CA1156879A

CA1156879A - Method and apparatus for developing latent electrostatic images

Info

Publication number: CA1156879A
Application number: CA000346257A
Authority: CA
Inventors: Ronald Swidler; Kenneth W. Gardiner
Original assignee: Savin Corp
Current assignee: Savin Corp
Priority date: 1979-02-23
Filing date: 1980-02-22
Publication date: 1983-11-15
Also published as: FR2449908B1; GB2041790B; FR2449908A1; JPS55115066A; CH641576A5; GB2041790A; DE3006781A1

Abstract

Abstract of the Disclosure Our invention comprises an applicator roller positioned closely adjacent but not touching a photoconductive surface on which there is a latent electrostatic image to be developed. The surface of the latent image and the surface of the applicator move at synchronous speed and are spaced from each other. The applicator roller is adapted to be wetted with an electrophoretic type of developer liquid. The volume of developer liquid on the roller surface is controlled to within narrow limits. The developer liquid carries charged toner particles enabling the developer liquid to jump the gap between the applicator roller and the latent electrostatic image on the photoconductive surface. Our method and apparatus is such that substantially only the latent electrostatic image is moistened by developer liquid. The control of the volume of developer liquid on the applicator roller may be accom-plished by providing the surface of the applicator roller with re-entrant portions of predetermined capacity. This volume may also be controlled by using a smooth-surfaced applicator roller together with a closely-adjacent but spaced reverse doctor roller adapted to meter the thickness of the film on the smooth applicator roller. Post-transfer electro-phoresis is aided either by a field electrode biased to a potential of the same polarity as the polarity of the latent electrostatic field or by a corona discharge device biased to a potential opposite to that of the latent electrostatic field of the developed image. Owing to the fact that the developer liquid has a lower vapor pressure than the liquid component of liquid developers of the prior art and a higher ratio of toner particles to carrier liquid than the developing liquids of the prior art, substantially no evaporation of the carrier liquid occurs with our method and apparatus and our copies, when transferred to a carrier sheet, are substantially dry.

Description

3'7~

Title of the Invention FOR DEVELOPING LATENT ELECTROS~ATIC IMAGE5 :
ac~round of the Invention S There are a number of methods in the art o~
developing latent elcctrostatic image~. These latent images may be formed by charging a photoconductive surface an~ then exposing the charged surface to light or ionizing radiations adapted to render the phctoconductor conductive in the area~
exposed to liqht or radiant energy and thus neutralize the areas subjected to light. The latent electrostatic image may be formed on a dielectric sheet by energizing ~tyli to form the desired elec~rostatic pattern on the dielectric sheet. The development of the latent electrostatic image was first accomplished by the electrical attraction of fina p~rticles to the latent image areas on the photocGnductive surface -- preferably, particles carrying electri~al charge3 oppoctite in polarity to the la~ent image charge~. These particles were usually dry, from which ~e process derived the name "xerograph~f". Later~ it was discovered that particles could be suspended in a dielectric li.quid and th~
latent electrostatic image could be developed by immer~ing it in the liquid. When dry powdcr was used as the developi~g medium, it was usually transferred to plain paper or othe~

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carrier sheet and then ixed by fu~ing~ Thi~ required heat, and the fine particles were formed of heat-fuqibl~ reain.
If a carrier ~heet, such a~ paper~ was coated with ~
photoconductive material, such as zinc oxide, no tran~er of the electrostatic image after development wa3 required and the development was made with a developing liquid carrying toner particles.

As the art developed, it waq convenient, in order to aliminate the necessity of fusing a powder-developed image, to develop the latent electrostatic image with a liquid developer and then transfer the developed image to ,9 plain paper. The developer liquid usually compri~ed a hydrocarbon carrier, such as Isopar-5~ or the like~
which were dispersed toner particles adapted to render the latent image visible. The immersing of the photoconductiv~
surface carrying the latent electrostatic image in a bath of liquid developer required wetting the entire ~urfac~ of the photoconductor. The amount of liquid developer remaining on the developed image could he largely removed by a doctor roller. }~owever, there was always a ~mall amount of developer liquid remaining on the entire photoconductive sur f ace. This remaining liquid, along with the developed ima~e, would be tranjferred to paper, or other carrier, and had to be dried on the paper. This resulted in a continual evaporatlon of the small amount of carrier liquid trallsferred rom the .

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L5~
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photoconductlve surface to the carrier paper. This evapora~
tion wa~ undesirable from several standpoints: Fir3t, th~
~ece~sity of drying the sheet not only ~equired some e~ergy, but the time involved Was such that the speed o ~opy~ng waq limited by this requirement. Furthermore, the ratio o~
carrier liquid to toner would constantly vary. If a battery of electrophotographic machines were used in ~ small ch~mber, the evaporation of the carrier liquid into that chamber would be objectionable.

The art recogni~ed that it would be desirable if a latent electrostatic image could be developed by applying developing liquid only to the image area, and not to the photoconductive surface on which the latent electrostatic image was formed. If this were possible, since a typical copy i3 only about five percent to seven percent image area~
thc potential evaporation would be correspondingly reduced.
I f a roller, such as a printing roller, were coated with developer and this roller were allowed to touch the photo conductor bearing the latent electrostatic image, both the photoconductor and the latent image would be covered w$th the developer liquid. ~his would leave developer liquid on the non-image arqa~, a~ well as the image areas, and reduce the contrast between the image areas and non-image or bac~ground area~ on the conductive surface, with a resul~ant ; ~5 unsati~factory copy. It was then conceived that~ if thc ~7~ .

roller were formed with re-entrant pDrtions and a mo~
concentrated developing liquid were used -- that is, th~
ratio of the toner particles to the carrier liquid ~ere increased, so that there would be more solids in the de-veloping liquid -- a gr~vure roller of this type would tone the latent electrostatic image without leaving developing liquid on the non-image areasO A wiper blade would remove the developer liquid from the land portions, while the re-entrant portions would carry the developer liquid. It was ~ound that wiping the land areas clean was more di~ficult than believed. The wiping blade did not completely wipe the land areas free of liquid, so that there would be 3tringing, and a sharp image did not resu:Lt. Furthermore, this qtringing increased as the speed of c:opying was in-lS creased, thus limiting the number of copies which could bemade in a given time.

The art also attempted to overcome the difficulties experienced in employing an applicator roller in contact with the drum by 3eparating the applicator roller from the photo-~0 conductive surface and permitting the electro~tatic field othe latent electrostatic image to pull the developing liquid from the applicator roller onto the photoconductivo drum.
Thesc attempts were unsuccessful, for that the toned imag~

was not sharp and free of imperfection3 owing to the un predictahility of what the developer liquid did when lt arrived on the photocond~ctor.

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~ t Field of the Invention Our inventio~ relates to a no~el method of developing latent electro~tatic images ~y a developing liquid, comprising a carrier liquid having toner particles diYper~ed therethro~gh, and avoiding the draw'oacks and disadvantages of the prior art which prevented the succes3ful ~mployment of developer~liquid applicator rollers.

~'''' .
Description of the Prior Art Gundlach 3,084,043 uses a belt having raised portions formi~g peaks and valleys. A layer of onductive ink is applied to the belt, which i~ then brought into ;~
contact with the photoconductor bearinq the latent electro-static image.

