US3105770A - Cascade development improvement - Google Patents

Description

1963 E. H. LEHMANN ETAL 3,

CASCADE DEVELOPMENT IMPROVEMENT Filed April 15, 1960 INVENTOR. ERNEST H. LEHMANN ROBERT W. GUNDLACH A T TORNEY image degradation.

United States Patent CASCADE DEVELOPMENT IMPROVEMENT I Ernest H. 'Lehmann, Rochester, and Robert W. Gnndlach, .Spencerport, N.Y., assignors to Xerox Corporation, a

corporationof New York I Filed Apr. 15, 1960, Ser. No. 22,439

ld'Claims. (1. 11717.5)

This application is concerned with xerography and moreparticularly with improved development means and methods for use in xerognaphic machines.

In the xerographic process an electrostatic latent image is formed on a photoconductive insulating member and is developed 'or made visible by the attractionthereto of finely divided pigmented material. The most widely used automatic xerographic machine's employ a photoconductive insulating member in the form of a cylinder with a horizontal axis and use the so-called cascade type of development in which a developer material is poured or cascaded over the surface of the xerogr-aphic cylinder. As is well known, cascade developer generally comprises a mixture of granular particles having a diameter in the range of from about 20 mesh to about 200 mesh and much smaller pigmented resin powder particles generally smaller than 50 microns which electrostatically adhere to the larger particles. Since the smaller particles called toner are adherent upon the larger particles called carrier, the developer mixture flows and otherwise behaves substantially as a granular rather than a powdered material. As the developer mixture is poured over the latent image bearing xerographic drum, toner particles are selectively separated from the carrier and deposit on the drum in image configuration. I I

Presently available machines of this generalcharacter employ a xerographic drum having a diameter of about 15 inchesandoperate at a linear speed of about 20 feet H 3,11%,770 Patented Oct. 1, 1963 2 In the drawings: SE16. 1 is a schematic cross-sectional view of a conventional xerographic machine; j FIG. 2 is a cross-sectional schematic view of a machine incorporating development means according to the I present invention; and

FIG. 3 is a cross-sectionalschematic view of a portion of a modified machine according to the present invention.

' FIG. 1 is a cross-sectional schematic view of a xero-- graphic copying machine constructed according to the principles of the prior art. At the heart of the machine is a cylindrical xerographic member 10 rotatable about its axial shaft 30 by a motor 31 which is connected tothe axial shaft 30 by an endless belt 32, and having on its outer surface a layer of photoconductive insulating material 1 1 such as a layer of vitreous selenium. A corona charging device 12 is positioned adjacent to cylinder 10 and is adapted to deposit a uniform electrostatic charge thereon. Cylinder 10 is then exposed to a light image projected through film strip '13 by lamp I14 and lens 15. Film strip 13 is moved in the direction shown by motor means, not shown, in order to synchronize theprojected image with the motion of cylinder 10. The action of light on photoconductive insulating layer 1 1 causes the layer to become temporarily more conductive, thereby forming on'the layer an electrostatic charge pattern corresponding to the original image on film strip 13. Cylinder =10 then passes by the development means which forms on the cylinder a powder deposit corresponding to V the electrostatic latent image. The development means per minute. Other machines having an 8-inch diameter drum are developed at surface speed of 10 feet per minute. It isobviously desired for some applications to increase thespeed of operation on machines and to make greater speeds available with machines employing even smaller drums. As machine speed is increased with present developing apparatus, image density begins to fall off and other forms of image degradation appear; which are associated with incomplete development; As

the diameter of the xerographic cylinder is decreased, these eiiects set in atlower and lower speeds, since the time spent by any portion of the cylinder in contact with the flowing developer stream is progressively. reduced.

