GB2080735A - Electrostatographic process and apparatus - Google Patents

Abstract

A photoconductive sheet supported on a drum 20 of electrophotographic apparatus is developed with toner at a developing station 25 and then advanced to a first fixing station 27 where the toner image is fused with the sheet still in position on drum 20 by means of radiant heat from a lamp 60 followed by pressure from a roller 28. When used for electrophotographic duplicating the drum 20 rotates through a first relatively low speed cycle where the photoconductive sheet is charged by corona station 21, imaged at 32, developed by a first developing station 25 and fixed by station 27. In subsequent high speed duplicating cycles of the drum 20 the sheet charged at station 21 is flood exposed at 22 and developed at a second station 24. The developed sheet is rotated to a transfer roller 26 where the toner image is transferred to copy sheet material and is fixed in a copy fixing unit 31. The combination of heat and pressure in the fixing of the master image enables the fixed toner to retain the necessary insulation properties, and there is no warm-up period before fixing can be begun. <IMAGE>

Description

SPECIFICATION Electrostatographic process and apparatus The present invention relates to a method of forming a developed image of an original document by electrostatography and to an apparatus which enables the aforesaid method to be carried out. The invention is particularly, but not exclusively, applicable to electrostatographic duplication or printing.

United States Patent No. 2576047 (Schaffert) which dates from early in the development of electrostatography describes a method of electrostatic printing in which a drum bearing an electrically charged image layer formed on an electrically insulating or non-conducting material supported by an electrically grounded conductive surface is developed by means of a toner powder, after which the toner image is transferred by competitive electrostatic attraction to copy sheet material which travels in a path adjacent to a portion of the periphery of the drum. Although Schaffert envisaged making his printing plate by electrostatography, the printing plate had to be imaged and developed separately in advance of use.

U.K. Patent Specification No. 1210666 (Addressograph-Multigraph) describes a development of this basic technique to provide a method of electrostatic duplicating (i.e., the production of multiple copies from a single original without repeating the full sequence of steps required to produce an original image). One method which is described in this reference involves the mounting of photoconductive sheet onto a rotatable drum, charging the drum, exposing it and developing it by means of a particulate toner having a high resistivity thermoplastic resin binder, and then fusing the toner image by means of a radiant heater. The imaged photoconductive sheet can then be used in subsequent cycles of the drum to produce multiple copies by repetitive electrostatic charging, flood exposure to discharge the non-imaged areas of the sheet and development.

However, the use of a radiant heater alone involves a large amount of thermal energy being supplied to a restricted area of the sheet and it is difficult to obtain a fixed toner image having the required charge acceptance and dark decay properties. Our experimental results show that the use of a radiant heater alone gives a fixed toner image having poor charge acceptance and dark decay properties probably as a result of thermal degradation of the resin.

Although it is possible to fuse large black regions, it is difficult to achieve acceptably accurate and reproducible fusing of line images and printed character images to give fixed images in which the fused toner has the correct electrical properties. A further disadvantage of the arrangement shown in Patent Specification No. 1210666 is that no means is provided for cleaning the master during the duplicating cycles, and accordingly a progressive build up of toner material on the master sheet will lead to its progressive deterioration. Afurther U.K. Patent Specification No. 1272087 also by Addressograph Multigraph describes a development of the machine shown in Specification No. 1210666 and it is significant that the master preparation unit is separate from the rotatable drum which is employed to produce the copies.Another recent proposal for electrostatographic duplicating is shown in West German Offenlegunsschrift 2724105 but again the toner is fused onto the master image by means of radiant heat alone. Furthermore, the developed image passes through a brush cleaner before it reaches the fusing unit and it is believed that serious deterioration of the toner image would thereby be caused.

Despite the many proposals which have been made over a 20 year period, no fully satisfactory electrostatographic duplicator has hitherto been devised.