Willmott 3,232,190 discloses passing a film bearing dry toner particles sufficiently close to a latent electro~tati~ image that the dry particles will jump the gap and tone the image. This requires using or ~ixing of the toner particles after transfer to the carrier sheet.

Ca3siers et al 3,3~3,209 discloses the use o~
an applicator roller formed with grooves to apply conductivo ink to'a latent electrostatic imag~. The applicator roller is in contact with the photoconductor and the toning t~Xes place by selective wcttln~O

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Ca~siers et al 3,486,922 disclose~ the develop-ment of a lat~nt electrostatic image with an aqueous con-ductive developing liquid. The applicator roller is i~
contact with the surface bearing the electrostatic ima~o.
The surfaca is hydrophobic, so that only the image will b~
toned, and not the backgrou~d area~ of the water ~epellant surface.

Damm 3,560,204 di3closes the use of a water-soluble acid or basic dyestuff as a self-fixing ink. ~he surface hearing the latent electrostatic image pa3ses close to but not touching a rotating drum having a layer of the ink4 A field exists between the applicator drum and an electrode behind the image, so that an unsupport~d column of ink having the shape of the image jumps the gap and form~
a~ inked image. Damm also disclose3 that nonpolar liquids, ; such as toluene or nitrile silicone, can be used and point~
out that they do not jump the gap as rapidly as water-ba~ed ~olutiona.
;
Smith 3,667,428 discloses the use of an applicator roller having a re~ularly patterned surface adapted to transfer developer material to a latent electro3tatic image.
The applicator surface has a doctor blade for removing exce~s developer. The developer is described as comprising polar and nonpolar liquid~ and dry-powder electroscopic materials.
25 rh2 applicator roller iB in contact with the photoconductlv~
surface.

; -6 Whittaker 3,772,012 discloses a method of revers~
development. This is accompli3hed by biasing the developer applicator to substantially the same potential a~ the maximu~
potential on the imaging surface. Development i~ obt2ined by placing the applicator surface sufficiently close to the electro3tatic imaging surface, such that the polar liquid developer is pulled from the recessed portions of the applicator surface to the imaging surface in reversal image configuration. ..

Smith et al 3,~39,032 discloses the use of applicator rolls in contact with a photoconductive drum bearing a latent electrostatic image for app].yins a liquid developer to the latent electrostatic image to develop it.

Deshayes et al 3,886,900 sho~s the ~se of an applicator roller to form a dynamic pool, or meniscus, to apply a liquid developer to a latent electrostat~c image.

Fantuzzo 3,974,554 discloses the use o~ a roll fo.r applying fluid to develop latent electrostatic ima~es, formed with a pattern of volute and convex groovas and lands, which roll is in contact with the latent electrostatic image~ `

Nakano et al Patent 3,991,711 disclo~es a roller for applying electroconductive ink to a latent elcctrostatl~
image, the roller being formed of an electroconductive ~56~

elastomer and positioned in contact with the photoconductiv~
surface.

Matkan Patent 4,021~586 discloses 1n applicator roller adapted to reciprocate into and out of a liquid S developer ~ath, A coating electrode in the bath i8 adap~ed to electrodeposit toner particles from the bath onto the roller while in the bath, The roller is then lifted out of the developer and into contact with the photoconductive surface bearing the latent electrostatic image which i2 mounted on a rotating drum, A corona is adapted to depre~s carrier liquid contained in the toner depoqit from the applicator roller before it contacts the photoconductive surface O

Summar~ of the Invention In general~ our invention contemplate~ a method and apparatus of employing liquid toners on plain paper copying machines, in which the development of the latent electrostatic image on the photoconductive surface i~ so controlled as to tone only the charged 1mage areas o~ the photoconducti~e surface, we employ a developer liquid with a higher concentration of toner than ~ feasible in the developing liquids used in copying machine~ of thq prior art in a composition in which the carrier liquid and th~
toner concentrate, or solids, is such that th~ layer o~

developing liquid on the toned image i~ apprDximately ~rom three to seven microns thick in the damp state. The developor liquid has a carrier9 such as a liquid hydrocarbon, which has a higher vapor pressure at ambie~t temperature than the carrier liquid hydrocarbons customarily employed in liquid-develope~ copying machines to reduce evaporation of the carrier component of the liquid developer.

Our invention comprises the provision of ~n applicator roller positioned closely adjacent but not touch-ing a photoconductive surface. The roller i8 d~iven in~uch direction and at such speed that, at the close3t approach of the two surfaces, the relative motion between them i~ substantially zero. The gap 'oetween the applicator roller and the photoconductive surface may vary between one and one-half mils and four mils. The applicator roller is adapted to be wetted with developer liquid to produce a film of developer liquid on its surface. The a~ount of liquid on the applicator roller must be uniform and con trolled within narrow limits as a function o~ the gap between the applicator roller and the photoconductive surface and the conductivity of the liquid developer. I
the film on the applicator roller is too thick, the copi~s will be quite wet, and if the film is not thicX enough, the image will not be properly toned. If the liquid d~-veloper does not have su~ficient conductivity, it will not ., 9 jump the qap sufficiently; if it i5 too conducti~e, lt will partially discharge the latent electrostatic image and impair satisfactory electrophoretic development and result ~n poos copy quality. The prior art used conductive liquids to ton~
by jumping the gap. Wc rely on electrophoresis, which takes place on the pho~oconductor during and after tran~fer of the developer liquid from the applicator roller to the surface of the photoconductor. Our improvement is such that columns of developer, corresponding precisely to the form of the latent electrostatic image, do not occur. Small amounts of developer liquid, only approximately corresponding to the fields of the latent electrostatic image, jump the gap and electro-phoresis continues aEter the developer liquid has left the gap and arrived onto the photoconductive surface. This post-transfer electrophoresis enables us to obtain sharply defined images while subjecting the latent electrostatic image to small amounts of developer liquid for short period3 of time. rrhis enables us to achieve rapid operation of apparatus capable o~ carrying out our invention and thus produce a larger number of copies within a given period of time than is possible with machines of the prior art. Further-more, the copies produced will ~e sharp and substantially dry, owing to the fact that the entire surface of the photo-conductor has not been wetted with the developer liquid.
The amount of liquid on thc applicator roller can be con trollcd by providing the applicator xoller with re-entrant portion~ of a predctermincd capacity. Tlle applicator roller 6~
can be wiped with a doctor blade to level the amount of liquid in the re entrant portions of the applicator roller.
However, we have found it advantageous to use a smooth applicator roller in combination wi~h a doctor roller which rotates in the same direction as the direction of the rotation of the applicator roller. By varying the speed of the reverse doctor roller, we can control the volume of liquid on the applicator roller within narrow limits. Light liquid hydrocarbons are normally dielectric. A polar compound or charge director must be added to the developer liquid for imparting a charge to the toner particles, or the manufacture of the toner particles per se must be such that they have the proper polarity and further such as to render the developer li~uid conductive to the desired extent, in order that the developer liquid will jump the gap. This conductivity must not be so great that the developer liquid will partially discharge the latent electrostatic image after transfer.
The post-transfer electrophoresis is aided by a development electrode adapted to enhance the fields of the latent electrostatic image. The image can be further sharpened before it is transferred to a carrier sheet, such as plain paper, by flooding the developed image with light, as shown in applicant's U.S. Patent NoO 4,233,381, issued November 11, 19800 Not only is offset to the carrier sheet aided by light flooding but the image density on the carrier sheet is increased. Alternatively, the post-transfer electrophoresis may be augmented by a corona ~ 1 ms/~

o~

discharge over the toned image of a polarity oppo~ite to tha polarity of the latent electrostatic image.

