US3246629A - Apparatus for developing electrostatic images - Google Patents

Description

April 19, 1966 SHELFFO ETAL 3,246,629

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES 3 Sheets-Sheet 1 Filed June 18, 1963 INVENTORS QYK/Z n IQN April 1966 L. E. SHELFFO ET AL 3,246,629

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES 3 Sheets-Sheet 2 Filed June 18, 1965 Hll llll

April 19, 1966 s o ET AL 3,246,629

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES Filed June 18, 1963 3 SheetsSheet 3 United States Patent C) 3,246,629 APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES Loren E. Shelifo, Palatine, and John A. Harmon, Arlington Heights, Ill., assignors, by mesne assignments, to Addressograph-Mnltigraph Corporation, a corporation of Delaware Filed June 18, 1963, Ser. No. 288,705 8 Claims. (Cl. 118-637) This invention relates generally to electrostatic printing apparatus, and more particularly, to an improved magnetic brush apparatus for developing electrostatic latent images.

Electrophotographic processes involve converting a light image into an electrostatic charge pattern on a suitable photoconductive insulating material, such as, for example, Zinc oxide .particles suspended in a resin binder. The electrostatic image is developed into a visible image by bringing the charged surface into physical contact with a suitable developer powder. The necessary contact between developer powder and the charged surface is accomplished by the use of the well known magnetic brush type applicator. The developing powders that are employed are also well known in the art and generally comprise dyed or colored pigmented thermoplastic powders, known in the art as toner particles, which are mixed with more coarse carrier particles, such as, for example, iron powder. Developing powders can be formulated so that the toner carries a negative or positive charge. A typical positive developer is formulated from carbon black pigmented polystyrene resin toner admixed with iron, magnetites or ferrites. In any case, the toner and the carrier are selected so that the toner particle acquires the .proper charge with respect to the latent electrostatic image. When the developer brush is brought into rubbing contact with the charged surface of the insulating layer of the electrophotographic member, the greater attractive force of the charged image causes the toner particles to leave the carrier particles of the brush and adhere to the imaged portion. While there has been described developer powders capable of acquiring a charge of a specific polarity, developing powders can be used with this invention in which the toner and carrier do not acquire a specific polarity in relation to one another. An important constituent in the formulation of these developing powders is a lubricating material which prevents a premature agglomeration of the developer mix so that the carrier particles and the toner particles are equally distributed throughout the mass of material. Such a developer mix is disclosed in US. application Serial No. 221,888, filed September 6, 1962, and assigned to the same assignee.

Apparatus for applying the developer to the latent electrostatic image is set forth in US. Patent 3,003,462 wherein the magnetic brush technique is employed. A magnetic brush typically consists of a non-magnetic, rotatably mounted tube or cylinder having fixed magnetic means mounted inside. The tube can be arranged to rotate with part of its surface immersed in a supply of the developing powder, or powder can be supplied to the tube surface by other means. Said powder is a mixture of iron carrier particles and electroscopic toner particles hereinafter referred to as the developer mix or mix.

The particles of mix are magnetically attracted to the surface of the tube and arrange themselves in the form of a brush as the developer mix comes Within the infinence of the field generated by magnets within the tube. The bristles of mix conform to lines of magnetic flux, standing erect in the vicinity of the poles and lying substantially fiat between the poles. The electrophotographic sheet (e.g., paper) bearing a latent electrostatic of a magnetic brushapparatus.

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image thereon is brought into physical contact with the brush. The attractive force of the electrostatic charge on the sheet, in image areas, is greater than the force which holds toner to the carrier particles. Thus, toner leaves the brush and deposits in image areas.

The rotating tube continually picks up developer mix from the trough and returns part or all of this material to the trough within one revolution. This mode of operation assures that fresh mix is always available to the copy sheet surface at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer pick-up, brush formation, brush contact with the copy sheet, brush collapse, and finally mix release.

Additional means can be provided for doctoring oif the excess mix that is picked up on the surface of the tube and prior to brush formation. A doctor blade or a smoothing bar interposed before the brush contacts the paper controls the thickness of the layer or mix on the tube, thus contributing to more uniform brush formation.

In practice, the applicator roll rotates in the same direction as the travel of the sheet (e.g., paper) but at 1.5-5 times the paper speed so that a renewed brush continually contacts the imaged surface.

The same principles and design considerationsas outlined above apply to both positive (or direct) and negative (or reversal) image reproduction. Various types of toner powders and magnetic carrier particles are known in the art, and it is in the particular selection of these materials for developer mix preparation that either form or image reproduction can be accomplished.

