US3379553A - Continuous tone development method for xerographic printing - Google Patents

Description

CONTINUOUS TONE DEVELOPMENT METHOD FOR XEROGRAPHIC PRINTING FIG. 2

I0 I IITI 7 FIG. 4

1 FIG. 6

W I8 I I I I I2 7 II J I I I I I I I I I I M- W. DOWLEY Filed March 20, 1964 I 0% III III I I I III III 9 m fl++++++++++++++++ \III/ IIII III /II /III II IrIIr\ IIIIIIIIIIIIIIIIII IIII III A rii 23, 1968 FIG I FIG. 3

FIG. 5

FIG. 7

7 FIG. 8

INVENTOR. MARK W. DOWLEY BY Z 9 ATTORNEY United States Patent 3,379,553 CONTINUOUS TONE DEVELOPMENT METHOD FOR XEROGRAPHIC PRINTING Mark W. Dowle-y, San Jose, Calif., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Mar. 20, 1964, Ser. No. 353,440 1 Claim. (Cl. 11717.5)

ABSTRACT OF THE DISCLOSURE A method of continuous tone xerographic development wherein the phenomenon that toner attraction is governed by electrical field distribution is recognized and utilized such that the electrical field distribution of an insulating member is made to be essentially that of a free body in space by applying a very high potential to the conductive backing plate of the same polarity as the charge on the insulating surface.

This invention relates to xerography and electrophotography generally and more particularly to a method of continuous tone development.

In xerography, plates having a photoconductive insulating material overlying a conductive backing member are sensitized by placing a uniform electrostatic charge on the surface of the photoconductive insulating layer while the plate is kept in darkness. A latent image is then formed on the surface of the insulating layer by exposing it to a light pattern. Exposure to light causes areas of the photoconductive insulating layer to become conductive and charges on the surface are thus dissipated. Areas not exposed to light continue to act as insulators and, therefore, retain their electrostatic charges. The electrostatic charge pattern formed may be developed by bringing electrostatically charged particles into contact with the surface area carrying the charge pattern or the pattern may be otherwise utilized. A developed image composed of particles deposited in conformity with the electrostatic charge pattern may be transferred from the surface of the plate to sheets or webs through use of electrostatics or other methods known to the art and may be there permanently affixed.

Continuous tone copies without distortion of the original are in almost all instances desired. Continuous tone development in xerography is, however, rarely achieved due to a phenomenon which occurs in the case where an insulating member carrying a charge is backed by a conductor as in the usual xerographic situation. Upon developing a xerographic plate, where a large area of continuous toning should occur, unless later discussed precautions are taken, emphasized development takes place along the edges of the large area and little or no toner is attracted to the center portions. To prevent such development, development electrodes are normally used. A development electrode is positioned at a slight distance from the photoconductive insulating layer during development. The development electrode is composed of a conductive material and is either biased or maintained at the potential on the conductive backing member. This will cause an increase in the lines of force extending outwardly from the plate member creating electrostatic fields which, when developed, will produce continuous tone copy.

Several problems are, however, attendant to the use of a development electrode. For instance, the development electrode, since it is spaced at a slight distance from the sensitive layer, tends to build up on its surface a coating of developer material. While this coating remains thin, the image which is developed is generally not affected; however, a coating of developer material on the develop- 3,379,553 Patented Apr. 23, 1968 ment electrode interposes a new surface between the sensitive layer and the development electrode. If the particles carry electrostatic charges, this coating may effectively change the bias potential on the electrode, which substantially decreases the beneficial aspects of the development electrode during development. This coating also adds additional and uncontrollable problems. The lack of control following heavy disposition is to some extent attributed to the fact that the coating which forms is generally uneven and follows unknown and different patterns. A heavy coating may also drop agglomerates of developer powder to the plate surface resulting in a non-uniform distorted development of the electrostatic image pattern. Moreover, such a coating may cause irregular and uncontrolled flow patterns of the developer material being presented to the plate surface which will cause streaking of the final developed image.

These problems are well known in the art. Several systems have been developed in an attempt to overcome the problems attendant to the use of development electrodes. Thus, in a patent to Crumrine et al., 2,784,694, a segmented development electrode is provided, the segments of which are rotatable for brushing by an associated brush to effect cleaning. This patent also states that another apparatus developed for preventing this buildup of toner on a development electrode was one wherein the development electrode was caused to constantly move back and forth transversely across the xerographic plate. Brushes were provided to engage the surface of the electrode when removed from its operative position relative to the xerographic plate for cleaning the powder from the surface. Another apparatus provided is shown in a patent to Hayford, 2,844,123, wherein a continuous belt develop-ment electrode is provided which rotates past the sensitive surface and then by a vacuum cleaner-brush arrangement whereby toner is cleaned from the development electrode. Another apparatus more recently devised is that covered in a patent to Clark et al., 2,952,241. The apparatus of this patent is designed to overcome the problem of a rigid electrode which would not allow toner to pass between the electrode and the sensitive surface in the case of toner buildup. Thus, the development electrode is made flexible to permit toner particles to pass between it and the image bearing surface thereby avoiding undesirable pileups of toner.