One object of our invention i5 to provide a novel method for using liquid developers in plain paper copying machines, and the like, in which the development of the image on the photoconductive surface is so controlled a~
to tone substantially only the charged i~age areas of the latent electrostatic image, and to provide novel apparatu~
for carrying out our method.

Another object of our invention is to provide such apparatus having an applicator roller which serve~ as a development electrode.

Still another object of our invention is to provide such applicator roller having means for controlling the volume of liquid on the same.

A further object of our invention is to provide a mathod of developing latent electro~tatic image~ on a photoconductive surface, such that electrophoresi~ taXe~
place, not only during transfer of developer liquid rom the applicator roller to the photoconductive 3urface,but also after transer has been accomplished.

An additional object of our invention is to provide a method of employing liquid developers to davelop latent electrostatic image~ ln whlch the ratio of 901~d3 to a carrier liquid in the liquid developer i9 higher than had hereto fore been employedO

Another object of our invention is to provlde a method and apparatu3 employing liquid developers in which a developing liquid having a carrier-liquid component of low-vapor pressure may be employed.

Still another object of our invention i~ to provide a method and apparatus for dynamically controlling the volume of liquid on an applicator roller of an electr~-photographic copying assembly employing liquid developer~.

A further object of our invention is to provide a toning system for an electrophotographic proces~ and apparatus in which an applicator roller for applying developer liquid i~ spaced from and does not touch the photoconductive ~urface bearing the latent electro4tatic image.

An additional object of our invention is to provide an electrophotographic method and apparatus employ-ing a field electrode for enhancing thc field of the latent electro3tatic image to a~sist in post-transfer elect~o-phoresi~O

A further object of our invention is to employa corona discharge device to augment development of the latent electrostatic image on the photoconduct~r.

Other and further object~ of our ~nvention wSll appear from the following d~Qeription.

Brief Description of the Drawin~3 S In the accompanying dra~ings, which form part of the instant specifLcation and which are to be read in conjunction therewith, and in which like xeference numeral~
are used to indicate like parts in the variou~ views:

FIGUR~ a diagrammatic view o a photocopy-ing mæchine adapted to carry out tho method of our invention, ~hown with parts in 3ection.

FIGURE 2 is a side elevation viewed along thn line 2-2 of FIGURE 1, ~howing the applicator rollnr and the reverse doctor roller.

FIGU~E 3 is a side elevation, similar to FIGU~$

2, showing the applicator roller dr.ive~ from tho photo~
conductive drum with the doctor roller and wip~ng blad~
a~sembly removedO

FIGURE 4 is a diagrammatic perspecti~e ~ie~, drawn on an enlarged scale showing a portion of the surface of an applicator roller provided with re-entrant portions formed by knurling.

FIGURE 5 is a frasmentary sectional view, drawn o~ an enlarged scale, ta~en along the line 5-5 of FIGURE 1, sho~ing hemispherical re-entrant portions on an applicator roller.

FIGURE 5a is a sectional view similar to FIGUR~
5, showing circumferential groove~ forming re-entrant portions on the applicator roller.

FIGURE 6 is a top plan view of FIGURE 5.

PIGURE 6a is a top plan view oE FIGURE 5a.

FIGURE 7 i~ a top plan view, drawn on an rnlarged scale, showing grooves running axially of the applicator roller to form the re-ei-trant portionsO

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FIGURE 8 is a diagrammatic view of a photocopying machine similar to that shown in FIGURE 1 with parts show~ ln section adapted to carry out a modified method o augmenting electrophoresis in the development of the latent el~otrostatic image.

Description of the Preferred Em~odiment ~ ore particularly, referring now to the drawing~, a conductive dru~ 2, provided with a layer 4 of photo-conductive material, is mounted by apertured discs 6 upon a Rhaft 8 and keyed thereto by key 10 for rotation therewith in a counterclockwise direction indicated by the arrow (A). Tho photoconductive drum and surrounding assemb].y are mounted in a lightproof housing (not shown), as is known in the ~rt, Thc shaft 8 may be grounded or biased to any desired direct current voltage from a source (not shown), as is known in the art, A corona discharge device 12 is adapted to imposa a charge on the photoconductor 4 as it passes tho s~me. If the photoconductor i3 selenium, th~ charge ~ill hav~ a posltive polarity. If the photoconductive material is resin-bound zinc-oxidQ, or an organic photoconductive material 3uch a~
polyvinyl carbazole, or the like, the charge will ba nagative.

After the photoconductor receive3 a charga, it passe3 lens 14, fitted in th~ housing and adapted to projact 7~

an image which i8 de~ired to be reproduced upon the charged photoconductive s~rface. Since a photoconduetor 1~ ~n insulator in the dark and a conductor in th~ lght, the impingement of the image pattern of light and shade upon th~
photoconductor will permit the areas of light to eonduet tha charge throuqh the photoconductor to the conductivo drum 2, thence through the disc 6, to the shaft 8. A latent ~leetro-~tatic image will thus bP formed on the surfaee of tho photo-eonduetor 4. Thi3 image will have a high potential in th~
order of eoo to 1000 volt3 above that of shaft 8, though ~t ~ay vary over wide limitq. A tank 16, formed of insulating material, i~ adapted to contain a developing liquid 18. An applicator roller 20, which may be made of a conduetiv~ xesin such a~ buna N, is mounted adjacent the photocondueto~.
Buna ~, as i9 known in the art, is a synthetie rubb~r produced by the copolymerization of butadiene and aeryloni trile. The applicator roller may be made of synthetie resin or of metal. If made of dielectric synthetie resin, thin line3 of the latent electro~tatic imago will bo toned, ~ut large areas will not. with a metal roller, whil~ ton~ng will tak~ plac~ ove~ a wide range of imago areaY, arci~g may oeeur over area~ cf high image inten~ity, re~ulting in copie~
showing horizontal white lines whor~ the areing occurred.
Buna N is pr~ctical because its conducti~ity can be closely controlled, and it i9 resistant to hydr~carbon oils, which are the carrier liquids f~r the toners used in our proces~.

The surface of the roller may be smooth or may be text~red -- that is, provided with re-entrant portion~
adapted to hold a volume of liquid developer. PIGURE 4 show~ a knurled surface; FIGURES S and 6 show a 3urface formed with depression~; FIGURES Sa and 6a show a roll~r formed with circumferential grooves concentric with the axi3 of the roller and FIGURE 7 shows axial groove parallel to the axis of the roller.

The conductivity of the roller is important and optimum resulta have been obtained when the surface of the roller is smooth and polished and the roller material ha~
a resistivity of 10 ohm-centimeters to 10 ohm-centimeter~.
Optimum performance with minimum breakdown occurs when th~
resistivity of the roller material lies between 10 ohm-centi~eters and 5(108 ohm-centimeters). The resi~tivlty ~f the roller i~ important, as will be pointed out hereinafter.

The gap between the applicator roller and tha photoconductive surface, at the closest approach, indlcated by the opposing arrow3 ~), lies between one and one-half mils and four mils. We have obtained good performance when the physical gap batween th~ roller and the photoconduct~ve surface is approxi~ataly two mil~.

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In our m~thod, the developin~ liquid jump~ from th~
appllcator roller to the laeent electro tatic ~mage owln~ to thc electro~tatic field~ of the electro3tatic imag~. ~n araas where the fields are inten3e, breaXdown may occur, discharging the latent electrostatic image along a line of close~t approach of the appli~ator roller to the photo-conductiv~ surface. We have discovered that the occurrence o~ thi~ breakdown can be reduced dramatically by making the roller ~lightly conductive, as pointed out above. The conductivity can be controlled by loading the buna N rubber, or other hydrocarbon oil-resistant synthetic resin, with disseminated carbon particles to impart a degree of conductance to the material of which the roller i~ made.

The quantity of developing liquid which negotiates the gap is important, We have found that the volume of the developing liquid on the applicator roller should lie ~etween 10 billion and 30 billion cubic micron3 per squaro inch.
~hi~ can be obtained by controlling the re-entrant portion3 if the appl.icator roller i3 textured. The textured roller i~ simply wip~d by a doctor blade (not shown), a3 i8 ~nown in the art, to levcl the quantity of liquid remaining in tho re-entrant portion~ of the textured roller. In thil manner, we are assured that a predetermined quantity of develop~r liquid will be present on the applicator roller for transfer over the gap between the applicator roller and the ~ .
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photoconductLYe surface for developing the latent electro-~tatic image on the photoconductive surface.

Preferably, the applicator roller is driven to move at the same speed as the surface of the photoconductor.
Precise equivalence of speed is not necessary~ It is important that the relative motion between the applicator roller and photoconductive surface be smooth and not varying.
The applicator roller can be driven at a slightly greater speed than the velocity of the photoconductive surface bearing the latent electrostatic imaye~ Too great an increase in speed, however, tends to flood the electrostatic image. If the roller is driven at a slower speed, the image tends to be undertoned.

Referring again to FIGURE 1, a doctor roller 22 lS is positioned to rotate with a shaft 24 by a belt or chain 26 driven by a pulley or sprocket wheel 28 which is actuated by a variable speed motor 30. The direction of rotation o~
the doctor roller 22 is the same as that of the rotation o~
the app1icator roller 20. A wiper blade 32 removes excess developer fluid from the doctor roller and keeps it clean.
The doctor roller is spaced by rings 34 from the applicator roller 20 by a distanco of between one mil and five mil3.
The amount of liquid on the applicator roller, as pointed out above, should lie botween 10 billion and 30 billion cubic microns per squaro inch. ~his amount of liquid ls eas~ly metered by varying the rotational speed of the doctor roller ;, . , -20- ~

~nd tho gap ~C) b~ween ~he doctor roller and th~ appllcator roll~. A clo~e gap will pcrmit a ~lower rotational ~p~a~
.
of th0 doctor roller; ~ wider gap b~tween tho ~ppllc~or roller and thc doctor roller r~quire~ ~ greater veloc~ty of S rot~tion of th~ doctor roller. ~he doctor rollar can b~
made to rotate at various speed3 by th~ variable speed moto~ 30. At a higher sp~ed, the volume of developing liquid on the applicator roller is 30 reduced that the latent ~l~ctro~tatic image on th~ ,ohotoconductive surface i9 lightly toned and very dry. If the ~peed of the doctor roll~r i~
r~duced too much, images that are quit~ wet aro achiovedO
The np~d 3ho~ld be such that the correct volume of doveloping fluid i~ applied to the doctor roll~r. Thi~
spoed lie~ between 100 r~m and 200 rpm and dep~nd3 on th0 gap (3) between the applicator roller and th~ photoconductive ~urface being used and the specific developing liquid be~ng employ~d. The smooth-surfaced applicator ro'Ller, together with a variable-Ypeed doctor roller, produce~ a mo~t flo~lbl~
combination and ha~ ~ number of advantagesO Th~ quantlty Of tOnCr on the applic~tor roller can be ~asily controllod.- A
wlper blade 36, bearing again~t the ~urfac~ of thc ~ppllc~t~r roll~r, keep~ the applic~tor rollcr claan. With a tcxtur~d ~oll~r, the volume of the develop~r liquid i~ flxcd und no ~dju3t~ent of th~ roller ~urfac~ ~an bo madc for variatlona in tho dev~loper liquid. Furth~nmorc, ther~ i~ a t~nd~ncy :~
or th~ re-~n~rant portion~ of a-textur~d rollor to.ac~ulro d~po~it~ which ch?ngc tho volumet~ic c~p~city~of t~-llquld:

-2~-applicator.

Referring again to FIGURE 1, the applicator roller may be driven from a variable speed motor 38 through à~h~lt or chain 40, driven through pulley or sprocket wheèi 39 to rotate at such speed that the relative motion between the 3urface of the applicator roller and the surface of th~
photoconductor is close to zcro. By driving the applicator roller faster, more developer liquid will be presented for developing the latent electrostatic image. By driving the~
applicator roller slightly slower, less developer fluld will be presented for negotiating the gap to develop the latent electrostatic image. If a textured roller i9 used, the control of the speed of the applicator roller enables us to make adjustments for variations in the developing liquid and changes in the capacity of the re-entrant portion~ owing to collection of sediment in them.

The applicator roller is biased either above ground or below ground by a potentiometer, indicated ~ener~lly by reference numeral 42t adapted to apply a bias through a conducto~ 44 to a shaft 46 with which the applicator roller rotates. Since zero relative motion between the ~urface of the applicator roller and the surface of the photoconductive drum ls usually optimum, we can easily achieve this by the - - : - : . -, ; . , ~
construction shown in FIGURE 3. The applicator roller 20 iR
, . , .. .~ . , .
formed wi~h flanges 21 at each end. These flanges bear . .
against the photoc~nductor 4 and are secured to the roller ~22-or rotation therewith, Thc roller is mounted for rotatio~
with t~e shaft 46, and i3 insulated from ground so that the bia~ on shaft 46 can be adju~ted by potentiometer 42. It will be seen that ~he friction between the flanges 21 and the surface of the photoconductor upon which they bear causes the roller to rotate such that the relat~ve motion between the surface of the applicator roller and the surface o~ the photoconductor on drum 2 is ~ero.

Referring again to FIGURE 1, the applicator roller 20 is shown as independently rotatable in respect of the photoconductive surface. The shaft 46 of the applicator roller carries end discs 50 which are secured to the inner races 52 of ball bearings, the outer races 54 of which bear against the photoconductive surface, as can be seen by reference to FIGVRES 1 and 2. The shaft 46 extend3 through the bearings 47 loosely supported on the machine frame 51 and carries a pulley or sprocket wheel 53, adapted to be engaged by the belt or chain 40. Springs 49 connected between bearings 47 and the frame urge outer ring~ 54 into engagement with the surface 4. As pointed out above, the relative speed of the applicator roller to the photoconductl~e surface may be adjusted by varying the speed o~ motor 38.

The doctor roller 22 is similarly spaced from tho applicator roller by a ball bearing in which the ring~ 34 form the outer races. The inner rings 33 of the ball bearlng assemblies are mounted on reduced diameter end roll portion~

.

secured to the shaft 24 of the doctor roller, which e~tends through the housing 51 and which carries a pulley or sproc~et wheel 23. The shaft of motor 30 drives a pulley or ~procket ~heel 29. The belt or chain 26 extends between the member 23 carried by the shaft 24 and member 29 carried by the motor shaft. The variable-speed motor 30 is adapted to rotate the doctor roller 22 in the same direction as the direction of rotation of the applicator roller at any desired speed.

