Printing – Processes – With heating or cooling
Patent
1990-08-21
1991-11-26
Wiecking, David A.
Printing
Processes
With heating or cooling
101467, B41L 3514
Patent
active
050674048
DESCRIPTION:
BRIEF SUMMARY
The invention is directed to a method and to an apparatus for printing by inking a latent thermal image.
Non-mechanical printing processes that are based on the electrophotographic or magnetophotographic principle are notoriously known and have been successfully employed. Such a printing process working according to the principle of electrophotography is disclosed, for example, by U.S. Pat. No. 4,311,723. A latent charge image is thereby produced on an electrostatically charged, photoconductive material, whether it is a photoconductor drum or a photoconductive belt, by a selective discharge with a light source modulated image-like. This charge image is then inked in a development station with electrically charged colorant particles (toner) and is subsequently transferred onto a recording medium, for example onto an endless paper web or onto an individual sheet, in a transfer printing station.
Such a charge image is developed either by applying dry toner or liquid toner.
In dry toner development, the charge of the colorant particles is triboelectrically generated by friction at what are referred to as carrier particles, usually iron, steel or ferrite, that also see to conveying into the proximity of the charge image on the basis of their magnetic adhesion to a rotating magnetic drum.
The charge of the colorant particles can also be produced by different methods, for example corona charging or by the polarization of the toner particles in the electrical field of the latent charge image itself. It is standard to employ a toner particle size of about 5 through 10 .mu.m in order to obtain an acceptable relationship between the undesired adhesion forces of the toner particles to the photoconductor, carrier or conveyor means and the desired electrical switching forces.
In the liquid methods, the toner particles that are electrically charged by, for example, chemical charge separation, move in an insulating, organic carrier liquid, for example ISOPAR (Trademark of Exxon). Since undesired adhesion forces are compensated better, the colorant particles can thereby be significantly smaller.
A certain granularity of the image with negative consequences for extremely fine characters and for the transition region between image patterns and backgrounds (edge sharpness) arises in dry toner development due to the required particle size. Further, local electrical development fields cause an especially high particle application in these transition regions that can lead to visible image disturbances (over-tonering) and, consequently, to a poorer adhesion of the toner on the recording medium as well.
The elimination of the carrier liquid from the printer means together with the recording medium and the extremely great sensitivity of the particle application to fluctuations in the toner concentration in the carrier liquid are disadvantageous in the liquid developing processes.
The magnetographic principle is based on producing a latent magnetic image on a permanently magnetizable carrier medium. A defined iron part allows the single-component toner powder to adhere to the carrier medium magnetized in accord with the image. The transfer printing ensues with the assistance of pressure or magnetic field.
The granularity of the toner powder is disadvantageous in the electrophotographic processes. The iron/ferrite additive, moreover, makes it more difficult to produce brilliant hues.
U.S. Pat. No. 4,514,744 discloses an electrostatic copier means, whereby an opaque, thin layer is first applied onto a belt-shaped photoconductor. This layer is then removed character-dependent with the assistance of a thermal writing means, for example by evaporation. After the photoconductor is exposed, the photoconductor is inked with toner and a transfer printing onto recording medium ensues. The opaque layer is then removed from the photoconductor.
An object of the invention is to provide a nonmechanical printing method and a printing means that enable a printed format having high resolution and high color saturation to be produced with high printin
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Frunder Henning
Wiedemer Manfred
Funk Stephen R.
Siemens Aktiengesellschaft
Wiecking David A.
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