It is well recognized in the art that the developing step in direct and reversal electrostatic printing is extremely critical. The final print quality is no better than the quality achieved in the development step. Proper development employing the brush technique depends upon the structure of the brush. Preferably, each bristle of magnetic material (eg, iron) should be firmly set into the underlying mass of developer mix immediately adjacent the tube surface. Control of brush length and flexibility is important. Intimate contact with the copy sheet must be assured, yet extreme stiifness can cause abrasion marks in the finished copy. Good image resolution, density, and contrast depends upon proper brush texture. V

In the past, the rotational force of the applicator roll against the slower moving paper disrupted brush formation and caused bristles to slip and slide along the periphery of the applicator tube. This has placed a practical speed limitation on the rate of travel of the copy sheet relative to the brush surface. Such a limitation in the amount of brush action obtainable is reflected not only in the quality of the final print but also limits the rate of print production of the machine. The problems of slip and brush rupture become more pronounced when lubri' cants are incorporated in the developer mix. Said lubri: cants ofifer distinct advantages in that they provide improved mix circulation in the trough, prevent mix clumping and stagnation, and through better agitation maintain constant the ratio of toner powder to iron carrier from point to point in the body of the mix.

Some slight improvement has been realized by going to larger magnets; that is, increasing the magnetic field. However, this gives rise to other problems. It causes more developer mix to cling to the surface of the tube when it is out of range of the magnetic field, thereby preventing the introduction of fresh mix from the supply trough to the roll.

Prior art (Bliss US. Patent 3,040,704) has suggested using a roughened external surface on the roller or drum Applicator rolls with roughened surfaces have not met the needs of brush development and fall short of achieving the main objects of this invention. Random abrasion of the roll surface is ineffectual because the size and number of bristle sites created in this manner is limited, and the relatively fine grooves thereby formed are readily polished down by the communitive action of the developer mix.

Attempts to roughen the surface by cutting grooves, serrations, or knurls in the tube surface have been inadequate since the number of bristle sites are restricted by this technique. The brush subsequently formed upon the surface of a tube roughened in this manner takes on the definite pattern of the grooves or knurls, thus defeating an object of this invention, namely, the formation of a thick, uniform, continuous brush surface. Furthermore, the machining operation is relatively expensive, particularly as one attempts to increase the number of irregularities per unit of surface area. Also, the preferred tube materials are non-magnetic metals such as aluminum, brass, and other soft alloys whose grooved or knurled surface is rapidly worn down by the abrasive action of a developer mix.

It has been found that a separate coating on the surface of the non-magnetic tube, comprising a layer of particulate matter, provides a base or stratum on which can be produced a thick bristle-like brush formation capable of producing dense visible images at relatively high developing speeds. To advantage, such powdered materials as glass, ceramic and plastic materials, and ferrous metals (e.g., powdered iron) have proven to be particularly successful as coating materials.

It is an object of this invention to provide an improved magnetic brush applicator to develop electrostatic images.

Another object of this invention is to provide a magnetic brush which uniformly applies developer powder to all portions of the electrostatic image to be developed.

A further object of this invention is to provide an improved magnetic brush applicator capable of greater rotational speeds permitting more rapid development of the electrostatic images.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description in connection with the accompanying drawings in which:

FIGURE 1 is a front view in elevation of the developer apparatus partially broken away showing the developer powder applicator roll positioned in the trough;

FIGURE 2 is a sectional view of the developer apparatus taken on a plane indicated by the line 2-2 on FIGURE 1;

FIGURE 3 is a plan view of the developer apparatus showing the feed path of the developer powder;

FIGURE 4 is a perspective view of an applicator roll with a stratum of particulate material shown in greatly enlarged scale, applied to part of the surface thereof; and

FIGURES 5a and 5b are enlarged photomicrographs showing the character of the untreated and treated roll surfaces of the invention respectively.

The magnetic brush applicator to be described is of special value in developing electrostatic images created on zinc oxide photoconductive type layers, but it can be adapted equally well to the development of electrostatic images on recording members in which a coating of selenium forms a photoconductor. In the preferred embodiment of this invention, the coated surface of the applicator roll forming the stratum on which is formed the deeloper brush is produced by adhering a layer of irregular particulate matter thereto. Such a layer can be permanently bonded to the tube surface by a number of common techniques. The irregularly shaped particles can be applied in a matrix of bonding adhesive, by spraying, coating, flocking or other known methods. Fusible particulate matter can be affixed to the tube surface by flame or hot-melt spraying techniques.

For purposes of illustration there is described herein,

by way of example, apparatus for the development of a positive image on a zinc oxide photoconductive recording member; however, the apparatus may also be employed with equal success for the development of reversal images.

Referring to FIGURES 1 through 3 of the drawings, the developer apparatus of this invention comprises an applicator roll, designated generally by the reference numeral 11, journaled for rotation between a pair of sideplates 12 on which is suitably fixed a container 13 in the form of rectangular trough and charged with a supply of developer powder 14. The applicator roll 11 is adapted to have its peripheral surface in intimate contact along its entire length with the supply of developer powder 14. The trough is extended at one end to provide a separate supply section or hopper 15 in which is received fresh toner or developer mix for storage. Means for conveying and mixing the developer is provided in the form a pair of drive screws 18 and 19 which communicate with the supply section. Drive screw 18 conducts the toner or developer from the hopper 15 to the trough 13, thoroughly mixing it in with the main body of the developer in the trough, and the other drive screw 19 returns the spent developer that falls off the applicator roll to the supply section of the trough.

The assembly of the applicator roll 11 shown in FIG- URE 1 constitutes a non-magnetic tube 21 supported at either end by threaded bushings 22 and journaled for rotation between the sideplates by bearing members, such as, for example, ball bearing mounts 23. A shaft 24 made of a magnetizable material such as Armco metal, is concentrically mounted within the tube 21 and serves as a fixed mounting support for a permanent bar magnet 25 and a soft iron pole or cross-piece 26. The permanent magnet 25 and the soft iron cross-piece 26 are magnetized as an assembly producing a magnetic field having its polarity oriented in a path perpendicular to the longitudinal axis of the cross-piece. The path of magnetic flux, for example, as shown in FIGURE 2 emanates from the top of the magnetic assembly, returning to the cross-piece through the air. The brush formation follows the lines of magnetic flux with the bristles tending to be normal to the path of the electrophotographic recording member. It will be understood that a magnetic means just described is a prior art technique and that the present invention is concerned with the surface treatment of the applicator roll 11 used in combination with the prior art developer apparatus.

The applicator roll has fixed to an extension shaft a driving sprocket which can be driven from any suitable source of power to rotate the roll with its periphery moving in intimate contact over the body of magnetic carrier particles and toner powder disposed in the trough 13. As shown in FIGURE 2, the trough is formed with its upper edge or lip 13a positioned ap roximately /4 52 to 3;; of an inch from the periphery of the applicator roll, thus forming a narrow space between the periphery of the roll and the exit end of the trough which controls the thickness of the layer of the developer mix deposited thereon.

As shown in FIGURE 4, the surface of the tube 11, comprised of a non-magnetic material such as aluminum, copper or brass, and of suitable thickness and diameter, is treated to provide a separate layer of irregular particulate material 32 (in this case, particles of stainless steel) bonded to the periphery thereof by a layer of adhesive 33. The irregular or roughened surface may be imparted by adhering a thin layer of finely ground particles. Materials such as sand, glass, Carborundum (silicon carbide), ferrous or non-ferrous metals or admixtures thereof are mixed with a suitable hinder or adhesive. Successful bonding has been achieved with epoxy resins and neoprene cement. Other bonding agents such as phenolic and acrylic resins are suitable.

The size of the finely divided particles forming the layer on the surface of the applicator roll or tube should be in the range from 20 to 500 microns (.0008 to .02

inch) along the largest dimension. An average particle size of 150 to 300 microns is preferred. The average thickness of the layer or stratum containing the particles (including adhesive) on the surface of the tube or roll should be in the range from .003 inch (75 microns) to .02

inch (500 microns). The particles need not be uniform insize and can vary over a ten-fold or larger range, for example, from 20 to 300 microns or from 50 to 500 microns, with normal distribution of number of particles in any range of sizes. Because the particles are fine, they appear to the naked eye to be of uniform size but when viewed under a microscope it is seen that they vary widely in size. The particles are preferably of irregular shape with sharp or jagged edges, as represented in FIGURE 4. These particles can be in the form of fiat chips like snow flakes or pieces of a broken pane of glass, but they can also be of spherical shape such as fine glass beads or particles of sea sand.

