JP2006047839A - Method for forming image and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming an image for developing an image on a sheet by using a two-component developer comprising toner and carrier, wherein a toner deposition amount in a unit area of a line image is appropriately controlled compared to a toner deposition amount in a solid image. <P>SOLUTION: The developer used shows the following properties: that when the relation between a charge amount Q/M per unit weight of a toner and the toner concentration Tc (%) representing a weight proportion of the toner in the developer is represented by an expression with numerical values A and B, the absolute value Q/M of the toner is ≥15 μC/g at the toner concentration Tc satisfying B≤0.035 and B×Tc<0.54. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming method and an image forming apparatus using an electrophotographic system.

  As an image forming apparatus using an electrophotographic image forming method, a laser beam printer, a copying machine, and the like are known. The main image forming method is that after charging the photoconductor, exposure according to the image data is performed to form a charge distribution corresponding to the image pattern on the photoconductor, and the toner is developed according to the charge distribution. As a result, a visualized toner image appears. Thereafter, the toner image is transferred to paper and fixed on the paper by heat fixing to complete printing. As the development, a two-component development method is known in which toner, which is a colored particle having a particle diameter of about 5 to 10 μm, and a carrier, which is a magnetic particle having an average particle diameter of 30 to 100 μm, such as ferrite, magnetite, and iron powder are mixed. Yes. On the other hand, a one-component development method in which toner contains magnetic powder and does not use a carrier is known.

The developer used in the two-component development system charges the toner by the friction between the toner and the carrier when the toner and the carrier are mixed. Since the toner charged according to the charge formed on the photoreceptor adheres, an image can be formed. For this reason, giving an appropriate charge amount to the toner is an important element for forming an image having an appropriate darkness. Further, since the toner charge amount is stable, the image density is also stabilized. Since the toner is consumed, it is necessary to replenish the toner in the developing machine, and it is necessary to quickly charge the replenished toner. As a two-component developer that improves these performances, a developer having a desirable relationship and characteristics between the toner charge amount and the mixing ratio of the toner and the carrier (or the toner concentration that is the weight ratio of the toner in the developer) is disclosed. Examples are known. (For example, Patent Document 1) Further, an example describing what kind of idea the toner density should be determined is known. (For example, Non-Patent Document 1)
Furthermore, in the two-component development method, the amount of toner adhering per unit area differs between a line image and a large area image (solid image), and the adhering amount of the line image is generally large. there were. These are called edge effects, and toner adheres to the line image more than necessary. In particular, since characters and the like are a collection of line images, if the edge effect is strong even if the area where the toner is attached is small, the amount of toner attached increases. When you actually trace the surface of such an image with your finger, you will feel the toner rising. If the amount of adhesion of the line image due to the edge effect is excessive, the toner is wasted, so the number of sheets that can be printed with the same amount of toner is reduced and the printing cost is increased. As for the amount of toner adhered per unit area, it is desirable that the amount of image density required for a solid image is the same as the amount adhered to a line image. In order to obtain such characteristics, it is effective to narrow the development gap, and a device for narrowing the development gap has been devised. (For example, Patent Document 2 and Patent Document 3)
The adjustment of the development conditions that have been conventionally performed is such that a solid image density of a wide area is appropriate and a toner adhesion amount is obtained, and development bias, toner density, peripheral speed ratio, The development gap has been adjusted, but measures such as edge effects have been insufficient.

Japanese Patent Laid-Open No. 8-248672

JP 56-161565 A JP 2001-343832 A Nobuyoshi Hoshi, Masaho Anzai, Effective carrier coverage of two-component developer, Journal of Electrophotographic Society, Vol. 25, No. 4 (1986)

  The problem to be solved by the present invention is that, in an image forming apparatus using a two-component developer, the toner adhesion amount per unit area of a line image is too much as compared with the solid image toner adhesion amount.

  The main feature of the present invention is to use a developer that is less dependent on the toner concentration of the toner charge amount of the two-component developer and that becomes a predetermined charge amount or more at a predetermined toner concentration.

  According to the present invention, the amount of toner adhered per unit area can be adjusted to the same amount as that of a solid image with a large area, so that wasteful toner consumption can be suppressed. Costs can be reduced, and excessive toner adhesion to line drawings is reduced, so that scattering is reduced and image quality can be improved.

