US 3749059 A
Description (OCR text may contain errors)
United States Patent 11 1 6/1967 Oliphant et a]. 118/637 Sato July 31, 1973' [5 APPARATUS FOR DEVELOPING 3,551,146 12/1970 Gundlach 118/637 ELECTROSTATIC LATENT IMAGE 3,405,683 IO/1968 .Ions et al 118/637 3,577,259 5/1971 Sato et al 118/637 Inventor: Mmmlchl a Asakl-shl. Sanama, 3,409,358 11/1968 Fauser 118/637 Japan 3,599,605 8/1971 Ralston et al.... 118/637 3,368,894 2 1968 Matkan et al.... 96/1  Asslgnee- 3,554,161 1/1971 Blanchette 118/8 j Kanagawa, Japan [-22] 1 Filed: *5' 1970 "Primary Examiner-+Mervin Stein  AppL- No: 67,473 I Assistant Examiner-Leo Millstein v 1 Attorney'-'Addams & Ferguson  Foreign Application Priority Data Aug. 27, 1969 Japan 44 67795  ABSTRACT  Cl 118/637 117/37 gg f d An apparatus for developing an electrostatic latent image on an insulating layer having an electroconduc- 2; 'igbg z q tive backing layer by spacing the-layer from a develop- 1 0 l ing electrode in a developing liquid where either or I 37 355/10 both the developing electrode and the electroconductive backin la er are rounded via resistance in the  References Cited range of o s 12 g UNITED STATES PATENTS 3,328,193 12 Claims, 14 Drawing Figures i I I v 4 Pmmtnww 3.749.059
sum 1 or 4 lllllllllllllllllll INVENTOR Mama/aw 6:470
PATENIEUJUUI I915 I 3.749.059
SHEEI 2 0F 4 F7; 6 a; r------ Hy 7 v IN VENTOR PATENTEU M3 1 .973
sum 3 or 4 Z ll/lj/ll/ INVENTOR J mo APPARATUS FOR DEVELOPING ELECTROSTATIC LATENT IMAGE BRIEF EXPLANATION OF THE DRAWING tribution of electriccharge onthe surfaceof theelectrophotographic material illustrated in FIG. 1.; FIG. 3
is adiagram showing thedistribution of electric field in the; direction perpendicularto the surfaceofsheetzas derived from:theqdistribution ofel'ectric charge illustrated in FIG. 2 in the; absence. of a developing electrode; FIG. 4 isa-diagram showingthedistribution of electric field as derived' when a. developing electrode is grounded directly. and-.- when itis. grounded via agroundingresistance; FIG. 5 is a side elevation in longitudinal section offthe developing. electrode and the sheet-shaped" electrophotographic material grounded via the grounding resistance; FIG& 6. is adiagram' showing the, distribution of 1 electric: charge derived where there is residual electriccharge on the: sheetsurface; FIGS; 7'and8- arediagramsshowing the distribution of electric field" as derived where the developing electrode. is grounded: directly and it is grounded -via'thegrounding resistancerespectively in the case 'ofthe 'diagram'of FIG. 6;,FIG. 9,.throughFIG; ll and FlG. l4-are side elevations in longitudinal-section of devices employed for: puttingthis invention to working; and FIG. 12 and FIG. 1-3 are side elevations-in longitudinal section of the structure of bearingof theroller illustrated. in FIG. 11'.
DETAILED DESCRIPTION OF THE INVENTION This invention-relatesto an apparatus for developing electrostatic latent image; More specifically, this invention relates. to. a liquid developingyapparatus so improved as to prevent theoccurrence. of'fogging (a phenornenonv which obscures the. background to uniform density) due to residual electric charge andfiof streaks in the image.
