|
(11) | EP 0 056 313 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Improvements in and relating to electrophotographic copying |
| (57) method of producing a right-facing photocopy using a reflectoelectro photographic
process involving a single step. The method involves positively charging a coated
side of a sheet of copy paper (52) which is itself transparent, locating the charged
sheet over a document (50) to one copied with the uncoated side in contact with the
document, illuminating the upper, coated side by a lamp (46) moving on the charged
paper and applying toner (56) thereto and forming and fixing thereon an image of the
document. The apparatus for performing the method is conveniently contained in a handheld
housing containing inter alia a roll (16) of the coated paper which is unrolled through
charging, exposing and development stations as the housing is moved across the document.
A paper for use in the method and apparatus includes an electrically insulating backing
layer. |
Field of invention
[0001] This invention concerns electrophotographic copying and in particular an improved copying medium on which electrophotographic copies can be made, an improved method of producing such copies based on a reflex exposure technique and apparatus for performing the improved method.
Background to the invention
[0002] Electrophotographic copying using a so-called reflex exposure technique is known. In methods involving such techniques, a sheet of paper having one face coated with a photoconductive medium such as zinc oxide is initially charged electrostatically so that the coated surface bears a relatively uniform electric charge and the charged surface is then located in contact with a document to be copied. Light is shone through the charged paper for a controlled period of time and the relatively uniform electric charge on the sheet of copying paper is converted into an electrostatic charge pattern corresponding to the inverse of the printing on the document. This electrostatic pattern can either be transferred to another sheet of similarly coated paper so as to produce a right facing electrostatic pattern for subsequent development using finely divided toner in liquid suspension or as a fine powder. Alternatively the original sheet of paper bearing the inverted electrostatic image can itself be developed using conventional toner in liquid form or as a dry powder and the resulting inverted copy used as a new master in place of the document in a second and similar stage to the overall process. If care is taken the overall result using the two stage reflex process is almost equivalent to that in which the electrostatic charge is transferred and its only disadvantage is one of speed in that it is usually twice as long and in its double consumption of consumables unless the first image is formed on a re-usable plate or drum in place of the first sheet of paper.
[0003] The present invention seeks to provide an improved method of photocopying based on an improved paper by which a right facing copy can be produced using a reflex technique involving only a single stage of charging exposure and development. The resulting method has thus all the advantages of reflex copying coupled with the advantages of speed and minimal consumption of consumables associated with drum and plate exposure techniques hitherto known.
[0004] It is also an object of the present invention to provide an improved paper for use in this method and improved apparatus for performing the method.
Prior art
[0005] U.K. Patent Specification 995491 discloses an electrostatic recording material comprising an insulating dielectric layer, typically of zinc oxide (although other dielectric materials are specified) and as a support therefore, a GLASSINE-type or'modified GLASSINE-type paper.
[0006] Modification of such paper is by the addition of fillers and/or dyes to reduce the transparency of the paper.
[0007] The addition of sodium silicate to such paper is also described, for the purpose of increasing the uniformity of conductivity through the paper and to improve its mechanical strength,surface smoothness and density.
[0008] Methods of photocopying using such papers are also described in which an electrostatic image is formed by first charging the dielectric layer and then exposing it reflectrographically through the GLASSINE support to produce a so-called mirror image copy of an original placed in contact with the dielectric surface.
[0009] The present invention starts from this known state of the art and provides an improved method and apparatus for producing a right facing copy and an improved paper for use in such a method and apparatus.
Definition
[0010] As used herein the expression "copy paper" is intended to mean any sheet or film substrate carrying an appropriate chemical coating of a dielectric photo- conductive material and capable of being employed in electrophotographic processes such as described herein. The expression is not intended to be limiting to materials which are conventionally throught of as papers but is intended to include any plastics or paper or film or fabric or composite thereof. The paper may be in the form of separate sheets or on a roll from which it is drawn off as required.
[0011] A preferred material for the substrate is a paper manufactured by Wiggins Teape Limited and sold under the Trade Name GLASSINE. A preferred form of this material is so-called bleached GLASSINE type N802.