In an attempt to remedy the difficulties asociated with the use of radiant heat we have carried out a number of experiments in which the toner was fused onto the image bearing master by means of a heated fusing roller urged into line contact with the imagebearing master sheet when in position on the drum of the machine. The combination of heat and pressure applied to the toner by the roller would bring about, so it was hoped, satisfactory toner fusing without use of excessive pressure or use of temperatures so high as to cause deterioration of the charge acceptance properties of the fused toner image. However, it was found that the temperatures to which the surface of the roller had to be raised were so great that the elastomeric coating of the roller was effectively destroyed even though the coating was formed of a heat-resistant material.

Furthermore, despite the use of adhesive treatments the temperature was such that the toner transferred to the fixing roller, from which it could not be removed conveniently.

Surprisingly, we have found that if radiant heat is directed at the imaged developed master sheet before it enters the nip between an unheated fixing roller and the drum or belt, the toner can be fixed efficiently onto the image bearing master sheet. The resin binder of the toner does not need to be heated to a temperature at which thermal degradation destroys the necessary insulation properties thereof and the pressures applied by the fixing roller remain low enough not to place unacceptable loads on the drum. Furthermore, there is no need for any warmup period to raise the temperature of the fixing roller surface.

In one aspect the invention provides a method of forming a developed image of an original document by electrostatography which comprises: supplying a photoconductive sheet to a drum or belt which rotates to cause the sheet to travel successively past a plurality of stations arranged about the periphery of the drum or belt; imaging the sheet by moving it past a charging station arranged to apply a first electrostatic charge to the sheet, and an exposure station at which an electrostatic latent image corresponding to the original document is formed on the sheet, developing the latent image on the sheet by applying particles of a fixable toner thereto; and fixing the developed toner image with the sheet in position on the drum or belt by supplying radiant heat thereto by means of a heater and applying pressure to the toner image by means of a fixing roller urged into line contact with the sheet supported on the drum.

Advantageously an insulating toner is used and the toner is fused onto the sheet to give a fixed image having a charge acceptance potential of at least 180 volts with a dark decay of less than 25% after 0.5 seconds. The method maythen comprise the further steps of withdrawing the fixing roller from drum or belt contact, switching off the heater and making duplicated copies during succeeding rotations of the drum or belt by moving the photoconductive sheet past the charging station at which a second electrostatic charge is applied to the sheet, a flood exposure station at which the non-imaged areas are discharged to leave a duplicating electrostatic charge on the fixed toner image, a copy developing station (which may be the same as or separate from the master developing station) arranged to form a transferable toner image on the charged toner image, a transfer station in which copy sheet material is conveyed synchronously past the drum or belt to make line contact therewith and the toner image is transferred to the copy sheet, and a cleaning station at which residual toner is removed from the photoconductive sheet prior to its return to the charging station whereby progressive image deterioration is minimised; and during duplicating rotations of the drum or belt passing the copy sheet from the transfer station to a fixing station at which the transferred toner image is fixed thereto.

The photoconductive sheet applied to the drum or belt may comprise a photoconductive zinc oxide layer on a paper support. The charging station may apply a negative electrostatic charge to the photocondutive sheet during the master imaging cycle and may apply positive electrostatic charge onto the photoconductive sheet during subsequent duplicating cycles. Alternatively the charging station may apply a negative electrostatic charge to the photoconductive sheet during both the master imaging cycle and subsequent duplicating cycles.

The fixable toner applied to the photoconductive sheet preferably has a d.c. bulk resistivity greater than 1010 ohm/cm. The imaged and developed photoconductive sheet may be fixed by passage through the nip between the drum and a fixing roller exerting a pressure on the photoconductive sheet of 1.1 to 3.3 pounds per linear inch, preferably 1.5 to 2.6 pounds per linear inch, and preferably the fixing roller has a deformable surface of elastomeric material including a surface layer having non-stick properties (i.e., made of low surface energy material). Thus it may include a silicone rubber sheath surface coated with an organo-fluorine polymer, and it can be coated with a silicone oil to prevent toner on the photoconductive sheet offsetting onto the fixing roller.

