US2892446A - Apparatus for developing electrostatic image - Google Patents

Description

June 30, 1959 QLDEN 2,892,446

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE Filed Oct 30, 1956 Z Sheets-Sheet l I Hy] V/B/M-ZURY MECHANISM IN V EN TOR.

E? Z- BY Roger 61' flldez'z ATTOHNEK June 30, 1959 R. G. OLDEN Filed Oct. 30, 1956 APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE 2 Sheets-Sheet 2 INVENTOR.

Rage!- fi. Ola e22 BY ATTOIf/IEY United States Patent 2,892,446 APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE Roger G. Olden, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application October 30, 1956, Serial No. 619,203 5 Claims. (Cl. 118-637) This invention relates to electrostatic printing, and more particularly to an improved means for applying developer powder to a record, such as a continuously moving web, bearing an electrostatic charge image.

An electrostatic printing process is a process for producing a visible reproduction, or copy, which includes the steps of converting a light image to a latent electrostatic charge image and converting the latent charge image into a visible image.

A typical electrostatic printing process may include first producing a uniform electrostatic charge upon the surface of a photoconductive insulating material, such as selenium, anthracene, or zinc oxide dispersed in an electrically-insulating film-forming vehicle, carried on one surface of a record. Then, the charged surface of the photoconductive material is exposed to a light image through a photographic transparency or by other suitable means. The portions of the surface irradiated by light rays become discharged while the remainder of the surface remains in a charged condition, thereby forming a latent electrostatic image corresponding to the light image. The latent electrostatic image is developed by applying a colored developer powder which is held electrostatically to selected areas of the surface. The visible powder image thus formed may be fixed directly to the photoconductive surface by fusing.

When the record carrying the photoconductive surface is an elongated strip or web upon which it is desired to make a plurality of reproductions at a rapid rate, a problem arises in presenting a sufiicient quantity of developer powder to the surface of the web to permit development at the desired rate.

A desired developer mixture includes a developer powder and a granular carrier chosen with regard to the tribeelectric series to produce charges of opposite polarities in the developer and carrier particles when they are mixed together. The polarity of'the developer powder will be opposite to that of the charge image. The carrier particles are generally larger than the developer particles so that a number of developer particles will adhere to each carrier particle after mixing. The specific problem then is to continuously present new carrier particles to the charge image surface so that sufiicient developer particles will be available for transfer from the carrier to the charge image surface.

An object of this invention is to provide an improved apparatus for applying developer powder to the surface of a continuously moving record.

Another object of this invention is to provide an improved apparatus for mixing developer powder and granular carrier particles to produce electrostatic charges of opposite polarities and for presenting the mix to the surface of a continuously moving record.

A further object of this invention is to provide an improved apparatus for presenting a continuous supply of freshly mixed developer powder and granular carrier particles to the surface of a continuously moving record.

Apparatus in accordancewith the present invention includes a trough for containing a mixture of developer and carrier particles. A path is provided through the trough to permit the passage of a movable record upwardly through the trough contiguous to one wall of the trough. One surface of the record is directly contacted by the mixture. An agitating means is provided in the trough for mixing the developer and carrier particles to maintain a substantially homogeneous mixture and to produce charges of opposite polarities in the developer and the carrier. A mixture of iron filings and developer powder is especially suited for use with apparatus of this invention since it is readily confined to the area of developer application.

The novel features of the invention, as well as additional objects and advantages thereof, will be understood more fully from the following description when read in connection with the accompanying drawings, in which:

Figure 1 is a diagrammatic view illustrating an electrostatic printing system embodying the present invention;

Figure 2 is a view in front elevation, partially broken away, of a developing apparatus in accordance with the present invention; and

Figure 3 is a view taken on the line 33 of Figure 2 looking in the direction of the appended arrows.

Referring now, particularly, to Figure 1 of the accompanying drawings, a system for electrostatically printing on a continuous web 11 is illustrated diagrammatically. The web 11 is fed from a supply roll 13 and is drawn through the various units of the system by means of idler rollers 12, 14, and 16 and by means of drive rollers 15 and 17 driven by a motor 19. The web is then collected on a take-up roll 21 which may also be driven by the motor 19.

The web is first passed through a corona charging unit 23 in which a uniform electrostatic charge is applied to one surface of the web. The corona charging unit 23 includes a grounded drum 25, around which the web passes and a plurality of wires 27 spaced from the drum and being under high potential with respect to the drum.

