EP0916113B1 - Process for optimising a half-tone reproduction on a photoconductor of electrophotographic printers and copiers - Google Patents

Abstract

In order to optimise half-tone reproduction by optimising toner deposit intensity in electrophotographic printers and copiers, a toner grid is generated on a photoconductor before and during a printing or copying process, once basic values have been set, and the optical density of the toner grid is determined. A bias potential is then lowered with respect to a momentary value when the optical density exceeds a set value and the bias potential is higher than a minimum value. The photoconductor is discharged, cleaned and charged again before returning to the initial state.

Description

The invention relates to a method for optimizing a Halftone display by optimizing the Toner deposition intensity on a photoconductor from electrophotographic printing and copying equipment.

On by means of electrophotographic printing or Copies and printouts are made by the Users set extremely high quality requirements. Around It is to meet these high requirements required, the permissible tolerance ranges at reduce electro-photographic processes.

In electrophotographic printing and copying equipment for example, single sheets or continuous paper printed by preferably on a drum trained photoconductor a latent image is generated. To do this, a photoconductor is placed on a defined one Charging potential and then be charged in Depends on the method used in the expression areas appearing white or black. The then exposed areas have a Charging potential lower discharge potential. The generated latent image is then developed by in Depends on the method used on the exposed or unexposed areas of toner applied so that these areas appear black in the printout.

The toner commonly used is preferably by two-component toner, which consists of a Carrier component and micro toner exists. The Two-component toner, in turn, is dependent on that applied procedures positively or negatively charged.

The image developed on the photoconductor is then opened Transfer paper or another record carrier and then in a fuser by heating in the Melted record carrier or with this through at Melting of the toner image resulting adhesion forces connected.

After transferring the image from the photoconductor to the The photoconductor becomes a record carrier in preparation for Generation of the next image cleaned and complete discharged.

The due to fluctuations in the process parameters of pressure and Differences in copiers caused in The degree of blackening of different printouts will be particular then clearly if a large number of identical pictures is generated one after the other. Are these pictures for example, pictures with gray areas that are fine Have gray value gradation, so are already low Fluctuations in the gray value perceived by the human eye.

Such fluctuations can be caused, for example, by Process parameters, such as the charging potential to which the Photoconductor is charged at the beginning of each printing process is, by the discharge potential, which certain areas of the photoconductor after exposure, and by Variations in the exposure intensity caused become. The charging and / or discharging potential of the In particular, the photoconductor can be from the manufacturing batch that Operating time, the temperature and the cyclical load of the Dependent on the photoconductor.

A toner deposition intensity, i.e. the amount of toner that is in an area to be blackened deposited on the photoconductor is essentially from the humidity and the Toner concentration in the two-component toner, i.e. the Mixing ratio between microtoner and carrier component, dependent. In addition, the toner deposition intensity is from the triboelectric excitation state of the Two-component toner depending, for example, in turn on the temperature, humidity, duration of use, the intensity and duration of the mixing of the Two-component toner and the amount of the mixture supplied fresh toner is dependent.

To limit the parameter fluctuations described above in electrophotographic processes it is known that To keep the photoconductor temperature constant, the charging potential of the photoconductor, the depth of discharge, i.e. the Difference between the charging and discharging potential of the Photoconductor to regulate the exposure energy for generation of the discharge potential to keep constant that Regulate the toner concentration in the two-component developer or for example to carry out a toner mark control. Here, toner brands are understood to mean areas which are arranged outside the printed image on the photoconductor, and exposed to process monitoring and control and be developed.

The rules described above for reducing the Parameter fluctuations are either individually or in Combinations carried out. The above effects individually Control procedures only a partial stabilization, however for the high demands in terms of the required Print quality, especially with a halftone display is sufficient.

DE-A1-38 43 672 describes a method for optimizing a toner deposit intensity in analog working Known copiers, in which depending on one optical density of a toner mark a bias potential and / or a toner concentration is changed. As a toner brand an unscreened "image pattern" with full-surface coloring used.

By using a toner brand with Full-surface coloring in connection with the analog function the copier is this regulation to optimize a Halftone image is not suitable.

On the other hand, if a coarsely screened toner mark is used, unlike the full-face toner brand, both blackened as well as non-blackened areas, one can stable regulation of the toner deposition intensity only achieved when high-resistance two-component developers is used. Because high-resistance two-component developers not when using modern conductive developer brushes This method can be used in modern Printing or copying devices cannot be used.

It continues to regulate electrophotographic Process in electrophotographic devices generally known with the help of sensors to scan raster toner marks and in Depending on their optical density, process parameters Taxes. For example, in US-A-4999673 the integral density of a toner mark with a densitometer and correspondingly the toner concentration or another Process parameters changed. US-A-4949105 Toner brands developed in a known manner and their integral Density used to control process parameters. This Principle is using different toner brands also known from US-A-5400120.

The well-known toner brands are specially designed to control one individual process parameters designed or suitable. to Regulation of the various parameters of the overall process are different, differently designed Toner brands used.

The object of the invention is a method for optimizing the halftone display in electrophotographic Printing and copying equipment by setting the Process parameters influencing the print image depending to be provided by a raster toner brand at which by scanning the raster toner mark all the printed image influencing essential parameters are detected so that it is possible by setting a few parameters optimal halftone reproduction with high resolution to reach.

According to the invention, this is a method for Optimize a halftone display by optimizing the Toner deposition intensity in electrophotographic printing and Copying devices by the features in claim 1 or 9 reached. Advantageous further developments are the subject of to claim 1 or 9 directly or indirectly related claims.

In the method according to the invention, a photoconductor is used creates a halftone toner mark with fine halftone elements and their integral optical density averaged over the surface determined. Depending on the optical determined in this way A bias voltage is changed and / or a density Toner concentration changed.

By changing the between the photoconductor and the Bias voltage present at the developer station can be applied to the corresponding places affected amount of toner become. Is used in printing devices that after "Discharged Area Development" (DAD process) work, i.e. where the areas are colored with toner that previously exposed, the bias voltage for example increases, the one attracted by the photoconductor increases Amount of toner. Thus, by appropriately regulating the bias voltage within the specified limits Development process applied to the photoconductor Amount of toner and thus the optical density of a toner brand to be influenced.

Another option is toner depositing intensity or to influence the optical density of a toner mark, is a variation of the toner concentration, i.e. of Mixing ratio of microtoner and carrier component. Depending on the determined optical density thus, for example, by adding microtoner to the Two-component developer the toner deposition intensity a photoconductor. However, this is too take into account that increasing the toner concentration, compared to changing the bias voltage, a more sluggish Process is because both components are mixed and combined into one corresponding triboelectric Must be brought into the excitation state. Therefore is to increase the toner concentration a longer lasting impact on the Toner deposition intensity.

It should also be taken into account that the toner concentration simply by adding toner to the two-component developer can be increased, but decreasing the Toner concentration only through more complex "printing" can be achieved. Under "Print out" a Perform multiple print or copy operations understood, it is colored as possible Pictures should act to get one as quickly as possible large amount of toner from the two-component developer remove.

In the method according to the invention Possibilities of changing the bias voltage and the Toner concentration linked together, creating a Fluctuations in the toner deposition intensity or the optical Density of a toner brand and thus the quality of one Halftone is kept within extremely small limits becomes. The method according to the invention thus has in particular the advantage that already through the targeted change of two parameters fluctuations of others on the system acting parameters are compensated. This is about it is the process parameters described above, such as for example, the batch of photoconductor whose Duration of use, the temperature and the cyclical load of the Photoconductor and humidity.

According to a preferred embodiment of the invention for example the bias voltage increases once the optical Density of the toner mark has fallen below a target value. At the same time, however, the optical density of the toner mark may have not fallen below a minimum value and the bias voltage must be below a predetermined upper limit lie.