FIGURES 5a and 5b illustrated untreated and treated applicator rolls, respectively. FIGURE 5a represents prior art rolls and FIGURE 5 b represents this invention. FIGURE 5a is a photomicrograph of the surface of an extruded aluminum tube without surface treatment. The lines represent microscopic flawsin the surface thereof. FIGURE 5b is a photomicrograph of the same surface after treatment in accordance with this invention by application thereto of a layer of epoxy resin adhesive and a layer of fine stainless steel particles (20 to 500 micron size) embedded therein. It is seen that the stainless steel particles provide a vast number of points on which magnetic particles are collected by magnetic attraction to form a thick, uniform brush without bare or bald spots.

The invention is further disclosed by means of the following examples which are provided for purposes of illustration. It will be readily understood that other particulate materials and adhesives can be used within the scope of the invention.

Example 1 To a degreased aluminum tube stock having an outside diameter of 1.75" a layer of adhesive such as an epoxy resin (e.g., Epoxy 820-D sold by the Shell Chemical Co.), or an acrylic ester polymer (e.g., Acryloid B20 sold by Rohm & Haas Co.), is applied uniformly over all the outside surface. While the adhesive is still wet, the tube is rolled in a bed of iron particles ranging in size from .001" to .006". The iron particles are partially embedded in the adhesive, being substantially exposed above the adhesive layer. After the adhesive had set, the resultant tube had created thereon a layer of uniform thickness thereacross ranging from .003" to .02. To some extent the type of adhesive used controls the thickness of the coating.

Example 2 Instead of iron particles, the tube prepared as in Example l, with the undried adhesive thereon, was rolled in silica sand having a particle size range of from .001" to .006.

Example 3 The tube was prepared as in Example 1 but with a layer of glass comprising particles largely in the range of .005" to .007".

Example 4 The technique of flame spraying the degreased tube produces a coating that affords greater control of a uniform thickness. Metallizing apparatus such as is available from the Metco Co. of Westbury, New York can be used to apply a layer of metal particles of stainless steel to the tube. The final thickness of the layer produced in this manner ranges from .006" to .02 in thickness.

It will be readily appreciated that a plurality of discrete particles is bonded to the tube in each of the Examples 1-4, affording a greatly increased surface area on which the developer mix can be deposited in a uniform and continuous manner so as to present a uniform, thick brush-like formation to the surface of the electrophotographic member being developed. As one observes the character of the surface in the enlarged photograph of FIGURE 5b, it becomes apparent that the irregular. iron carrier particles of the developer mix provide a vast number of bristle sites not present in prior art devices. Iron particles, or equivalent materials which are highly permeable to the magnetic field, are used to advantage since the discrete array of particles direct the magnetic lines of force so that said particles align themselves along lines which are generally normal to the surface of the roller.

In magnetic brushes of the prior art bald spots are interspersed among the bristles. However, the portion of the tube having been treated in accordance with this invention provides a thick, continuous, uniform bristle formation. The deficiencies in using an untreated tube have been clearly demonstrated, but it should also be pointed out that an improperly treated surface will prevent the applicator roll from turning easily in the body of the developer mix. Improper application of the particulate material can result in high power requirements for brush rotation. An excessively coarse surface is undesirable too because it would exert a grinding action on the mix and upset the particle size balance. It has been found under a given condition of power input to turn a developer roll, in contact with the body of developer mix, that a properly coated tube is one in which a comparison of the dynamic torque of the coated roll with an uncoated roll does not result in more than a 10% increase in power consumption to achieve the same rotational speed. Coatings containing particles not greater than 500 microns achieve this result.

Feeding of the photoconductive copy sheet is effected by two sets of feed rollers suitably spaced on opposite sides of the applicator roll, which feed the copy sheet into tangential contact with the magnetic brush on the applicator roll. These feed rollers are made up of soft nap or pile material which is of sufiicient firmness to move the sheet without scratching or marring the surface, as disclosed in the Schulze Patent No. 2,894,744. In order not to impart a competing charge to the surface of the paper as it moves through the feed rollers, the nap material is selected so that it is triboelectrically compatible with the paper and with the particular toner powder. A positively charged roller is formed of such materials as polyacrylonitrile (Orlon), polyester resin (Dacron), vinyl resin (Saran) fiber or acetate rayon; and a negatively charged roller may be of wool, nylon, glass wool or animal fur.

