Title:
CONTROL OF ELECTROSTATIC DEFORMATION OF THERMOPLASTIC FILM
United States Patent 3730621


Abstract:
Apparatus for producing images by electrostatic deformation has a light source for both exposing the deformable member to an image and heating the deformable member to the point where it is deformed by the electrostatic charges. A photocell receives rays passing from the light source through the deformable member or reflected from the deformable member and generates an electrical signal to terminate the heating of the film at the appropriate time.



Inventors:
SULLIVAN W
Application Number:
05/110214
Publication Date:
05/01/1973
Filing Date:
01/27/1971
Assignee:
XEROX CORP,US
Primary Class:
Other Classes:
355/68, 355/83, 399/132, 430/50
International Classes:
G03G16/00; (IPC1-7): G03G15/00
Field of Search:
355/3,9,41,68,83 96
View Patent Images:



Primary Examiner:
Matthews, Samuel S.
Assistant Examiner:
Hayes, Monroe H.
Parent Case Data:


This application is a continuation-in-part of Applicant's prior copending application, Ser. No. 777,959, filed Oct. 17, 1968, now abandoned which in turn is a continuation of Applicant's application, Ser. No. 506,646, filed Nov. 8, 1965, now abandoned.
Claims:
What is claimed is

1. A control device for use in regulating the development of images formed by electrostatic deformation of a plastic surface including

2. the device of claim 1 wherein said heat terminating means includes a shutter interposed between the film and the illuminating and heating means which is capable of shielding the film from the radiation directed onto the film for heating.

3. The device of claim 2 wherein said means to support said thermoplastic film is transparent to the radiation for indicating the degree of deformation and said photoresponsive means is positioned to intercept radiation passing through said film and support means.

4. The control device of claim 2 wherein said means to support said thermoplastic film is an opaque means and said photoresponsive means is positioned to intercept radiation reflected from the surface of said film.

5. The control device of claim 1 wherein said heat terminating means is a translating member adapted to direct the radiation for heating away from said film.

6. The device of claim 1 wherein the illuminating and heating means includes an infrared lamp.

Description:
This invention relates to a method and to a control apparatus and particularly to a method and apparatus to control the deformation of plastic films by electrostatic forces.

More specifically, the invention herein relates to a method and apparatus used to control the production and resolution of "frost" images produced on plastic films by electrostatic forces. The process of plastic deformation by electrostatic force requires that an electrostatically charged plastic surface be softened by either heating or exposure to solvent vapors so that the softened surface may be deformed by the forces of the electrostatic charges on the surface of the plastic. The deformation of the film is directly related to the viscosity of the film after it is heated. The deformation or "frosting" may be controlled by controlling the softening of the film. If the heat is controlled by the deformation of the film, then all the films will be deformed to the same extent.

In conventional xerography, a photoconductor, such as selenium mounted on a conductive substrate, is uniformly charged by a corona discharge device and exposed to a light image of the copy to be reproduced. The exposure to the light image renders the photoconductor conductive in areas exposed to light, and thus portions of the electrostatic charge are discharged from the surface of the photoconductor. The electrostatic charge remaining on the photoconductive surface is in image configuration of the copy being reproduced. The latent electrostatic image remaining on the photoconductive surface is developed by contacting the surface with a thermoplastic resinous powder referred to as toner. The toner is attracted to the photoconductive surface by the electrostatic charge and produces a powder image of the copy being reproduced. The powder image is then transferred to a support material and bonded to the support material by either a heat or vapor fuser. The art of xerography is well known and completely described in many patents such as for example, U.S. Pat. No. 3,062,109.

Frost xerography differs from conventional xerography primarily in the nature of the development step. In frost xerography a photoconductor is used to convert variations in lighting into variations in charge density on the surface of a plastic film. The charge density is then impressed on the surface of the film. When the film is heated, the softened plastic is deformed by the electrostatic forces to yield a rippled or frosted image. The rippled image may be viewed by light scattering optical techniques. The frosting effect consists of random depressions or ripples in the thermoplastic which have a fairly constant frequency depending on the characteristics and thickness of the plastic. After the surface is frosted, light will pass through non-image or smooth areas of the plastic whereas it will be deviated or scattered by refraction and deflection in the frosted areas. The extent of this light scattering will depend on the topography of the frosted surface. In general, higher charge density causes deeper frost pockets or depressions so that the stepped angles of the "valleys" cause scattering to greater angles away from the normal. Scattering is sufficient for viewing frost images even in conventional slide projectors. A complete description of the "frost" process is found in U.S. Pat. No. 3,196,011 issued July 20, 1965.

In the reproduction of frost images, the viscosity of the film surface is important only as it effects the final image; therefore, the viscosity of the plastic film should be controlled by the image produced on the film. By regulating the exposure of the plastic film to heat, the viscosity of the film and thus the degree of deformation and the resolution of the frosted image can be accurately controlled. A small infrared source may be used to heat the film to lower the viscosity of the thermoplastic to a point where the stresses exerted by the image charges deform the surface.