[0012] It is believed that the substrate may have to possess an electrical resistance which bears some relationship to the resistance of the photoconductive film applied thereto and/or to the process of charging which is to be employed in the method to be described. In general it is believed that the resistivity of the substrate material should be high although experiments have led to the applicant to believe that materials having lower resistivities than GLASSINE can still be employed provided certain measures are taken in the method of use to be described.
[0013] Since the photoconductive layer adds significantly to the weight of the resulting paper, the substrate may to advantage be selected so as to have the smallest possible weight per unit area consistent with the strength etc., of the required copy. Satisfactory results have been obtained using 37 gram GLASSINE paper and it is believed that lower weight papers could be used if required.
Summary of the invention
[0014] According to one aspect of the present invention a method of producing a right facing electrophotographic copy comprises the steps of:-
(1) exposing the photoconductive film covered face of a sheet of copy paper, to an electrical field and inducing an electrical charge thereon,
(2) locating the copy paper in contact with a document which is to be copied with the uncoated face of the copy paper in contact with the document,
(3) illuminating the covered surface of the substrate with light by direct illumination, and
(4) separating the copy paper from the document and developing the electrostatic image thereon using a finely divided toner.
[0015] Where the photo-conductive film material is zinc oxide it has been found that the charge induced thereon is preferably of positive polarity.
[0017] It has been found to be advantageous if the copy paper, during charging, is located adjacent a conductive surface which itself is electrically insulated from its surroundings (including earth) and one side of the source of high potential used to create the field for charging the photoconductive layer, is connected to a charging electrode, spaced from the paper, and the other side is connected via a high resistance link to the conductive plate.
[0019] The value of the high resistance link may be established by experiment. Where the substrate is GLASSINE and the photocohductive material is zinc oxide, good results have been obtained using a resistance of 200 Megohms.
[0020] The rear, uncoated, face of the substrate may be electrically insulated from the conductive plate by a thin layer of insulating material. This may for example be in the form of a separate sheet or film between the paper and the plate.
[0021] Where the substrate requires the use of an insulating sheet between it and the conductive plate, during charging, it has been found that after a succession of charges, the insulating sheet becomes charged itself to an extent sufficient to inhibit further useful charging of copy paper placed thereon. To this end the insulating sheet must itself be replaced or discharged if the imaging efficienty of the arrangement is to be maintained.
[0022] In a preferred embodiment the insulating sheet may be an endless belt of plastics material or the like and the belt is discharged by moving it around a closed part adjacent one or more conductive electrodes located around its path.
[0023] The discharging electrodes may be earthed or have applied thereto an electric potential opposite that built up on the belt.
[0024] According to another feature of this aspect of the invention, the photoconductive layer may be charged through an intermediate insulating film or layer which may be separate from the copy paper and interposed between the charging electrode and the photoconductive layer. The insulating film or layer may for example be laid on the photoconductive material or may be applied to the said material by way of an additional film for example by spraying or other technique during the manufacture of the copy paper.
[0026] According to another feature of this aspect of the invention, a sheet of transparent material such as plastics or glass may be located in contact with the charged photoconductive surface during the exposure of the latter to light.
[0027] This last mentioned sheet of transparent material filters out the wavelengths of light normally absorbed by the photoconductive material employed. Alternatively the filtering materials may be such as to form a dichroic filter typically a red or blue or green filter.
[0028] The substrate may be intrinsically an electrical insulator or have applied thereto an electrically insulating layer.
[0029] Attempts have been made hitherto by the applicant and others-to obtain right facing electrophotographic copies using a single stage reflex technique but all attempts so far have been unsuccessful or have produced only very inferior copies. Applicant believes that the failure of prior attempts to produce such copies lies in the fact that all such previous methods have failed to use an appropriate polarity charge on the face of the photoconductive layer on which reflected light impinges and have furthermore required the reflected light from the document to pass through the photoconductive material before the appropriate charge pattern of the right facing image can be formed.
[0030] In the present invention, the light for illuminating the document has to pass through the photoconductive material which is thus preferably at least translucent but thereafter only has to pass through the substrate material first to the document and thereafter from the document before it is available to form the charge pattern in the appropriately polarised surface of the photoconductive material.
