A METHOD FOR IMPROVING THE PRINTING QUALITY AND REPETITION ACCURACY OF ELECTROGRAPHIC PRINTERS AND A DEVICE FOR ACCOMPLISHING THE METHOD

Abstract: Method and device to improve printing quality and the repetition accuracy of electrographical printers, in which are produced a latent electric charge pattern of electrical signals by means of an electrode matrix or the like, which respectively opens and closes passages between electrodes for exposing electric fields for attraction of pigment particles (11) against an information carrier (7). The electrodes (1, 2) of the electrode matrix in the area about one or several open passages (3) are screened electrostatically against closed passages, by means of at least two electrically isolated electrodes (1, 2).

Description: A Method for- Improving the Printing Quality and Repetition Accuracy of Electrographic Printers and a Device for Accomplishing the Method The invention refers to a method and a device to improve the printing quality and the repetition accuracy of electro- graphical printers, in which are produced a latent electric charge pattern of electrical signals by means of an elec¬ trode matrix or the like, which opens respectively closes passages between electrodes for exposing electric fields for attraction of pigment particles against an information carrier. BACKGROUND OF THE INVENTION In the international patent application PCT/ SE88/00653 is shown a method to develop pictures and text with pigment particles on an information carrier, directly from computer generated signals, without the need for these signals to be intermediately stored at temporary conversion to light energy which is the case in photo conductive printers eg. laser printers. These problems have been solved by bringing the information carrier into electric cooperation with at least a screen or lattice-shaped, preferably an electrode matrix, which through control in accordance with the con- figuration of the desired pattern at least partly opens and closes passages through the matrix which is galvanicallly connected to at least a voltage source. Through the passages thus opened is exposed an electric field for attraction of the pigment particles against the information carrier. This method (in the following called the EMS- concept), as it is described in the above patent application, however may result in that the produced print does not show high quality enough, particularly at repeated and continous use. The EMS-concept refers to electrode matrices in which passa¬ ges or meshes through the matrix are defined and separated by simple electrodes which results in that the potential of every single electrode substantially influences the charac¬ teristics of the electric field on the pigment particles symmetrically in passages adjoining the electrodes. This results in that the development, that is the attraction of pigment particles (in the following called toner), not only will occur in the mesh which is surrounded by electrodes, the potential of which is intended to completely or partly •open said mesh (in the following called "black" voltage), but also expose passages in adjacent meshes. In electrode matrices with several mesh lines, meshes surrounded by simple electrodes will develop full- dots with intended extension and position as well as half- and quarter- dots surrounding the full-dots. This manifests itself as an unsatisfactory edge definition and in certain cases as a "blurr" on the printed page. Indeed its possible to change the potential of the adjacent electrodes, which are intended to close the passage in the adjacent meshes (in the follo¬ wing called "white" voltage) and hereby reduce the problem with the undesired half- and quarter- dots by skew setting the above mentioned symmetrical influence on the electrical field. This however leads to that the potential difference between electrodes with "white" voltage and electrodes with "black" voltage increases (in the following called contrast voltage), which in turn increases the manufacturing costs for the control electronics as well as the electrode matrix. These problems stated above are not limited to the EMS- Concept but are also present wholly or partially in several electrographic printer concepts, where passages are created in an electric way. Common to all problems described here and drawbacks of known technique is that the printing quality and thereby the readability is influenced in a negative direction, with reduced competitiveness and lower consumer value as a re¬ sult. THE OBJECT OF THE INVENTION AND MOST IMPORTANT FEATURES The object of the invention is to create a method which gives the EMS and other electrographic printer concepts, high quality prints with good readability even during such circumstances when the device operates continously without maintenance and service . These problems have been solved by the electrodes of the electrode matrix in the area about one or several open passages being electrostatically screened against closed passages. DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail with reference to accompanying drawings on which embodiments are shown. Fig. 1 shows a section of an electrode matrix the meshes of which are defined with double electrodes. Fig. 2 shows the position and form of the equipotential lines in a two dimensional lateral view of the electrode matrix according to fig. 1 in the electric field produced by a certain voltage setting of the electrode matrix. Fig. 3 shows a woven net with double electrodes in perspec¬ tive. Fig. 4 shows an electrode matrix with double electrodes produced as a conductor pattern on a carrier. DESCRIPTION OF EMBODIMENTS In the drawings the reference numeral designates an elec¬ trode, in the continuation called a print electrode, the extension of which is substantially parallel to the movement direction of the paper. With 2 is designated a second elec¬ trode, which in the continuation is called a transversal electrode, in the same electrode matrix, and the extension of which is substantially tranversal against the movement direction of the paper and with 3 one of many passages or meshes, between the electrodes 1, 2 through which transport of toner takes place at the development. With 7 is designa¬ ted an information carrier, eg. a sheet of paper, with 8 an equipotential line in an electric field between a developer roller 9 for transport of pigment particles 11 (also called toner) from a container 14 in the proximity of the electrode matrix, and with 10 a background electrode, which can be a so called plate electrode. 12 designates a carrier for the electrode matrix and/or its pattern of connecting conductors and electric resistors 13. By applying several parallel electrodes 1 , 2 than one which surrounds every mesh, the cross coupling or the crosstalk between two adjacent meshes 3 will be substantially reduced, since every conductor acts like a screen for electrostatic field lines. Fig. 1 shows an electrode matrix with double electrodes 1, 2 in both electrode directions. The appearance of the electric field can be illustrated by equipotential lines 8. ' Fig. 2 gives an example on this calculated by a numerical method (the finite element met- hod). In fig. 2 equipotential lines, which represent a potential, which in relation to the charge of the toner par¬ ticles has an "attracting" influence on the toner, have been marked with solid lines 8a. Further equipotential lines which represent a potential, which in relation to the charge of the toner particles has a "repelling" influence on the toner, have been marked with dashed lines 8b. The toner particles 11, which for the sake of clarity only have been marked in the right part of the picture, in this example are negatively charged. All electrodes except for two have "white" voltage of - 400 V. Between the two remaining elec¬ trodes which have a "black" voltage of 0 V, is intended to produce a dot in zone D on the paper 7. Fig. 2 shows clearly that the earlier mentioned and undesired crosstalk which is present at single-wired electrode matrices is no longer troublesome. At A in fig. 2 where developing is intended to take place, equipotential lines 8a penetrate down through the mesh 3 and will thereby increase the field to the extent necessary for the toner to lift from developing roller (not shown). However at B, where no development is intended to be carried out, the lines 8a have been "forced" up in a direc¬ tion from the toner particles 11 and "substituted" by "bloc- king" equipotential lines 8b. The appearance and form are equal for the process in the mesh at B as in the mesh to the right in the picture. Fig. 3 and 4 shows examples on devices according to the invention. The invention is not limited to the above described embodi¬ ment. Thus it is possible to apply the invention on other developing and pigment particle systems than those shown herein, eg monocomponent toner with carrier. Parts of the invention are also useful when the electrode matrix is placed behind the " paper in a way that is described in eg. PCT/SE88/00653.