Furthermore, once the optical density of the toner mark is the Target value exceeds and below an upper limit the bias voltage is reduced. Lowering the bias voltage is only performed when the bias voltage is greater than a predetermined lower limit.

If in the process steps described above optical density of the toner mark the upper limit exceeds the toner concentration of the Two-component developer reduced by toner is printed out. The toner concentration is corresponding increases as soon as the optical density of the toner mark falls below the lower limit.

In a further preferred embodiment of the invention is at the time of increasing the toner concentration or shortly after this time the current bias voltage set a target value to prevent the optical Density the upper limit due to the time delay Effect of increasing the toner concentration in the short term exceeds.

Such an overshoot of the optical density can also can be avoided by, for example, between the Minimum value of the bias potential and the maximum value of the bias potential additional comparative values are provided.

According to the invention, neither the bias voltage nor the Toner concentration changes when the optical density of the Toner mark corresponds exactly to the required setpoint.

Fig.1a

einen Verlauf der optischen Dichte in Abhängigkeit von Druck- oder Kopierzyklen bei Verwenden einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens;

Fig.1b

einen Verlauf einer Bias-Spannung in Abhängigkeit von Druck- oder Kopierzyklen bei Verwenden einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens;

Fig.1c

Zeitpunkte einer Tonerförderung in Abhängigkeit von Druck- oder Kopierzyklen bei Verwenden einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens;

Fig.1d

einen Verlauf der Tonerkonzentration in Abhängigkeit von Druck- oder Kopierzyklen bei Verwenden einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens;

Fig.2

ein Ablaufdiagramm einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens, und

Fig.3

beispielhaft eine Ausschnittsdarstellung einer Raster-Tonermarke zum Verwenden bei dem erfindungsgemäßen Verfahren.

The invention is explained in detail below with reference to the attached drawings. Show it:

1a

a course of the optical density as a function of printing or copying cycles when using a preferred embodiment of the method according to the invention;

1b shows

a curve of a bias voltage as a function of printing or copying cycles when using a preferred embodiment of the method according to the invention;

Figure 1C

Points in time of toner delivery as a function of printing or copying cycles when using a preferred embodiment of the method according to the invention;

Fig.1d

a course of the toner concentration as a function of printing or copying cycles when using a preferred embodiment of the method according to the invention;

Fig.2

a flowchart of a preferred embodiment of the method according to the invention, and

Figure 3

an example of a detail of a halftone toner mark for use in the method according to the invention.

In the following, the. 1a to 1d The method according to the invention is described in principle. In Fig.1a To 1d is an example of a control start if the start is too high Toner concentration described.

At a time t ₀ , the optical density OD has a maximum value OD ^max . Since the optical density OD should ideally be reduced to a desired value OD _s and in the present example the toner deposition intensity on a photoconductor and thus also the optical density OD of a toner mark can be reduced by keeping the bias voltage V _B as low as possible in the present case the bias voltage V _B at time t _{0 has} a lower limit value V _B ^min .

In FIG. 1c, depending on the printing or copying cycles, a toner request is shown, which is not carried out at time t ₀ , since, as can be seen from FIG. 1a, the optical density is above the target value OD _s . For clarification, the toner concentration in the two-component developer is also shown in FIG. 1d as a function of printing or copying cycles.

Up to a point in time t _1, one or more printing or copying cycles are now carried out with a minimum bias voltage V _B ^min (FIG. 1b) without toner delivery (FIG. 1c). As a result, the optical density OD drops to a value below the target value of the optical density OD _s . The toner delivery (Fig.1c) is not yet activated. The toner concentration (FIG. 1d) also decreases in accordance with the optical density OD.

Since at time t ₁ the optical density OD is below the target value of the optical density OD _s , the bias voltage V _{B is raised} by one step above the minimum value V _b ^min (see FIG. 1b), thereby reducing the toner deposition intensity on the photoconductor to increase. As can be seen in FIGS. 1a and 1b, the bias voltage V _B is increased by a further voltage step as soon as the optical density OD of the toner mark has dropped below the target value of the optical density OD _s .