[0031] On the other hand'the applicants proposals are somewhat surprising in that it is well known that for example zinc oxide photoconductive layers should be negatively charged in order to produce good copies and applicants method requires the exact opposite namely a charge of positive polarity to be applied to the coated surface of the copy paper. A negative polarity charge has been found to result in copies of totally unacceptable quality.
[0032] There are occasions when a white on black copy is required from a black on white document and vice versa. Such a copy will be referred to as a "reversed copy" and the method of the invention can be adapted to produce a so-called reversed copy if during development a charged and exposed copy paper with the invention is placed in close proximity to a conductive member which is electrically floating relative to earth potential. The member may be in the form of a plate or roller and a preferred material for the same is aluminium.
[0033] According to a second aspect of the present invention apparatus for performing the method of the invention comprises an electrostatic charging station including a corona discharge, a high voltage source for powering same and applying a positive potential to a sheet of copy paper located in the charging station, an exposure station to which the charged copy paper can be moved, which exposure station includes a light source for illuminating the charged coated surface of the copy paper, the apparatus being adapted to expose the other side of the copy paper as it is stretched across the exposure station to allow it to be brought into contact with a document to be copied, means for developing the electrostatic charge pattern remaining on the charged surface of the copy paper after exposure at the exposure station, which development means include finely divided toner means for fixing the developed image onto the copy paper.
[0034] The source of high voltage may be a conventional high voltage generator using the example a transistor oscillator circuit and appropriate transformer or may comprise a piezo electric crystal with means for stressing the crystal and subjecting it to considerable forces to produce (in manner known per se) a high voltage across the faces of the crystal. Where the source produces high voltage pulses first of positive polarity and then of negative polarity, rectifying circuit means or switch means may be provided to ensure that pulses of only one polarity are supplied to the charging electrode.
[0036] In addition or alternatively voltage multiplying circuits may be employed to increase the potential available for charging the paper.
[0037] Where an insulating film has to be provided between the rear of the copy paper whilst it is being charged, and a conductive member such as a conductive plate, the insulating film may be in the form of an endless belt of plastics material or the like with means for removing charge therefrom so as to eliminate charge build up during successive chargings.
[0038] In an alternative arrangement, where a separate insulating sheet is required between the rear of the copy paper and the conductive plate during charging, the insulating sheet material may be in the form of a backing sheet lightly adhereing to the rear of the copy paper and rolled up therewith on a drum or attached to the rear of separate sheets to be fed from a hopper, and after the copy paper has passed through the apparatus, and a copy has been formed thereon, the insulating backing sheet is peeled from the copy and may be discarded. So as not to interefere with the exposing step, the insulating material must be transparent and non-light dispersing.
[0039] According to a third aspect of the present invention an improved paper for use in a method according to the invention comprises:-
(1) a substrate of light transmitting material in the form of a film or sheet.
(2) a film or coating of photoconductive material applied directly to one face of the substrate.
(3) a layer of electrically insulating material applied directly to the other face of the substrate.
[0040] By using a water repellant, non-water absorbing material for the insulating material mentioned above, so materials can be used for the substrate which tends to absorb water vapour and then have a tendancy varying electrical characteristics depending on the humidity at the time.
[0041] The invention will now be described by way of example with reference to the accompanying drawings in which:-
Fig.l is a cross section to an enlarged scale of a copy paper constructed in accordance with the invention,
Fig.2 is a cross section also to an enlarged scale through another copy paper also constructed in accordance with the invention,
Fig.3 is a flow diagram showing how a photocopy is produced using the reflex technique of the invention and,
Fig.4 shows part of a modification of the system shown in Fig.3 in which an insulating layer of material is placed between the copy paper and the conductive plate during charging.
Detailed description of illustrated embodiments
[0042] In Fig.l the cross section shows a copy paper having a substrate of GLASSINE type N802 paper as manufactured by Wiggins Teape Limited designated by reference numeral 10 having applied to one face thereof a thin layer of zinc oxide 12. As distinct from conventional zinc oxide coated papers, the substrate 10 is not rendered conductive nor is a conductive resin or film located between the substrate 10 and the zinc oxide layer 12.