Referring again to FIGURE 1, the tank 16 i5 provided with a pipc l5 connected to a source of developing liquid ~not shown) to supply developing liquid to the tank.
A level control (not shown), known to the art, is adapted to keep the level of developing liquid in the tank at a pre-determined level. As can be seen by reference to PIGURE l, l~ the applicator roller 20 is mounted so that its lower portion dips into the developer liquid in the tank and i5 adapted to carry liquid from the tank to adjacent the latent electro-static image on the photoconductor, as heretofore described, l'he composition of the developing liquid i~
lmportant. If it is too conductive, it will partially discharge the latent electrostatic image. Xf it is not conductive enough, suffici~nt liquid developer will not negotiate the gap and underdevelopment will result. Tho developing liquid which we employ differs from developing liquids of the prior art. in that the concentration of toner in the developing liquid is greater , than heretofore employed and the carrier liquid i~ a hydrocarbon oil having a lower vapor pressure than thc hydrocarbon liquids used in known liquid-developer copying machine3. Most liquid developers of the prior art use Isopar-G as the carrier liquid, This is a narrow cut of isoparaffinic hydrocarbons having an initial boiling point of 319F. and an end or dry point of 345F. It has a flash point above 100F. However, it will completely evaporate, if left exposed, in 2200 seconds, as measured by Method 11 of the Federation of Societies for Paint ~echnology. As one ob~ect of our invention, we desire to reduce the e~aporation of the carr~er hydrocarbon liquids and, accordingly, employ a higher-boiling isoparaffinic hydrocarbon such as Isopar-M, which has a boiling range batwaen 4looF~ and ~85F. and a lower vapor pre~sure. It~
flash point is 170F., and it takes 3500 seconds to e~aporate only 10%, as measured by Method 11 of the Federation of Societies for Paint Technology. It i3 sub~
'~ stantially a light ~ neral oil. Ot~ r light mineral oil~, such as "Marcol 52"~and "Marcol 62'~ are highly refined white mineral oils, free of unsaturated compound3 and possessed of a high degree of chemical stability. They are odorle3s and tasteless and meet all the ~tandards of internal, medicinal, and co~metic uses. They are -25~

~ ~ ~ 6 ~ J ~

manufactured by HuJnble Oil and Reining Company and have a visco~ity in Saybolt seconds, at 100P., of 51 and 61 re-spectively. These oils conform to the requirements of FDA
Regulation 124.1146. "Marcol 52"~ha3 a specific gravity of .827 at 60F., and "Marcol 62"~has a specific gravity of .834 at 60F. These light mineral oils are used by us as the carrier liquid in which concentrated toner particles are disseminated. Development of the image takes place by electrophoresis -- that is, the migration of the toner particle~ through the carrier liquid to the latent electro-static image, which is a pattern of charge on the photo-conductive surfaco corresponding to the liqht and shade pattern projected onto the charged photoconductor. The carrier liquid~ have very low vapor pressure, and the Qmall amount of carrier liquid which negotiates the gap carrie~
toner particle~ which develop the image ~y electrophore~is.
What happens is that droplets of toner liquid negotiatc t~e gap and arrange themselves, roughly, according to the field~
of the electrostatic image. This electrophoresis continue3 after the gap has been jumped and a small amount of liquid carrier ~emains on the photoconductor, permitting this electrophoresis to occurO When the developed imag~ i~
transferred to a carrier sheet such as paper, the carrier liquid disperses immediate1y into the paper and, though the image is damp on the photoconductor, it is substantially dry S and smear-free on the paper to which it is tran~ferred, a~
will be described more fully hereinafter. It is to be under-stood that ~hile we prefer to employ the carrier liquidq herein particularly described, any appropriat~ dislectric liquid which does not dissolve the toner particles may be used.

In our alectrophoretic type of toner, we employ as much as 5% to 30% by weight, in respect of the weight of the carrier liquid, of toner solids. Liquid hydxocarbon~
are dielectric and will not jump the gap. A polar compound, such as a charge director, must be added to the developer liquid in order to make our process operative with the applicator roller out of contact with the photoconductor.
As described in U. S. Patent 3,669,886, these charge directors a~e well known to the art. They are polar com-pounds, soluble in the hydrocarbon carrier liquid, and create an electrostatic charge on the micron pigment (toner) particles which are adapted to migrate by electro-phoresis during the development process. The charge , director should be such as to impart a negative polarity wh~n the photoconductor is adapted to be charged po~itively and to impart a positive polarity when the photoconductox ls . .~ _ :~15~

. .
r9ed nega~i~el~ Char9 di p gmen~ particles are of 5u h p liquid conductive in d It i 9 known a l l m i 11 i ng o~ 1 ong_ cha i griDg l~illing 9nd they parted to long-chain pol or by adding a select p. The carrier liquid tner particles make th d ~he conductivi~y of th a 9iVen gap- ~or a wi~
q ed- By way of example h a tWo-mil gap, a Cond o l~ooo,OOO pico mho9 i ls go great, ths lstsnt el Y diSCharged, If the COnd elPer liquid will not;
thg imag9 Will not be properly dsvmlopod.

g ran~fer of ths dsvslOp f to the phutoconductive p t, which was initigtsd d i oner p9rticlss Out9id~ th -28.

~5~

latent electrostatic image move into and within the bou~daries of the latent electrostatic ima~e. This eleetrophoresi3 i~
enhanced, in one form of our method, by the u~e of a field electrode 60, which is biased by a direet-current souree 3ueh as battery 62. The polarity of the bias on the development electrode 60 is the same as the eharge on the photoeo~duetor ~- that is, the charge of the latent eleetrostatie image. If the photoconductor is of selenium, the charge will be po~itive:
if an organic photoconductor or cadmium sulphide i~ used a3 the photoconductor, the polarity will be negative. We have observed that the field electrode, when grounded, provide~ a ground plane which is effective in aiding eleetrophoresis even when the bias i9 absent. The gap, indicated by referenee letter (D), between the field electrode and the photoeonduetive ~urface should be as small as practieable. The funetion o the field electrode is to increase the projection of the eleetrieal field of the latent electrostatic image to a greater distanee from the surface of the photoconductor. The gap, however, s`nould not be so close that it will touch the image. Sinea the developing liquid transferred to the photoconduetive drum i~ never more than one or two mils ln depth, if the eleetrode is po~itioned between twelve and fifteen mils, there will be no eontact of the field electrode 60 with the latent eleetroqtatie image on the photoconductive drum 40 There i~ another eon-~ideration which must be taXen into account. The distane~between the surface of the photoeonductive dru~ and tho fi~ld ~29 electrode -- that is, the gap (D) -- should be le~3 than te~
times the thickness of the photoconductor. If $t i9 greater than this, the increased resolution produced by the field electrode will be reduced. The optimum spacing is egual to the thicXness of the photoconductive layer. This, of course, is not possible since the photoconductive material on the drum is usually quite thin. The function of the field electrode is different from the classical development electrode used in developm~nt systems; that iq, it is different from the function of the applicator roller 20 which acts as a development electrode during transfer of the developing liquid from the applicator roller to the photoconductive surface. The development electrodes o the prior art have for thcir purpose the prevention of the deposit of toner on the bac~ground or non-image area3 of the photoconductor. This takes place, o course, only when the developing liquid is in contact both with thQ electrostatic image on the photoconductor and the development electrode.
In our method and apparatus9 the electrophoresis takes place chiefly after transfer. The development electrode enhances the electric ficlds, which are normally small fringe fi~lds at the edge of the charged areas forming the latent electro-static image. The field electrode, in our process, appliQs a voltage of the samc potential as the charged image, but o8 a higher value than the highest charge of the latent electro-static image. Our development clectrode cr~ate3 field , patterns on the surface of the photoconductor which are conducive to assisting the mobility of the toner particles in the small amount of developing liquid which has been transferred to the photoconductive drum and which lies out-side of the charged image patterns. This enables electro-phoresis to continue on the photoconductor in an expeditious and efficient manner during the post-transfer period and enables development to continue to a greater e~tent than otherwise. The development electrode 60 should be as long as possible, since this will enable the electrophoresis continue to the greatest extènt and produce a sharper developed image.
A low-level illumination lamp 64 -- such as described in U.S. Patent No. 4,233,3~1, referred to above --is positioned so that the photoconductive surface and the developed image will be flooded with light after :the enhancement electrode 60 has been passed by the image. This not only sharpens the image, as described by Landa, by discharging the background or nonimage areas of the image, but also enables a more complete transfer of the developed image to the carrier, such as paper, ko which the image will be transferred.
The use of a higher-boiling mineral oil leaves a ms ~