It can be seen, therefore, that the prior art systems have accepted the problems attendant the use of development electrodes and attempted to live with them by means of the various systems above described.

It is and object of the present invention to provide a novel method of xerographic development.

Another object of the present invention is to provide a new method of xerographic development wherein continuous tone copies are provided.

Another object of the present invention is to provide a new method of xerographic continuous tone development which does not necessitate a development electrode.

Another object of the present invention is to provide a new method of continuous tone development which provides results far superior to those obtained through use of conventional development electrodes.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which:

FIG. 1 is a view which aids in understanding the phenomenon on which the subject method is based;

FIG. 2 is a view representing the electric field distribution in and around 'a uniformly charged insulator backed by a grounded conductive plate;

FIG. 3 is a view illustrative of the charge pattern in and around a randomly charged insulator backed by a grounded conductive plate;

FIG. 4 is a graphical representation of the electric field distribution in and around the insulator and plate combination of FIG. 3;

FIG. 5 is a view illustrative of the toner deposition which would occur if the plate and insulator combination of FIG. 3 were toned;

FIG. 6 is a view representative of the electric field distribution of 'a dielectric sheet charged in conformity with the charge pattern of FIG. 3 and wherein the insulating sheet is isolated in space;

FIG. 7 is a view illustrative of one method of separating a flexible insulating member from a rigid conductive base member; and

FIG. 8 is a view illustrative of 'a method of simulating isolation of a member in space to provide electric field distribution normal to its face.

Briefly, in accordance with the subject invention, a method of continuous tone xerographic devclopment is presented wherein the phenomenon that toner attraction is governed by electric field distribution is recognized and utilized such that the electric field distribution of an insulating member is made to be essentially that of a free body in space either by separation of the charged insulating member from the conductive backing plate or by applying a very high potential to the conductive backing plate of the same polarity as the charge on the insulating surface.

One view expressed with respect to what causes the edge effect (i.e., the emphasized deposition of toner at the edge of a large charged area and not in the center) is that this behavior is due to the greater field gradient, or charge density at the edges of the electrostatic image, which are not present at the center of a large uniformly charged area. It is submitted that a more accurate view is that electrically charged bodies experience a force when in an electric field and in xerography charged toner particles are attracted to and are retained by regions of the xerographic plate which have a sufiiciently strong component of the electric field of the correct polarity perpendicular to and above the plate.

Furthermore, the density of toner adhering to a region of the plate will be proportional to the vertical component of electric field in that region, i.e., the greater the electric field, the greater the density of toner deposited. For the same reasons, in the cascade development technique, the deposition of toner on the xerographic plate will depend on whether the toner particle experiences a force, due to the electric field of the electrostatic image, sufficient to overcome the attractive force of the carrier. In other words, toner will be deposited if the electric field above the image is greater than that of the carrier. Tests of an electrostatic image before application of toner to determine image quality should be concerned with a measurement of the electric field at the surface of the plate and not with the voltage or charge density which may or may not be relevant to the development process. Electric fields strength, however, is always relevant.

The truth of the above may be proven by recourse to fundamental definitions and concepts of electrostatics. According to Coulombs Law, a charge q experiences a force F when in the vicinity of another charge Q2. The force F is given by the equation F=q q /41ree r (1) wherein F is in newtons if (1 and :1 are in coulombs, r the separation between the charges is in meters, 6 is the dielectric constant of the medium and c the permitivity of free space equal to (1/3671') 10- farad/meter.

The electric field E at the position of q due to :1 is defined by the equation traction between carrier and toner particles originates in the mutual transfer of electric charge which occurs when the carrier and toner, both neutral originally, are brought into contact. This phenomenon, triboelectrification, is 'a result of the greater electron afiinity of one material over the other. The attractive force could be represented by Eq. 1 with g and r the separation of the charges. Experimentally, it is found that a certain electric field strength is required to remove the toner from the carrier.

When a charged conducting sphere 1 is dipped into a dish 2 of carrier plus toner 3, as illustrated in FIG. 1, it

is found that the density of toner 4 adhering to the sphere is proportional to the field strength at the surface of the sphere v/r where v is the voltage of the charging battery 5 and r the radius of the sphere. For spheres of different r'adii, equal densities are found for equal field strengths not for equal voltages. Roughly, the toner density p on a conducting sphere after dipping is given by an expression of the form where E is the field strength at the surface of the sphere, E is the threshold field below which no toner adheres to the sphere and c is a proportionality constant.

