Xerographic Developing Apparatus

Abstract

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 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 development system results in excellent copy quality with reduced background development.

Parent Case Text

This is a continuation-in-part of copending application Ser. No. 332,852 filed on Feb. 15, 1973, now abandoned.

Description

BACKGROUND OF THE INVENTION

In the art of xerography as disclosed in U.S. Pat. No. 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 reproduced 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 electrostatically charged, finely divided material such as an electroscopic powder, that is brought into 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 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. In connection with these various developing systems, it is known that a conductive control electrode as, for example, disclosed in U.S. Pat. Nos. 2,808,023, 2,777,418, 2,573,881 and others, is highly effective in influencing electrostatic gradients to develop images having varying charge gradients and having relatively large solid image areas. At the same time, when developing images generally devoid of solid areas and consisting primarily of lined-copy images, superior results are generally obtainable without the electrode in place.

Another important development technique is disclosed in U.S. Pat. No. 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 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 electrically conductive donors respond best with solid areas in a manner comparable to the control electrode. Accordingly, prior attempts to provide development flexibility on a practical basis for development of any kind of 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 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 member 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 photoconductor 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 techniques 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. Skidding the toner by as much as the width of the thinnest line will double the amount of toner available for 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 "transfer 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 effect 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 across 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 appropriate 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 electrostatic latent image, it becomes much more effective than an insulating or highly resistive unbiased donor for developing images having 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 devoid of solid areas and gradations in tone and consisting primarily of line copy images, substantially greater image exposure latitude can still be obtained by developing with the donor in its inherently more resistive state without the benefit of the corona bias applied thereto.

A number of transfer type development systems were advanced in which background development was minimized. In U.S. Pat. No. 3,232,190 to Wilmott, a 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 spacial disposition 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. 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 particles onto the charge image pattern, the charged toner was also motivated onto the uncharged or background areas of the pattern, thereby resulting in a background development.

In U.S. Pat. No. 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 electrostatic 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 afforded 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. 332,851, (internally designated as D/3234) filed on Feb. 15, 1973, now abandoned, there is described a donor development system in which a high frequency bias is applied between a spacially disposed image bearing surface and a donor. The bias is created by applying the voltage from an alternating current power supply between the plate and donor at frequencies of from about 10 to 3,000 kilocycles/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 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.

OBJECTS OF THE INVENTION

It is the object of this invention to describe a novel development system using a noncontacting donor.

A further object of this invention is to describe novel donor developing apparatus which enables development between a space gap formed between said donor element and image-bearing surface.

It is also an object of the present invention to describe a novel donor developing method.

BRIEF DESCRIPTION OF THE INVENTION

The above and other objects of the instant invention are attained by providing a donor member that is adjacent and in spaced relationship to a photosensitive plate and providing means for applying a pulsed bias to the donor member. The applied pulse is a combination of a short intense electrical pulse to release toner from the donor and start it towards the photoreceptor and a nominal bias to prevent background development. The instant pulsed bias development system makes possible good images over larger gap widths than those possible with application of a continuous bias. The instant invention results in excellent continuous tone development and line copy having little background development.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further features and advantages of the present invention will become 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.

FIG. 1 is a cross-sectional view of a continuous automatic xerographic copying machine utilizing the developing technqiue of this invention.

FIG. 2 is a graphic illustration of the characteristics of the controlled pulsation technqiue utilized in the instant invention.

FIG. 3 is a cross-sectional view of the development system of the present invention illustrating the particular mechanism thereof.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now specifically to FIG. 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 photoconductive insulating peripheral surface on a conductive substratus 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 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 consisting 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 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 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 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 photoreceptor utilizing a pulsed electrical field to establish the proper field relationships whereby optimum line and solid development is effected with a minimum of background deposition. Any type of pulse generating source, including combinations 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 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 FIG. 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 .times. 10.sup..sup.-3 inches, while the copper grid layer would be in the order of 5 .times. 10.sup..sup.-4 inches in thickness. The typical grid pattern on a donor member of this type generally has from about 120 to 150 lines per inch with the ratio of insulator-to-grid surface areas being about 1.25 to 1.0.

In order that a donor member function in accordance 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 electrical insulator or at least possess sufficient high electrical resistance of approximately 10.sup.12 ohm-cm or greater. This is not to be considered an absolute limitation, since the resistivity requirement will become less than about 10.sup.11 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 suitable for this purpose include Teflon, polyethylene terephthalate (Mylar), and polyethylene.

In carrying out a preferred method of development within the purview of the present invention, a microfield donor of the type described above is used as member 50 of FIG. 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 FIG. 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 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 negatively using a negative corona (see 71 of FIG. 1). As the toner passes peripherally adjacent the spacially disposed photoconductive layer having the electrostatic image disposed thereon, a square pulse of certain potentials (see 61 of FIG. 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 FIG. 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 V.sub.a which operates for a time T.sub.a, and a positive part described as development transfer, defined by a potential V.sub.d which operated for a time T.sub.d. The number of times per second a pulse cycle is repeated is defined as the repetition rate R, where

R = K/T.sub.a + T.sub.d.

Where the activation and development times are given in microseconds (1 sec. = 1,000,000 microseconds), and k is a proportionality constant, 1,000, the repetition rate is given in kilo-Hertz (KH.sub.z). 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 seen in FIG. 2 any definition of parameters of a square pulse have to account for an activation potential V.sub.a, an activation time T.sub.a, a development potential V.sub.d, 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). Activation times T.sub.a between 10 and 200 microseconds and development times T.sub.d between 100 and 500 microseconds (repetition rates between about 1 1/2 and 10 kiloHertz) give improved results. Best results are obtained with spacings between 2 and 7 mils, activation times between 30 and 70 microseconds, and development times between 100 and 180 microseconds (repetition rates between about 4and 8 kilo-Hertz). Typical times are 50 microsecond activation time and 150 microsecond development time, resulting in a repetition rate of 5 kiloHertz.

The activation potential at spacings of from 2 to 7 mils is about -150 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 (V.sub.a) are from about -150 to -450 volts. The development potential varies from about +400 to +800 volts. Any combination of V.sub.a and V.sub.d can be used, the preference being that the peakj amplitude of the pulses bias, i.e., the difference between V.sub.a and V.sub.d, not exceed 800 volts.

While not to be construed as limiting, a general description of possible mechanism occurring at the development interface, i.e., the space gap between the donor and photoconductive surface, is shown in FIG. 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 backgkround 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 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 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 maintained by plastic shim stock placed on the edges of the plate. Nominal spacings of from 2 to 7 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 1/2 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 made 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 apparently 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

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 spacially 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

2. The apparatus of claim 1 wherein the spacial gap measures from about 2

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

4. The apparatus of claim 3 wherein the difference between the activation

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

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

7. The apparatus of claim 5 wherein the activation and development time

8. The apparatus of claim 1 wherein the donor member is in the form of a

9. The apparatus of claim 5 wherein the cylindrical donor comprises an aluminum substrate and an enamel surface layer containing an etched layer

10. The apparatus of claim 6 wherein the grid contains 120 to 150 lines per inch.