Certain known steps may be taken to increase the efficiency of xerographic development, but these are of limited elfcctiveness. The "rate at which developer material is poured over the xerographic cylinder may be increased, but beyond a certainfflow rate the excess. de-

veloper never-contacts the cylinder and is therefore ineffective incausing development. Similarly, the speed of development may be increased by raising the concentration of, tonertin the; developer from the usual 1% to about 2%,,but-further increases merely cause serious chines adaptable for faster operation than has"her eto-- onlyyand'it is apparent that; many well'kno'wn mo-dificafore been possible. -It is a further object to provide for 7 improved machine concepts allowing for the production of more efficient and more compact xerographic mgchines thanhave previouslybeen known by the 1 art. Ad-

ditional objects will become apparent in connection with the following description and claims.

comprises elements 21' through 25 inclusive and will be discussed subsequently. Cylinder 10 then passes into contact witha synchronously moving web of paper 16 which is maintained in contact with the cylinder by guide rolls 17. "A coronadevice '18 is positioned and adapted to apply electrostatic charge to the back surface of paper 16 at the position shown, thereby causingthe powder patternon cylinder '10 to betransferred to paper '16. A heating device 19 is provided to permanently fuse the inrage pattern onto paper 16 and a rotating brush 20 is also provided to remove any residual powder from cylinder 10' before reuse thereof."

' The developmenttmeansj itself includes a collection trough 21 which is partially filled. with a xerographic deplate 24. The developer then slides and rolls down guide plate '24 onto cylinder 10 andreturnsto collection trough 21 after'sliding and rolling over part of the surface of cylinder 10. Each'and every one of theelements' de;

scribed above is wellknown inthe patent literature as well as being widely used in commercial xerographic apparatus; Accordingly, these elements willnot be further' described. They are shown for illustrative purposes tions andsubstitntions may be made therefor;

Certain features of FIG. 1 areparticularly pertinent tothe present invention. It will be notedthat developer 22 falls away from cylinder at a point whichis about 7i) degrees removed from thetop of the cylinder rather than at the degree point as might be expected. This is because of the horizontal momentum acquiredfby the developerin slidingand rolling against the inclined surv face of cylinder .10. It will likewise'be noted that the developer initially contacts cylinder 10 at a point about 40 degrees fremoved'from the, top of the cylinder. The

plointof developer impingement, unlike the point of de-.

veloper separation, is to some extent under the control I of the equipment designerfilt has been found, however,

that the point of impingement cannot be brought much closer to the top of cylinder than shown without causing an undesirable pile-up of developer at the point of impingement. It has also been found that introducing the developer at a higher point than shown results in carrier sticking to the surface. This is called blocking in the art and is believed to occur since the developer, when introduced at a higher point, tends to travel at the same rate as the drum, and the image fields. on the drum tend to hold onto the drum carrier particles. Further, no improvement in the developed image takes place if the developer is introduced at a higher point. This may occur since the developer does not effectively develop while it is relatively stationary with the drum, and it does not start to move at a different speed until it attains the position shown as the point of introducing developer in this figure. In other words, in this form of apparatus the developer must be introduced to the cylinder at a point which is su-fiiciently inclined so that the developer immediately begins to slide and roll downhill in order to avoid blocking and in order to obtain best quality development. In consequence of these limitations on the point of developer impingement and separation, it will be appreciated that only about 30 degrees, or A of the periphery of cylinder 10 is available for development and that the time available for developing any given point of a cylinder is only of the time required for a single revolution. Since the time required for a single revolution becomes progressively smaller as the cylinder diameter decreases or as the linear speed increases, it becomes progressively more diflicult to secure adequate development in xerographic machines if smaller or faster than present sizes or speeds.

FIG. 2 is a schematic cross-sectional view of a xerographic machine incorporating the present invention. Elements 10 through 22 and 30 though 32 of this figure represent precisely the same elements shown in FIG. 1. Conveyor 23 in this figure moves the developer relatively further than in FIG. 1 and discharges the developer onto a guide plate 24 which in turn discharges the developer onto chute 25. Since chute 25 is positioned at a very steep angle, the developer picks up considerable speed in sliding or falling down the chute before it is deflected by the nadius section at the bottom of the chute and discharged to cylinder 10 approximately tangentially thereto in this embodiment at a point about 30 degrees before the top of the cylinder. The momentum of the developer at the point of impingement is surficient to cause the developer to slide and roll uphill past the top of the cylinder and then to slide and roll downhill, leaving the cylinder at the normal 70-degree point. The developer in this embodiment is thus in contact with cylinder 10 for approximately 100 degrees of are as compared to about 30 degrees for the conventional arrangement shown in FIG. 1. For any given combination of cylinder size and speed, there is thus provided approximately 3 /3 more time for development than is provided by the prior art arrangement. While it might be expected that at least a portion of the developer material would slide down the left side of cylinder 10, it has been found in practice that it does not. I

