US 3891434 A
An electrostatic copier for use with zinc oxide coated copy paper in which a developing fluid having toner and particles is stored in a fluid absorbing cloth or the like. The cloth is placed against the exposed coated paper side. Toner particles migrate to the coated side only from cloth areas overlying a given paper zone. Cloth areas adjacent such a zone supply few or no toner particles so that the zone is uniformly toned without forming dark borders and light centers.
Description (OCR text may contain errors)
United States Patent [1 1 Taege et al.
1 POROUS MATERIAL [N A LIQUID DEVELOPMENT METHOD  Inventors: Herbert Taege,
Oberheidkamperstrasse, Bergisch-Gladbach; Friedrich Fliis, Lindenstrasse 28, 5161 Merzenich Krs. Duren; Giinther Mertens, Buchloerstr. 26, Kaufbeuren, Allgau, all of Germany  Filed: Dec. 8, 1972 121] Appl. No.: 313,295
 US. Cl. 96/1 LY; 117/37 LE; 118/637;
 Int. Cl. 603g 13/10; 603g 13/22  Field of Search 96/1 LY; 117/37 LE; 118/637; 355/10  References Cited UNITED STATES PATENTS 3,096,198 7/1963 Schaffert 117/37 LE 3,102,045 8/1963 Metcalfe ct al. 96/1 LY X 3,124,482 3/1964 Olden 118/637 3,133,484 5/1964 Wright 117/37 LE X 3,247,007 4/1966 Oliphant.... 117/37 LE 3,343,956 9/1967 Wright 96/1 LY X 1111 3,891,434 1 June 24, 1975 3,592,678 7/1971 Honjo et a1 117/37 LE 3,634,867 1/1972 McConnell 1 17/37 LE X 3,669,073 6/1972 Savit et a1 118/637 3,776,723 12/1973 Royka et al. 96/1 LY FOREIGN PATENTS OR APPLICATIONS 2,038,341 2/1972 Germany OTHER PUBLICATIONS Crawford, Developing Electrostatic Charge Patterns," IBM Tech. Discl. Bu1l., Vol. 8, No. 4, Sept. 1965, p. 527.
Primary ExaminerRo1and E. Martin, Jr. Attorney, Agent, or FirmTownsend and Townsend 5 7] ABSTRACT An electrostatic copier for use with zinc oxide coated copy paper in which a developing fluid having toner and particles is stored in a fluid absorbing cloth or the like. The cloth is placed against the exposed coated paper side. Toner particles migrate to the coated side only from cloth areas overlying a given paper zone. Cloth areas adjacent such a zone supply few or no toner particles so that the zone is uniformly toned without forming dark borders and light centers.
4 Claims, 5 Drawing Figures 1 POROUS MATERIAL IN A LIQUID DEVELOPMENT METHOD BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for electrostatically copying employing a zinc oxide coated substrate such as paper. The zinc oxide coated side hereinafter coated side" of the paper is first negatively charged and, thereafter, selectively discharged in proportion to the light intensities of an image directly or indirectly directed on to the coated paper side. The coated side is then contacted with a carrier fluid that has positively charged toner particles.
Known electrostatic copying systems briefly submerge the exposed paper in a toner bath. It has been observed that black areas of copy papers developed in this manner have border zones which are blacker or darker than more centrally located portions. To obtain a more even toning, it has been proposed to position grids in the toner bath opposite the coated side to increase the supply of toner particles at the coated side. It has further been proposed to employ electrodes which reduce the time in which toner particles are attracted to the coated side.
These prior art attempts have made the toning more uniform. However. especially with respect to the reproduction of gray areas, the discussed prior art copying systems are not satisfactory.
SUMMARY OF THE INVENTION The present invention provides a method and apparatus for obtaining copies in which even relatively large dark areas exhibit uniform toning and which provide a true reproduction of gray's.
The method of the present invention comprises the steps of positioning a cloth soaked or the like with the toner fluid against the zinc oxide coated side of the copy paper to thereby contact that side with the toning fluid and prevent a centripetal attraction of the toner particles to the electrostatically charged coated side of the substrate.
