CA1057590A - Application of pulse bias across gap between donor and imaged member - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus for developing a latent xerographic image is disclosed. The development device comprises a toner supporting donor member adjacent, and in spaced relationship to, an image retaining member. Means are also provided to apply a pulsed electrical bias to the donor member to introduce an electrical field in the gap between the donor and image retaining member whereby the electroscopic particles are made more readily available to the charged image thereby resulting in fine image development. The pulse applied across the gap can be of two different frequencies to insure either good line copy quality or faithful tonal reproduction of an original. The instant donor development system enable reproduction of line and pictorial images.

Description

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BACKGROUND OF THE INVENTION
In the art of xerography~as disclosed in U.S. Patent

2,297,691 to Carlson, a xerographic plate comprising a layer of photoconducting and insulating material on a conducting backing is given a uniform electric charge over its entire surface and is then exposed to the subject matter to be repro-duced usually by conventional projection techniques. This exposure results in discharge of the photoconductive plate whereby an electrostatic latent image is formed. Development of the latent charge pattern is effected with an electrostati-cally charged, finely divided material such as an electroscopic powder, that is brought into surface contact with the photo-conductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. There-after, the developed image may be fixed by any suitable means to the surface on which it has been developed or may be trans-ferred to a secondary support surface to which it may be fixed or utilized by means known in the art.
In any method employed for forming electrostatic images, they are usually made visible by a development step.
Various developing systems are well known and include cascade, brush development, magnetic brush, powder cloud and liquid developments, to cite a few. One other important development technique is disclosed in U.S. Patent 2,895,847 issued to Mayo. This particular development process employs a support member such as a web, sheet or other member termed a "donor"
which carries a releasable layer of electroscopic marking particles to be brought into close contact with an image bear-ing plate for deposit in conformity with the electrostatic image to be developed. Development processes of this type are ` 1~575~(~

termed transfer development.
One form of transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles will be transferred from the layer to the imaged areas. In one transfer development technique, the layer of toner particles is applied to a donor mernber which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In the closely spaced position, particles of toner in the toner layer on the donor member are attracted to the photoconductor by the electrostatic charge on the photo-conductor so that development takes place. In this technique the toner particles rnust traverse an air gap to reach the imaged regions of the photoconductor. The present invention relates to this type of transfer development, i.e., the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to effect development.
In U.S. Patent 3,232,190 to Wilmott, a space gap transfer type development system is disclosed in which the charged toner particles are typically stored on a donor member and development is accomplished by transferring the toner from the donor to the image regions on the photoconductive surface across a finite air gap caused by the spatial disposition of said donor and image surface. Activation of the toner particles, i.e., removal from the donor surface, and attrac-tion onto the image regions (development) was primarily due to the influence of the electrostatic force field associated with the charged photoconductive plate surface. E~or this reason, the spatial positioning of the two coacting members (donors and photoconducting surface) in relation to each other 1~57S9O
was critical. Should the members be in too close proximity, excessive background development occurs, ~hile too great a distance results in inade~uate development.
In an attempt to alleviate the criticality of the spacing between the donor and the photoconductor, a bias poten-tial was introduced to aid the motivation of the toner to the charged image areas. Therefore, in U.S. Patent 2,289,400 to Moncrieff-Yeates, there is disclosed an out of contact transfer development system in which a continuous and uniform force field is established within the transfer zone and assists the electro-static force field associated with the charged imaging element -during activation and development. The application of this type of electrical force field cannot, however, simply permit the toner particles to be transported over a wider gap. Because the force field is continuous and uniform, no additional control is a~forded over the development process. Therefore, the electrostatic force field associated with the latent image still remains the predominant mechanism by which the toner particles are both activated and attracted to the imaged area of the photoconductive surface.