Preferably there is supplied to the drum or belt a photoconductive sheet dimensioned so that a dead space is defined between the leading and trailing edges thereof, and the fixing roller is urged from its retracted position into line contact with the sheet on the drum or belt for fixing of the developed image in timed relationship to the rotation of the drum or belt such that initial contact therebetween occurs in said dead space.

In a further aspect the invention provides an apparatus for forming a developed and fixedelec- trostatographic image of an original document comprising a drum or belt arranged to receive and discharge a photoconductive sheet and to rotate the photoconductive sheet successively past a plurality; of stations disposed about its periphery, said stations including: a corona charging station arranged to apply an electrostatic charge to the photoconduothre sheet, an imaging station at whish a latont electrostatic image corresponding to an original do cument to be copied is formed on the photoconductive sheet synchronously with rotation of the drum or belt, a first developing station arranged to apply particles of a fusable insulating toner to the electrostatic image formed on the photoconductive sheet; and a first fixing station arranged to fix eExe developed toner image onto the photoconductive sheetwith said sheet in position on the drum or belt and including a fixing roller which is urged into li-ne contact with the photoconductive sheet on, the drum or belt to apply pressure to a developed toner image and further including a heater positioned adjacent the fixing roller and arrangedto direct radiant heat onto the photoconductive sheet before ft enters the nip between the fixing roller and the drum or belt When the aforesaid apparatus is used for electrostatographic duplication, the imaging station and first fixing station are arranged to operate during a first cycle of the drum or belt to form a developed and fixed toner image on the photoconductive sheet which is retained on the drum or belt and said stations are disabled when said first cycle is complete with the fixing roller retracted from contact with the photoconductive sheet, and said apparatus additionally comprises also disposed about the periphery of the drum or belt; a flood exposure station arranged for illuminating the surface of the photoconductive sheet, a second developing station (which may be the same as or separate from the first developing, statiqnl arranged to apply fusible toner particles to an electrostatically charged and fixed image on an otherwise discharged photoconductive sheet to form a trarsferrable toner image;; a transfer station at which copy sheet material iss urged into line contact with sheets of photoconductive material on the drum or belt during passage of' the copy sheet material synchronously with rotation of the drum or belt from a copy sheet,; supply through a nip between the drum or belt and a roller of other transfer means electrically biased with a polarity similar to that applied by the corona charging means to effect transfer of the toner therefrom; and a second fixing station arranged to receive copy sheet material conveyed from thetransfer stafion and to apply heat and or pressure thereto to fix transferred toner images to the ctgry sheet, the corona charging fiP'dX & ,j,5:t2'.*d" dv,,,'''.;p'- ment tra nsfer qEd sexed .,? arranged to operate during second and subsequent rotations of the drum or belt to form electrostatographically duplicated images on the copy sheet material.The apparatus then preferably includes a cleaning station disposed after the first fixing station with respect to the normal direction of rotation of the drum or belt and arranged to remove residual toner from the photoconductive sheet during duplicating cycles in advance of return of the photoconductive sheet to the corona charging station whereby progressive image deterioration is minimised without adversely affecting the fixed image on the photoconductive sheet. The drum or belt may be driven by drive means arranged to rotate said drum or belt at a first speed during a first cycle in which the photoconductive sheet is imaged and to rotate said drum or belt at a second speed faster than said first speed during succeeding duplicating cycles.