The present invention utilizes a photoelectric detector positioned to detect the changes in optical density of the film as the frost development process takes place. A light source provides both the infrared radiation used to heat and plasticize the film and the visible source of light used for optical monitoring of the development process. The output of the photocell can be used to control plasticizing of the film in the development process, and thus the development can be terminated at any preset limit. This arrangement provides automatic compensation for small inconsistencies in the processing conditions of the film due to coating variations, variations in the charging potential and variations in ambient temperature conditions. The apparatus is extremely simple and flexible. It is possible to control the density of the frost image with a simple adjustment to the photo-detecting device, and there is no physical movement of the image during developing and no additional heat sources or heat sensing devices required to control the temperature of the film.

It is therefore an object of this invention to broadly control the surface deformation of plastic films by electrostatic charge.

It is also an object of this invention to control the viscosity of plastic films in the reproduction of frost images, relative to the degree of development of the image.

It is also an object of this invention to directly control automatically the development of frost images.

It is a further object of this invention to directly measure the degree of development of frost images on plastic films and to regulate a heat source used to soften the plastic film.

These and other objects of this invention are attained by means of an infrared heating source used to soften a transpatent plastic film having an electrostatic image thereon and a light-sensitive device which reacts to changes in the optical characteristics of the film surface to directly control the exposure of the film to the heating source.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view showing one embodiment of the invention;

FIG. 2 is a schematic illustration of a second embodiment of the invention; and

FIGS. 3 through 6 are schematic representations of the steps used in creating a frost image.

Referring to the drawings, there is shown in FIGS. 3 through 6 the process for deformation of thermoplastic film referred to herein as "frost imaging" and completely disclosed in the cited U.S. Pat. No. 3,196,011. In the process, as disclosed, a transparent thermoplastic layer 8 of a material such as a rosin ester approximately 2 microns thick is coated on a photoconductive layer 10 overlying a conductive base 12. The photoconductor 10 is composed of a transparent material such as a charge transfer complex of an epoxy polymer and Lewis acid such as disclosed in U.S. Pat. No. 3,408,181. The layer 10 is generally characterized as being a good electrical insulator capable of maintaining a surface charge in the dark, of becoming substantially more conductive when illuminated by visible light, x-rays or other forms of activating radiation.

The conductive base 12 is also composed of a transparent material such as glass coated with a transparent layer of tin oxide and commonly referred to as NESA glass. As seen in FIG. 3, the layer of thermoplastic resin 8 is electrostatically charged by moving a corona charging device 14, which is connected to a high voltage power supply 15, across the surface thereof. Corona charging devices are well known in the xerographic art, and suitable ones are described, for example, in U.S. Pat. Nos. 2,777,957 and 2,836,725. Other methods of applying a uniform potential onto an insulating surface are known and may be employed. In accordance with the conventional xerographic practice, a potential of about 800 volts, generally positive, may be applied to the layer 8. The total potential difference applied is divided between layers 10 and 8, inversely as the ratio of their capacitances per unit area. Most of the potential will generally appear across the photoconductive insulating layer 10 because, even though it has a higher dielectric constant than layer 8, it is sufficiently thicker so as to have a lower capacitance per unit area.

After the uniform electrostatic charge is applied to the surface of layer 8, the surface is exposed to a pattern of light and shadow of the image to be reproduced in the thermoplastic layer. The exposure step is illustrated schematically in FIG. 4 by the projector 16. It should also be noted that since the support layer 12 is transparent, exposure may be made through the member 12. The electrostatic charge on the surface of layer 8 induces a negative charge to migrate to the interface between the conductive substrate and the photoconductive insulating layer. Where struck by light, photoconductive insulating layer 10 becomes electrically conductive and permits the charge found at the interface between the layer 10 and support member 12 to migrate to the interface of layer 10 and layer 8. Such charge migration does not lower the electric field in layer 8 but does locally lower the potential at the layer 8.

The next step, as shown in FIG. 5, is to again charge the surface of layer 8 to a uniform potential which is usually the same potential as was applied in connection with FIG. 3. In areas of previous exposure, and thus of internal charge migration, the surface of layer 8 accepts additional charge as indicated in the Figure. The electric field on layer 8 is now greatly increased in regions corresponding to illuminated areas, and the electrostatic energy in those areas is likewise increased, while unexposed areas retain only the original charge. If desired, plate 10 may again be recharged to a uniform potential. This produces a somewhat higher field in layer 8 in exposed areas; and by repeating this process several times, it is possible to create in the exposed areas of layer 8 an electric field virtually equal to the entire potential applied to the surface thereon divided by the thickness of the layer 8. Nevertheless, a single exposure step is all that is ordinarily required or desired. Instead of recharging to the same potential that is applied in FIG. 3, layer 8 may be brought to zero potential which creates smaller fields or may be recharged to the opposite polarity which creates the greatest field in unilluminated areas.