It should be clear from the above that whether toner :0 will be attracted to and adhere to a xerographic plate depends on whether there is a sufficiently strong component of electric field strength perpendicular to the surface of the plate. More particularly, the density distribution of toner will depend directly on the distribution of electric field strength over the surface of the plate. As shown in FIG. 2, a uniformly charged dielectric layer 6 is backed by a grounded conductive plate 7 appears, in so far as its electric field distribution 8 and 9 is concerned, very much like a parallel plate condenser, i.e., the field 9 is strong between the plates and is essentially zero outside the plates except at the edges where fringing fields 8 protrude beyond the plates. With this field distribution, it is expected and confirmed that no toner is deposited on the surface when development is attempted other than at the edges. A photoconductive plate 10, however, as illustrated in FIG. 3, which, having been uniformly charged and then exposed in some regions to light, will have charged areas 11 and 12 and uncharged area 13 corresponding respectively to those areas which were not ex- 30 posed and those which were. The electric field distribution of the charged areas 11 and 12 will again correspond approximately to that of a similar sized parallel plate capacitor while the uncharged area 13 will have no electric field. FIG. 4 illustrates the vertical field intensity ;5 (E) at the surface of the plate-photoconductor combination of FIG. 3 while FIG. 5 illustrates the toner density 7) after development. It will be noted that large uniformly charged areas 11 develop only at the edges 15 and 16 (the edge effect) due to the fact that only at the edges 14 and 15 is there appreciable electric field strength.

on development. This sharp edge phenomenon is consistent with the postulate that the toner is attracted in proportion to the electric field strength and not in proportion to voltage contrast or field gradient.

From these considerations the subject method of obtaining field intensity near the surface of the photoconductive material which is proportional to the charge density and which results in uniform or continuous tone development of charged areas evolved. As shown in FIG. 7, the photoconductive member 1-8 is peeled from the backing plate 19 and removed therefrom. In this isolated condition, as illustrated in FIG. 6, the electric field distribution, as illustrated by the arrows 20, of the charge on the photoconductive surface is radically different from that of the combined photoconductive layer It) and conductive plate 7a illustrated in FIG. 3. The electric field distribution of an isolated charge area is such that the electric field strength outside the surface is proportional to the charge density on the surface, and thus, uniform toner deposition on a uniformly charged surface can be obtained thereby eliminating the edge effect. Note the illustrated electric field distribution where the photoconductive layer 18 has been peeled from the backing plate 19.

Application of a potential to the conductive plate in such a Way as to repel the photoconductive layer will aid in overcoming the electrostatic attractive force to facilitate separation.

Another method of simulating or providing the effect of a charged insulator arranged in space to accomplish results nearly as good as the peeling technique is that illustrated in FIG. 8 wherein the conductive base plate 21, which supports the photoconductive surface 22 is raised to a high electric potential of the same sign as the charge originally deposited on the photoconductive plate. This has the effect of changing the electric field intensity pattern at the surface from that of a parallel plate condenser, as illustrated in FIG. 3, to that of two adjacent layers of similar charge, as illustrated in FIG. 7. The result is that the electric field intensity in the region directly above a charged area is substantially increased and, hence, toner will be deposited there producing a uniform toner development or development proportional to the charge density.

'In the above described manner, there has been provided a novel method which is conductive to continuous tone development in xerographic printing. This method requires neither the conventional development electrode used in the prior art to obtain continuous tone development nor does it consequently require any of the peripheral means utilized in conjunction with the development electrode to assure proper functioning. Instead, realization of the phenomenon that toner attraction is due not to field gradient, but due to the electric field distribution along with two methods of providing electric field distribution normal to the surface of the member to be toned, is considered to be, in light of the state of the art, a new and novel, useful method of continuous tone development in xerographic printing.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of continuous tone printing comprising a the steps of:

forming a latent electrostatic charge pattern on one side of an insulating member while the other side thereof is backed by a conductive member,

applying a potential of suificient magnitude of like sign to that of said electrostatic charge pattern solely to said conductive backing member to cause the electric field distribution attendant the electrostatic charges to include components substantially perpendicular to and rising above said one side of said insulating member, the electric field intensity in the region directly above said charged pattern being thus substantially increased due solely to said applied potential,

cascading a mixture of solid toner and carrier particles to said electrostatic charge pattern while said potential is applied, said toner particles being of opposite sign to that of said electrostatic charge pattern,

whereby said toner particles adhere to said charge pattern forming a visible image of continuous tone quality.

References Cited UNITED STATES PATENTS WILLIAM D. MARTIN, Primary Examiner.

0 E. J. CABIC, Assistant Examiner.

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