Preferably the first fixing station includes an electrical heating lamp and reflector arranged to direct a beam of radiant heat onto the photoconductive sheet just before it enters the nip between the fixing roller and the drum or belt. It also preferably includes drive means independent of the belt or drum and arranged to drive the fixing roller at a surface speed slightly less than the surface speed of the belt or drum during the master imaging cycle, and a uni-directional clutch arranged to release when the fixing roller is in contact with the surface of the belt or drum.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of an electrostatographic duplicating machine in vertical section along the line of travel of the copy sheet material; Figures 2 and 3 are diagrammatic views of the drum and master fixing station of the machine of Figure 1 in the operating and stand-by positions respectively; Figures 4(a) to 4(j) show successive stages in the master imaging cycle and in the duplicating cycles carried out by the machine shown in Figure 1; and Figure 5 is a block diagram of the master imaging and duplicating process.

An electrostatographic duplicating machine comprises a light-tight casing having at one end a paper tray 11 which supports a stack of copy paper sheets and at its other end has a copy receiving tray 14 in which copy sheets bearing duplicated images are received. Mounted to the casing above the copy paper tray 11 is a master feed chute 12 down which photoconductive zinc oxide coated paper master sheets are fed. The top of the machine is provided with a transparent document table 13 which is slidably supported for longitudinal movement in either direction on guide rails at the top longitudinal edges of the casing.

A rotatary drum 20 is mounted within the casing with its axis perpendicular to the direction of adv ance of the copy paper from the paper tray 11 to the receiving tray 14. it may be rotated by means of an electric motor (not shown) via a gear train (not shown). Advantageously the drum drive is arranged to operate at a first relatively slow speed during the master imaging cycle and at a second substantially higher speed during copy imaging cycles. Master sheets for use in duplicating a particular document may be fed from the master feed chute 12 by rotation of a master feed roller 45 until they are taken up on the periphery of the drum 20 by clamping means (not shown).After use of a master the clamping means releases it, and an automatic ejection system (not shown) of conventional type removes the spent master from the drum 20 and transports it out of the machine casing. Rotation of the drum 20 causes a master to travel past a plurality of processing stations arranged in planetary manner about the periphery of the drum.

A corona charging station 21 includes corona charging wires which during a master imaging cycle are arranged to supply a negative electrostatic charge onto the photoconductive master sheet and during subsequent duplicating cycles supplies a positive electrostatic charge thereto.

The master sheet then passes to a master imaging station generally designated by the reference number 32 at which a light image of a document to be copied is projected onto it. In the arrangement shown, a document to be copied is supported on the top face of the transparent document table 13 which is driven by drive means (not shown) longitudinally of the machine synchronously with rotation of the drum 20. A strip lamp 44 and associated reflecting surfaces 44a and 44b illuminate the docucment which is conveyed by the document table over a slit in the light box of the optical system generally designated by reference numeral 23. The optical system includes an objective mirror 41 which receives light from successive strips of the document conveyed past it on the table 13.The light path passes to an intermediate mirror lens 42 and thence back to an intermediate mirror43a and a final mirror 43b which projects the focused inverted strip image onto the master on the drum. It will be appreciated that other optical systems can be used, for example, an optic strip such as a Selfoc self-focussing lens array or a strip lens array. Following the master imaging station in the normal direction of rotation of drum 20 is a flood exposure station 22 which is a strip lamp directed transversely of the drum 20 and is illuminated during duplicating cycles of the machine to discharge the non-imaged areas of the master sheet.

The master sheet then passes through a copy developer station 24 in which a single component magnetic toner is applied to the surface of the developed and fixed image bearing master sheets in duplicating cycles by means of magnetic brush development. The toner used may be of medium resistivity and may have a d.c. bulk resistivity of I 107 ohm cm or above. In master imaging cycles the copy development station is disabled. The master sheet then passes to a master development station 25 in which a two-component magnetic developer including particles of a toner composition having a d.c.

bulk resistivity greater than 1012 ohm cm can be applied to a latent electrostatic image on the master sheet by means of magnetic brush development.