The present invention is based on the realization that in known electrostatic copying systems the toner reduction towards the center of larger dark areas results from the fact that the border areas of the darkened surfaces, which adjoin white areas, have a greater supply of and attract more toner particles than more centrally located zones of darkened areas. The charged paper attracts the toner particles centripetally, that is it attracts toner particles from toner overlying dark as well as light surfaces. By masking the coated side with a cloth, a like supply of toner particles for deposit in the various zones of darkened areas on the copy paper is assured. A cloth which counteracts the centripetal motion satisfactorily and which provides a sufficient supply of toner particles for the darkening of charged paper zones must exhibit significant capillary action. in addition, it requires a smooth surface to prevent the formation of traces or marks of the webbing structure on the coated side.
It is preferred to immerse the substrate in a developing or toner bath while the cloth or the like masks the coated side of the substrate.
To assure that the cloth supplies sufficient toner particles to the coated side the cloth is preferably pressed or biased against the coated side.
Excellent results have been obtained when the side of the cloth facing away from the copy paper is masked against the toner or developer bath so that no additional toner particles from the bath can be absorbed by the cloth while toner particles are deposited on the copy paper. While the cloth is biased against the coated side the masking of the cloth on the one side and the paper itself on the other side of the cloth form a cavity that is filled by the cloth and that cannot accept additional particles from the bath. In this manner, the capillaries of the cloth or the like supply a quantity of toner particles to the coated paper side which is a function of the electrostatic charge of the various paper zones.
The present invention further provides an electrostatic copying apparatus or copier for performing the above described methods. The copier has electrodes or a corona for charging the coated paper side, means for exposing the paper and means for transporting it past various work stations, including the developing station of the copier. The developing station includes a tank filled with developing or toner fluid for submerging the exposed copy paper therein.
In accordance with the present invention, those portions of the transport means of the copier which pass the paper through the toner bath include two longitudinally movable, opposing bands that extend through the bath and are under tension. The copy paper is retained between the bands. The band facing the coated copy paper side is constructed of material having large capillary action and a smooth surface. It further includes means submerged in the bath for applying pressure against the band contacting the coated paper side over a relatively large pressure area.
The pressure applying means preferably comprises a rotatably mounted roll or cylinder having a continuous, smooth surface over which the bands retaining the copy paper are looped.
Good results have been obtained by looping the bands over roughly one-half the periphery of the roll.
Particularly good results have been obtained by constructing the band facing the coated paper side of material which has a slight negative charge when imersed in the toner bath. Such a material, for example, is Velounyl-Double Jersey.
Before the negative recharged band arrives at the pressure roll, the negative charge causes an oversaturation of the capillary structure with positive toner particles. When this over-saturated band is biased against the electrostatically charged, coated paper side an over-supply of toner particles is present. The number of toner particles attracted by the coated side corresponds to the relative magnitude of the electrostatic charge of each paper zone contacted by the band. The slight negative charge of the band assures that toner particles are retained by those band portions which overlie only slightly charged copy paper zones. Thus, if intensive light exposure of a coated side zone reduce the electrostatic charge of that zone close but not completely to zero, the zone remains light or untoned. The slight negative charge assures that toner particles are only supplied to those zones which have a larger negative charge caused by gray or black areas of the projected image.
During the transfer of toner particles to the zinc oxide layer on the copy paper the roll completely masks the band from the toner bath. This assures that the cloth fully fills the cavity defined by the roll on the one side and by the paper on the other side so that no additional toner particles can transfer from the toner bath to the band and/or the copy paper.
The method and apparatus of the present invention enable the reproduction oflarge, uniformly toned dark areas adjacent even large light or untoned areas. The invention also assures a correct reproduction of grays.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates an electrostatic copier constructed in accordance with the invention;
FIG. 2 is a fragmentary, enlarged schematic representation of a developer station constructed in accordance with the invention;
FIG. 3 is an enlarged, fragmentary representation of the electrostatic charging of the copy paper;
FIG. 4 is an enlarged, fragmentary representation of the exposure of the charged paper to light and the corresponding variations in the electrostatic charge of various paper zones; and
FIG. 5 is a fragmentary, side elevational view and illustrates the intimate contact between the capillary cloth and the coated copy paper side as they are looped over the pressure roller.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, and electrostatic copier constructed in accordance with the invention comprises a paper supply roll 1 which has a zinc oxide coated side 32 and from which a paper band 2 is spooled off. A cutoff shear 3 severs individual paper sheets from band 2. Transport devices 4, 5, 6 and 7 convey the sheets through the copier to discharge chute 8. The sheets are conveyed through a corona between two opposing electrodes 9 and 10. Electrode I0 is opposite coated paper side 32 and is charged to 12 KV while electrode 9 is at ground potential or zero volts.