[0043] Fig.2 shows an alternative arrangement in which a similar zinc oxide layer 12 is formed on a substrate 10 which again may be GLASSINE type N802 paper as previously described. On the underside of the latter, is a transparent plastics film 14 which can be peeled off if required. The paper together with the film is passed through the charging stage and exposure stages of apparatus to be described and after passing through a toner bar for example the plastics film can be pulled away from the rear of the copy paper substrate 10. This can be to advantage since toner and dispersant can cling to the rear of the copy paper after passing through the toner bath and if the rear of this paper is in fact in the form of a detachable film then any toner etc., clinging thereto can be discarded together with the film leaving the back of the copy clean.
[0044] Fig.3 of the drawings is a flow diagram illustrating one embodiment of apparatus for performing the method of the invention. The apparatus comprises a roll of coated copy paper designated by reference numeral 16 with feed rollers 18,20 and 22. Beyond the rollers 22 is located a guillotine 24 or other knife device for cutting the paper which is passed through the line.
[0045] The paper is coated with zinc oxide and as previously described with reference to Fig.l the zinc oxide is supported on a substrate of GLASSINE paper of the type described. The coated paper is rolled with the zinc oxide surface outer most so that when unrolled as in Fig.3., the upper surface of the paper is the coated surface.
[0046] The first operation is to charge the coated surface of the paper and this is achieved using a corona charging electrode typically in the form of stretched wire 26 which is electrically connected via the conductor 28 to the positive output. of high tension generator 30 to be described. The other side of the generator is connected through a high resistance 32 and conductor 34 to a conductive plate typically a metal plate designated 34 and located on the underside of the paper where the latter is stretched under the charging electrode 26. The conductive plate 34 is supported on two mounts 36 and 38 from a perspex bed 40. The latter serves as a mounting and also to electrically insulate the plate 34 from earth and can of course be any suitable material.
[0047] The high tension generator 30 may be an alternating current supply in which event a diode 42 is connected between the two output terminals to inhibit negative going pulses so as to maintain a positive only potential on electrode 26.
[0048] The value of the resistance 32 is adjustable as designated and is selected as appropriate to the material of the substrate etc.. Where 37 gramm, GLASSINE paper is used, the value of the resistor 32 is typically 200 Megohms.
[0049] The electrode 26 may include an earthed reflector plate typically parabolic in shape although this is not shown in the drawing.
[0050] Beyond the charging stage in the direction of the arrow 44 is located the exposure region. Here the charged paper passes below a lamp 46 in a reflecting housing 48 and below the stretched paper is located the document 50 which is to be copied.
[0051] The document 50 and the stretched copy paper designated by the reference numeral 52 are either maintained stationary or moved in synchronism with no relative movement therebetween at least over the illuminated section of the copy paper path. An electrostatic image of the printing if any on the document is formed in known manner by reflection and absorbtion of light and the relatively uniform charged pattern obtained from the charging electrode 26 is replaced by a charge pattern corresponding to the printing or other markings on the document 50. The charge pattern is such that charge remains in those regions of the coated surface which correspond to dark lines or dots or areas in the original document while as the charge is to a large extent dissipated in all other regions of the photoconductive layer.
[0052] The electrostatic image can be replaced by a visible image by a development stage in which the paper bearing the electrostatic charge pattern is passed through a bath 54 containing a liquid dispersant 56 containing in suspension finely divided toner powder.. Typically the latter is black so that black dots, lines and areas appear on the coated surface corresponding to the areas remaining charged after the exposure stage.
[0053] After the complete document has been exposed and the copy paper has been pulled through the toner bath until the whole of the document is now visible beyond the nip rollers 22, the guillotine or knife means 24 is operated so as to remove the exposed and developed length of paper bearing the final copy.
[0054] There will always be a lead of some few inches of paper which has not been exposed and developed and this can be trimmed from the cut length of paper to leave the final photocopy of the actual document.
[0055] Where a film of insulating material has to be located between the rear of the paper and the conductive plate 34, this may be wound on the reel 16 with the paper in accordance with the embodiment of Fig.2 and the insulating film can be stripped from the back of the copy paper after it has been cut by the knife or guillotine 24.