In operation, the trough is filled with developer mix to a level surrounding the lower peripheral portion of the applicator roll. The electrophotographic recording member with a latent electrostatic image thereon is fed by the rolls into tangential contact with the applicator roll. As the applicator roll rotates in the trough, it picks up the developer mix comprising the carrier and toner particles on its surface and is doctored to a A; layer by the doctor blade. As the roll approaches the position where the magnetic field is relatively greatest, that is, at the top of its rotational path, the particles stand erect, forming a thick bristled brush. The brush rubs against the sheet and the toner particles preferentially adhere to the latent electrostatic image to develop a visible image thereon. As the roll rotates, a portion of the particles fall off the periphery of the roll into the trough as they move out of range of the magnetic field. In order to facilitate the removal of some of the developer powder, a mechanical scraping device is provided in the form of a wire that scrapes off additional quantities of the developer powder. Not all of the particles are removed; enough remain attracted to the roll to pick up toner and/or carrier particles to provide a renewed surface.

While the construction of the applicator roll has been described in detail, it is to be understood that many changes will occur to those skilled in the art. The detailed description provided herein and the specific examples and construction are not intended to limit the invention without departing from the scope of the invention as defined in the appended claims.

We claim:

1. In apparatus for developing a latent electrostatic charge image of a photoconductive recording member comprising a rotatably mounted hollow applicator roll of nonmagnetic material, magnetic means fixedly disposed within said roll for creating a magnetic field in the path of the periphery of said roll, a trough charged with a body of developer mix and means for bringing the developer mix into contact with the periphery of said applicator roll, the improvement comprising a stratum of irregularly shaped particles bonded to the periphery of said applicator roll, said stratum having an average thickness of 300 microns and said particles being in the size range from 20 to 500 microns, whereby the developer powder on said stratum forms a thick brush-like configuration when in said magnetic field.

2. Apparatus defined in claim 1 wherein the particles are comprised of ferrous metal.

3. Apparatus for developing a latent electrostatic charge image on a photoconductive recording member comprising a rotatably mounted hollow applicator roll of nonmagnetic material, magnetic means fixedly disposed within said hollow roll for creating a magnetic field in the path 01': the periphery of said roll, a trough charged with a body of developer powder, said applicator roll mounted in said trough with the periphery of said applicator in contact with the body of developer powder, means for transporting said photoconductive recording member into contact with said applicator roll, said roll turning in the direction of the movement of said photoconductive recording member and at a speed ranging between 1.5 and 5 times the linear velocity of said member, the periphery of said roll having a continuous stratum bonded thereto comprising iron particles ranging in size of from 20 rnicrons to 500 microns and said particles being in the size range from 20 to 500 microns, whereby the developer powder on said stratum forms a thick brush-like configuration when in said magnetic field.

4. Apparatus defined in claim 3 wherein said stratum is comprised of particles having an average size from microns to 300 microns.

5. Apparatus defined in claim 3 wherein said applicator roll turns at a rate approximately three times the linear velocity of the recording member.

6. Apparatus defined in claim 3 wherein the thickness of said bonded stratum is in the range of from .003 to .02".

7. An applicator apparatus for applying developer mix to an electrostatically charged sheet comprising a rotatable hollow cylindrical tube of nonmagnetic material, a stationary magnet disposed within said hollow tube, and a stratum of irregularly shaped particles bonded to the outer surface of said hollow tube, said stratum having an average thickness of 300 microns and said particles being in the size range from 20 to 500 microns.

8. An applicator roll as defined by claim 7 wherein the particles are comprised of magnetizable metal.

References Cited by the Examiner UNITED STATES PATENTS Re. 18,111 6/1931 Woodward 29-132 1,218,324 3/1917 Severy 11731 X 2,062,317 12/1936 Joseph 29132 X 3,003,462 10/1961 treich 118--637 3,040,704 6/1962 Bliss 118637 3,081,737 3/1963 Frantz 118637 3,088,386 5/1963 Sugarman 118637 X FOREIGN PATENTS 330,323 6/1930 Great Britain.

CHARLES A. WILLMUTI-I, Primary Examiner.

WILLIAM D. MARTIN, Examiner.

Claims (1)

1. 7. AN APPLICATOR APPARATUS FOR APPLYING DEVELOPER MIX TO AN ELECTROSTATICALLY CHARGED SHEET COMPRISING A ROTATABLE HOLLOW CYLINDRICAL TUBE OF NONMAGNETIC MATERIAL, A STATIONARY MAGNET DISPOSED WITHIN SAID HOLLOW TUBE, AND A STRATUM OF IRREGULARLY SHAPED PARTICLES BONDED TO THE OUTER SURFACE OF SAID HOLLOW TUBE, SAID STRATUM HAVING AN AVERAGE THICKNESS OF 300 MICRONS AND SAID PARTICLES BEING IN THE SIZE RANGE FROM 20 TO 500 MICRONS.

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