The master sheet than passes to a transfer station where it encounters copy sheets 50 which are advanced from the paper tray 11 by copy sheet feed rollers 46 and appropriate sheet guides and forwarding rollers. During a duplicating cycle, a transfer roller 26 is urged into line contact with the surface of the drum 20 as shown in solid lines in Figure 1 and the copy sheets 50 pass through the nip between the roller 26 and the drum 20 at a speed of advance which is synchronised with the speed of rotation of the drum. During duplicating cyclesthetransfer roller 26 is biased with a polarity and electrical potential such as to effect transfer of the transferable toner image developed at the copy developing station 24 from the master sheet to the copy sheet 50.During the master imaging cycle the transfer roller 26 is retracted away from contact with the drum as shown in the chain dotted lines. Sheets leaving the transfer station come from the drum 20 to a copy fixing unit 31 where the toner image is fixed to the copy sheets by passage through the nip of pressure rollers.

Following the transfer station in the normal direction of rotation of drum 20 is a master fixing station 27 (shown in more detail in Figures 2 and 3) which includes a fixing roller 28 which is urged into line contact with the drum 20) for master fusing but may be retracted therefrom as shown by the arrow. The roller 28 applies pressure to a toner image on the mster sheet developed by the master developing station 25. The fixing station further includes an infra-red lamp 60 and reflector 61 arranged to direct infra-red radiation onto the developed photoconductive sheet just before it reaches the entry side of the nip between the roller 28 and the drum 20.Underthe combined action of the infra-red radiation and the pressure exerted by the roller 28 the toner image is fused to the master sheet, the conditions of fixing being such that the fused toner resin possesses the appropriate charge acceptance and dark decay properties. The roller 28 may typically be of two inches (5 cms) outside diameter with a stainless steel core surrounded by a silicone rubber sheath the sheath being overcoated by a PFA (perfluoroalkoxy polymer), FEP (fluorinated ethylenepropylene copolymer) or other "Teflon" overcoatings depending upon the particular material selected. The roller 28 may be supplied with a thin coating of silicone oil from an applicator roller 50 which prevents the toner on the master sheet offsetting onto the fixing roller. The infra-red heater 60 is a non-segmented tungsten halogen filament lamp of power 2000 watts.The pressure necessary to fuse a heat-fusible toner to the master sheet is of the order of about 1.1 to 3.3 Ibs.

per linear inch preferably 1.5 - 2.6 Ibs. per linear inch.

The master fusing station 27 is also provided with independent drive means for effecting rotation of the roller 28. The roller is driven via a drive 54 at a surface speed slightly less than the normal surface speed of the drum 20 during the master imaging cycle so that when the roller 28 is moved into engagement with the drum 20 a uni-directional clutch disengages and the roller follows the drum.

The master sheet fitted to the drum 20 has a dead-space between its leading and trailing ends and the rotation of the drum 20 and movement of the roller 28 are inter-related so that the roller 28 first contacts the drum 20 within the dead space. Move- ment of the roller 28 between inoperative and drum contact positions may be made in response to ah electrical signal derived from an encoder disc (not shown) on the pivot shaft ofthe-drum 20. The lamp 60 and reflector 61 need not be retract ble with the movement of the roller 28 but are controlled by suitable switch means so that haMp 60 Is switched on only when required, since response is substantially immediate and no warm-up time Is required.

Following the master fixing station 27-in the normal direction of rotation of the drum 20 is a cleaning station 29 which is oparstive during dupli- cating cycles of the drum. The-statIon 22 includes a rotatory brush 30 which makes line cOntact with the master sheet and is effective to remove therefrom particles of toner from the 'copy development station 24 which have failed to transfer to the copy sheets 50 at the transfer station. The brush 30 may bee @ fibre brush or it may be a magnetic roller brush.The removal ofthesetoner particles Is- important to prevent build-up of toner on the master sheet during successive duplicating cycles whtch'woutd have the effect of reducing the quality of thecopy. It is possible that the toner from the master fixing station 25 may still be soft by the time it reaches the cleaning station 29, and accordingly during the master imaging cycle of the drum 20 the station 29 may be made retractable out of-drum contact. Frorri the station 29 the master sheet has completed a full cycle and returns to the corona charging station 21.