[0056] Fig.4 shows an alternative charging region for the system shown in Fig.3. Here the copy paper 52 does not have an insulated backing layer but nevertheless requires one between it and the conductive plate 34. To this end an endless belt of appropriate insulating plastics material 58 is stretched around two rollers 60 and 62 at least one of which is driven so as to rotate the endless belt 58. Typically the belt is rotated in the same direction and at the same speed as the paper is to move through the system.
[0057] By arranging a discharge electrode or a number of such electrodes as at 64, so any charge induced in the endless belt 58 is removed as the belt passes the discharge electrodes or electrode 64.
[0058] The high tension source 30 of Fig.3 may to advantage be a manually or battery operated device so as to allow the unit to be portable. Where a manual device is provided, this may either be in the form of a highly geared generator and appropriate high tension transformer circuit or more preferably a squeeze unit incorporating a piezo electric crystal which when subjected to a squeezing force develops a high tension voltage across two faces of the crystal, As is well known, such cyrstals develope an equal and opposite voltage when the strain is released and to this end a rectifying diode such as 42 is provided to remove unwanted polarity spikes from the output. Although not shown a full wave rectifying bridge circuit may be employed in the alternative so as to increase the available direct current energy.
1. A method of producing a right facing electrophotographic copy comprises the steps
of:-
(a) exposing the photoconductive film covered face of an area of copy paper (as herein defined), to an electrical field and inducing,an electrical charge therein,
(b) locating the copy paper incontact with a document which is to be copied with the uncoated face of the copy paper in contact with the document,
(c) illuminating the covered surface of the substrate with light by direct illumination, and
(d) separating the copy paper from the document and developing the electrostatic image thereon using a finely divided toner.
2. A method as claimed in claim I in which the photoconductive film material is or
includes zinc oxide and the electrical charge induced thereon is positive.
3. A method as claimed in claim 1 or 2 in which the copy paper is, during- charging,
located adjacent.a conductive surface which itself is electrically insulated from
its surroundings (including earth) and one side of the source of high potential used
to create the field for charging the photoconductive layer, is connected to a charging
electrode spaced from the paper and the other side is connected via a high resistance
link to the conductive plate.
4. A method as claimed in claim 3 wherein an insulating sheet is located between the
copy paper substrate and the said conductive surface.
5. A method as claimed in any of claims 1 to 4 in which an insulating film is located
between the photo- conductive layer of the copy paper and the charging electrode,
at least during charging.
6. A method as claimed in any of claims 1 to 5 in which a sheet of transparent material
is located to contact with the charged photoconductive surface during the exposure
of the latter to light.
7. A method as claimed in claim 6 in which the sheet of transparent material filters
out the wavelengths of light normally absorbed by the photoconductive material.
8. A method as claimed in claim 6 in which the sheet of transparent material comprises
a dichroic filter.
9. Apparatus for performing the method of claim 1 comprising:-
(a) an electrostatic charging station including a corona discharge.
(b) a high voltage source for powering same and applying a charge to an area of copy paper, located in the charging station,
(c) an exposure station including a light source for illuminating the charged area of the copy paper and adapted to present the other side of the area for contact with a document to be copied, and
(d) means for developing the electrostatic charge pattern remaining on the said area after exposure.
10. Apparatus as claimed in claim 9 further comprising means for fixing the developed
image onto the copy paper.
11. Apparatus as claimed in claim 9 or 10 to which the voltage source produces a positive
polarity charge on the paper.
12. Apparatus as claimed in any of claims 9 and 11 which further comprises a conductive
plate adjacent the rear of the paper at the charging station.
13. Apparatus as claimed in claim 12 wherein an endless belt of insulating sheet material
is provided with means for moving same around its closed path so that it passes between
the rear of the copy paper and the conductive plate, and means is provided for removing
charge from the belt to prevent charge build-up during successive chargings.
14. Copy paper for use in the method of claim I comprising:-
(a) a substrate of light transmitting material in the form of a film or sheet.
(b) a film or coating of photoconductive material applied directly to one face of the substrate, and
(c) a layer of electrically insulating material applied directly to the other face of the substrate.
15. Copy paper as claimed in claim 14 in which the film or coating of photoconductive
material is or includes zinc oxide.