In copending application Ser. No.192,003 filed February 7, 1974 there is described a transfer development system which utilizes a spaced donor-receptor system in combination with a pulsed bias of different polarities to effect development of imaged areas while preventing deposition on background areas. m e donor and photoreceptor preferably operate at spacings between 2 and 7 mils while the frequencies of the pulse are from 4 to 8 kilo hertz, the negative polarity operating between 30 and 70 microseconds.

5'~'5~0 In xerographic development, generally two types of quali y reproduction are desirable. There is line copy quality in ~hich reproduction of the letteriny is more important than ~aithulness of reproduction of the original. This occurs in the case of a withered document with a grey background. The second type of development is pictorial, i.e., reproduction of a grey or colour gradient. In this instance faithfulness of reproduction is esseniial, a perfect example being photographs.
While the aforementioned development system (Serial No. 192,003) is satisfactory for line copy, it is not effectively adaptable for pictorial reproduction. The instant invention presents a transfer development system which enables control of the tonal response of the development system 50 as to be capable of both line copy and pictorial reproduction.
BRIEF DESCRIPTION OF THE INVENTION
-In accordance with one aspect of this invention there is provided an apparatus for developing a latent electrostatic image recorded on an image retaining member comprising: (a) a donor member for supporting a uniform layer of electroscopic developing material adjacent to the image retaining member, the donor member and the image retaining member being spatially dis-posed as,to create a space gap between both members; (b) means for charging the developing material on the donor member to a polarity opposite to that of the image retaining member; (c) means to move the donor member and the charged toner thereon to the zone where the gap exists; and (d) means to selectively introduce high and low frequency pulse biases across the gap, the pulses being comprised of an activation potential segment in which electroscopic particles are released from the donor member and a development potential segment of different polarity in which the electroscopic particles in non-image areas are attracted towards the donor thereby preventing particle deposi-tion in the non-image areas.

_5_ 1(3S7590 In accordance with another aspect of this invention there is provided a method of development of a charge pattern on a photoconductive surface of an apparatus including a donor member for supporting a uniform layer of electroscopic develop-ing material adjacent to the image retaining member, the donor member and the image retaining member being spatially disposed as to create a space gap between both members, and means to selectively introduce high and low frequency pulse biases across the gap, the method comprising the steps of: (a) loading the donor with toner; (b) charging the developing material to a ` polarity opposite to that of the photoconductive surface; and (c) selectively applying a high or low frequency pulse bias across the gap, the respective pulse being comprised of an activation potential segment in which electroscopic particles are released from the donor member and a development potential se~ment of different polarity in which the electroscopic particles in non-image areas are attracted towards the above thereby preventing particle deposition in the non-image areas.
BRIEF DESCRIPTION OF THE DR~WINGS
This invention will become more apparent upon consideration of the following detailed disclosure, along with specific embodiments of the invention, especially when taken in conjunction with the accompanying drawings herein.
Figure 1 is a cross-sectional view of a continuous automatic xerographic copying machine utilizing the developing technique of this invention.
Figure 2 is a graphic illustration of the character-istics of the controlled pulsation technique utilized in the instant invention.
Figure 3 is a cross-sectional view of the development system of the present invention illustratiny the particular mechanism thereof.

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DETAILED DESCRIPTION OF THE DRAWINGS