The sequence of operations performed by the machine is shown in Figure 4(a) to 4(j). A master sheet 51 comprises a photoconduc2ive insulating zinc oxide layer 52 coated on a paper or other support 53. In initial passage of the sheet 51 through the corona charging station 21 a negative charge is applied to the surface of the photoconductive layer.

Exposure at the master imaging station 32 discharges the non-imaged areas of the master sheet leaving a latent electrostatic image as shown in Figure 4(b). Rotation of the drum 20 then brings the imaged master sheet to the maker development station 25 where, as shown in Figure 4(c), it is developed with a fusible insulating toner.The developed master sheet than passes to the' fixing station 27 where the master fixing roller 28 and lamp 60 operate to fuse the toner onto the master sheet while leaving it with the charge ralaining properties previously described (Figure 4(d)). The master fixing roller 28 is then withdrawn from line contact with the drum 20 and the lamp 60 Is switched 'off after which the machine is ready for duplicating cycles. During the master imaging cycle the drsm 20 travels relatively slowly, typically at about 12 r.p.m.

When the master fixing station has been retracted the machine is ready for subseqi ntduplicating cycles in which the drum speed can be substantially higher, typically from 30 to 100 r.p.m. The master sheet is recharged positively or negatively at the corona charging station 21 tFl - reA(e)-'showing a positive charge) after whii -Ms flciaposeci by illumination of the strip lamp at the @@ood exposure station 22 to leeave positive charges @@@@@@ on the regions of fused toner, the positive charges in the non-imaged areas having leaked away.Rotation of the drum 20 then carries the master sheet to the copy development station 24 where a monocomponent magnetic toner is applied by magnetic brush development to give a transferable toner image (Figure 4(9)). The master sheet then travels round to the transfer station at which a bias is applied to the roller 26 to transfer over the toner image to the copy sheets 50 as described above (Figure 4(h)). The master sheet then passes on to the cleaning station 29 where residual particles of toner are removed by the brush 30 without adversely affecting the properties ofthetoner image fixed on the master and then returns to the corona charging station 21 for a further cycle. The copy sheets from the transfer station are fixed by means of the copy fixing unit 31.The copy fixing unit 31 may also operate by combined heat and pressure and may also include an infra-red lamp as previously described with reference to the master fixing station 27.

The sequence of operations is illustrated as a block diagram of Figure 5 which is believed to be self expianatory.

Various modifications may, if desired, be made to the process and apparatus described above without departing from the invention. For example, before the master is fixed, there may be one or more proofing cycles so that the exposure and/or the amount of toner applied by the master development unit 25 may be modified to get the best possible imaged master. In this case the master sheet is charged exposed, developed and a proof copy made before the image is fused onto the master. When adjustments give a satisfactory proof, the complete master and fixing cycle is performed followed by duplicating as hereinbefore described. Although it is believed to be desirable to charge the master sheet positively during the duplicating cycle, negative charging may also be used. In the duplicating cycles, either a positive or negatively charged image can be developed by toners which are conducting. The charging characteristics of high resistivity toners may be selected for development of either positively or negatively charged images. Instead of using a transfer roller, other known transfer means such as conventional corona means could be used to transfez the developed toner image from the master sheets to the copy sheet material.

Claims (25)

1. A method of forming a developed image of an original document by electrostatography which comprises: supplying a photoconductive sheet to a drum or belt which rotates to cause the sheet to travel successively past a plurality of stations arranged about the periphery of the drum or belt; imaging the sheet by moving it past a charging station arranged to apply a first electrostatic charge to the sheet, and an exposure station at which an electrostatic latent image corresponding to the original document is formed on the sheet, developing the latent image on the sheet by applying particles of a fixable toner thereto; and fixing the developed toner image with the sheet in position on the drum or belt by supplying radiant heat thereto by means of a heater and applying pressure to the toner image by means of a fixing roller urged into line contact with the sheet supported on the drum.