.In a particular embodiment in accordance with FIG. 2, cylinder 10 was six inches in diameter and covered with a thin layer of vitreous photoconductive insulatingselenium. Chute 25 was approximately nine inches in vertical extent and inclined from the verticalby about 25 degrees. The lower end of the chute was curved with a radius of about 2 /2 inches and discharged developer material against the cylinder approximately tangentially thereto and at a point about 30 degrees before the top of the cylinder. The developer was a commercially available material available from Xerox Corporation, Rochester, New York, and had a toner concentration of about 2% by weight. The developer flow rate was 10 pounds per minute per linear inch of drum length. With these conditions, it was possible to obtain high quality image development at speeds in excess of feet per minute.

Certain generalities are applicable for developing apparatus of this type. An important factor is that the developer should be directed to the cylinder approximately tangentially and in the direction of rotation of the cylinder. This prevents blocking and minimizes abrasion of the delicate drum surface. A critical factor is that the developer contact the drum with a velocity sufficiently greater than that of the cylinder so that the developer will continue to slide and roll uphill over the surface of the cylinder until it is past the top of the cylinder. If the developer velocity is insufitcient the developer may, under the influence of electrostatic forces from the cylinder, become stationary with respect to the cylinder near the top thereof and travel with the cylinder in this region rather than sliding or rolling overr. The simplest way to impart the desired velocity to the developer is to allow it to fall down a steep chute and then negotiate a bend as shown in FIG. 2. An inclined, rather than a vertical chute has proven more satisfactory because the developer closely follows the surface of the chute and readily negotiates the bend at the bottom of the chute. It has been found that where a vertical chute, such as a two-sided vertical chute, is employed that some of the free falling developer piles up where it first contacts the bend and that some of the developer leaving the chute has insufficient velocity as a result. Nevertheless, vertical chutes can be employed by careful designing of the deflecting portion. The bend at the bottom of the chute should be as gradual as possible in order to smoothly deflect the developer without slowing it down excessively. Although the chute is preferably inclined from the vertical, it should nevertheless be sufliciently steeply inclined so that the developer readily accelerates in falling down the chute. The height of the chute, of course, should be sufficient to impart the necessary velocity to the falling developer particles.

As has already been pointed out, a preferred region for directing the developer to the cylinder is at about 30 degrees before the top of the cylinder. As this point of contact is moved further away from the cylinder top, it becomes increasingly more diflicult to make all the developer flow over the top and it has also been found that negligible improvement in development is obtained. On the other hand, the point of contact may be moved in the direction of rotation of the cylinder. This does reduce the developing zone contact and thus reduces the rate of copy output, but as should be apparent, for some applications it may be desirable to obtain less output rate than the ultimate. Any extension of the developing zone beyond that illustrated as the prior art in FIG. 1 would, of course, require accelerating particles to the drum surface to cause them to move relative to the drum all along the developing zone as discussed above. The 30-degree position is the preferred position for rapid development.

As the point of contact of the developer material with the surface of the drum is moved closer to the conventional position of the prior art, a lower rate of output is obtained than if the feed point is positioned as illustrated in FIG. 2 or FIG. 3. However, the chute length may be shortened proportionately in relation to the position the developer is fed to the drum as one moves from the position illustrated in FIGS. 2 and 3 toward the position illustrated in FIG. 1. Also pertinent to different feed points is the fact that the conveyor of FIG. 2 may be made shorter as one moves from the position in FIG. 2 toward that of FIG. 1. For example, in a particular embodiment of this invention it was found that employing a 15-inch diameter drum and feeding developer substantially tangentially to the topmost point of the drum, a chute having a length of about 2% inches was adequate to produce sufficient acceleration to feed developer to and across the drum surface at a greater rate of speed than the linear speed of the drum. As should be apparent, the length of -the chute will varytdepending on thespeed desired in the developer'flowing to the drum surface and the speed of the developer as it makes con-tact with the drum surface necessarily will depend upon the linear speed of the drum itself. Accordingly, in this latter example where the speed of the drumwas at 20 feet per minute, the chute is shorter than would be necessary if the drum speed were 70 feet per minute. Similarly, in the example discussed above in which the drumspeed was 75 feet per minute and the drum itself was 6 inches in diameter, if the drum speed were slower or if the diameter of the drum itself were greater, then the chute length could be reduced. 7