These steps are carried out up to a time t ₂ . Since at time t ₂ the optical density OD of the toner mark lies below the target value OD _s , but the bias voltage V _B can no longer be increased, since it has already reached a maximum value V _B ^max (FIG. 1b), the Bias voltage V _{B kept} at the maximum value V _B ^max .

If, for example at a time t _3, the optical density OD falls below a predetermined minimum value of the optical density OD ^min , the toner delivery is activated, as can be seen in FIG. 1c. The bias voltage V _B is kept at its maximum value V _B ^max at time t ₃ . As can be seen from FIG. 1d, the toner concentration increases from time t ₃ .

Due to the increasing toner concentration, the optical density OD (FIG. 1) also increases and at a time t _{4 is} already above the target value OD _s ; the bias voltage V _B (Fig.1b) is therefore reduced again.

Since the toner concentration rises up to a point in time t ₅ (the end of the toner delivery) (FIG. 1a), at point in time t ₅ the optical density OD is above the maximum value OD ^max . Such overshoot could be counteracted, for example, by supplying a smaller amount of toner, by adding bias comparison voltages between the maximum and minimum bias voltages, or by automatically lowering the bias voltage V _B at or shortly after the time of toner delivery.

As can be seen in FIG. 1b, the bias voltage V _{B is} reduced until the optical density OD falls below the target value OD _s . In accordance with the time period between t ₁ and t ₂ , the bias voltage V _{B is} increased or maintained as a function of the value of the optical density OD from a time t ₆ . From time t ₆ , the method according to the invention thus proceeds analogously to the above steps carried out from time t ₁ .

2 shows a flow chart of a preferred embodiment of the method according to the invention for optimizing a toner deposition intensity in electrophotographic printing and copying devices. In preparatory steps 1a to 1c, after a start of printing, the photoconductor is charged to a charging potential and only exposed with a set or regulated exposure energy, so that a discharging potential reaches a predetermined target value. In step 1c, the bias potential is set to a standard value V _B ^s .

Step 2 asks whether the printing or Copier is in print mode or not. If the printing or copying device is not yet in Printing operation, but for example still in one Warm-up phase, so there is a on the photoconductor Halftone toner mark exposed and then developed (Step 3). During printing, in addition to Raster toner mark also a printed page on the photoconductor exposed and developed (step 3 ').

In step 4, the optical density OD of the screen toner mark generated in step 3 or 3 'is measured. If the decision in step 5 is "yes", since the optical density OD of the halftone toner mark corresponds to the desired target value OD _s , further regulation of the toner deposition intensity or the optical density OD is not necessary, so that steps 6 to 10 or 6 ' to 10 'are omitted and steps 11a to 11c are carried out, in which the photoconductor is cleaned, the photoconductor charge is deleted and the photoconductor is subsequently recharged. Following steps 11a to 11c, step 2 is returned to for the next printing process.

If the decision made in step 5 is "no", ie the optical density OD of the halftone toner mark does not correspond to the target value OD _s , it is determined in step 6 whether or not the optical density OD of the halftone toner mark is greater than the target value OD _s . If the target value OD _{s is} greater, step 7 examines whether the optical density OD of the halftone toner mark is greater than a maximum value OD ^max or not. If the optical density is greater than the maximum value OD ^max , the toner concentration is reduced in step 8, for example by printing out toner.

If the optical density OD in step 7 is not greater than a maximum value OD ^max , it is determined in step 9 whether or not the bias potential V _{B is} greater than a minimum value V _B ^min .
If the bias potential V _{B is} greater than the minimum value V _B ^min , the bias potential V _{B can} be reduced in step 10 to reduce the optical density OD or the toner deposition intensity.

However, if the bias potential V _B is not greater than a minimum value V _B ^min , steps 11a to 11c are carried out, ie the photoconductor is cleaned, the photoconductor charge is deleted and the photoconductor is then recharged. Following steps 11a to 11c, step 2 is continued again.