h~

damp developed lmage on ~he photoconductor, though the amount of carrier liquid is small and the dampne~s slight.
A carrier material such as paper 66 is fed by roller3 63 past a pressure roller 70. we have found that the pres~ure roller 70 will enable the transfer of the damp image from the surface of the photoconductor to the carrier sheet 66 if the carrier shect is absorbent, such as paper. This transfer will be aidcd by biasing the roiler as i~ known in the art. If the carrier sheet is a transparent film o}
synthetic resi.n which is not absorbent, a corona 72 i9 adapted to charge the back of the carrier sheet to effect transfer of the image from the photoconductive surface to the carrier sheet. The polarity of this charge should be the same as the charge of the developed electrostatic image.
If selenium i3 used, the charge will be highly positive: lf cadmium sulphide or an organic photoconductor i9 used, the charge will be negative. The developed image will be pull~d from the sheet onto the carrier~ A picX off blade 74 ensureY
that the caxrier sheet 66 will leave the photoconductor for feeding by roller 76 to a tray or other copy-handling devlce, such as a collator or the liXe. In order to ensure that the surfaca of the photoconductor is clean, a cleaning roller 78 wipes the surface of the photoconductor drum and a wiper blade 80 completes the drum-cleaning operation.

..

Referring now to FIGURE B, in an alternative embodiment of our inven~ion after the developer liquid ha~
been applied to the image by roll 20 and while the image i8 still damp we subject it to the action of a corona 82 having a polarity which is opposite to that of the electro-static image to enhance the electrophoretic action ~hich was initiated upon the transfer of the developer liquid from the applicator roll to the latent image on the photocond~c-tive surface. We have discovered that this greatly sharpens the resultant copy.

Further in the form of our invention illustrated in FIGURE 8, after the image has been developed, we transfer it to the sheet of copy material 66 by passing the sheet 66 from the register rolls 68 into the nip between a transfer roller 84 and the surface 4. We form the roller 84 from a deformable conductive material~ which may for example be buna ~ rubber, to which sufficient carbon particles have been added to give it the required degree of conductivity.
Any suitable means, su~h for example as a battery a8, provides a source of biasing potential which i8 applied to the shaft 86 of the roller 84. Under the combined action of the bias provided by battery 88 and the pres~ure of roll 84, the developed image is transferred from the surface 4 to the sheet 66, which then i8 picXed off the drum 3urfaca by pickoff 74 and delivered to a tray or other paper handling Oqllipment a8 i8 well known in the art.

In operation~ the photoconductive drum is rotated by a motor or o~her drive means (not shown) to rotate i~
the direction of arrow (A). When a copy is desired to be made, the drum is charged by corona device 12 and then exposed to an image of the document of which a desired copy is to be made, projected by lens 14 on the surface of the photoconductor. This produces a latent electrostatic image.
The applicator roller 20 is rotating at a speed such that there is substantially no relative motio~ between the photoconductor and the applicator roller. This may be biased as desired further to reduce the amount of developer liquid on the non-i~age areas of the photoconductor during transfer of developer liquid. Such bias, owing to our post-transfer electrophoretic method~ is not as important a9 iS the case in the copying machines of the prior art using liquid de-velopers. The closest portion o~ the applicator roller 20 i8 spaced from the surface of the photoconductor 4, carried by the drum 2, to form a gap, preferably by a distance o~
two mils. The doctor roller 22, which is wiped by wiper blada 32, i8 driven at the proper speed by motor 30 and in a reverse direction, so that the volume of developer liquld on the applicator roller is proper to produce a plea~ingly developed image having the desired contrast. The applicator roller dips into the tray 16 which carries the developing liquid 18, the wiper blade 36 keeping the applicator roller ~3~-.

clean. The applicator roller may, if desired, be indepen-dently driven by mo~or 38 at speeds such that the relative motion between the applicator roller and the surface of the photoconductor is not ~ero, but such that the surface of the applicator roller moves faster than the surface of the photoconductor. The developing liquid is, as described, formed of a carrier liquid having a lower vapor pres~ure than the carrier liquids of the prior art, and having a concentration of solidst with respect to the carrier liquid, greater than the prior art. The conductivity of the de~eloper liquid is closely controlled by the addition of polar com-pounds or sharge directors, such that it is conductive enough to permit sufficient developer liquid to jump the gap, but not so conductive as to partially discharge the latent electrostatic i~aqe on the photoconductive surface.
After the developing liquid jumps the gap, development oCCUr9 by po3t-transfer electrophoresis, though some development may take place during transfer. To enhance the electro-phoretic development, we employ a field electrode 60, which is biased by the same polarity a3 the polarity of the latent electrostatic image and posi~ioned as closely as possible to the photoconductive surface, without permittlng the developer to touch the field electrode. This) field electrode enhances the electrical fields which are normally small-frin~e fields at the edge of the charge areas and a3sists electrophoresis by increasing the mobility o~ the 7~i toner particles in the carrier liquid lying outside tha charged image patterns~

After the developed image passes the field electrode 60, the background areas, or non-image areas, are discharged by a low-level illumination lamp 64 which flood~
the developed image. This sharpens the image and enables a denser image to be transferred to the carrier sheet such as paper. If paper or other absorbent carrier is used, a slightly damp imagc will transfer by pressure alone~ The small amount of carrier liquid in the damp image will be rapidly absorbed by the paper, and the copy on the paper will be dry without the necessity of using heat.

The operation of the form of our invention illustrated in FIGURE 8 is the same as that of the form of our invention illustrated in FIGURE 1, up to the point at which the surface 4 leave~ the applicator roller 20.
Following this point, the image to which the developer ha~
been applied is subjected to the action of the corona 82 which greatly sharpens the image. Subsequently, regi~ter rolls 68 feed a sheet of copy material 66 into the nlp between roll 84 and the developed image bearing surface 4.
Under the combined action of the pressure of roller 84 and the bias provided ~y battery as, the image is transferred from surface 4 to the sheet which then is picked off the drum and delivered to the user.