It should be recognized that the gravitational acceleration of the developer down a chute is not the only way of imparting the desired velocity and direction to it. BIG. 3 shows another embodiment of means which can be employed to apply developer to a cylinder in accordance with the present invention. A hopper 26 is loaded with developer 22 which it discharges onto the upper surface of an endless belt conveyor 27 which is supported between rollers ZS. Conveyor 2 7 is rapidly driven in the indicated direction by a motor, not shown. The developer particles are carried to the right at high speed by the conveyor 27 and, due to their momentum, fly off the conveyor at the right end thereof and travel on to strike cylinder it). A guide bar 29 may also be used as shown to deflect the developer particles toward a more tangential contact toward a cylinder. 7

The utility of the present invention extends beyond the particular embodiments and applications heretofore described. Thus, the invention may be employed with machines which use a photoconductive insulating web partially wrapped around a cylinder as well as with machines wherein the photoconductive insulating matenial is solidly attached to a cylinder. It may be used with continuously rotating cylinders or stationary cylinders. The invention may also be employed with machines wherein developable electrostatic charge patterns are created by means other than the exposure to a light pattern of an electrically charged photoconductor. The invention may also be employed in connection with methods and apparatus wherein magnetic images are developed by oascading thereover magnetically attractable granular material. As a further point, the invention is not limited to any particular type of developer material such as the two component type which has been employed herein for illustrative purposes only. These and other modifications as will occur to those skilled in the art are intended to be encompassed within the scope of the following claims.

What is claimed is:

1. In a method of developing a xerographic latent electrostatic image on a cylindrical image bearing member rotating about its horizontal axis with cascading granular electroscopic developer material in which the developer material contacts the periphery of the cylinder while moving in the same direction as the periphery of the cylinder at a point where if the developer were merely dropped against the cylinderit would travel with the cylinder at the same rate of speed as the cylinder, the improvement comprising accelerating the developer to a rate of speed prior to contacting the developer to the cylinder so that the developer contacting the cylinder has a velocity greater .than the cylinder during its passage along the cylinder while cascading against the cylinder and developing the latent image thereon.

2. Xerographic apparatus for developing an electrostatic latent image on the periphery of a cylindricalupper segment of an image bearing member comprising a rotating horizontally journaled cylindrical member, said apparatus comprising a steeply inclined chute, means to introduce granular xerographic developer into the top of said chute whereby the developer is accelerated in falling down said chute, and a smooth bend near the bottom of sad chute whereby the direction of said developer is changed without substantially altering the velocity thereof,

to contact the image bearing member with a sufiicient velocity to cause said developer to move at a greater rate of speedfthan, said image bearing member as it passes thereacross in contact therewith.

3. Apparatusaccording to'claim 2 including means to collect the developer material after passage of the developer material across the image bearing member and including means to refeed the developer material from'said means to collect developer to the top of said chute.

4. Xerographic apparatusfor developing anfelectrostatic latent image on a horizontally journaled cylindrical rotating image bearing member, comprising a steeply inclined chute, means to introduce granular xerographic developer into the top of said chute whereby the developer is accelerated in falling down said chute, and a smooth bend near the bottom of said chute whereby the direction of said developer is changed without substantially altering the velocity thereof, the bottom of said chute being positioned to discharge developer against the periphery of said image bearing member at a speed in excess of the peripheral speed of said image bearing member and substantially tangentially thereto in the direction of rotation thereof and at a point in the rotation of said image bearing member prior tothe top thereof.

5. Apparatus according to claim 4 in which said chute is sufliciently long and at a sufficiently steep angle to accelerate the developer to a sufficient extent to cause the developer to travel over the top of said cylindrical image bearing member.

6. Apparatus according to claim 5 in which the bottom of said chute is positioned to discharge the developer material at a point about 30 degrees before the top of the cylinder.

7. Apparatus according to claim 6 in which said chute is inclined about 25 degrees from vertical and is at least about 9 inches high and in which said latent image bearing member has a 6-inch diameter.

bend near the bottom of said chute has a radius of curva ture of about 2 /2 inches.