Photoconductive insulating layers used'in the fieldof electrophotography commonlyprocess more or less residual electric potential. When aphotoconductive insulating layer-is electrically chargeduniformly and subsequently exposed to. light throughan image, theelectric chargeis neutralized in thearea where the layer has been exposed to light but the charge is retainedintact on the photoconductive insulating: layer in the area where the layer has escaped exposure to' light. The electriccharge-to be neutralized increases with the in-- tensity of light to which the layer is exposeda'lhus, an electrostatic latent image conformingv to the. image which is represented in terms of light intensity is producedonthephotoconductiveinsulating layer. ln'actualit-y, however, thedegreeof neutralization of electric chargestops growing proportionally with the rise of lightintensity when the light intensity exceeds a certain level, beyond: which the: degree of neutralization growth is lowered." When the .exposureto light through theimageisstopped before completeneutralization of electric charge while the intensityof-Iight for illumination 'isatits maximum ,':electric charge is suffered to re* main even in areas where .electric charge .wouldotherwise be neutrali'zed completely. Consequently, there is observedthe so called-phenomenon of residualtelectric potential. When the exposure to light of high-intensity is continued until perfect elimination of residual electric potential, electric charge is neutralized excessively in areas where electric charge. should not be neutralized beyond a certain level. Thus, it becomes no longer possible to produce an electrostatic latent image faithful to the original image, namely, properly proportional to the intensity of light used for exposure. In reality, therefore, there is followed a practice by which exposure is effected to such extent astoallow residual electric charge to occur to some extent and subsequently eliminate fogging due to residual electric charge by a suitable means. If the residual electric charge is'devel-. oped faithfully, the background portion of image is foggeduniformly to produce a very unsightly image.
Heretofore, there has been proposed a practice ofapplying DC voltage to a developing electrode from an external source so as to give rise to electric potential equalling to the residual electric charge and consequently prevent the charge-from producing fogging.
The conventional method and apparatus has been found defective in that toner particles are accumulated on'thedevelopingelectrode and tend to comeinto con,- tact withthe electrostatic latent image and, as a result, smear the image. The loss of toner particles because 0 such accumulation has been heavy.
This invention relates to an apparatusfor developing an image free from fogging due to residualcharge and faithful-to the original without requiring any external power source for the neutralization of residual poten tial.
One moredefect in liquid development is occurrence of streaks in developed image. Next steps are usually involved in makingaprint by electrophotography; neg.- atively charging an electrophotographic material, c.g., a paper bonded a photoconductive layer consistingof zinc-oxide pigment in a resin binder, exposing it to form a latent electrostatic image, and developingithe latent image with positively charged fine particles dispersed in insulating liquid. In this developing process, the flow of liquid developer produces streaks. The streaks are conspicuous inthe'case the flow of liquid developer is fast or the potential of the latent image is high. This phenomenon easily occurs when the liquiddeveloper-is' poured into the space between the surface bearing a latent image and the developing electrode held close" to the surface.
In a developing apparatus wherein an electrostatic latent image formed on an insulating layer of an electrophotographic material is disposed so as to-confront a developing electrode with a minute opening and via a developingliquid havingfinely divided charged particles suspended in an electrically insulating liquid, this invention is characterized by conferringupon either or both of the developing electrode and the electroconductive layer of electrophotographic material such magnitude of grounding resistance'as to permit prevention of excessive deposition of the aforementioned charged particles-in the highlight zone of electrostatic latent image (the zone where the surface potential is smallest).
When the effect of developing electrode is main- I tained at an extremely high level throughout the period of development, even the smallest amount of residual potentialdue toresidual charge is developed faithfully to produce fogging. Therefore, the spirit of this invention resides 'in' conferring an adequate magnitude-off grounding resistance upon either or both of the devel- 3 oping electrode and electroconductive layers of electrophotographic material, whereby toners are prevented from depositing in the zone of the least charge density within the electrostatic latent image (namely,
further detail by referring to the drawing.
FIGQl shows a condition under which electric charge exists uniformly only in the right half portion of surface on the electrophotographic sheet 10. Numeral 11 denotes a'photoconductive insulating layer which may be made of any known substance. Examples are amorphous selenium and photoconductive zinc oxide powder blended respectively with an insulating resin. Numeral l2 denotes an electroconductive base. The materials usable therefore include metal plate, plastic film treated so as to acquire electroconductivity, and paper. In the course of development, the base 12 is usually grounded. The latent image of FIG. 1 represents a case wherein positive charge is distributed uniformly in the right half portion of the upper side of the photoconductive insulating layer 11 while negative charge of an equal magnitude exists in the half portion of the lower boundary.