If the decision in step 6 is "no", ie the optical density OD of the halftone toner mark is not greater than the target value OD _S , steps 7 'to 10' are carried out instead of steps 7 to 10.

At step 7 ', it is determined whether or not the optical density OD is less than a minimum value OD ^min . If the optical density is less than the minimum value OD ^min , the toner concentration is increased in step 8 ′ by adding toner to the two-component developer. However, if the optical density OD is not less than the minimum value OD ^min , a decision is made in step 9 'as to whether the bias potential V _{B is} less than a maximum value V _B ^max . If the decision is "yes", then the bias potential V _{B is raised} in step 10 '.

However, if the bias potential V _B has already reached the predetermined maximum value V _B ^max , steps 11a to 11c are carried out as described above. Following step 11c, the optimization process is also repeated again from step 2.

3 shows a detail of a Raster toner mark as in an electrophotographic Printer with a resolution of 600 dpi and an LED character generator Application.

The halftone toner mark is made of MIP micropixels Macropixels MAP built. A micropixel MIP with the Edge length a (in this case 42 µm) defines the smallest colorable stain that is used with the LED character generator the image of a single LED light spot on the photoconductor. The length a becomes also called micropixel pitch. Multiple micropixels MIP form a macropixel MAP (1,1) (basic cell). In which shown example there is a macropixel MAP with the Edge length b of 8 x 8 = 64 micropixels, i.e. the Edge length b of the macropixel is 8 x 42 µm = 0.336 mm.

With the micropixels MIP, a raster structure S is now represented in the macropixel MAP, which corresponds to a fine gray value in the gray value scale (halftone display). This fine structure S is dimensioned such that in the macropixel MAP (1,1) at least any micropixel is not colored with toner.
Depending on the development process used (reverse development, positive development), either the structure S is colored and the surrounding micropixels remain toner-free or vice versa. In the example shown with reverse development in which the charged photoconductor is discharged depending on the character, the structure S remains toner-free.

Because in electrophotographic printing the electrical Coloring conditions in the direction of rotation of the photoconductor drum and are different across it, it is according to the Representation of Figure 3 in the macropixel MAP (1,1) advantageous to form a linear structure S with a defined extent in the X direction (abscissa) and in the Y direction (Ordinate). This makes it possible to build one Screen toner mark from a variety of macropixels MAP (1,1) to MAP (n, n) the macropixels e.g. MAP (1.1) and MAP (1.2) or MAp (2.1) and thus the structure S in each case by 180 ° to show rotated to each other. So there is one Toner brand with particularly sensitive in the X or Y direction Structure.

In the illustrated embodiment, there is Toner mark from 15 x 15 macro pixels with a total edge length of 15 x 0.336 mm = 5 mm, of which only 4 Macropixels are shown. The size of this toner brand is however any. It depends on the location and the type of Scanning.

Instead of a raster tone mark, built up from square ones Macro pixels, a halftone tone mark can also be used individual lines extending in the Y direction. The lines then have a width in the X direction the width a of a micropixel and any length. On Line macropixels then have a width b in the X direction and a length in the Y direction which can be a multiple of b.

a. aus Wiederholungen von Makropixeln (Grundzellen) besteht,

b. ein Makropixel MAP mindestens eine Dimension größer ist als das Mikropixel-Rastermaß a (also mindestens 2 x a) und kleiner als 0,5 mm,

c. das im Makropixel enthaltene Muster. S derart aufgebaut ist, daß es mindestens ein nicht mit Toner eingefärbtes und mindestens ein mit Toner eingefärbtes Mikropixel aufweist.

In general, therefore, for a toner mark suitable for the optimization of halftone images, the following applies:

a. consists of repetitions of macropixels (basic cells),

b. a macropixel MAP is at least one dimension larger than the micropixel grid dimension a (i.e. at least 2 xa) and smaller than 0.5 mm,

c. the pattern contained in the macropixel. S is constructed such that it has at least one micropixel not colored with toner and at least one micropixel colored with toner.