  Next, an image forming apparatus to which the present invention is applied will be described with reference to FIG.

  The surface of the photoreceptor 1 rotating in the clockwise direction (arrow direction) is uniformly charged by the charger 2. The light exposure device 3 such as a laser beam or LED flickers in accordance with image data, and exposes the surface of the photoreceptor 1. On the photosensitive member 1, the portion irradiated with the light irradiated from the exposure device 3 becomes conductive and the surface charge disappears.

  The developing device 4 holds a two-component developer composed of toner and carrier, and is conveyed to a region facing the photoreceptor 1 as the developing roller 41 having a magnet therein rotates. A developing bias voltage in which an alternating current is superimposed on a direct current is applied to the developing roller 41 from a developing bias power supply 48, and toner having the same charging polarity as that of the photosensitive member 1 has an electric field between the photosensitive member 1 and the developing roller 41. Due to the action, it adheres to the place where the charge on the surface of the photoreceptor disappears. The toner image formed on the photoreceptor 1 by development is transferred to the paper 7 by the transfer device 5. Although not shown, the toner image transferred onto the paper 7 is melted by heating with a fixing device and fixed on the paper 7. Thereafter, the toner remaining on the photosensitive member 1 is removed by the cleaning device 6, and thereafter image formation is similarly performed. The toner removed by the cleaner 6 is collected by the toner hopper 42 and used again for development.

  When the toner is consumed by the development, the toner concentration in the developer of the developing machine 4 is lowered and the output value of the toner concentration sensor 44 is changed. The output value of the toner density sensor 44 is compared with a predetermined value stored in a storage unit provided in the control unit 46. When the controller 46 detects that the toner density has decreased below a predetermined value, the controller 46 drives the toner supply roller 43 to supply toner from the toner hopper 42 into the developing device 4. When the output value of the toner density sensor 44 reaches a predetermined value, the control unit 46 stops the supply roller 43 so that the toner density in the developing device 4 does not become excessive.

  A portion where the photosensitive member 1 and the developing roll 41 face each other is a portion where development is performed, and the interval between the photosensitive member 1 and the developing roller 41 is referred to as a developing gap. Further, the ratio of the moving speed of the peripheral surface of the photoreceptor 1 and the moving speed of the peripheral surface of the developing roll 41 is referred to as a peripheral speed ratio.

In the embodiment of the present invention, in order to adjust the toner adhesion amount per unit area of the line image to the toner adhesion amount of the solid image, the adjustment of the development gap, the development bias, the peripheral speed ratio, and the development bias voltage is performed by adjusting the toner charge amount. By using a developer that is less dependent on the toner concentration, the toner charge amount and the toner concentration are adjusted together.
(Example)
First, the relationship between the charge amount Q / M per unit toner weight of the developer and the toner concentration Tc representing the toner weight ratio will be described with reference to FIG.

  FIG. 1 is a graph showing the toner density Tc characteristics of the toner charge amount Q / M for three types of developers. As the developer used, a black toner of styrene acrylic resin having an average particle diameter of 8.5 μm is commonly used. The carrier has the following specifications for each developer.

  In the carrier of developer de-1, the core material is magnesium ferrite, the average particle size is 40 μm, silicon resin is coated, and carbon black is added to the coating material.

  In the carrier of developer de-2, the core material is magnesium ferrite, the average particle diameter is 40 μm, an acrylic resin is coated, and carbon black is added to the coating material.

The carrier of developer de-3 has a core material of magnetite, an average particle size of 40 μm, a silicon resin different from the carrier of developer de-1 is coated, and carbon black is added to the coating material. The charge amount was measured with a suction method apparatus (Trek Japan 210HS equivalent product).
The toner charge amount Q / M tends to decrease as the toner concentration Tc increases. The decrease in Q / M accompanying an increase in Tc is not necessarily reduced at a constant rate with respect to Tc, and the rate of decrease differs depending on the developer. As the toner concentration Tc increases, there are developers such as de-2 in which the toner charge amount Q / M rapidly decreases, and developers such as de-1 and de-3 in which the Q / M gradually decreases. The relationship between the toner charge amount Q / M of these developers and the toner concentration Tc can be described by one equation even if the rate of change with respect to Tc is different. This content is described in detail in Patent Document 1. In Patent Document 1,
[Equation 1]
Q / M = A / (1 + B × T / C)
It is explained that it is expressed in the form of T / C is a weight mixing ratio of the toner and the carrier, and is between the toner concentration Tc.
[Equation 2]
Tc = T / C / (1 + T / C) × 100
There is a relationship. In Patent Document 1, A and B are obtained by applying the relationship between the toner charge amount Q / M and the mixing ratio T / C to Equation 1.