2. A method according to Claim 1, wherein the toner is fused onto the sheet to give a fixed image having a charge acceptance potential of at least 180 volts with a dark decay of less than 25% after 0.5 seconds.

3. A method according to Claim 2, further comprising the steps of withdrawing the fixing roller from drum or belt contact, switching off the heater and making duplicated copies during succeeding rotations of the drum or belt by moving the photoconductive sheet past the charging station at which a second electrostatic charge is applied to the sheet, a flood exposure station at which the non-imaged areas are discharged to leave a duplicating electrostatic charge on the fixed toner image, a copy developing station (which may be the same as or separate from the master developing station) arranged to form a transferable toner image on the charged toner image, a transfer station in w-hich copy sheet material is conveyed synchronously past the drum or belt to make line contact therewith and the toner image is transferred to the copy sheet, and a cleaning station at which residual toner is removed from the photoconductive sheet prior to its return to the charging station whereby progressive image deterioration is minimised; and during duplicating rotations of the drum or belt, passing the copy sheet from the transfer station to a fixing station at which the transferred toner image is fixed thereto.

4. A method according to Claim 3, wherein the photoconductive sheet supplied to the drum or belt comprises a photoconductive zinc oxide layer on a paper support.

5. A method according to Claim 4, wherein the charging station may apply a negative electrostatic charge to the photoconductive sheet during the master imaging cycle and may apply a positive electrostatic charge onto the photoconductive sheet during subsequent duplicating cycles.

6. A method according to Claim 4, wherein the charging station may apply a negative electrostatic charge to the photoconductive sheet during both the master imaging cycle and subsequent duplicating cycles.

7. A method according to any preceding claim, wherein the fixable toner applied to the photoconductive sheet has a d.c. bulk resistivity greater than 10l2 ohm/cm.

8. A method according to any preceding claim, wherein the imaged and developed photoconductive sheet is fixed by pasage past a source of radiant heat followed by passage through the nip between the drum and a fixing roller exerting a pressure on the photoconductive sheet of 1.1 to 3.3 pounds per linear inch.

9. A method according to Claim 8, wherein the fixing roller exerts a pressure of 1.5 to 2.6 pounds per linear inch on the photoconductive sheet.

10. A method according to any preceding claim, wherein the developed and imaged photoconductive sheet is fixed by means of a fixing roller having a deformable surface of elastomeric material including a surface layer having non-stick properties.

11. A method according to Claim 10, wherein the fixing roller includes a silicone rubber sheath, surface coated with an organo-fluorine polymer.

12. A method according to any preceding claim, wherein the surface of the fixing roller is coated with silicone oil to prevent toner on the photoconductive sheet offsetting onto the fixing roller.

13. A method according to any preceding claim which comprises supplying to the drum or belt a photoconductive sheet dimensioned so that a dead space is defined between the leading and trailing edges thereof, and the fixing roller is urged from its retracted position into line contact with the sheet on the drum or belt for fixing of the developed image in timed relationship to the rotation of the drum or belt such that initial contact therebetween occurs in said dead space.

14. A method of duplicating an original document by electrostatography substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15. Apparatus for forming a developed and fixed electrostatographic image of an original document comprising a drum or belt arranged to receive and discharge a photoconductive sheet and to rotate the photoconductive sheet successively past a plurality of stations disposed about its periphery, said stations including:: a corona charging station arranged to apply an electrostatic charge to the photoconductive sheet, an imaging station at which a latent electrostatic image corresponding to an original document to be copied is formed on the photoconductive sheet synchronously with rotation of the drum or belt, a first developing station arranged to apply particles of a fusable insulating toner to the electrostatic image formed on the photoconductive sheet, and a first fixing station arranged to fix the developed toner image onto the photoconductive sheet with said sheet in position on the drum or drum including a fixing roller which is urged into line contact with the photoconductive sheet on the drum or belt to apply pressure to a developed toner image and further including a heater positioned adjacent the fixing roller and arranged to direct radiant heat onto the photoconductive sheet before it enters the nip between the fixing roller and the drum or belt.