FIG. 2 is a graph illustrating the distribution of positive charge on the layer 11 as shown in FIG. 1. In the graph, the origin represents the boundary between the zone containing electric charge and the zone devoid of electric charge as shown in FIG. I. It is assumed that the electric charge is uniformly distributed at a density of 0'0.
.FIG. 3 is a graph illustrating the composition of electric fields distributed perpendicularly to the plane of latent image as formed, in the absence of a developing electrode, within the electric field close to the latent image which is produced by distribution of electric charge as shown in FIG. 2. The distribution of electric charge shown in FIG. 2 has an entirely different shape from the distribution of electric field shown in FIG. 3. It is universally known that the phenomenon of edge effect results consequently. What should be noted at this point is the fact that even at the position satisfying 0' 0 in the diagram of FIG. 2, a negative electric field (with the electric field lying outward from the plane of latent image taken as a positive field) occurs in FIG. 3. This shall be referred to hereinafter as opposite electric field.
FIG. 4 is a diagram illustrating the distribution of electric fields corresponding to those of FIG. 3 where a developing electrode is disposed to confront the surface containing the latent image at a fixed distance. In FIG. 4, the curve A represents a case wherein the grounding resistance of developing electrode (hereina after expressed as R") is very small (R 0), while the curve B represents a case wherein the grounding resistance is very large (R 0, for example). When R becomes as small as this, a fair agreement is shown between distribution of electric charge and that of electric field. Yet, there still remains a very minor magnitude of opposite electric field. Particularly, the opposite electric field in the neighborhood of edge (the boundary of distribution of electric charge, namely, the origin of the graph of FIG. 4) is much larger than that at a distance. The opposite electric field becomes muchnitude of grounding resistance R fixed at an extremely high level. Negative charge is induced through electrostatic induction in the areaof the developing electrode confronting the positive charge of latent image. In the other portion of the developing electrode, positive charge substantially equivalent to the induced negative charge is induced. Because of a large value of R, the positive charge is not immediately discharged to the ground. This positive charge can move about with considerable freedom within the developing electrode. Of the negative charge distributed as opposite charge of latent image in the boundary of the electroconductive layer 12, the magnitude of negative charge equivalent to the negative charge induced on the developing electrode acquires freedom of mobility and eventually confronts the positive charge induced on the developing electrode where there exists no electric charge of latent image, as shown in FIG. 5. Consequently, an opposite electric field is formed at this portion and properties as represented by the curve B of FIG. 4 are believed to be acquired.
, FIG. 6 illustrates the distribution of electric charge as obtained where there exists a residual charge 0- This means that the electric charge a 0 remains where the magnitude of electric charge should desirably become 0-,, 0. The distribution of electric charge shown in FIG. 2 represents an ideal case, while the distribution of electric charge which is actually obtained by charging and exposure to light is as shown in FIG. 6.
FIG. 7 is a diagram which is contrasted to that of the distribution of electric charge in FIG. 6 and which corresponds to FIG. 4. The diagram represents a distribution obtained when the distance between the developing electrode and the latent image surface is fixed to comparatively small and the value of R is very small. As is evident from FIG. 7, the electric field at the portion where the residual charge 0', exists constitutes a positive field except in the neighborhood of the boundary of charge distribution. In the vicinity of boundary, the edge effect persists to induce negative electric field. When the distance between the developing electrode and the surface of latent image becomes very small (to several 10s of u, for example), this edge effect becomes greatly decreased. When a liquid developer (having a microfine charged particles or toner particles with negative charge suspended in an insulative liquid) is supplied between the opening between the latentimage surface and the developing electrode while the electric field is distributed as shown in FIG. 7, the toner particles are deposited in the portion of positive field. The consequence is that there occurs fogging due to residual electric charge.
