CA1041292A

CA1041292A - Xerographic developing apparatus

Info

Publication number: CA1041292A
Application number: CA192,003A
Authority: CA
Inventors: James M. Hardenbrook; Paul G. Andrus
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-02-15
Filing date: 1974-02-07
Publication date: 1978-10-31
Also published as: JPS5030537A; US3866574A

Abstract

IMPROVED XEROGRAPHIC DEVELOPING APPARATUS

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 spacedrelationship to, an image retaining member. Means are also provided to apply a pulsed electrical bias to the donor member to intro-duce 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 electric field applied across the gap is a result of a pulsed bias applied in such a manner so as to enable toner to deposit on the electrostatic image and to reduce deposition in non-image areas of the xerographic plate. The instant donor develop-ment system results in excellent copy quality with reduced background development.

Description

BACKGROUND OF T~:E INVENTION
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In the art of xerography as disclosed in U.S. Patent ~,297,691 to Carlson, a xerographic plate comprising a layer of photoconducting and insulting material on a conducting backing is given a u~iform electric charge over its entire surface and is then exposed to the subject matter to be reproduced usually by conventional projection techniques. This exposure results in discharge of the photoconductive plate whereby an electrostatic ~j latent image is formed. Development of the latent charge pattern is effected with an electrostatically charged, ~inely divided I material such as an electroscopic powder, that is brought into i surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic . . - .
latent image. Thereafter, the developed image may be fixed by any suitable means to the surface on which it has been developed or may be transferred 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 fewO In connection with these various developing ~ systems, it is known that a conductive control electrode as, for ;~ example-, disclosed in U.S. Patents 2,808,023, 2j777,418, 2,573,881 and others, is highly effective in influencing electrostatic gradients to develop images having varying charge gradients and ~ having relatively ~;arge solid image areas.

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At the same time, when developing images generally devoid of solid areas and consisting primarily of lined-copy lmages, ; superior results are generally obtainable without the electrode in place.
Another important development technique is disclosed in U.S. Patent 2,895,847 issued to Mayo. This particular develop~
ment 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 wlth an image bearing plate for deposit in conformity with the electrostatic image to be developed. In donor or transfer development of this type, the electrical properties of the donor are a factor for development in response to the area characteristics of the latent charge image. Specifically, electrically insulating donors respond best with line copy, while electrlcally conductive donors respond best with solid areas in a manner compa~able to the control electrode. Ac~ordingly, prior attempts to provide development flexibility on a p~actical basis for development of any kind o~ image, such as solid area versus line copy, have met with difficulty. This has resulted in limitations on the usual copying system and has necessitated selectivity with regard to particular materials to be reproduced.
As mentioned above, transfer development broadly in-volves bringing a layer of toner to an image 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 member which is capable of retaining the particles on its surface and then the donor member i5 brought into close proximity to the surface of the photoconductor. In the closely spaced position, partlcles of toner in the toner layer on the donor member, are attracted to

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LZ~2 the photoconductor by the electLostatic charge on the photo-conductor so that development takes place. In this technique the toner particles must traverse an air gap to reach the imaged regions of the photoconductor. In two other transfer techiques the toner-laden donor actually contacts the imaged photoreceptor and no air gap is involved. In one such technique, the toner-laden donor is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in a single rapid step. In another such technique, the toner-laden donor is skidded across the xerographic surface. Skiddingthe toner by as much as the width of the thinnest line will double the amount of toner available or development of a line which is perpendicular to the skid direction and the amount of ` skidding can be increased to achieve greater density or greater .
area coverage.
It is to be noted, therefore, that the term "trans~er development" is generic to development techniques where (1) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to e~fect development, (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded acroæs the imaged surface to effect development. Transfer development has also come to be known as "touchdown development".
In connection with transfer type development, it is known that by applying a controlled bias to a donor member characterized by appropriake electrical resistance while in contact with a plate being developed, that the donor functions to effect results-similar to a control electrode described above~
That is, by applying a bias potential to the rear surface of the donor member when presenting developer into contact with an , ~4~
electrostatic latent image, it becomes much more effective than an insulating or highly resistive unbiased donor for developing images haviny relatively large solid areas, as well as the various gradations of charge commonly associated with continuous tone images. At the same time, when developing images generally de-void of so]id areas and gradations in tone and consisting primarily of line copy images, substantially greater image ~- exposure latltude can still be obtained by developing with the donor in its inherently more resistive state without the benefit ! 10 of the corona bias applied thereto.
number of transfer type development systems were advanced in which background development was minimizedO In U.S.
Patent 31232,190 to Wilmott, a transfex 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 spacial dieposition of~said donor and image surface.
Activation of the-toner particles, i.e., removal from the donor surface, and attraction onto the image regions (development) was primarily due to the influence of the electrostatic force field associated with the charged photoconductive plate surface.

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- For this reason, the spacial positioning of the two coacting members (donors and photoconducting surface) in relation to each -other was critical. Should the members be in too close proximity excessive background development occurs, while too great a distance results in inadequate development.
- In the application of an electrical field to a transfer ; development system, a problem of background development arose.
This was due to the fact that, while applying a bias across the ; development zone enhanced the deposition of the electroscopic -- 4 ~

2gz particles onto the charge image pattern, the charged toner was also motiva~ed onto the uncharged or background areas of the pattern, thereby resulting in a background development.
In U.SO Patent 2,289,~00 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 electrostatic force field associated with the charged imaging element during activation and develop-ment. The application of this type of electrical force field cannot, h~wever, simply permit the toner particles to be trans-ported over a wider gap. Because the force field is continuous and uniform, no additional control is afforded over the develop-:~ ment 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 Canadian application Serial Number 191,594 :~ there is described a donor development system in which a high fsequency bias is applied between a spacially disposed image ~bearing su~face and a donor. The bias is created by applying : the voltag~ from an alternating.~current power supply between theplate and donor at frequencies of from about 10 to 3,Q00 kilo-; ~ cycles/sec. while the gap between the donor and image retaining member can be up to about 7 mils (1 mil equals 1/1000 of an inch).
While such a system results in good quality line . , :,-:, ' '.

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copy images, it has been found that superior quality in both line and continuous tone images can be attained utilizing a square pulse signal having proper frequencies and duty cycle voltage amplitudes in a transfer development system.
As can be ascertained from the above, the art of xerographic development, and in particular transfer development, would be significantly advanced if a pulsed bias could be used to improve both line and continuous tone quality in transfer development.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with 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 devel-oping material adjacent to the image retaining member, said donor member and image retaining member ~eing spatially disposed as to create a space gap between both members;
~) means to introduce a pulse bias across said gap, said pulse being comprised o~ an activation potential segment in which ,~ : 2a electroscopic particles are released from the donor member and a development.potential segment o 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.
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. ~ ~ ' - . 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.
30 Figure 1 is a cross-sectional view of a continuous ; automatic xerographic copying machine utilizing the developing .
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~ 0 technique o this inventionO
Figure 2 is a graphic illustration of the charactex-: istics of the contrslled pulsation techni~ue utilized in the . instant invention~
Figure 3 is a cross-sectional view of the developm~nt system of the present invention illustrating the particular mechanism thereof~

DETAILED DESCRIPTIO~ 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 likeO The apparatus includes the xerographic plate 10 in th~ form of a cylindri~al drum which comprises the photocon-ductive insulating peripheral surface on a conductive s~bstratus -~ ~ aboveO The drum is mounted on an alxle 15 ~or rotation, and ; driven by a motor 1~ thxough belt 17 connected to pulley 18 .
: secured to the shaft or axle 15.
Positioned adjacent the path of motion of the surface of tha drum 10 is a charging element 20 comprising, for èxample.
20 a positive polarity corona discharye electrode consisting of :

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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 pro-jection lens 25 or other exposure mechanism as is conventional in the art, preferably operating with slit projection methods to . . .
focus the movlng lmage 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 develop-ing station~is a transfer station 31 adapted to transfer a deyeloped image from the surface of the drum to a transfer web 32 that is advanced from supply roll 33 into contact with the surface o~ the xerographic drum at a point beneath a transfer 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 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 in-clude, 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.

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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 des-cribed, which revolves about a center axis 51. Rotation of the donor is effected by means of an axle 51 being driven by a motor ... ~
S5 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 i 10 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 ins~ant invention.
Between the donor member 50 and the drum 10, there is maintained a spacial gap 70 of from about 2 to 20 mils (1 mil equals 1/1000 of an inch). The actual development step within ` the purview of the instant invention is achieved maintaining a gap of between 2 to 7 mils between the rotating donor and photo-I ZO receptor utilizing a pulsed electrical field to establish the i ~ proper field relationships whereby optimum line and solid develop-ment is~effected with a minimum of background deposition. Any type of pulse generating source, including combinations of D.C.
s~urces, which will effect the requisite pulsing (to be discussed hereinafter) will be s~itable within the purview of the present . . . .
invention.
; Adjacent 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 _ g _ Z~ ;
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 in-vention 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 orm af a grid pattern. The enamel layer would have a thlckness of about 2 x 10 3 inches, while the copper grid layer would be in the order of 5 x 10 4 inches in thickness. The typical grid pattern on a donor member of thls type general'ly has from about 120 to 150 lines per inch with the ratio of nsulator-to-grid surface areas being about 1.25 to 1Ø
In order that a donor member function in accordance with the instant invention, it must first be characterized by : , .
sufflcient strength and durability to be employed for continuous recycling, and in addltion should preferably comprise an electri-. , .
cal insulator or at least possess sufficient high electrical resistance of approximately 1012 ohrn-cm or greater. This is not to be considered an absolute limitation, since the resistivity ,; .
~ requirement will become less than about 1011 ohm-cm and below .. . .
with reduced time period of exposure between the particular in~
crernental area of the donor and the xerographic plate. Hence, the use of donor material of too low a resistivity permits ex-cessive 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 suitable for this purpose include Teflon*, polyethylene terephthalate (Mylar*), and i polyethylene.
In carrying out a preferred method of development with-in the purview of the present invention, a microfield donor of the type described 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 `~ tonex 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 additional steps such as 20~ 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 ~ubstrate. 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 negatively using a negative corona (see 71 of Figure 1). As the toner passes peripherally adjacent the spacially disposed photoconductive layer havlng *trade marks the elec~rostatic image dispos~d 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 and facilitates continuous tone development.
Referring now to Figure 2, the pulse cycle contemplated in 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 posi~ive part described as development transfer, defined by a potential Vd which operated for a time Td. The number of times per second a pulse cycle is repeated is defined as the repetition rate R, where i, -R = k _. Where the activation and development times are T + T
a d given in microseconds (1 sec. = 1,000,000 microseconds), and k is a proportionality constant, 1000, the repetition rate is given i~; in kilo-Hertz (KHz). A zero volt reference is used for all -; voltage levels. In reality, the pulse is not perfect in shape;
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~ 20 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 seen in Figure 2 any definition of parameters of a square pulse have to account for an activation potential Va, an activation time Ta, a development potential Vd, and a repetition (or frequency) rate. These parameters may be varied to accommodate donor-photoreceptor spacings of from 2 to 20 mils (1 mil = 1/1000 of an inch). Activ~tion times Ta betweQn 10 and 200 microseconds and development times Td between 100 and 500 microseconds (repetition rates between about 1 1/2 and 10 kilo-" ~

Hertz) give improved results. Best results are obtained with spacïngs between 2 and 7 mils, activation times between 30 and 70 microseconds, and development times between 100 and 180 micro-seconds (repetition rates between about 4 and 8 kilo-Hertz).
Typical times are 50 microsecond activation time and 150 micro-second development time, resulting in a repetition rate of 5 kilo-Hertz.
The activation potential at spacings of from 2 to 7 mils is about -lS0 volts or greater (i.e. -150 volts, 200 volts, etc.). The development potential at these spaces is about +400 ~ volts or greater (~450 volts). Ranges of the activation potential ;~ ~ (Va)l are from about -150 to -450 volts. The development potential varies from abou* +400 to +800 volts. Any combination of Va and Vd can be used, the preference being that the peak amplitude of ~,~ the pulse bias, i.e., the difference between Va and Vd, not exceed 8QO volts.
While not to be construed as limiting, a general des-oription o possible mechanism occurring at the development ; lnterface, i.e., the space gap between the donor and photoconduc-tive surface, is shown in Figure 3. As shown, the blas level during the activation portion of the pulse is such that the negative toner particles experience a field force in thé direction of-the photoreceptor 10 comprised of a substrate 11 and photo~
conductiue.layer 12~ This force is in addition to the force pro-duced by the potential on the photoreceptor and, fox 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 imaged element. Therefore, the actual position of the toner particle 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 background areas.
The experimental work carried out in devaloping 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 tray of toner -~ and then charged negatively. The actual transfer development ;` step was completed by rolling the donor over a halogen doped selenium plate. The donor-to-photoreceptive spacing was main-`'~ tained by plastic shim stock placed on the edges of the plate.
Nominal spacings of from 2 to 7 mi:Ls were used on most tests~
~ Since the primary objective of the experimentation was to in-; ~ vestigate development variables, the charging and loading functions -` 20 were kept reasonably constant. Typical toner layers were 2 to i~ 2 1j2 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 ~uality measure-ments were made on semimicro densitometer systems and pulse variabIes were set and monitored on an oscilloscope at all phases : . .
of experlmentation.
Since many-chages could be made, the above invention and many apparently widely different embodiments of this in-vention could be made without departing from the scope thereof, it is intent that all matter contained in the drawings and speci-fications should be interpreted as illustrative and not, in any sense, limiting.

Claims

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 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, said donor member and image retaining member being spacial-ly disposed as to create a space gap between both members;
(b) means to introduce a pulse bias across said gap, said 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 donor thereby preventing particle deposition in the non-image areas.

2. The apparatus of Claim 1 wherein the spacial gap measures from about 2 to 7 mils.

3. The apparatus of Claim 1 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.

4. The apparatus of Claim 3 wherein the difference between the activation and development potentials is no greater than 800 volts.

5. The apparatus of Claim 1 wherein the activation potential takes place from periods of about 30 to 70 microseconds and the development potential from periods of about 100 to 180 microseconds.

6. The apparatus of Claim 5 wherein the activation and development time segments of the pulse result in a repetition rate of from about 4-8 kilo-Hertz.

7. The apparatus of Claim 5 wherein the activation and development time segments of the pulse result in a repeti-tion rate of 5 kilo-Hertz.

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

9. The apparatus of Claim 8 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.

10. The apparatus of Claim 'wherein the grid contains 120 to 150 lines per inch.

11. The apparatus of any one of claims 1, 2 or 3 wherein said pulse is applied to said donor member.

12. The apparatus of any one of claims 4, 5 or 6 wherein said pulse is applied to said donor member.

13. The apparatus of any one of claims 7, 8 or 9 wherein said pulse is applied to said donor member.

14. The apparatus of claim 10 wherein said pulse is applied to said donor member.