Claims: CLAIMS 1. Method to improve printing quality and the repetition accuracy of electrographical printers, in which are produced a lateht electric charge pattern of electrical signals by means of art electrode matrix or the like, which opens re¬ spectively closes'passages between electrodes for exposing electric fields for attraction of pigment particles (11) against an information carrier (7), c h a r a c t e r i z e d t h e r e i n, that the electrodes (1, 2) of the electrode matrix in the area about one or several open passages (3) are screened electrostatically against closed passages. 2. Device for performing the method according to claim 1 at ele trographic printers, in which are produced a latent electric charge pattern of electric signals by means of an electrode matrix or the like, which is arranged to open and close passages respectively between electrodes for exposing of electric fields for attraction of pigment particles (11) against an information carrier (7), c h a r a c t e r i z e d t h e r e i , that the electrodes of the electrode matrix (1, 2) are arranged in a square shaped pattern, forming passages (3) between the grossed electrodes, and that the passages (3) of the electrode, matrices are separated from most closely situated passages by means of at least two from each other elctrically isolated electrodes (1, 2).

Inventors: LARSON OVE, LARSON, OVE
Applicants: ARRAY PRINTERS AB
Application year: 1990
Main classification: G06K15
IPC codes: B41J2, B41J29, G03G15, G03G17, G06K15