Referring now specifically to Figure 1, there is illustrated a continuous xerographic machine adapted to form an electrostatic reproduction of a copy onto a paper sheet, web or the like. The apparatus includes the xerographic plate 10 in the form of a cylindrical drum which comprises the photo--6a-~()S759(1 conductive insulating peripheral surface on a conductive sub-stratus above. The drum is mounted on an axle 15 for rotation, and driven by a motor 16 through belt 17 connected to pulley 18 secured to the shaft or axle 15.
Positioned adjacent to the path of motion of the surface of the drum 10 is a charging element 20 comprising, for example, a positive polarity corona discharge electrode con-sisting of a fine wire suitably connected to a high-voltage source 22 or potentially high enough to cause a corona discharge from the electrode onto the surface of the drum 10. Subsequent to the charging station 20 in the direction of rotation of the drum, is an exposure station 23 generally comprising suitable means for imposing a radiation pattern reflected or projected from an original copy 24 or to the surface of the xerographic drum. To effect exposure, the exposure station is shown to include a projection lens 25 or other exposure mechanism as is conventional in the art, preferably operating with slit projection methods to focus the moving image at the exposure slit 26.
Subsequent to the exposure station is a developing station, generally designated 30, as will be further described below for rendering the latent image visible. Beyond the developing station is a transfer station 31 adapted to transfer a developed image from the surface of the drum to a transfer web 32 that is advanced from supply roll 33 into contact with the surface of the xerographic drum at a point beneath a trans-fer electrode 34. After transfer, the web desirably continues through a fusing or fixing device 35 onto a take-up roll 36 being driven through a slip clutch arrangement 37 from motor 16. Desirably, electrode 34 has a corona discharge operably '. _ ~ _ ~ ~b ~.~.~ :. r~?~A _~ ~.~_ .~.~ ~___. ___~_ ._~._.W __~. . __ ________ _.__ _ _ _ _ _. _ __ . _ _ _ . _ ._ . . _____ _.. __.. _._~._ ._.~ ~.. _ . ~ ~__. ..
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connected to a high-voltage source 40 whereby a powder image developed on the surface of the drum is transferred to the web surface. Fusing device 35 primarily fixes the transferred powder image onto the web to yield a xerographic print. After transfer, the xerographic drum 10 continues to rotate past a cleaning station 41 in which residual powder on the drum's surface is removed. This may include, for example, a rotating brush 42 driven by a motor 43 through a belt 44 whereby the brush bristles bear against the surface of the drum to remove residual developer therefrom. Optionally, further charging means, illumination means, or the like, may effect electrical or controlled operations.
Operative at the developing station 30 is a donor member 50 in the form of a cylindrical roll, as will be further described, which revolves about a center axis 51. Rotation of the donor is effected by means of an axle 51 being driven by a motor 55 operating through a belt 56, preferably to drive the cylinder in the same direction as the surface rotation of the drum. The speeds of the donor member and drum may be substantially the same or the donor member can travel at speeds as high as 5 to 10 times as fast as the peripheral speed of the drum to effect a greater development in imaged areas. Also affixed to donor member 50 is a pulse generator source 61 for applying the pulsed bias potentials of the instant invention.
Between the donor member 50 and the drum 10, there is maintained a spatial gap 70 of from about 5 to 20 mils (1 mil eguals 1/1000 of an inch). Preferred spacings, within the purview of the instant invention, are from about 5 to 10 mils between the rotating donor and photoreceptor utilizing a pulsed electrical field to establish the proper field relation-ll35~S90ships. Any type of pulse generating source, including com-binations of D.C. sources, which will effect the requisite pulsing (to be discussed hereinafter) will be suitable within the purview of the present invention.
Adjacent to one portion of the path of motion of the developer donor member 50 is a powder loading station which may, for example, comprise a developer hopper 57 containing a quantity of developer product 58 which may be a form of a toner or electroscopic powder. The hopper opens against the donor member whereby the cylinder passes in contact with the developer supply and is contacted uniformly with the toner powder as the donor passes through the developer. Other loading mechanisms may, of course, be employed including a dusting brush or the like, as is known in the art.
While the donor member of Figure 1 has been described in the terms of a cylindrical element, it is to be understood that said donor may be in the form of web, belt, or roll, or any other structure capable of operating within the purview of the instant invention. A preferred donor element of the present invention is a microfield donor consisting of a milled aluminum cylinder over which a thin layer of insulating enamel is placed, on which enamel layer there is a thinner layer of copper etched in the form of a grid pattern. The enamel layer would have a thickness of about 2 x 10 3 inches, while the copper grid layer would be in the order of S x 10-~ inches in thickness. The typical grid pattern on the donor member of this type generally has from about 120 to 150 lines per inch with the ratio of insulator-to-grid surface areas about 1.25 to 1Ø
In order that a donor member function in accordance 1~S'~590 with the instant invention, it must first be characterized by sufficient strength and durability to be employed for continuous recycling, and in addition should preferably comprise an elec-trical insulator or at least possess sufficient high electrical resistance of approximately 1012 ohm-cm or greater. This is not to be considered an absolute limitation, since the resisti-vity requirement will become less than about 1011 ohm-cm and below with reduced time period of exposure between the particular incremental area of the donor and the xerographic plate. Hence, the use of donor material of too low a resistivity permits excessive penetration of charge from the corona discharge source into the donor within the time of contact. As a result, as the low resistivity donor advances from charged to uncharged areas of the electrostatic latent image, the charges induced into the bulk of the donor causes excessive deposition of toner in these uncharged or background areas. At the same time, however, for development speeds giving shorter contact times, materials of lower resistivity may be used. Materials found æ suitable for this purpose include Teflon, polyethylene tere-phthalate (Mylar), and polyethylene.
In carrying out a pre~erred method o~ development within the purview of the present invention, a microfield donor of the type descrihed above is used as member 50 of Figure 1. Generally, the four basic steps in carrying out a development process are loading the donor with toner, corona charging the toner (see corona charging element 71 of Figure 1), passing the toner to the electrostatic latent image on the photoconductive surface, and cleaning residual toner from the donor member so as to allow repetition of the process. In the actual practice of development of most machines, there are e ~rks 1~:)5'7~