The next step is to temporarily soften deformable layer 8 so that it becomes physically alterable by the mechanical forces associated with the electrostatic pattern thereon. Any softening method may be employed, provided that it does not increase the electrical conductivity of layer 8 sufficiently to cause the electrical charges thereon to leak away or become dissipated. The most common methods of softening are either to expose layer 8 to an atmosphere of solvent vapors or to heat it. The latter method is illustrated in FIG. 6, wherein the plate is shown in position to be heated by element 18. As the material of layer 8 is softened, it is able to flow in response to the electrostatic forces acting upon it. As shown in FIG. 6, the layer of surface 8 in areas of high field develops a microscopically uneven or frosted surface. This local deformation causes layer 8 to take on a milky appearance in proportion to the amount of illumination received in different areas and thus represents a form of continuous tone reproduction.

The next processing step is to reharden layer 8 thereby freezing the frost surface pattern in place. This can be accomplished by literally removing the source of heat, terminating the heat at its source, interposing a shield or shutter between source and imaged surface, removing the imaged surface from the heat source, etc. Excessive softening or excessively prolonged softening of layer 8 is to be avoided because it may cause a loss of image pattern. Deformation of the layer 8 is caused by electrostatic forces exerted by charges at the surface of the layer, excessive softening of the layer permits these charges to flow through the layer, thereby destroying the image-producing forces. Certain materials with sharp melting points, such as certain waxes, are therefore poorly suited for layer 8 because it is impractical to bring them to a viscous as opposed to watery condition.

Referring now to FIGS. 1 and 2, there are shown a pair of embodiments of control devices which directly regulate the frosting or amount of deformation of the thermoplastic coated photoconductive member 20. The member 20 is supported by support members 22 and processed in the manner previously described to produce an electrostatic charge pattern on the surface of the plastic in image configuration. At this point in the process, in order to soften the plastic material and permit the frosting of the surface, the plastic is subjected to the heat of an infrared lamp 24. The heat produced by the lamp 24 softens the image and permits the necessary deformation. Visible light from the lamp 24 passes through the member 20 as shown in FIG. 2 and is focused by a lens 26 into a photoelectric detector 28. In this manner the photoresponsive means directly detects the degree of deformation of film 20. As previously described, the member 20 in this embodiment is composed of a transparent thermoplastic member overlying a transparent photoconductor which is mounted on a conductive transparent base of the NESA glass type. In the embodiment shown in FIG. 1, the visible light from the lamp 24 is reflected off of the member 20 through the lens 26 directly to the photoelectric detector 28. In this embodiment, the member 20 may be composed of a transparent thermoplastic layer mounted on an opaque photoconductive material such as selenium and on any conductive base rather than a transparent conductive base. The major difference between the two embodiments is merely that the light used to control the degree of frosting on the member 20 in one case passes through the member 20 to the photoelectric detector 28 and in the other case is reflected off of the member 20 into the photoelectric detector 28.

Prior to heating and deformation by the electrostatic forces, the surface of member 20 is smooth and even, and the light rays pass directly through the member to the photoelectric detector 28. As the heat from the lamp softens the surface of the member 20 and the electrostatic forces start to deform the surface, the light rays from lamp 24 tend to become diffused and deflected away from the photocell 28. As the degree of frosting or deformation of the plastic continues, a greater amount of light is deflected away from the member 28 and the change in light intensity is recorded by the photocell and visibly indicated on a meter 30. In FIG. 1, the amount of light reflected from the surface of the member 20 into the photoelectric detector 28 also changed and is recorded on meter 30.

The change in the light density, as recorded by the photocell 28 and registered on the meter 30, can also be recorded on a chart recorder 32. A control circuit which is preset to limit the density of the frost image is indicated as a circuit box 34 and is electrically connected to drive a shutter 36 physically positioned between the lamp 24 and the frost film member 20.

The purpose of the shutter 36 is to provide one preferred means of accomplishing instant termination of the transmission of heat from the source to member 20. With the shutter 36 interposed between the lamp 24 and the member 20, the closing of the shutter immediately prevents any rays from the lamp 24 from reaching the member 20, and thus effectively terminates the frosting of the film 20.

Instead of employing shutter member 20 to immediately terminate the heating of member 20, a pivot or translating member 40 can be utilized to accomplish the same result. The pivot member 40 is responsive to a signal from circuit box 34 and acts to move lamp 24 in the direction shown by the arrow. This immediately terminates the heating of member 20.

As another alternative means for terminating the heating of member 20, a rotatable member 42 is also responsive to a signal from circuit box 34 when the predetermined density of the frost image has been obtained. The rotatable member 42 rotates support member 22 and member 20 away from lamp 24 thereby immediately terminating the heating of member 20.

It is to be understood that the alternative means for terminating the heating of member 20 as shown in FIG. 1, can also be used in the arrangement shown in FIG. 2.

As shown herein, the control circuit to operate the lamp 24 and the shutter 36, pivot member 40 or rotatable member 42 can be set with a microswitch 38 positioned to be actuated by the pin holder or recorder 32 at a specific density of frosted image or a particular reading of the photocell 28. It can also be used as an override limit switch which would prevent overheating of the film. When the switch 38 is tripped, a relay in the control circuit 34 turns off the lamp 24 and closes the shutter 36, actuates pivot member 40 or rotatable member 42. With this arrangement, all images produced on the members 20 have the same optical density.

While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth; and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.