It will be seen that we have accomplished the objects of our invention. We have provided a novel method of employin~ liquid toners in plain-paper copying mach~nes, in which the development of the image on a photoconductive surface is so controlled as to tone substantially only the charged image areas of the latent electrostatic image withou~ wetting the uncharged or background areas. We hav~
provided a novel apparatus for carrying out our method~
which apparatus is equipped with an applicator roller which applies liquid developer to the latent image through a gap.
This applicator roller is provided with means for either statically or dynamically controlling the volume of liquid presented by the applicator roller to the latent electro-static image, separated from the roller by a gap. The static contxol means may involve a wiper bLade and a textured roller having a predetermined cubic volumetric capacityO The applicator roller may be a smooth roller~
used in association with a doctor roller adapted to revolve in the same direction as the direction of rotation of the applicator roller and at speeds controlled to dynamically meter the volume of liq~id on the applicator roller.

Our method is such that electrophoresis takes place, not only during thc period of transfer of the developer liquid from the applicator onto the photoconductive surface, but chiefly after transfer ha~ been accompllshedO

-~7-Our method employs liquid developers having a higher rati~
of 301ids to carricr liquids than has heretofore been employed and a dielectric carrier liquid having a lower vapor pressure than the carrier liquids of the prior art.

In our proce~s, the conductivity of the develop~r liquid is controlled by employing toner particl~s which have polar characteri~tics or charge directors. The condu~tivity of the developer liquid compo~ition as a whole i3 such that 1t is high enough to jump the gap in connection with which our process is employed9 but not so high as to partially discharge the latent electrostatic image. Our field electrode, or a post-toning corona, enhances the field of the latent electrostatic image and asgists in the post-transfer electro-phoresis. We may alternatively employ a post-toning corona to augment electrophoresis.

It will be understood that certain features and ~ubcombinations are of utility and may be employed without reerence to other features and 6ubcombinations. Thi3 i8 contemplated by and is within the scope of our claim~. It i3 further obviou~ that various changes may be made 1n detail~ within the scope of our claims without departing from the spirit o our invention. It is, therefore, to be understood that our invention i3 not to be limited to th~
specific detail~ sho~n and described.

2S llaving thus de3cribed our invention, what we claim3 is: ~i

Claims

The Claims

1. In a method of developing latent electrostatic images wherein a latent electrostatic image is formed on a photoconductive surface, the steps of applying a developer liquid comprising dielectric liquid hydrocarbons having charged toner particles disseminated therethrough and having a conductivity from 50,000 pico-mhos to 1,000,000 pico-mhos to the latent electrostatic image over a gap whose distance from the surface of the developer liquid to the surface of the latent electrostatic image lies between one-half mil and three mils, said applying step being performed from a layer of developing liquid having a volume between 10 billion cubic microns per square inch and 30 billion cubic microns per square inch, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, increasing the velocity and extent of said electrophoresis by subjecting the initially developed electrostatic image to an electric field of the same polarity as the charge of the latent electrostatic image, then discharging the non-image areas of the charged photoconductor by illuminating the developed image, and then transferring the developed image to a carrier sheet.

2. In a method of developing latent electrostatic images wherein a latent electrostatic image is formed on a photoconductive surface, the steps of forming a layer of developing liquid comprising liquid hydrocarbons having toner particles disseminated therethrough and a minute amount of a polar compound adapted to charge said toner particles whereby to impart a conductivity to the developing liquid between 50,000 pico-mhos and 1,000,000 pico-mhos upon a support having resistivity between 107 ohm-centimeters and 109 ohm-centimeters, controlling the thickness of the layer of developing liquid on said support to between one and two mils, applying said liquid developer on said support to said latent electrostatic image over a gap whose distance from the surface of said layer to said photoconductive surfaco lies between one-half mil and three mils, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing the extent of said electrophoresis by subjecting the initially developed electrostatic image to an electric field of the same polarity as the charge of said electrostatic image.

3. In a method of developing latent electrostatic images wherein a latent electrostatic image is formed on a moving photoconductive surface, the steps of applying developer liquid comprising dielectric carrier fluid having charged toner particles disseminated therethrough by a moving applicator whose surface moves at substantially the same velocity as the photoconductive surface, spacing said toner applicator from said photoconductive surface to form a gap therebetween, forming a film of liquid developer on said applicator, metering the thickness of the film on said applicator to a predetermined thickness as a function of the distance of said gap and the conductivity of said developing liquid and the potential of the field of said electrostatic image such that the developing liquid will jump said gap, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing said electrophoresis by subjecting the developing latent electrostatic image to a corona discharge of a polarity opposite to the polarity of the latent electrostatic image.

4. In a method of developing a latent electrostatic image wherein the latent electrostatic image is formed on a moving photoconductive surface, the steps of applying developer liquid comprising a dielectric carrier fluid having charged toner particles disseminated therethrough from a rotary applicator revolving at a rate such that its surface moves at a relative velocity equal to or slightly greater than the velocity of said moving photoconductive surface, spacing said toner applicator from the photoconductive surface to form a gap therebetween, forming a film of liquid developer on said applicator, dynamically metering said film to a predetermined thickness as a function of the conductivity of the developing liquid the distance of said gap and the potential of the field on said electrostatic image such that the developing liquid will jump said gap, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing said electrophoresis by subjecting the developing latent electrostatic image to an electric field of the same polarity as the charge of said electrostatic image.

5. In a method of developing a latent electrostatic image wherein the latent electrostatic image is formed on a moving photoconductive surface, the steps of applying developer liquid comprising a dielectric carrier fluid having charged toner particles disseminated therethrough from a rotary applicator revolving at a rate such that its surface moves at a relative velocity equal to or slightly greater than the velocity of said moving photoconductive surface, spacing said toner applicator from the photoconductive surface to form a gap therebetween, forming a film of liquid developer on said applicator, dynamically metering said film to a predetermined thickness as a function of the conductivity of the developing liquid the distance of said gap and the potential of the field on said electrostatic image such that the developing liquid will jump said gap, permitting the development of the latent electrostatic image to occur by electrophoresis of the toner particles in the developing liquid which has jumped the gap, and increasing said electrophoresis by subjecting the developing latent electrostatic image to a corona discharge of a polarity opposite to the polarity of the latent electrostatic image.

6. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electro-phoresis of the charged toner particles and then subjecting the damp and developed electrostatic image to an electric field of the same polarity as the charge of said electrostatic image.

7. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electro-phoresis of the charged toner particles and then subjecting the damp and developed electrostatic image to a corona discharge of a polarity opposite to the polarity of the charge of said electrostatic image.

8. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electro-phoresis of the charged toner particles, then subjecting the damp and developed electrostatic image to an electric field of the same polarity as the charge of said electrostatic image, and then transferring said developed image to a carrier sheet.

9. In a method of developing latent electrostatic images with a liquid developer comprising a dielectric carrier fluid having charged toner particles disseminated therethrough, the steps of applying said developer liquid to the latent electrostatic image to develop the same by electro-phoresis of the charged toner particles, then subjecting the damp and developed electrostatic image to a corona discharge of a polarity opposite to the polarity of the charge of said electrostatic image, and then transferring said developed image to a carrier sheet.