additional steps such as agglomerate toner removal and corona discharging of the donor member, which steps are auxiliary to the development process.
In loading a microfield donor of the type described above, a bias is applied to the grid which establishes strong electrical fringe fields between the copper grid and the grounded aluminum substrate. As the donor is rotated through a bed of vibrating toner, these fields collect toner on the donor in both grid and the enamel insulator areas. In the next process step this layer of toner is then charged nega-tively using a negative corona (see 71 of Figure 1). As the toner passes peripherally adjacent to the spatially disposed photoconductive layer having the electrostatic image disposed thereon, a square pulse of certain potentials (see 61 of Figure 1) is applied by the pulse generator at the donor to effect development. The overall effect of the pulsed bias is an oscillating negative and positive potential between the xerographic plate and the donor and the xerographic plate whereby toner is intermittently driven (activated) into the space gap, thereby being made readily available to the charge image, and attracted away from background areas.
Referring now to Figure 2, a pulse cycle contemplated within the purview of the instant invention is demonstrated.
Basically, the single pulse cycle is considered in two components, namely, a negative part described as activation and defined by an activation potential Va which operates for a time Ta~ and a positive part described as development transfer, defined by a potential Vd which operated for a time Td. The negative segment of the pulse is termed an activation potential because the toner has been charged negatively, as described above, and l(~S7S9~
therefore releases upon the negative potential on the donor.
The number of times per second a pulse cycle is repeated is defined as the repetition rate R or frequency, where R = k Ta+Td Where the activation and development times are given in micro-seconds (1 sec. = 1,000,000 microseconds), and k is a pro-por$ionality constant, 1000, the repetition rate is given in B ~ hertz (KHz). A zero volt reference is used for all voltage levels. In reality, the pulse is not perfect in shape; however, rise times are small enough so that they can be neglected. In utilizing the microfield donor elements described above, the pulse is usually applied to both the grid and aluminum substrate.
As can be appreciated from Figure 2, four independent parameters, negative amplitude, negative pulse duration, positive amplitude, and frequency, offer an infinite combination of development conditions. However, the present invention relates to the advantage of being capable of utilizing both high and low frequencies to control tonal response, the remaining para-meters being relatively fixed or defined. Additionally, spacings of from about S to 20 mils can be set at both instant high and low pulse frequencies. It has been found that high k;lo pulse frequencies on the order of about 18-22 ~i~ hertz results in extended tonal range of development, i.e., development of the gradient scales of gray are possible. On the other hand, pulsing at low frequencies of from about 2 to 5 ~ hertz yeilds strong black-white separation, i.e., excellent line copy reproduction. The use of a transfer development system having this dual capability would enhance any xerographic reproduction device.
As mentioned above, definition of parameters of a square pulse have to account for an activation potential Va, ~ S'75~V
an activation time Ta~ a development potential Vd, and a repetition (or frequency) rate. While all these parameters may be varied to accommodate donor-photoreceptor spacings of from 5 to 20 mils (l mil = l/lO00 of an inch), generally activation times Ta between 5 and 100 microseconds at frequency rates of from 2 to 5 and 18-22 ~i~e hertz give optimum results in the subject invention. Best results are obtained with spacings between 5 and 10 mils, activation times between 5 and 50 microseconds at the above cited frequencies. Typical times are from 20 to 50 microseconds activation time at the lower frequencies and from 5 to 25 microseconds at the higher fre-quencies.
The activation potential at spacings of from 5 to 20 mils is about -150 volts or greater (i.e., -150 volts, -200 volts, etc.). The development potential at these spacings is about +400 volts or greater (+450 volts, etc.). Activation potentials (Va) can be from about -150 to -1400 volts while development potential varies from about +400 volts to +1000 volts. The greater values of Va and Vd indicated are used at the larger values of the spacing between the donor and the photoreceptor. ~he peak value of the activation potential ~a is limited in part by the onset of an electrical breakdown phenomenon in the air gap 71 between the donor and the photo-receptor. The peak value of development potential Vd must be chosen such that the thickness of the electroscopic powder deposit on the developed image is sufficient for the ultimate use of the imaging process; i.e., the final copy must be adequate.
While not to be construed as limiting, a general description of possible mechanism occurring at the development l(~S~5~10 interface, i.e., the space gap between the donor and photo-conductive surface, is shown in Figure 3. As shown, the bias level during the activation portion of the pulse is such that the negative toner particles experience a field force in the direction of the photoreceptor 10 comprised of a substrate 11 and photoconductive layer 12. This force is in addition to the force produced by the potential on the photoreceptor and, for this reason, the image areas produce a higher activation force than the non-image or background areas. The duration of the activating field is important in that a fraction of this time is spent breaking the toner-donor bond, while the remainder is used to drive the toner toward the image element. Therefore, the actual position of the toner particles in the gap is dependent upon the forces applied, as well as the time duration of the activating force. A similar analysis can be applied to what happens during the actual development part of the cycle.
The bias levels which are established during the development part of the pulse are such that a negative toner particle in the gap experiences a field force away from the photoreceptor.
By means of this mechanism, toner not caught up in the field caused by the imaged areas is drawn onto the donor away from the non-image or bac~ground areas.
To recapitulate, the present invention relates to a transfer development system which is capable of controlling tonal response utilizing high and low frequency pulse biasing.
It has been found that the low frequency pulse engenders a strong contrast or unfaithful response in low density areas of an image, thereby being excellent for line copy. The high freqllency pulse results in faithful reproduction of low density areas (grays) and, therefore, is excellent for pictorial ~(~5'7S~O

