US 3600210 A
Description (OCR text may contain errors)
United States Patent @fice 9,392/ 7 Int. Cl. G03g 13/16, 13/20 US. Cl. 11717.5 7 Claims ABSTRACT OF THE DISCLOSURE Positive xerographic transparencies are produced by transferring the image-forming thermoplastic powder (toner) particles from a photosensitive surface to a transparent film, heating the image and the film to a temperature only sufficient to soften the thermoplastic toner particles without damaging the transparent film and then, in a separate operation, heating and pressing the imagebearing film to fuse the particles to the film and to provide a sharp transparent copy.
In xerographic copying a latent image of the matter to be copied is formed as a residual electrostatic charge on a photosensitive surface. When oppositely-charged particles of a thermoplastic powder are distributed over the surface they adhere to the charged area and are then transferred to paper, on to which they are fused by heat to form a permanent copy.
Xerographic copying has the advantages that it is a dry process and that it gives a positive print of the original image. It has, of course, been recognised that these advantages would be equally desirable in the production of positive transparencies, which could then be used, for example, in optical projectors to assist lecturers; but in spite of this, it has not previously been possible to make transparencies by xerographic copying with the common form of xerographic apparatus, in which the photosensitive surface is a drum having an electrically conductive backing coated with vitreous selenium. Attempts to make such copies have encountered the difiiculty that heating to the temperature required to fuse the particles damages or destroys the transparent sheeting, while at lower temperatures the particles do not adhere permanently.
According to the invention, charged particles are transferred from a photosensitive surface, in the manner described above, to a transparent film, which is then subjected to radiant heating sutficient to soften the particles and cause them to adhere to the film without damaging it, after which the film is heated under pressure to fuse the particles and fix them to the film. The use of a second heating stage, with pressure, to fuse the particles adhering to the film avoids damage to the film. The film must, of course, withstand heating to the temperature required to soften the particles used. I have found that the film may advantageously consist of triacetate film and that suitable thicknesses for the film are from .004 to .006". I prefer to use a thickness of 0.005". With such a film, good results are obtained if the temperature for softening the particles in the image-transfer stage is between 90 and 95 C. and if the fusion under pressure is carried out at a temperature of about 85 C. for a period of about 30 seconds. In the image-transfer stage, too low a temperature will result in an image with insufficient adherence to the film so that it cannot be handled without damage to the image. Too high a temperature will cause the film to begin to buckle. The fusing temperature can vary within a wide range provided that the period for which the temperature and pressure are maintained is suitably chosen.
3,600,210 Patented Aug. 17, 1971 As an example, we have found that a fusing temperature of C. for a triacetate film requires not less than 25 seconds for satisfactory fusion and fixing of the image; and if the period is longer than about 45 seconds, the image may transfer partially to the pressure plate or drum. At 100 C., the period should be between 10 and 20 seconds. Generally speaking, the temperature should be not less than 50 C. and not greater than 100 C. (preferably between 80 C. and C.) and the time not longer than 1 minute.
The heating under pressure is conveniently performed" by pressing the power-carrying side of the film against a polished heated metal surface or platen, for example in a photographic dry mounting press. Alternatively, this heating under pressure may be carried out by passing the film between heated polished metal rollers. It is important to apply a sufficiently uniform pressure over the whole area of the film to prevent the film from buckling. We find that when the film is heated in this way the particles of powder fuse together as well as to the film base and that the image is thereby densified and sharpened almost to the extent that is achieved in a conventional photographic image.
A useful feature of this method is that although the initial heating of the transparent sheet fixes the image sutficiently to permit the sheet to be transported through the xerographic copying machine and to be handled without damaging the image, nevertheless portions of the image can easily be removed by light abrasion (for example, with a rubber eraser), or by the use of a suitable solvent. This allows the user to retouch the image, i.e. remove unwanted matter before the final heating under pressure to firmly fix the image. This unwanted matter may, for example, be background speckling or may be unwanted portions of the images. Even when the image has been fixed, further additions may be made to it by hand using suitable opaque inks or paints.
After one image has been produced and fixed on the sheet as described above another image may be applied in the same manner.
Copies made by xerography on a transparent sheet in this way may be used in various ways, for example as transparencies in an optical projector; as masters for high-speed dyeline printing; and as intermediate masters for the production of plates for photo-offset lithography.
As an alternative to triacetate film, the transparent sheeting known as Melinex can be used.
1. A method for producing positive xerographic transparencies comprising the steps of, forming a latent image of matter to be copied as a residual electrostatic charge on a photosensitive surface, developing the image by distributing oppositely-charged particles of thermoplastic powder over said surface whereupon said particles adhere to the charge areas, transferring said image-forming particles to a transparent film, heating said film and said particles suificiently to soften said particles so as to cause removable adherence thereof to said film without fusing the particles or damaging said film and thereafter further heating said image-bearing film under uniform pressure applied to substantially the whole of the film area to fuse said particles and fix them to said film whereby the image is densified and sharpened while the film is prevented from buckling.
2. The method according to claim 1, wherein the temperature in said heating operation to soften said particles is conducted in the range of about 90 C. to about C. and wherein said particles are fused under pressure in a temperature range of about 50 C. to about C. for a time of about one minute to about 10 seconds.
3. The method according to claim 2, wherein said particles are fused under pressure at a temperature of about 85 C. for about 30 seconds.
4. The method according to claim 2, wherein said transparent film is a triacetate film.
5. The method according to claim 2, wherein said heat and pressure employed to fuse said particles are applied to said image-bearing film by means of a heated platen.
6. The method in accordance with claim 1 in which, following the first heating step and prior to the fusion step, the image is retouched by selective removal of image-forming particles.
7. A method in accordance with claim 1, in which after one image has been produced and fixed on the transparent film, another image is applied thereon.
References Cited UNITED STATES PATENTS 3,317,317 5/1967 Clark 1l7l7.5 3,408,216 10/1968 Mott et al. 1l7--.5
4 3,060,051 10/1962 Johnson et al 11717.5 2,554,663 5/1951 Cowgill 118101 3,196,765 7/1965 Walkup 961 3,322,537 5/1967 Giaimo 11717.5 3,227,549 1/1966 Ullrich 1178.5 3,411,932 11/1968 Malone et al. 11717.5
FOREIGN PATENTS 1,049,783 11/1966 Great Britain.
OTHER REFERENCES Xerography and Related Processes, Dessauer and Clark, 1965, pp. 403, 492, 493.
WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner