US2889234A - Xerographic development process - Google Patents

Description

June 2, 1959 J. J. WALKER XEEOGEAPHIC DEVELOPMENT PROCESS Filed July 13, 1955 FIG.I

I: n INVENTOR.

IG 2 JAMES J. WALKER A 'ATTORNEY' United States Patent 1 2,889,234 XEROGRAPHIC DEVELOPMENT PROCESS James J. Walker, Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc., Rochester, N.Y., a

corporation of New York Application July 13, 1955, Serial No. 521,668 1 Claim. (Cl. 117-17.5)

This invention relates to a method and apparatus for the development of electrostatic latent images.

In xerography it is usual to form an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposures to a pattern of activating radiation. Other means of forming electrostatic latent images are set forth in U.S. 2,647,464 to lames P. Ebert and U.S. 2,576,047 to Roland M. Schatfert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopicparticles corresponding to the electrostatic latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case, this visible image in turn may be transferred to a second surface to form a xerographic print.

It is at once evident that this process has exceptional flexibility and lends itself to a limitless number of end uses. On such application, which is particularly challenging in the strictness of its requirements, is the transfer of the visible image formed by the electroscopic particles to a conductive plate and fixing the particles to the plate to form thereby a lithographie plate. Such plates vary in size from the dimensions of a postage stamp up to those used in making maps, which plates are generally 2x3 in size. Furthermore, irrespective of the size of the image developed, an extremely quality of resolution is required throughout, often as high as 150 lines per inch or better.

One particularly troublesome defect accompanying the transfer of the visible image formed by the electroscopic particles to the conductive plate is termed hollow lines. A line segment appearing on the original as a solid line, on transfer to the conductive plate, appears as two very narrow parallel lines with a white or unprinted area in between. Because of this appearance the defect has been given the name of hollow lines. A most unusual feature of this defect is that the visible image formed on the photoconductive insulating surface by the deposited electroscopic particles reproduces the solid lines of the origi- 'eliminates this diltculty.

The developing material commonly used in xerography consists of a finely pulverized pigmented resin powder called a toner mixed with a coarser granular material called a carrier to which the powder loosely adheres by the electrostatic charges generated by contact between the powder and the granular carrier. When the developer mixture has tumbled over the exposed xerographic plate, as by tilting the plate to different angles while in contact with the developer mixture, the powder particles are attracted to the image from the granular material to produce a visible powder image. This method of development is known as ,cascade development and is set forth in U.S. 2,618,552. Applicant has now found that if, in this basic process of cascade development, the developing material-rather than being permitted to ow in uniform parallel lines across the surface of the xerographic plate-is made to follow a zig-zag course while cascading over the surface carrying the electrostatic latent image, the resulting image formed by deposition of the toner on the electrostatic latent image conforms faithfully to the original to be reproduced without loss of detail and, furthermore, an image developed in this way, on transfer to a conductive plate, is free from hollow lines.

By the term xerographic plate as used herein is meant, of course, a conductive backing member having a coating or layer of a photoconductive insulating material on at least one side. lf desired, the photoconductive insulator may be protected by a thin iilm of an insulating resin such as polystyrene, a cellulose ester, a polyvinyl acetal, etc. The backing member may be of any conductive material having the desired structural properties for the use intended such as, for example, a metallic member, plate or the like as of brass, aluminum, zinc, etc. or, optionally a nonconductive member of desired structural properties, such as glass or plastic having a conductive coating as of tin oxide, silver, etc., or a fibrous material, such as paper, having therein conductive material, such as water, particles of carbon, metal or the like, to render the material conductive. Representative photoconductive insulators include vitreous selenium, sulphur, anthracene, mixtures of selenium and sulphur, various photoconductive phosphors such as zinc oxide, cadmium sulde and the like. These may be applied to the conductive backing member either as a continuous layer or as discreet particles in a resinous binder.

The general nature of the invention having been set forth, `there will now be presented a more detailed description, in illustration but not limitation, of the invention in the following specication and drawings in which:

Fig. l is a semi-diagrammatic representation of apparatus according to one embodiment of the invention, and

Fig. 2 is a diagrammatic view of development mechanism according to another embodiment of the invention.

The present invention is particularly intended and adapted for the development of electrostatic latent images on relatively large xerographic plates such as are commonly used in the reproduction of full-size maps. Images of this sort require an unusually high degree of fidelity and resolution in their reproduction.

Illustrated in Fig. l is a development system presented semidiagrammatically comprising a developer tray 10 movably positioned on slides 11 and 12 wherein it is yieldably secured by means 13, as springs or the like. Means 14 to vibrate the tray are also provided in this case, consisting of an armature 15 and a solenoid 16 with a contact lever 17 which is loaded by spring 18. The slide and tray assembly is free to rotate about axis made by the two slide means 11 and 12.

ln operation, a surface bearing an electrostatic latent image to be developed is placed face up in the tray 10. When the image is on a photoconductive insulating layer, as is the case when a xerographic plate is used, the entire unit must be handled in a darkroom to prevent dissipation of the charge in the image areas. The image-bearing member is secured to suitable means, as pins, to the developer tray 1b. The tray is then positioned at a slight angle to the horizontal with a supply of developer, such as disclosed in 2,618,552, distributed along that end which is raised from the horizontal. The spring 18 is then adjusted to obtain the desired rate of vibration and the vibrating means 14 actuated by closing switch 19. Current ows through solenoid 16 forcing armature 15 back against contact lever 17. This causes contact lever 17 to rotate on pin 21 breaking the circuit at contact point 20. With the force exerted by the armature 1 5 on assaasa 3 the upper arm of contact lever 17 removed by the breaking of the circuit at contact point 20, spring 18 rotates contact lever 17 around pin 21 striking amature 15. The upper arm of contact lever 17 drives armature 15 against tray causing' it to vibrate while simultaneously the lower varm of contact lever 17 restores current to coil 16 by making contact with contact point 20. The cycle repeats until switch 19 is opened. The time for completing one cycle may be adjusted either by varying the tension on spring 18 or by varying the point of contact between contact lever 17 and contact point 20. The developer then cascades across the image-bearing surface in a zigzag path and collects on the lower side of the developer tray 10. In this manner the image-is developed.

Example 1 The apparatus of Fig. 1 was used. The image to be developed was on a Xerographic plate consisting of a layer of vitreous selenium on an aluminum backing and measuring 11 X 14" in size. The plate was secured to the developer tray 10 by pins and the tray tilted about ten degrees to the horizontal. A two-component developer prepared in accordance with U.S. 2,618,551 and available commercially from The Haloid Company under the name XeroX Toner and Developer was distributed at the far end of the upper side of the tray. The armature weighed two ounces and was moved by a hundred-watt electromagnet. The armature 15 traveled about onequarter inch. The spring 18 was adjusted to produce a frequency of about ten cycles per second. T he amplitude of vibration of the tray 10 was approximately 0.01. After the two-component developer was distributed, the switch 19 was closed. Development was complete in about one minute.

The image was then electrostatically transferred to a zinc plate using the apparatus of U.S. patent application Serial No. 491,344, tiled March l, 1955, by Matthews and Walker, with a potential of about 500 volts, and fused to the zinc plate in the same apparatus using trichloroethylene vapor. The image on the zinc plate was then examined and found to be characterized by exceptionally good quality, having good resolution, density, and relative absence of hollow lines.

In Fig. 2 the tray 30 contains a perforated screen 31 positioned in a slotted trough 32 at the upper end of the tray. The trough V32 connects to the surface of tray 30 by means of slot 39. The tray 30 is fastened to a tray support 33 which is fastened by yielding means as springs 34 to stand 35. Stand 35, in turn, is positioned by rack 36 so that the tray 30 is inclined at an angle to the horizontal. A spout 38 is at the bottom of the tray 30. An eccentric 40 driven by motor 41 is connected to tray support 33 by suitable means as a rod 42.

In operation, an image bearing member 44 having a photoconductive insulating layer 45 on a conductive back 46 is placed with the insulating layer 45 up in the tray 30 where it is secured by suitable means, as pins 37 and ange 43. Alternatively, the tray 30 may be so constructed as to firmly hold the image-bearing members to be developed therein. A supply of two-component developer prepared in accordance with U.S. 2,618,551 Vand available commercially from The Haloid Company under the name XeroX Toner and Developer is distributed `along the perforated screen in the slot developer trough.

The motor driving the eccentric is then actuated causing the tray to oscillate in the plane of the tray 30 and in a direction approximately at a right angle to the direction of motion of the powder particles over the imagebearing member. The result is to impart a zigzag pattern to the developer particles. These particles passing over the image-bearing member pass through the hole 38 and yare collected in suitable means as a jar, trough, or other container. The .developer so collected may then be redistributed over the perforated screen for the development of another image-bearing member.

4 Example `2 The apparatus of Fig. 2 was used. The electrostatic latent image to be developed was on a xerographic plate consisting of a layer of vitreous selenium on an aluminum backingV and measuring 11" X 14". Accordinglyfdevelopment wascarried out in the dark to prevent dissipa# tion of the electrostatic image. The plate `44 was placed in the tray 30 image side (i.e., the `side bearing thephotoconductive insulating layer 45) up. The stand 36 holding the tray 30 was so constructed that the tray was inclined at iive degrees to the horizontal. A supply of two-component developer prepared in accordancewith U.S. 2,618,551 and available commerically from The Haloid Company under the name XeroX Toner and Developer was distributed Valong the length ofthe perforated screen 31 in the slot developer'trough 32. The eccentric 40 was adjusted to displace the tray 30 about one-eighth inch while the motor 41 was adjusted to provide about thirty such oscillations per second. The motor 41 Iwas then actuated. The two-component developer cascaded over the image-bearing member 44 collecting at the bottom of the tray 30 where it passed through hole 38 to be collected in a jar. Development took about two minutes.

'Ihe image was then electrostatically transferred to a zinc plate using the apparatus of U.S. patent application Serial No. 491,344, filed March l, 1955, by Matthews and Walker, with `a potential of yabout '500 volts, and fused to the zinc plate in the same apparatus .using trichloroethylene vapor. The resulting image was characterized by good resolution, density, and by'freedom from hollow lines.

Example 3 Example 2 was duplicated except that the stand 36 was constructed to incline the tray 30 at an angle of ten degrees to the horizontal. In this case complete development of the 11" x 14" Xerographic plate was obtained in one-half minute. Again, the image on the zinc plate `was characterized by good resolution, density, and freedom from hollow lines.

While the present invention has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby'but it is intended to cover the invention broadly within the spirit and scope of the appended claim.

I claim:

In the process wherein an electrostatic latent image on a smooth, planar, electrically insulating surface is developed to yield an image of electrostatically attractable material electrostatically adhering on said surface by cascading across the electrostatic latent image a dry mixture of loose, movable finely divided particles of electrostatically attractable powder and separate granular carrier material, the individual granules of said carrier material being substantially larger Vthan the individual particles of said attractable powder, the powder and carrier having a triboelectric relationship of opposite polarity whereby the powder removably adheres electrostatically to the surface of the carrier granules the improvement comprising flowing the dry mixture over the said image bearing surface ina zigzag course.

References Cited in the file of this patent UNITED STATES PATENTS 238,991 Toye Mar. 15,1881 2,550,724 Sabel et al. May l, 1951 2,550,738 Walkup May 1, 1951 2,618,552 Wise Nov. 18, 1952 2,635,046 Sahel et al. Apr. 14, 1953 2,682,478 Howse June 29, 1954 2,705,199 Clark Mar. 29, 1955 Y 2,761,416 Carlson c Sept..4, 195.6

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