10. In an apparatus for making copies of a document by electrophotography, a rotatable conductive drum; a photo-conductor carried by said conductive drum; means for charging the photoconductor; exposing means for subjecting the charged photoconductor to the image of a document being copied to form a latent electrostatic image; the improvement comprising developing means including an applicator roller having a resistivity between 107 ohm-centimeters and 109 ohm-centimeters, means for mounting said applicator roller for rotation about an axis parallel to the axis of rotation of said drum so as to form a gap between the surface of the photoconductor and the surface of said applicator roller at their point of closest approach of between one and one-half mils and four mils, means for rotating said applicator roller so that the relative motion between the surface of the photoconductor and the surface of the applicator is substantially zero, means for applying developing liquid to said applicator roller, means for metering the thickness of the film upon the applicator roller such that it has a volume between 10 billion cubic microns per square inch and 30 billion cubic microns per square inch, a field electrode positioned adjacent the photoconductive drum in a position to subject a developed latent electrostatic image which has been subjected to the action of developer liquid to an electric field, means for imparting a potential to said field electrode of the same potential as the potential of the electrostatic image, and means for transferring the developed electrostatic image to a carrier sheet.

11. Apparatus as in Claim 10 in which said metering means comprises an applicator roller formed with re-entrant portions and a wiper blade for said applicator roller,

12. Apparatus as in Claim 10 in which said metering means comprises a doctor roller, means for spacing said doctor roller from said applicator roller, and means for rotating said doctor roller in the same direction as the direction of rotation of said applicator roller.

13. Apparatus as in Claim 10, including illuminating means for illuminating the developed electro-static image after it has been subjected to the field of said field electrode.

14. In an apparatus making copies of a document by electrophotography, a movable conductive support, a photoconductor carried by said support; means for charging the photoconductor; exposing means for subjecting the charged photoconductor to the image of a document being copied to form a latent electrostatic image; the improvement comprising developing means including movable developer applicator means.

means for moving said applicator means substantially in synchronism with said photoconductor so the relative motion between the photoconductor and the applicator is substan-tially zero, means for mounting said applicator means so that the surface of the same at its closest approach to the surface of said photoconductive means lies between one and one-half mils and four mils, means for applying developing liquid to said applicator means, means for metering the thickness of the film on said applicator means, a field electrode positioned adjacent the surface of said photo-conductor in a position to subject a developed latent electrostatic image which has been subjected to the action of the developer liquid to an electric field, and means for imparting a potential to said field electrode which is the same as the potential of said electrostatic image.

15. Apparatus as in Claim 14 in which said metering means comprises a doctor roller, means for spacing said doctor roller from said applicator means, and means for rotating said doctor roller in a direction so that its surface moves opposite the direction of movement of said applicator means.

16. In an apparatus for making copies of a document by electrophotography having a photoconductor, a station for electrostatically charging the photoconductor, an exposing station for subjecting the charged photoconductor to a light and shade image of the document to be copied to form a latent electrostatic image on the photoconductor, a liquid developing station to make the electrostatic image optically visible, and means for providing relative motion of the photoconductor with respect to the charging station, the exposing station, and the development station, the improvement which comprises a field electrode for subjecting the developed electrostatic image to an electrical field, means for applying a potential to said field electrode of the same potential as the potential of the electrostatic image, and means for moving the developed image past said field electrode.

17. Apparatus as in Claim 16 in which said developing station comprises a developer applicator roller and means for spacing said applicator roller from said photoconductor so that the surface of the photoconductor and the surface of the applicator roller at their closest approach lies between one and one-half mils and four mils.

18. In an apparatus for making copies of a document by electrophotography having a photoconductor, a station for electrostatically charging the photoconductor, an exposing station for subjecting the charged photoconductor to a light and shade image of the document to be copied to form a latent electrostatic image on the photoconductor, a liquid developing station to make the electrostatic image optically visible, and means for providing relative motion of the photoconductor with respect to the charging station, the exposing station, and the developing station, the improvement which comprises a corona discharge station, means for moving said optically visible electrostatic image while on the photoconductor part said corona discharge station, and means impressing a potential to the corona discharge station of a polarity opposite to the polarity of the visible electrostatic image.

19. Apparatus as in Claim 18 in which said developing station comprises a developer applicator roller and means for spacing said applicator roller from said photoconductor so that the surface of the photoconductor and the surface of the applicator roller at their closest approach lies between one and one-half mils and four mils.

20. In an apparatus for making copies of a document by electrophotography having a photoconductor, means for charging the photoconductor; exposing means for subjecting the charged photoconductor to a light and shade image of a document being copied to form a latent electrostatic image on said photoconductor; means for electrophoretically developing the latent electrostatic image with a liquid developer; means for transferring the developed image to a carrier sheet and means for moving the photoconductor part the charging means, the exposing means, the developing means, and the transfer means in succession; the improvement comprising interposing a field electrode between the developing means and the transfer means, and means for impressing a potential on said field electrode of the same polarity as the polarity of the developed latent electrostatic image on said photoconductor.

21. In an apparatus for making copies of a document by electrophotography having a photoconductor, means for charging the photoconductor; exposing means for subjecting the charged photoconductor to a light and shade image of a document being copied to form a latent electrostatic image on said photoconductor; means for electrophoretically developing the latent electrostatic image with a liquid developer; means for transferring the developed image to a carrier sheet; and means for moving the photoconductor past the charging means, the exposing means, the developing means, and the transferring means in succession; the improvement comprising a corona discharge device positioned adjacent the photoconductor between said developing means and said transferring means, and means for impressing a potential upon said corona discharge device of a polarity opposite to that of the developed electrostatic image.

22. In an apparatus for making copies of a document by electrophotography having a rotary conductive support, a photoconductor carried by said support, means for charging the photoconductor, exposing means for subjecting the charged photoconductor to a light and shade image of the document being copied to form a latent electrostatic image, the improvement comprising a rotary liquid-developer applicator roller, means for rotating said applicator roller substan-tially in synchronism with the photoconductor so that the relative motion between the photoconductor and the applicator roller is substantially zero, means for mounting said applicator roller so that its surface at its closest approach to the surface of the photoconductor lies between one and one-half mils and four mils, a supply of developing liquid, means for dipping a portion of said applicator roller into said liquid-developer supply, a doctor roller, means for positioning said doctor roller closely adjacent but out of contact with the surface of said applicator roller, and means for driving said doctor roller so that its surface moves in a direction opposite to the direction of movement of the surface of said applicator roller whereby to meter the thickness of a film of developed liquid on said applicator roller.

23. Apparatus as in Claim 22 including a wiper blade, and means for positioning said wiper blade to wipe the surface of said applicator roller after it has transferred developing liquid to said photoconductive surface.

24. Apparatus as in Claim 22 including a wiper blade, and means for positioning said wiper blade to wipe the surface of said doctor roller before its surface approaches the surface of said applicator roller.

25. Apparatus as in Claim 22 in which said means for rotating said doctor roller comprises variable-speed driving means.

26. Apparatus as in Claim 22 in which said applicator roller is provided with spacing flanges positioned adjacent opposite ends thereof for spacing its surface from the surface of said photoconductor, said flanges being in contact with the surface of said photoconductor.

27. Apparatus as in Claim 22 in which said applicator roller has a resistivity lying between 107 ohm-centimeters and 109 ohm-centimeters.

28. Apparatus as in Claim 22 in which said applicator roller is formed with a textured surface provided with re-entrant portions.

29. Apparatus as in Claim 28 in which the volume of said re-entrant portions lies between 10 billion cubic microns per square inch and 30 billion cubic microns per square inch.