16. Apparatus according to Claim 15, wherein the imaging station and first fixing station are arranged to operate during a first cycle of the drum or belt to form a developed and fixed toner image on the photoconductive sheet which is retained on the drum or belt and said stations are disabled when said first cycle is complete with the fixng roller retracted from contact with the photoconductive sheet, and said appartus additionally comprises also disposed about the pheriphery of the drum or belt;; a flood exposure station arranged for illuminating the surface of the photoconductive sheet, a second developing station (which may be the same as or separate from the first developing station) arranged to apply fusible toner particles to an electrostatically charged and fixed image o-n an otherwise discharged photoconductive sheet to form a transfer able toner image, a transfer station at which ropy sheet material is urged into line contact with sheets of photoconductive material on the drum or belt during passage of the copy sheet material synchronously with rotates1 of the drum or belt from a copy sheet supply throuWh a nip between the drum or belt and a roller of other.

transfer means electrically biased with a polarity.

similar to that applied by the corona charging means to effect transfer of the toner therefrom; and a second fixing station arranged to receive copy sheet material conveyed from the transfer station and to apply heat and or pressure thereto to fix transferred toner images to the copy sheet, the corona charging flood exposure second development, transfer and second fixing stations being arranged to operate during second and subsequent rotations of the drum or belt to form electrostatographically duplicated images onthe copy sheet material.

17. Apparatus according toClaim 16, wherein the first developing station contains a toner which on application to the phctoconductive sheet and fusing by the first fixing station gives a fused image having a charge acceptance potential of at least 180 volts and a dark decay of less than 2 after 0.5 seconds.

18. Apparatus according to Claim 17,further comprising a cleaning station disposed after the first fixing station with respect to the-normal direction of rotation of the drum or belt and arranged to remove residual toner from the photocondqctive sheet during duplicating cycles in advance of return ofthe photoconductive sheet to the corona charging station whereby progressive image deterioration is minimised without adversely affecting the fixed image on the photoconductive sheet.

19. Apparatus according to any of Claims 1818, wherein the drum or belt is driven by drive means arranged to rotate said drum or belt at a first speed during a first cycle in which the photoconductive sheet is imaged and to rotate said drum or belt at a second speed faster than said first speed during succeeding duplicating cycles.

20. Apparatus according to any of Claims 15 to' 19, wherein the first fixing station includes a fixing ' roller having a deformable surface of elastomeric material including a surface layer having non-stick properties.

21. Apparatus according to Claim 20, wherein the fixing roller includes a silicone rubber sheath overcoated with an organo-fluorine polymer.

22. Apparatus as claimed in any of Claims 15 to 21, wherein the first fixing station includes an applicator for silicone oil or the like arranged to form a thin coating of the oil on the fixiiWr411er to prevent toner offset onto the fixing ro rs

23. Apparahls-aa Apparatus as clailT < s 151 s 22, wherein thefiret: fxirg ' an' electrical heating lamp and reflector arranged to direct a beam of radiant heat onto the photoconductive sheet just before it enters the nip between the fixing roller and the drum or belt.

24. Apparatus as claimed in any of Claims 15 to 23, wherein the first fixing station includes drive means independent of the belt or drum and arranged to drive the fixing roller at a surface speed slightly less than the surface speed of the belt or drum during the master imaging cycle, and a unidirectional clutch arranged to release when the fixing roller is in contact with the surface of the belt or drum.

25. An electrophotographic duplicator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.