Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-04-22
2001-05-08
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06227655
ABSTRACT:
DESCRIPTION
1. Field of the Invention
This invention relates to a printing device useful in direct electrostatic printing (DEP). In DEP, electrostatic printing is performed directly from a toner delivery means on a toner receiving member by means of an electronically addressable printhead structure.
2. Background of the Invention
In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a receiving substrate, the latter not bearing any imagewise latent electrostatic image. In the case that the substrate is an intermediate endless flexible belt (e.g. aluminium, polyimide etc.), the imagewise deposited toner must be transferred onto another final substrate. If, however, the toner is deposited directly on the final receiving substrate, a possibility is fulfilled to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed in e.g. U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer;
a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer;
a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and
at least one row of apertures.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC-field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode. The printing device as described in the original Pressman patent is very sensitive to changes in distances from the toner application module towards said shield electrode, leading to changes in image density.
The problem of keeping this distance constant has been addressed in several ways.
In EP-A 675 417 it is disclosed to use a magnetic brush as toner delivery means, using a two-component developer (comprising toner and carrier particles), and to provide “long hairs” on said brush so that the hairs touch the printing structure. In that case slight deviations in distance between the surface of the toner delivery means and the printhead structure do no longer present problems, while in any case the hairs of the brush, made up by carrier particles and toner particles are in contact with the printhead structure. It was found that such a device could provide very good printing results, but yielded only adequate optical density in the print when the printing speed was not too high. The problem of varying image density, that can remain in a device according to EP-A 675 417, due to a varying distance between the surface of the magnetic brush and the printhead structure can further be decreased by adapting the electrical conductivity of the carrier particles used on the magnetic brush as described in EP-A 836 124.
For devices working at quite high printing speeds, the use of a charged toner conveyer (a CTC), whereon the toner particles can be deposited by a magnetic brush or any other means known in the art, presents advantages. But the problem of uneven density (white banding) in a direction perpendicular to the printing direction has to be solved.
In EP-A 740 224 a device is described in which the frequency of said density banding (in a direction perpendicular to the printing direction) due to the variation of the distance from the toner application module towards said printhead structure is diminished. To achieve this better evenness in printing, it is disclosed to give the toner bearing surfaces of the toner delivery means rather high rotational speeds. Since the surfaces that bear the toner particles rotate very fast and the distance between said toner bearing surfaces and the printhead structure is low, the particles are exposed to quite large shearing force. This high shearing force can give raise to agglomeration and/or deformation of the toner particles (especially when in the toner particles polymeric toner resins with low (<60° C.) Tg are used. Thus the printing apertures can be clogged by agglomerated or deformed toner particles, leading to images with missing dots and bad image quality.
In U.S. Pat. No. 5,552,814 it is disclosed to use a device wherein the CTC and the printhead structure are in close contact. Such a device does indeed decrease the banding in the direction perpendicular to the printing direction, but, as with the fast moving CTC's in EP-A 740 224 referred to above, the particles are exposed to quite large shearing forces. This high shearing force can give raise to agglomeration and/or deformation of the toner particles and thus to some clogging of printing apertures. To diminish that problem it has been proposed in U.S. Pat. No. 5,497,175 to provide a layer with very low coefficient of friction on the face of the printhead structure contacting the CTC or, in U.S. Pat. No. 5,539,438, to provide a layer with low coefficient of friction on the surface of the CTC. These layers may influence the charge or the chargeability of the toner particles and can thus, in some instances, negatively influence the printing quality.
In U.S. Pat. No. 5,448,272 an other approach to diminish the shearing forces on the toner particles in a DEP device wherein the CTC contacts the printhead structure has been disclosed. On the face of the printhead structure contacting the CTC a kind of guiding members are provided in the spacing between the printing apertures, and only these guiding members are in contact with the CTC. The guiding members are wedge shaped, with the point of the wedge against the toner feeding direction. In operation the guiding members, that keep the distance between the printhead structure and the CTC constant, “plough” through the layer of toner particles on the CTC and guide the particles to the printing apertures. A drawback of this device is the difficulty of manufacturing such a printhead structure with the desired accuracy for high resolution printers (50 dpi or higher). A high resolution printer necessitates a printhead structure with small apertures
Desie Guido
Dieltjens Günther
Joly Ludovicus
AGFA-GEVAERT
Baker & Botts L.L.P.
Barlow John
Gordon Raquel Yve He
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