CA1068172A

CA1068172A - Half tone development for touchdown system

Info

Publication number: CA1068172A
Application number: CA192,843A
Authority: CA
Inventors: Joseph Fantuzzo
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-04-16
Filing date: 1974-02-18
Publication date: 1979-12-18
Also published as: NL7404417A; JPS5010143A; GB1419926A; DE2403143A1; US3881927A

Abstract

ABSTRACT OF THE DISCLOSURE
A touchdown system for obtaining many distinctive steps in grey scale rendition when developing solid areas comprises a toner laden donor member and a toner layer pattern forming member. The pattern forming member has an electrode arranged in such a manner thereon that when a predetermined field is established between the electrode and the donor member, a predetermined pattern of peaks and valleys will be formed in the toner layer. When this patterned toner layer is presented to an electrostatic solid area latent image, the weakest charged portions of the latent image will attract only the toner from the portion of the layer which is closest thereto i.e., the peaks, thereby presenting a dot pattern development thereon. The strongest charged portion of the electrostatic latent image will attract toner not only from the closest portion or peaks but will also attract toner from the valleys which will fully develop the image where the charge is strongest.
m e charged pattern of the latent image which is between the weakest and strongest charge will attract a broadening pattern of toner from the peaks as well as from the valleys as the charge gets stronger.

Description

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, This invention is concerned with an improvement to a "touchdown development" system. The term "touchdown -development" as used in this application refers to the follow-ing system:
A toner laden rotating donor member (which may be a roller or an endless belt) is located adjacent to a moving photoreceptive surface bearing a latent electrostatic image thereon. The donor member is spaced from the photo-receptor so that a gap of about two to ten microns exists between the toner layer surface and the photoreceptive surface.
The toner on the donor member carries a charge in polarity opposite to the polarity of the electrostatic latent image.
As the donor member carries toner past the latent image, the ;
toner is attracted from the donor to the image to develop the image. ::
In developing solid areas with this touchdown system, there have been problems with achieving more than about three distinctive steps of grey scale rendition.
In accordance with this invention there is provided ' 20 a method for developing latent electrostatic images carried by a substrate, said method comprising the steps of loading a .
' donor member with a layer of toner, imparting a uniform '; electrical charge of predetermined polarity to said donor layer, subjecting the charged toner to an electrostatic field having an intensity varying in accordance with a predetermined grid-like pattern to create a corresponding pattern of hills and valleys - in said layer of toner, and presenting the patterned toner layer ;
of said doner member to said substrate for developing said images, whereby any gray scale content of said images tends to .: .
3~ be preserved.

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i It is an object of an aspect of this invention to provide a touchdown development system which provides for half-tone development of solid areas resulting in . '' ,' ' ' ', `.'` ' ., ~
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Other ob~ects of this invention will become apparent from the following description with reference - to the drawings wherein:
Figure 1 is a schematic view of a touchdown development system;
Figure 2 is a partial schematic view of a micro-field donor and a commutating system therefor;
Figure 3 is a view of a toner layer pattern .: . .
forming roll; and ; Figure 4 is a view of the donor roll with a ; toner layer thereon shown in section after a pattern has ' been formed therein.
, Referring to Figure 1, there is shown a xero-.. . . .
graphic reproduction system utilizing the concept of the present invention. In this apparatus a xerographic ; plate is in the form of a drum 10 which passes through ~tations A through E in the direction shown by the arrow.
The drum has a suitable photosensitive surface on which a latent electrostatic image can be formed. The plural-ity of stations about the periphery of the drum ~hich carry out the reproduction process are: charging station A, exposing station B, developing station C, transfer station D, and cleaning station E. Stations A, ~, D, and E represent conventional means for carrying out their respective functions and do not form a part of the present invention.
~t station A, a corona charging means 12 places a uniform electrostatic charge on the photocon-ductive material. ~s the drum 10 rotates, a light :. . . . .

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' ~1068172 - - pattern, by a suitable eY~posing apparatus 14, is :~ exposed onto the charged surface of drum 10~ The latent image thereby formed on the surface of the drum is developed by the application of toner particles at developing station C, which is described in greater detail below. ~fter the image is developed at station C, it passes through transfer station D, wherein the toner.
is transferred to a sheet 16. A corona charging device 18 is utilized in the normal way to facilitate transfer and fuser device 20 is provided to fuse the toner to :~ sheet 16. Following transfer of the developed image to . the copy sheet, the drum rotates through cleaning station E, comprising cleaning brush 22 which cleans .,4,, residual toner particles from the drum 10.
Referring to Figure 2, the apparatus includes a donor member 24 at the developing station C which comprises an aluminum drum 26 and a dielectric layer 28 separating the drum 26 from a copper grid pattern 30. There are a plurality of electrically isolated grid patterns 30 to allow a varying potential to be applied - thereto in accordance with a position of a particular ::. . portion of the donor member 24 with respect to the various stations located around it. This is accomplished by connecting the aluminum drum to ground by contacting a brush 34 with the inner surface thereof and with an electrical lead 36. Individual electrical connections are made from each grid pattern 30 to a programmed means which will permit each grid pattern to be placed at ground potential or to some desired charge potential.
This programmed means comprises a segmented slip ring . .

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37 and electrical contacts 38 which are shown schematically tn ~igure 2. In actual practice, a respective electrical lead 39 could be located in~ide of the donor member in ' electrical communication with a respective grid pattern -; 5 while the other end of the respective electrical lead ; 39 is connected to a respective contact 38 which is in ring 37. A voltage source 42 can supply the appropriate potential to each segment 40 of the slip ring.
The donor member 24 is rotatably mounted adjacent a toner reservoir 44 containing a supply of toner particles 46 in order that a portion of its periphery comes into contact with toner 46. This portion of the donor member 24 is properly biased to attract a layer 47 of toner particles thereto. The donor roll is also located so as to provide a small gap of approx-imately 2-10 microns between the surface of drum 10 and the outer surface of a toner layer carried by donor roll 24. After a particular portion of the donor roll is rotated through the toner reservoir, that portion of the donor rotates past a corona charging device 48 which ch~rges the toner particles 46 to the correct polarity.
Thereafter the charged toner layer 47 is rotated past A~ a toner layer pattern forming ~ gravure roll 50/
whereby a peak and valley pattern is formed in the toner layer 47, and then the toner layer 47 is presented to the latent image to develop the same.
Following development/ the donor roll is pre-pared for tonex reloading by exposing the residual toner thereon to a neutral~zing corona charge means 52 to make ,~ ' ' . . .

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easier the removal of the residual toner by way of a cleaning brush 54 equipped wi'th a vacuum means 56.

; Referring to Figure 3, the gravure roll 50 comprises ; a support drum 62 on which a copper film electrode 64 in the ' form of a grid pattern is fixed. The support may be either a ~--; conductive or a dielectric material. The roll 50 is so posi- ~
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,,~ tioned that outer surface of the grid pattern 64 is spaced 2 .,;., to 10 microns from the outer surface of the toner layer on the donor roll 24. A voltage potential Vl is applied to the grid 64. That portion of the donor opposite the gravure roll 50 is programmed to be at ground potential. The attractive field set up by the potential between the donor roll 24 and the grid 64 is predetermined so that a substantial amount of toner is removed from the toner layer and attracted to the grid 64 there-by leaving a predetermined pattern of individual peaks 66 .. ~ .
surrounded by a~continuous valley 68 in the toner particle layer as shown in Figure 4. m e valleys 68 correspond to the , ...................................................................... .
toner removed by the grid electrode 64.

When the donor 24 presents the patterned toner ,~ 20 layer 47 to the electrostatic latent image, the weakest charged ;; portion of the image will attract only the toner from the portion , . .
of the layer which is cLosest thereto, i.e., the peaks 66 thereby presenting a dot pattern development thereon. m e strongest charged portion of the electrostatic latent image will attract toner not only from the closest portion or peaks 66 but will also attract toner from the valleys 68 which will fully develop the image where the charge is strongest ,",'. '.

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the weakest and strongest charge will attract a broad-~' ening pattern of toner from the peaks 66 as well as from the valleys 68 as the charge gets stronger. Thus, one , 5 can see that there ~ill be many distinctive steps in ''7 grey scale rendition.
The developed image characteristics can be ; varied in accordance with the attractive field set up by the potential between the donor 24 and the gravure , . . .
, 10 grid 64 which can be in a range between the electro-!' ' .
static latent image background potential and the full potential on the photoreceptor. Thus, assuming a pos-; itive charge on the photoreceptor and negative charged ;~ toner, the potential on the~grid 64 can be in a range between ~100 volts to ~800 volts while the potential of the donor can be ground potential. As the lower potential range on the grid 64 is approached, less toner will be ; removed from the toner layer resulting in shallow and '~ narrow valleys, while as the upper potential range on . . .
, 20 the grid 64 is approached, more toner will be removed ` from the toner layer resulting in deeper and wider ~; - valleys. For all practical purposes, when the potential .~, . ~ .
of the gravure grid 64 is at background potential, the toner layer on the donor roll 24 is such that normal development takes place rather than half-tone development ~! and therefore this potential on the gravure roll could be utilized when half-tone development is not des~red.
~`, At this background potential, the gravure roll will act , . . .

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as a roll means for removing a substantial amount of toner from the layer 47 which would otherwise be attrac-ted to the background. When the gravure grid 64 is at a potential which is the full potential on the photoreceptor, only half-tone development will occur on the highest charged portions of the image. ~hus, if only half-tone development without much distinction in grey scale ren-dition is desired, the gravure grid 64 would be set at full photoreceptor potential.
From the above, it can be seen that when a , given electrode pattern is utilized for the graw re roll, the distinctive steps in grey scale rendition obtained by the method of this invention can be changed by varying the potential on the gravure roll. Also, by changing lS the electrode pattern on the gravure roll, the distinc-tive steps of grey scale renditio~ may also be varied.
The electrode may be any type of pattern desired as :..
long as ;t will form a pattern of peaks and valleys in the toner layer 47 on the donor ro~l 24.
While the donor roll 24 is described as a microfield donor roll, any type donor roll may be utilized.
; The key fac~or is that the donor roll must have a layer :: .
; of toner thereon and that a field be set up between the donor roll and the gravure roll ~hich will attract the particle~ to the gravure roll in such a manner that peaks and valleys are formed in the toner layer.
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While the photoreceptor 10 J the donor roll 24, and the pattern-forming member 50 have each been described as rolls, any one of them or all of them may be i.n the for of a bele.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for developing latent electrostatic images carried by a substrate, said method comprising the steps of loading a donor member with a layer of toner, imparting a uniform electrical charge of predetermined polarity to said donor layer, subjecting the charged toner to an electrostatic field having an intensity varying in accordance with a predetermined grid-like pattern to create a corresponding pattern of hills and valleys in said layer of toner, and presenting the patterned toner layer of said donor member to said substrate for developing said images, whereby any gray scale content of said images tends to be preserved.

2. The method of claim 1 further including the step of rotating said donor member to carry out said loading, charging, pattern creating and developing steps in sequence.

3. The method of claim 1 further including a post-development step of cleaning residual toner from said donor member in preparation for a subsequent cycle.