The reason why the streaks shall occur has been,
guessed as follows. When the liquid developer is applied to the surface of the latent image, for example,
applied to the left direction in FIG. 2, the toner particles, having negative charge in this example, are attracted to the positively charged right half area. Some 4.. urveA- 5, P r c s re car ied down rdr m he-area, and. attach even to the noeharged area. 'l hisis, significant in the casethe potential of charged area is the higher. In general, the counter'electrical field (repulsion field) which prevents the attachment of toner; particles appearson the no charged area. When the, counter field srweak he tonerpat iolesdoposit ontthe no ch ge 'otoai hough he enulsion fieldi s rong. in. the. ab-- n o; of deve pi g elec ode s howm in FIG. s Presen s: of e ping le tr de. mak the.-r p ..lsion oldz als nslat r c el s ronga sho n in-ElG.
fi sg ho; troak a-* I 1 RIG. 8;; is-a diagram. sim-ilar to that-of FlGj. 7"which corresponds to-FIG. 4.: Thediagram representsa-case.
8; hev lueotRx o arge-.-(cot, espom ingtolieou e.
Bi l 161 t the ppos te. leotriofield by and. heP s t v lo t i fihargeby .0, oanoele oh witharesultthat o positivefield;dev op :evenhe p tio t here l istsr sidualcharge e e oner par le positsat th sportion- It h l e-of ra opt a a-rhislt ore hr gho t thet ge of e pr on t there occurs hoodgel ffeot nd-t ere c nseql en y n uostho oallodihalm a phenomenon of lajcle. of toner deposition in the zoneof the low. charge density close to theboundary between thehighcharge densityland the low, charge idensity, Therefore, it must be. avoided .to. keepthe value of R? at a .high level,
ou ho t the.ontir meri dof ev l pm -1 h fore it becomes necessary ,to, lower thevalue-of B after lapseof ;a proper time. The Jengthof this time is not fixed but i varied by the or properties of electrophotographic materialand the resistanceof-liquid developer. Generally, the. time;is ..on ;theorder of several tens of econ s ut ay varyn e ange f om se eral eci mit o. v r l. m n tes tdosorint onr as b en made s. egards the ,gro -i ns o st noo en mpa ted o h dev pi g s trode, Similar;effect can"befljobtainod by perfectly ro ndinstho eve ping .olo t odo d allowing th ectr photosr ph o ma er al. (the i ernr e by meral IZ- inF IG. l) to acquire groundingresistance. It
is naturally permissible to. have both developing elec-a de n v lect oconductiv ayer- 1 l ph g aphic a er a cquir gt unding resistanoe. From he p a t alp rt fviow. f eedom, of opera ions inreased nd quip ne t. r mp' fi d y pr v grounding resistance for; the developing electrode. lnd., t,do Qfh skind an e utilized for any d f leqt opltot srophiomaterial. I Atspecifio means of o fer ing groundin esistan eon the electroconduc-v tive layer- 0f, electrophqtographic material, consists in noorporstina .s res stanc :l yo v onesth mlh .elec-trrr onduct v yer f electrophotoattttih o "tutorial or forming the aforementioned electroconductive vvlayer ne base. Possessed of r sistance FIG. 9 is .a side elevation in cross section of one; .pret od mbodim nt for, putti g the method .of this invention ,t o .working Nurneralflo\irt FIG. ;9 denotesandlessbo f r onv y na thes eotfshape l o roph Aeeordingly, developingjelectrode;intensistashi l-mn e iali10; wi hei tent i ge face he eo facin -i PP-Wa fd yi s su belt. 1 here m y, besod, flex ble,mota r el r:motalgauze belt; Plastic belt,- ubber". be t; an fabric-be t: T e ndless elt is i ri n a arqunda pai s tene s-9 ..,and 2i; he sheet. 10' is re o t ed on thebe t. 9 and co vey d om r to t The electroconductive layer 10 isgrounded either via the belt or independently. The sheet'carryingplane of the belt 90 is inclined by an angle of about 5 to:.20 with reference to the horizontal plane so as to facilitate the flow of developing liquid from rightto left. Thedeveloping, liquid 96. is stored in the container. scooped up by means of. a pump' which' is n'otiIlus trated; and applied to the nozzle 94..At asubStantially uniform distance upward from the sheetconveyingsurfaceof the endless belt 90, there is disposedaidevelop ing v electrode 93 of metal plate; grounded. via thegrounding resistance R. In onexworking example of the 1 device illustrated in FIG. 9 R-isflOflJhe length of d'eveloping electrode in the direction of travel is 20 cm,
-. the space between'the developingelectrode and.the; electrostatic latent image surface is 052mm; andthe conveying: speed of sheet is Zorn/sec.