The web is then passed through an exposure unit including a light source 31 and an optical system 33. In this unit the web is exposed to a light image produced through a photographic transparency 34 or other suitable means. The web is next passed through a developing unit 35 in which electroscopic developer powder is applied to the charge image. It is the developing unit 35 to which this invention is primarily related. The web next passes through a fusing unit 37 wherein the developer powder is fused to the web to form a permanent image. The fusing unit 37 includes a housing 39 enclosing a plurality of heating wires 41.

Referring now particularly to Figures 2 and 3 of the accompanying drawings, the developer unit 35 includes an elongated, closed trough having side walls 43 and 45 and end walls 47. The side wall 45 is folded under to form a bottom wall 49 of the trough and the bottom wall is again folded downwardly to form a portion parallel to and adjacent to the lower portion of the side wall 43. These last-named portions then form a narrow extension 51 of the trough. The side wall 43 extends upwardly short of the top of the trough. A trough cover 53 includes a downwardly extending portion 54 lying parallel to and slightly overlapping the wall 43. The portion 54 acts as a shield, the function of which will be described subsequently.

The web 11 is threaded through the trough, entering the trough through the lower extension 51, passing contiguous to the wall 43, and leaving the trough at the longitudinal opening between the walls 43 and 54. The web is threaded so that its photoconductive surface faces away from the wall 43.

Prior to the beginning of a printing operation, the trough is filledwith a developer mix 55 to a level of approximately one half the depth of the trough as indicated'in the drawings. The developer mix 55 will contain the desired proportions of a developer powder and a granular carrier. For purposes of the present description, the carrier consists of iron particles, such as iron filings, and this carrier is chosen for its desirable properties. It will be understood, however, that any suitable carrier may be used and that it need not be a magnetic material.

In order to provide a means for sealing the trough at the point of entry of the web 11, a magnetic structure is provided which encloses the extension 51 of the trough. This magnetic structure includes a pair of elongated pole pieces 57 and 59 which lie adjacent to the extension 51 on either side thereof and extend slightly beyond the ends of the trough. A pair of elongated magnets 61. and

63 bridge the pole pieces 57 and-59 at either end of the extension 51. These magnets are secured to the pole pieces in any suitable manner and are disposed so that their ends polarized north are contiguous to the pole piece 59, for example, and their ends polarized south are contiguous to the pole piece 57, for example. The pole pieces 59 and 57 are then polarized north and south, respectively, by magnetic induction.

The above described magnetic structure provides a unidirectional magnetic field in which the magnetic lines of force extend between the pole pieces and transverse to the web 11 which passes between the pole pieces. When the magnetic carrier particles are supplied to the trough, some of these particles are held by the magnetic field and effectively form a brush 65 of magnetic particles Within the opening of the extension 51. As the web 11 is drawn through the opening, this brush prevents the developer mix 55 from dropping through the opening. The gap, within which the magnetic brush is formed, is very narrow, being of the order of A; to /1 inch in width, for example. It will be understood that, in order for this magnetic seal or trap to function properly, the trough should be constructed of a non-magnetic material. In the event that the carrier particles are not of a magnetic material, the above described magnetic seal may be formed by employing granular magnetic particles for this purpose.

An agitator 67 is mounted in the lower portion of the trough to provide for a continuous mixing of the developer and carrier particles. This mixing not only producesthe desired electrostatic charges on the developer and carrier particles, but also maintains a homogeneous mixture of these particles and continuously presents new particles to the surface of the web 11. The agitator consists of a shaft 69 disposed horizontally and rotatably mounted in the end walls of the trough. Four vanes 71 are secured to the shaft 69 and extend longitudinally of the shaft substantially the length of the trough. These vanes are disposed radially about the shaft at 90 intervals and are shown simply as rectangular paddles. It will be understood that more or less of these vanes 71 may be provided and that they may have other configurations which may be more efficient in producing the desired mixing. A pulley 73 is secured to one end of the shaft 69 outside of the trough for driving the agitator 67.

A'hopper 75, Figure 1, for developer powder 77 is mounted on top of the trough. The hopper has side walls tapering to a rounded bottom portion and extends substantially the length of the trough. The rounded bottom portion of the hopper 75 comprises a screen 79 to permit the dispensing of developer powder from the hopper into the trough. The screen 79 may be mounted for vibratorymotion with respect to the hopper 75 and a suitable vibratory mechanism 81 may be mechanically coupled to the screen to dispense developer powder uniformly across the trough at a desired rate. The rate at which the developer powder is dispensed into the trough will be determined by the average rate at which the developer powder is'applied to the web 11.

-A rotary magnetic member 83 is mounted in the upper portion of the trough above the level of the developer mix 55. This rotary member serves to remove carrier particles which cling to the web 11 after development. This member includes a plurality of magnetized discs 85 fixed to a shaft 87 which extends longitudinally with respect to the trough and is rotatably mounted in the end walls 47. The discs 85 are mounted on the shaft so that they do not lie in planes perpendicular to the shaft axis. The effect of this arrangement is that, when the shaft is rotated, the discs Will appear to wobble on the shaft and will sweep all portions of the Web surface. The discs are ground so that their edges define a cylindrical surface concentric with the axis of the shaft 87. The shaft is disposed with respect to the path of the web 11 so that the discs do not engage the surface of the web, but are spaced slightly from the web. The discs 85 are all magnetized to the same polarity and act as magnets to attract from the web 11 any magnetic carrier particles which may have adhered to the web after passing through the lower portion of the trough. Use of the rotary member as a clean up member is described by way of example and any suitable magnetic member may be used.

An elongated magnetic pole piece 89 is mounted on the outside of the trough wall 43 parallel to the shaft 87 and directly opposite to the clean up member 83. A plurality of magnets 91 are secured to the pole piece 89 and are disposed to polarize the pole piece to one polarity along its entire length. The polarity of the pole piece 89 is opposite from the polarity of the discs 85 so that a concentrated unidirectional magnetic field passes through the web 11.

The clean-up member 83 is driven by a motor 93. A pulley 95 fixed to the shaft 87 is aligned with the pulley 73 of the agitator shaft 69. Power from the motor 93 is thereby transmitted to the agitator 67 by means of a belt 97 mounted on the pulleys 73 and 95. The clean-up member 83 may be rotated at a sufiicient speed to cause the magnetic particles which are attracted to and which adhere to the discs to be thrown off by centrifugal force. Where the magnetic field, strengthened by the pole piece 89, passes through the web 11, the magnetic particles are attracted from the web to the discs. When a magnetic particle contacts a disc, it adheres to the disc momentarily and then is thrown off due to the centrifugal force imparted to the particle. The shield 54, which extends downwardly from the trough top 43 lies inwardly of the Web 11 and prevents any of the magnetic particles, which are thrown from the discs, from again being deposited on that portion of the Web which has been cleaned by the clean-up member.

The operation of the apparatus will now be briefly described. The coated web 11 is first threaded from the supply roll 13 through the various units of the system, and through the drive rollers 15 and 17 to the take-up roll 21. After the web is threaded through the trough of the developer unit 35, the trough is filled to the proper level with a developer mix 55 consisting of an electroscopic developer powder and a granular magnetic carrier. The mix is prevented from passing through the trough by the magnetic brush 65. The proportions of the mix ingredients are predetermined for best developing efliciency. The developer mix in the trough directly contacts theweb surface. A supply of the developer powder 77 is provided in the hopper 75.

The web 11 is first driven past the charging unit 23 where a uniform electrostatic charge is applied to the photoconductive'surface. The web next passes the exposure unit where the charged surface is exposed to a light image to form a latent electrostatic image. In continuous operation, of course, a series of latent images will be formed.

The web next passes through the developer unit 35 wherein the developer powderand magnetic carrier have beenmixed by the agitator 67. Through this mixing, the developer powder particles have acquired a charge opposite to that of the charge image and opposite to that of the carrier particles. The agitator continuously presents new developer mix 55 to the surface of the moving web where the developer powder particles are attracted away from the carrier particles to selected areas of the web surface. The hopper 75 is actuated to provide a continuous supply of developer powder 77 to the trough to maintain the desired mix proportions. The rate at which developer powder is supplied is equivalent to the average rate at which it is deposited on the web. The agitator 67 maintains the developer mix 55 in a substantially homogeneous condition.

As the web passes through the developer mix 55, certain of the magnetic carrier particles may adhere to the web 11 due to the physical pressure involved. As the web passes the magnetic clean-up member 83, these particles are attracted away from the web surface and returned to the trough.

After development, the web passes through the fusing unit 37 wherein the developer powder is permanently fused to the web surface by means of heating and a permanent image is provided on the web.

A particular advantage of the developing apparatus described herein is that the web may be passed through the developer unit at a faster rate than in developer units known heretofore. This is possible because of the inherently large supply of developer powder which may be made available for transfer to the web surface. No intricate or rate limiting transport mechanism is required for presenting the developer powder to the web. The apparatus is simple in structure and eificient in operation.

What is claimed is:

1. Apparatus for developing a latent electrostatic charge image on a record receiving member comprising means for guiding and moving upwardly a record receiving member bearing a latent electrostatic charge image, a container, open at the bottom for holding a quantity of developer mix having magnetic particles and developer powder particles disposed to contact a record receiving member guided and moved by said first named means, means to agitate and to mix said particles in said container, means for providing a pathway through said container for said guided and moved record receiving member, means to shield one surface of said guided and moved record receiving member from said quantity of developer mix, and means to provide magnetic lines of force across said open bottom for causing the magnetic particles situated at said open bottom to align themselves thereat and form a brush adapted to provide support and seal for said developer mix.

2. Apparatus adapted to develop a latent electrostatic charge image on a record, said apparatus comprising a trough for containing a mixture of developer powder particles and granular magnetic carrier particles, said trough having a longitudinal opening at a lower surface, means to provide magnetic lines of force across said opening for causing the magnetic particles situated in said opening to align themselves across said opening and form a brush adapted to seal said opening and provide support to said mixture, means for moving said record upwardly through said longitudinal opening and along a path contiguous to one wall of said trough, and means for agitating said particles in said trough to produce charges of opposite polarity on the developer powder and the carrier and to maintain a substantially homogeneous mixture of said particles.

3. Apparatus adapted to develop a latent electrostatic charge image on a record, said apparatus comprising a trough for containing a mixture of electroscopic developer powder particles and granular magnetic carrier particles, said trough having a longitudinal opening at its lower wall, means providing a unidirectional magnetic field across said opening, said field being adapted to cause the magnetic particles situated in said opening to align themselves across said opening and form a brush adapted to provide a seal at said opening to thereby support said mixture, means defining a path for said record through said opening and contiguous to one wall of said trough, means for feeding said record upwardly through said trough, means for maintaining a flow of developer powder into said trough, and means for agitating said mixture in said trough to produce triboelectric charges of opposite polarity on the developer and the carrier and to maintain said mixture substantially homogeneous.

4. Apparatus adapted to develop a latent electrostatic charge image on a web, said apparatus comprising a trough for containing a mixture of electroscopic developer powder particles and magnetic carrier particles, said trough having a longitudinal opening at its lower wall, means providing a unidirectional magnetic field across said opening, said field being adapted to cause the magnetic particles situated in said opening to align themselves across said opening and form a brush adapted to provide a seal at said opening to thereby support said mixture, means defining a path for said web through said opening and contiguous to one wall of said trough, means for feeding said web upwardly through said trough, means for maintaining a flow of developer powder into said trough, means for agitating said particles in said trough to produce triboelectric charges on the developer and carrier particles and to maintain a substantially homogenous mixture of said particles, and means providing a unidirectional magnetic field at the upper portion of said trough adapted to pass through said web after it is contacted by said particles, said last named means being adapted to attract and remove any of said magnetic particles from said web and being positioned at sufficient distance above said mixture contained in said trough so as not to remove magnetic particles from said mixture.

5. Apparatus adapted to develop a latent electrostatic charge image on a web, said apparatus comprising a trough for containing a mixture of developer powder particles and magnetic carrier particles, said trough having a longitudinal opening at its lower surface extending substantially the length thereof, means providing a unidirectional magnetic field across said opening, said field being adapted to cause the magnetic particles situated in said opening to align themselves across said opening and form a brush adapted to provide a seal to said mixture at said opening, means defining a path for said web through said opening and contiguous to one wall of said trough, means for feeding said web upwardly through said trough, said charge image on said web facing inwardly in said trough, hopper means for maintaining a continuous feed of toner particles into said trough, means for agitating said particles in said trough to produce triboelectric charges of opposite polarity on the developer powder and magnetic carrier particles and to maintain a substantially homogeneous mixture of said particles, a rotary magnetic member mounted adjacent to said web path in the upper portion of said trough at sufiicient distance above said mixture contained in said trough so as not to remove magnetic particles from said mixture, and a magnetic member opposing said rotary member on the opposite side of said path to provide a relatively concentrated magnetic field passing through said path, said rotary member being adapted to attract and remove any of said magnetic particles from said web and to discharge them into said trough.

References Cited in the file of this patent UNITED STATES PATENTS 1,187,008 Nehring June 13, 1916 1,311,121 Haefely July 22, 1919 2,100,587 Chalker Nov. 30, 1937 2,761,416 Calson Sept. 4, 1956 2,784,109 Walkup Mar. 5, 1957 2,786,440 Giaimo Mar. 26, 1957

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