paper so that on the assumption that the charge distribution around the wire is not necessarily uniform, nevertheless the net effect of presenting the paper to the wire electrode in this way is to cause an integration of the charge over the area of the paper presented thereto over the time period during which it is presented thereto which tends to produce a uniform charge over the area of the flexible material. Where the flexible material is a coated paper such as zinc oxide coated paper, it is inserted into the aperture 12 so that the coated surface faces the wire electrode 28. It has been found by experiment that the voltage required to pro duce an acceptable level of charge on zinc oxide coated paper to perm photocopying thereon is considerably reduced if a corona charging uni of the type described and shown in Figs. 1 and 2 hereof is employed i place of a conventional charging unit. It is important that the flexible material adheres to the inside surface of the cavity and therefore the feed into the cavity should preferably be at a* slightly greater rate than the withdrawal traction from the cavity so that the paper is at all times ressed into the -_ -_ a__ __5_i- -5 _w_-B£__r,*-w-__*th_i-n-the -c v ^ ^ -_-Alte« tiv^y r ~^lthou h--no -^ho n, the two end caps 2k and 26 may be adapted to grip the edges of the flexible material within the cavity and drive means may be provided for rotating the end caps so as to draw the flexible medium into and around the interior of the cavity. Where this latter feature is incorporated, independent driving wheels (not shown) are preferably provided on the same axis as the wire electrode 28 so that it is not necessary for the wire to rotate " but the latter can remain stationary relative to the cavity.. Fig. 3-shows how- a-perforate drum 30 may- be situated within the housing 10 to provide, an annular guide for a sheet of paper 1*. The drum 30 is mounted for rotation within the housing 10. ϊig. k illustrates a preferred drive for feeding paper into and extracting paper from a charging unit embodying the invention. To this end three driven rollers 32, 3k and 36 are provided at the entrance/ exit and are- rotated in the directions shown by the arrows. Raper 1^ entering the nip between rollers k, 3 will be caused to pass around the interior of the housing 10 and will be pulled out through the nip between rollers 32, k. The feed shown in Fig. k may -be used in conjunction with a charg¬ ing device such as shown in Figs. 1 and 2 or with a device such as shown in Fig. 3. It is to -be understood that the circular extent of the wall of the housing 10 need not be of the order of 300° (i.e. almost full circle) as shown in. the. drawings, but may be. of much smaller circular extent i.e. so as to subtend for example 180 or thereabouts.

Claims: - - PATENT CLAIMS 1. A device for charging flexible sheet material such as paper comprising a housing containing a cavity at least the internal-surfac of which is conductive, an inlet and outlet to allow the flexible material to be introduced into and extend around the internal surface of the cavity, an elongate conductor extending through the cavity and electrically insulated from the conductive surface and a source of electric potential with one terminal connected to the con¬ ductive surface and the other to the elongate conductor. 2. A device as claimed in claim 1 in which the housing is of insulating material and the conductive surface is a film or coating or sheet of conductive .material. 3. A device as claimed in claim 1 in which the housing is of conductive material and the elongate conductor is stretched between insulating mountings at opposite ends of the housing. k. A device as claimed in any of claims 1 to 3 in which the inl and outlet comprise separate slits in the housing. 5. A device as claimed in any of claims 1 to 3 in which the inl and outlet are combined into a single slit into which the flexible material is fed and from which the flexible material leaves the housi 6. A device as. claimed in any of the preceding claims in which » the ends of the cavity are adapted to receive and grip the incoming paper and are rotatable relative to the remainder of the housing so a to provide a traction drive for the flexible material around the in¬ terior of the cavity. 7. A device as claimed in any of the preceding claims wherein t - - cavity is cylindrical and further comprising a cylindrical guide - , -—loeated-within.-*he-*--cavity, --the-*diaa-ete -of-the-guide--being-- ust less than the internal diameter of the cavity so as to leave a narrow gap around which the flexible material will pass. 8. A device as claimed in claim 7 ia which the guide is aper- tured to permit the -passage of electric charge from the conductor to the medium. 9. A device as claimed in claim 7 or claim 8 in which the guide is rotatable within the housing. 10. A device as claimed in any of the preceding claims wherein the source of electric potential comprises __ • piezo-electric crystal and means for stressing-or relieving the stress within the crystal so as to produce a high voltage thereacross. 11. A device as claimed in claim 10 further comprising rectif - ing means to ensure that charge of one polarity only is applied to the charging conductor. - 12. A device for- charging paper and like flexible material as claimed in any of the preceding claims further comprising three driven rollers adjacent the inlet and outlet, means for driving the rollers so as to form two nips one which will cause the flexible material to be drawn into the inlet and the other which will cause the flexible material leaving the outlet to be drawn away therefrom, one of the three said rollers being common to the two said nips. 13*. A device for charging paper and like flexible material con- εtructed arranged and adapted to operate substantially as herein de¬ scribed with reference to and as illustrated in the-accompanying drawings.

Inventors: PAYNE J
Applicants: PAYNE J
Application year: 1979
Main classification: G03G15
IPC codes: G03G15, H01T19