quality. Because the high and low frequencies are used select-ively, the use of a switch or control connected to the pulse generating device would be appropriate for use in a copy machine.
The experimental work carried out in developing the instant invention utilized simple bench-type apparatus. A
Xerox 813 size cylindrical donor containing a grid of 120 lines per inch was loaded by rotating through a vibrating tyary of toner and then charged negatively. The actual transfer develop-ment step was completed by rolling the donor over a halogen doped selenium plate. The donor-to-photoreceptive spacing was maintained by plactis shim stock placed on the edges of the plate. Nominal spacings of from 5 to 20 mils were used on most tests. Since the primary objective of the experimentation was to investigate development variables, the charging and loading functions were kept reasonably constant. Typical toner layers were 2 to 2-1/1 mils thick and were checked optically. The charge on the toner layer was monitored by reading the potential above the toner layer after charging. Then the image quality measurements were make on semimicro densitometer systems and pulse variables were set and monitored on an oscilloscope at all phases of experimentation.
Since many changes could be made, the above invention and many appaxently widely different embodiments of this invention could be made without departing from the scope thereof, it is intent that all matter contained in the drawings and specifications should be interpreted as illustrative and not, in any sense, limiting.

Claims (19)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for developing a latent electro-static image recorded on an image retaining member comprising:
(a) a donor member for supporting a uniform layer of electro-scopic developing material adjacent to the image retaining member, the donor member and the image retaining member being spatially disposed as to create a space gap between both members; (b) means for charging the developing material on the donor member to a polarity opposite to that of the image retaining member; (c) means to move the donor member and the charged toner thereon to the zone where the gap exists;
and (d) means to selectively introduce high and low frequency pulse biases across the gap, the pulses being comprised of an activation potential segment in which electroscopic particles are released from the donor member and a development potential segment of different polarity in which the electroscopic particles in non-image areas are attracted towards the donor thereby preventing particle deposition in the non-image areas.