9. A xerographic apparatus comprising a cylindrically formed xerographic plate journaled for rotation substantially about a horizontal axis, means to form an electrostatic latent image on the peripheral surface of said plate, and means to develop the electrostatic latent image, said developing means comprising a reservoir for containing electroscopic developing material, a vertically inclined chute disposed toward said plate and terminating in a substantially frictionless bend directed upwardly and substantially tangential to the plate periphery and extending in the same direction as the direction of rotation of said cylindrically formed xerographic plate, and means to feed developing material from said reservoir into an upper portion of said chute wherefrom the developing maup of a carrier and toner mixture into contact with the curved periphery of said cylinder along a line substantially tangential to the cylinder periphery at a speed in excessof the peripheral speed, of said cylinder and al-- lowing said developer material to fall out of contact with said cylinder at a point lower. than the point of initial.

contact between said developing material and said cylinder, developing substantially uniformly said latent electrostatic image by virtue of the rolling contact between said developing material and said cylinder from the point of initial developer contact to the point of developer separation.

11. The method according to claim 10 including bringing said granular developer material into contact with said cylinder periphery at a point in the rotation of said cylinder prior to its reaching its highest point and imparting sufficient velocity to said developing material to carry it over the highest point of said cylinder after it contacts said cylinder.

7 12. Apparatus for developing a latent electrostatic image comprising means to support a cylindrical member bearing a latent electrostatic image with the axis of rotation of said cylinder substantially horizontally oriented, means to rotate a cylinder on said support means in a first direction, feed means positioned along the path of rotation of a cylinder on said support to feed granular electroscopic two-component cascade type developing material into contact in said first direction with the surface of a cylinder on said support along a line which is substantially tangential to the periphery of the cylinder at the point where the developer and cylinder come into contact, said feed mean-s including means to impart a velocity to said developing material to cause said developer'to move at a greater speed than the peripheral velocity of the rotating cylinder at the time of contact between developer and the cylinder.

13. Apparatus according to claim 12 in which said feed means is located so as to direct said developing material into contact with the cylindrical member at a point which is about away from the top of said cylinder measured in a second direction opposite to said first direction.

14. A xerographic apparatus comprising a cylindrically formed xerographic plate journaled for rotation substantially about a horizontal axis, means to rotate said Xerographic plate about said horizontal axis, means to form an electrostatic latent image on the peripheral surface of said plate, and means to develop the electrostatic latent image, said developing means comprising a source of granular electroscopic material and means to direct a stream of said granular electroscopic developing material from said source into contact with the periphery of said cylindrically formed xerographic plate in the direction of rotation of said plate and along a line tangential to the periphery of said cylindrical plate at a speed in excess of the peripheral speed of said cylindrical plate.

References Cited in the file of this patent UNITED STATES PATENTS 2,705,199 Clark Mar. 29, 1955 2,779,306 Dunn Jan. 29, 1957 2,852,651 Crumrine et a1. Sept. 16, 1958 2,878,120 Mayer et a1 Mar. 17, 1959 2,884,348 Kulesza Apr. 28, 1959 3,013,342 Huber Dec. 19, 1961

Claims (1)

10. THE METHOD OF DEVELOPING A LATENT ELECTROSTATIC IMAGE ON A CYLINDRICALLY SHAPED MEMBER HAVING A HORIZONTAL AXIS AND ROTATING ABOUT SAID AXIS IN A FIRST DIRECTION COMPRISING, DIRECTING IN SAID FIRST DIRECTION TWO-COMPONENT GRANULAR ELECTROSTATIC DEVELOPER MATERIAL MADE UP OF A CARRIER AND TONER MIXTURE INTO CONTACT WITH THE CURVED PERIPHERY OF SAID CYLINDER ALONG A LINE SUBSTANTIALLY TANGENTIAL TO THE CYLINDER PERIPHERY AT A SPEED IN EXCESS OF THE PERIPHERAL SPEED OF SAID CYLINDER AND ALLOWING SAID DEVELOPER MATERIAL TO FALL OUT OF CONTACT WITH SAID CYLINDER AT A POINT LOWER THAN THE POINT OF INITIAL CONTACT BETWEEN SAID DEVELOPING MATERIAL AND SAID CYLINDER, DEVELOPING SUBSTANTIALLY UNIFORMLY SAID LATENT ELECTROSTATIC IMAGE BY VIRTUE OF THE ROLLING CONTACT BETWEEN SAID DEVELOPING MATERIAL AND SAID CYLINDER FROM THE POINT OF INITIAL DEVELOPER CONTACT TO THE POINT OF DEVELOPER SEPARATION.

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