FIG. 10 illustrates another preferredv embodiment of this invention, namely, a. device suitable for developing a. web-shaped electrophotographic material loofwhich;
is paid out of a roll. Theweb l00'retaining the electrostatic. latent imageon the. outside-iszdriven around-Jthe'" drum 10-1. The lower portionof the. drum "isheld: in contact with the liquid developer 104; and the develop. I ing electrode 102 zhavinga curved plane is spaced with a suitabletopeningfrorn the weband grounded-via I a resistanceR.
FIG. 11 is a sideelevation in cross section'of still an present invention-proves other example for which the particularly effective.
Numerals 11 191 through' 1 11 5 denote rollers which are made to rotate at a fixed speed irronesamedirection, and. numerals 112-1 through ll2-15:*are developing electroderollerswhich'areadapted .to rotate at a fixedspeed in oneandsame direction while :confront ing theaforementioned rollers. .The rollers -11 l and the rollers 112 .may be held in intimate contact or may be" separated slightly, Therollers 111 may. be .those made ofmetal or non-metal. lnorder thatthe rollers 1 12 may serve as developing electrode, they are requiredrto beielectroconductive on the surface or, if they are insular-"- ingon thesurface, they are required to be electrocon ductive in the regionclose to the surface. To be spe-w cific, metallic rollers; having the .surfacethereof ren--- dered insulating or semi-conductive, andinsulating rollers having the surface thereof covered with an electroconductive layer are also used. Numerals l 13-1 through 113-6 arenozzles used for sprayingthe rollers 112 .with the developing liquid. The developing liquid is supplied by a pump not illustrated and fed to the nozzles. The pairs of rollers 11] and rollers 112 seize and: transfer the sheet 10 containing the electrostatic latentimage on the upper surface at a fixed, speed toward rollers, '112 in the example shown in FIG: 11..
v In, FIG. 12, numeral 121-. denotes a side plate made' such as of metal so as to serve as the bearingof the roll ers. 0n the side plate; there is provided a notch-11$ for inserting the shaft 114 ,of rollers. To keep the shaft 1 I 4 of rollers from direct contact with the side plate 121, the resisting element 122 is disposed on the surface of the notch. A convenient resisting element for this purpose may be prepared by mixing silver or carbon particles into a resin or by blending electroconductive particles of silver or carbon into ceramic material.
FIG. 13 shows the shaft of rollers having a resisting element 13] disposed on the surface thereof instead of working on the side plate. As the resisting element, that which is similar to the resisting element 122 of FIG. 12 may be used.
In FIG. 12 and FIG. 13, if members 122 and 131 are insulating a resistor is connected between the side plate 121 and the shaft 114.
FIG. 14 illustrates still another preferred embodiment for which the present invention proves effective.
In FIG. 14, some numerals as those used in FIG. 10 are used to denote identical items. Numeral 140 is an electroconductive liquid, which may be water, an aqueous solution, or mercury. The liquid 140 is so selected that it will have a larger specific'gravity than the developing liquid 104 and it will not be miscible with the liquid developer. Numeral 141 denotes the electrode inserted into the liquid 140 to serve the purpose of grounding the liquid 140 via the resistance R. The electrode 141 is not required where the liquid 140 has a high electroconductivity, but is required when the electroconductivity of the liquid 140 is relatively small (of the order of 10 for example). Where an electroconductive liquid is used as the developing electrode (as disclosed in Japanese Patent Publication No. 44-6388,
for example), fogging tends to occur because the space between the latent image surface and the developing electrode is very small. Accordingly, the method of this invention is extremely effective.
The magnitude of the grounding resistance to be used for the present invention cannot be fixed indiscriminately because it varies with the properties of liquid developer, the properties of electrophotographic material, and the nature of latent image. Where letters are reproduced, for example, R may be fairly large. In reproducing an image of continuous toner, details in the highlight area cannot be reproduced when R is too large. The highlight area suffers from fogging if R is too small. It is, therefore, desirable to minimize the value of R within the range which is free from formation of fogging. Empirically, it has been found that R is desired to fall in the range of 10' 10 9.
Even if the developing electrode is insulated after the liquid developer is poured between the developing electrode and the electrophotographic material the developing electrode is earthed through the liquid developer. As the volume resistivity of the liquid developer is about 10"!) cm, on the assumption that the gap between the developing electrode and the surface of latent image is 0.] mm and that the facing area is 100 cm, the resistance of the liquid developer is What is claimedis:
1. Apparatus for developing an electrostatic latent image on an electrophotographic sheet of material optionally having an electroconductive backing layer, said apparatus comprising:
a developing electrode disposed adjacent to and spaced from said electrophotographic sheet of material;
means for providing a developing liquid between said developing electrode and said electrophotographic sheet of material to develop said electrostatic latent image;
resistive means for connecting either or both of said developing electrode and said electroconductive backing layer to -a reference potential, the resistance of said resistance means being in the range of 10 to 10 ohms, to prevent excessive background fogging and image streaking during development of said latent image.
2. Apparatus as in claim 1 wherein said reference potential is 0 volts.
3. Apparatus as in claim 1 including means for effecting relative movement between said electrophotographic sheet of material. and said developing electrode.
. 4. Apparatus as in claim 3 including a container for storing said developing liquidand where said means for effecting relative movement between said electrophotographic sheet of material and said developing electrode includes an endless belt driven around a pair of rollers, said electrophotographic sheet being disposed on said belt for movement with respect to said developing electrode and said endless belt being inclined by an angle of about 5 to 20 with respect to thehorizontal plane and where said means for providing'said developing liquid between said electrophotographic sheet and said developing electrode is disposed adjacent the upper end of said endless belt to introduce said developing liquid between said electrophotographic sheet and said developing electrode so that it flows therebetween and then to said container for said developing liquid which is disposed beneath the lower end of said endless belt. I
5. Apparatus as in claim 3 including a container for said developing liquid, said developing electrode being disposed in said developing liquid and where said means for effecting said relative movement between said electrophotographic sheet and said developing electrode includes a rotatably driven drum having disposed around a portion thereof said electrophotographic sheet, said portion being partially disposed in said developing liquid and adjacent said developing electrode. 1
6. Apparatus as in claim 3 including a first plurality of drive rollers disposed at one side of said electrophotographic sheet and where said developing electrode includes a second plurality of drive rollers respectively disposed adjacent said first plurality of drive rollers at the other side of said electrophotographic sheet, said first and second plurality of drive rollers moving said electrophotographic sheet therebetween.
7. Apparatus as in claim 6 including a shaft for each of said second plurality of rollers and a bearing for each of said shafts. Y
8. Apparatus as in claim 7 where said resistive means is disposed on the sides of said bearing and in electrical connection with said shaft and said bearing being connected to said reference potential;
9. Apparatus as in claim '7 where at least the outer portion of said shaft comprises said resistance means, said shaft being in electric contact with said bearing and said bearing being maintained at said reference potential. I 1
10. Apparatus as in claim 3 including a container for said developing liquid and for a second liquid having a larger specific gravity than and immiscible with said developing liquid and where said developing electrode is disposed in said second liquid and where said means for effecting relative movement between said electrophotographic sheet and said developing electrode comprises a rotatably driven drum having disposed around a portion thereof said electrophotographic sheet, said portion being disposed completely in'said developing liquid and partially in said second liquid whereby said developing liquid is dragged between said drum and said developing electrode due to the rotation of said drum to thereby effect said develp g a 1 g 11. Apparatus as in claim 3 including a container for said developing liquid and an electrically conductive liquid having a larger specific gravity than and immiscible with said developing liquid, said electrically conductive. liquid acting as said developing electrode and where means for effecting said relative movement between said electrophotographic sheet and said developing electrode comprises a rotatably driven drum having development of said latent image and an inclined elec? trophotographic material'transferring mechanism, said developing electrode arranged with a small space to the surface of the electrophotographic material and a noz- I zle arranged about the top of the developing electrode for supplying a liquid developer between the developing electrode and the electrophotographic material.
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