2. The apparatus of Claim 1, wherein the low frequency is from about 2 to 5 kilo hertz and the high frequency is about 18-22 kilo hertz.

3. The apparatus of Claim 1, wherein the high frequency is 20 kilo hertz.

4. The apparatus of Claim 1 or 2, wherein the spatial gap measures from about 5 to 20 mils.

5. The apparatus of Claim 1 or 2, wherein the activation potential is a negative polarity of greater than 150 volts and the development potential is a positive polarity of greater than 400 volts.

6. The apparatus of Claim 1 or 2, wherein the activation potential takes place from periods of about 5 to 100 microseconds.

7. The apparatus of Claim 1, wherein the donor member is in the form of a rotatable cylinder.

8. The apparatus of Claim 7, wherein the cylindrical donor comprises an aluminum substrate and an enamel surface layer containing an etched layer of copper in the form of a grid pattern.

9. The apparatus of Claim 8, wherein the grid contains 120 to 150 lines per inch.

10. A method of development of a charge pattern on a photoconductive surface of an apparatus including a donor member for supporting a uniform layer of electroscopic developing material adjacent to the image retaining member, the donor member and the image retaining member being spatially disposed as to create a space gap between both members, and means to selectively introduce high and low frequency pulse biases across the gap, the method comprising the steps of: (a) loading the donor with toner; (b) charging the developing material to a polarity opposite to that of the photoconductive surface; and (c) selectively applying a high or low frequency pulse bias across the gap, the respective pulse being comprised of an activation potential segment in which electroscopic particles are released from the donor member and a development potential segment of different polarity in which the electroscopic particles in non-image areas are attracted towards the above thereby preventing particle deposition in the non-image areas.

11. The method of Claim 10 which additionally com-prises the step of cleaning the residual developing material from the donor surface.

12. The method of Claim 10 or 11, wherein the low frequency is from about 2 to 5 kilo hertz and the high frequen-cy is about 18-22 kilo hertz.

13. The method of Claim 10, wherein the high frequen-cy is 20 kilo hertz.

14. The method of Claim 10, wherein the spatial gap measures from about 5 to 20 mils.

15. The method of Claim 10, wherein the activation potential is a negative polarity of greater than 150 volts and the development potential is a positive polarity of greater than 400 volts.

16. The method of Claim 10, wherein the activation potential takes place from periods of about 5 to 100 micro-seconds.

17. The method of Claim 10, wherein the donor member is in the form of a rotatable cylinder.

18. The method of Claim 17, wherein the cylindrical donor comprises an aluminum substrate and an enamel surface layer containing an etched layer of copper in the form of a grid pattern.

19. The method of Claim 18, wherein the grid contains 120 to 150 lines per inch.