Claims (14)

1. Method for optimising the halftone representation in electrophotographic printing and copying devices, in which,
   dependent on an optical density (OD) of a raster toner mark, determined integrally over the surface, a bias potential (V_B) and/or a toner concentration are modified, the toner mark consisting of repetitions of two-dimensional macropixels MAP, a macropixel (MAP) being at least one dimension larger than the micropixel raster dimension a and being smaller than 0.5 mm, and the macropixel (MAP) containing at least one micropixel (MIP) that is not inked with toner, as well as at least one micropixel (MIP) that is inked with toner.
2. Method according to claim 1, in which the bias potential (V_B) is increased with respect to the current value if the optical density (OD) falls below a desired value (OD_S) and is greater than a minimum value (OD^min), and the bias potential (V_B) is below a maximum value (V_B ^max).
3. Method according to claim 1, in which the bias potential (V_B) remains unmodified if the optical density (OD) corresponds to the desired value (OD_S).
4. Method according to claim 1, in which the bias potential (V_B) is lowered with respect to the current value if the optical density (OD) exceeds a desired value (OD_S) and is smaller than a maximum value (OD^max), and the bias potential (V_B) is greater than a minimal value (V_B ^min).
5. Method according to claim 1 or 2, in which the toner concentration is increased if the optical density (OD) is smaller than the minimal value (OD^min).
6. Method according to claim 5, in which the bias potential (V_B) is lowered during or shortly after the time of the increase of the toner concentration.
7. Method according to claim 1 or 4, in which the toner concentration is lowered if the optical density (OD) is greater than a maximum value (OD^max).
8. Method according to one of the preceding claims, in which additional comparison values are provided between the minimum value of the bias potential (V_B ^min) and the maximum value of the bias potential (V_B ^max).
9. Method for optimising a halftone representation in electrophotographic printing and copying devices, in which

   a) basic values are adjusted,

   b) before and during a printing or copying process, a raster toner mark is generated on a photoconductor, the toner mark consisting of repetitions of two-dimensional macropixels (MAP), a macropixel (MAP) being at least one dimension larger than the micropixel raster dimension a and being smaller than 0.5 mm, and the macropixel (MAP) comprising at least one micropixel (MIP) that is not inked with toner and at least one micropixel (MIP) that is inked with toner,

   c) an optical density (OD) of the raster toner mark, measured integrally over the surface, is determined,

   d) a bias potential (V_B) is lowered with respect to a current value if the optical density (OD) exceeds a desired value (OD_S), and the bias potential (V_B) is greater than a minimum value (V_B ^min),

   e) the photoconductor is discharged, cleaned and charged again, and

   f) the steps b) to e) are subsequently repeated.
10. Method according to claim 9, in which, instead of step d), in a step d₁) the bias potential (V_B) and the toner concentration are lowered with respect to the current value if the optical density (OD) is greater than a maximum value (OD^max), and the bias potential (V_B) is greater than a minimum value (V_B ^min).
11. Method according to claim 9, in which, instead of step d), in a step d₂) the bias potential (V_B) is increased if the optical density (OD) falls below a desired value (OD_S) and is greater than a minimum value (OD^min), and the bias potential (V_B) is smaller than a maximum value (V_B ^max).
12. Method according to claim 9, in which, instead of step d), in a step d₃) the bias potential (V_B) and the toner concentration are increased if the optical density (OD) is smaller than a minimum value (OD_min), and the bias potential (V_B) is smaller than a maximum value (V_B ^max).
13. Method according to claim 9, in which, instead of step d), in a step d₄) the toner concentration is lowered if the optical density (OD) is greater than a maximum value (OD^max) and the bias potential (V_B) is less than or equal to a minimum value (V_B ^min).
14. Method according to claim 9, in which, instead of step d), in a step d₅) the toner concentration is increased if the optical density (OD) is less than a minimum value (OD^min), and the bias potential (V_B) is greater than or equal to a maximum value (V_B ^max).

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