An exposure station 11 is downstream of the electrodes. An image of an original such as sheet containing typing or the like is projected on to sheet 12 via mirror 13 and one or more lenses 14. Transport device 6 then conveys the exposed paper to developing station 16.
Referring to FIGS. 1 and 2, developing tank 16 comprises a tank 17 filled with a liquid developer 18. The developer comprises a carrier fluid having dispersed therein positively charged toner particles. Two endless band loops l9 and 20 extend through the tank. At least a portion of the bands is submerged in the developer and the bands overlie each other while so submerged. A plurality of parallel guide rolls or cylinders 21, 22, 23, 24, 25, 26, 27 and 28 are provided for guiding the bands in endless loops through the tank and the developer. Rolls 21, 22 and 23 describe a triangle. Roll 21 has a relatively large diameter and is submerged in developer 18 while rolls 22 and 23 are disposed outside the tank above developer surface 33. A fourth roll 24 is arranged between rolls 22 and 23 and serves as a tensioning roll for band 20 which faces zinc oxide coated side 32 of the copy paper sheet.
Additional roll pairs 25, 27 and 26, 28 are positioned beneath rolls 22 and 23, respectively, for guiding band 19. The upper roll (27, 28) of each pair is disposed outside the tank above the developer surface while the lower roll and 26) of each pair is submerged in the developer. Means (not separately shown) is provided for rotating rolls 2I, 22 and 23 at a constant surface speed. Band 20, looped about these rolls, is thereby driven at the surface speed of the rolls. The diameter of roll 21, which serves as means for biasing the coated copy paper side against band 20 is sufficiently large so that band 20 contacts approximately one-half the circumference of the roll as illustrated in the drawing.
Referring to FIGS. 1 through 5, in operation copy paper 29 enters into gap 3I between rolls 22 and 27. The paper is oriented so that its zinc oxide coated side 32 faces band 20. As a result of its capillary action and its slight negative charge band 20 absorbs and becomes saturated with positively charged toner particles while it is submerged in developer 18 and as long as it does not come into contact with pressure roll 21. As band 20 loops over the downwardly facing portion of the pressure roll circumference band 19 biases the coated side 32 against and into intimate contact with band 20. This contact masks the band from the toner bath and prevents the absorption by the band of additional toner particles. Simultaneously it enables a transfer of toner particles from band 20 to the coated paper side as a function of and in proportion to the electrical charge of the various zones on the coated paper side. In the contact area between band 20 and the exterior surface of pressure roll 21, the band is in intimate contact with the roll to effectively form a cavity which is closed by the roll circumference on the one side and the coated paper on the other side. Thus, the cavity is formed and masks both the coated side and the band while toner particles transfer from the band to the coated side. A centripetal attraction of toner particles from the sourrounding developer bath is thus effectively prevented. The size of roll 21 and its rate of rotation determine the time period during which toner particles can be attracted by the copy paper. It is preferred to give the roll a relatively large diameter to form a relatively large cavity during the developing of the paper and to therebymore effectively prevent a centripetal attraction of toner particles to negatively charged paper zones.
I. An electrostatic copying method including the steps of negatively charging a zinc oxide coated side of a substrate, thereafter partially discharging said zinc oxide coated side in accordance with and as a function of the light intensity of an optical image obtained from an original to be copied, contacting said zinc oxide coated side with a fabric and submerging said substrate and said fabric in contact with said substrate in a developer which includes a carrier liquid and positively charged toner particles to tone certain zones of said zinc oxide coated side and to leave other untoned zones of the substrate, thereby to supply said zinc oxide coated side with toner particles from a portion of the fabric overlying a given zone of said zinc oxide coated side and substantially to prevent the attraction of toner particles from other portions of the fabric whereby substantially uniformly toned zones are obtained, said fabric being formed of a material which has a slight negative charge relative to said developer with which it is soaked.
2. A method according to claim 1 including the step of pressing the fabric soaked with said developer against said zinc oxide coated side of said substrate.
3. A method according to claim 2 including the step of masking a side of said fabric facing away from said 6 contact with the uncoated side of said substrate and then passing the assembly of webs and substrate into a developer bath and round a pressure roller with the further web under tension to compress said fabric web between said further web and said pressure roller.