The characteristic represented by the mathematical formula 1 is an exponential function,
[Equation 3]
Q / M = A × exp (−B × Tc)
Can be expressed in the form This is because the exponential function exp (-x) is
[Equation 4]
exp (−x) = 1−x + x × x / 2 + (third order and higher are omitted)
Therefore, when x is smaller than 1, it can be approximated by the second term on the right side of Equation 4,
[Equation 5]
exp (−x) ≈1−x
And then
[Equation 6]
1-x≈1 / (1 + x)
So, after all,
[Equation 7]
exp (−x) ≈1 / (1 + x)
Thus, the relationship between the toner charge amount Q / M and the toner density Tc can be expressed even in the form using the exponential function of Formula 3.

For the three types of developers, the toner density Tc of the developer is changed, the toner charge amount Q / M is measured, and the relationship between Q / M and Tc is calculated using the function of approximate curve fitting of the spreadsheet software. When applied to 3, it became as follows.
Developer de-1
[Equation 8]
Q / M = 29.22 × exp (−0.034 × Tc)
Developer de-2
[Equation 9]
Q / M = 38.04 × exp (−0.067 × Tc)
Developer de-3
[Equation 10]
Q / M = 22.06 × exp (−0.035 × Tc)
The relationship between the toner charge amount Q / M and the toner concentration Tc has been described above.
Next, the results of studying the development conditions for reducing the difference in the amount of adhesion between the line drawing and the solid will be described. In order to perform this study, evaluation was performed based on a printer DDP92 having a resolution of 600 dpi manufactured by Hitachi Printing Solutions.

First, the results of studying the effect of toner density will be described with reference to FIG. The developer is de-1, the development gap is 0.5 mm, the DC development bias is 350 V, and the peripheral speed ratio is 1.5.
FIG. 3 shows the relationship between toner density, line drawing, and solid adhesion amount. L1-2 is a repetitive line with a line width of 1 dot and a space of 2 dots, L2-4 is a repetitive line of a line width of 2 dots and a space of 4 dots, and L8-16 is a line width of 8 dots and a space between lines 16 It is a repeating line with dots. Solid is a solid image. Each print pattern was printed on an OHP film. The difference between the weight of the OHP film before printing and the weight of the OHP film after printing becomes the weight of the adhered toner. By dividing the weight of the toner by the printing area, it is possible to obtain a toner adhesion amount m / A (mass per Area) per unit area. These legends are the same in the following discussion. The line width of 8 dots is the line width of a character of around 10 points that is often used as a character size, and is also a line width that increases the toner adhesion amount due to the edge effect. Therefore, it is one of the features of the present invention to find out the conditions for bringing the adhesion amount of 8 dot lines and solid (solid) closer.
As shown in FIG. 3, when the toner density is increased, the increase in the toner adhesion amount is almost the same for both L8-16 (8 dot line) and solid (solid), and the ratio of the adhesion amount between the line drawing and the solid becomes close to 1. I understood. That is, increasing the toner density has an effect of bringing the amount of adhesion between the line image and the solid image closer.

  Second, the results of studying the effect of the development gap will be described. By narrowing the development gap, the edge effect is reduced. FIG. 4 shows the results of investigation using the developer de-1 at a toner concentration of 15%. The developing bias is DC 350V and the peripheral speed ratio is 1.5.

It was found that as the development gap was narrowed to 0.5, 0.4, and 0.2 mm, the solid adhesion amount increased and the line drawing adhesion amount decreased. It can be seen that when the development gap is 0.2 mm, the adhesion amount of solid (solid) and L8-16 (8-dot line width) will be the same, but the development gap of 0.2 mm requires higher mechanical accuracy and price. Becomes higher. In addition, when the development gap is narrowed, the amount of line drawing attached decreases, but this is because the scraping of the image is increased due to strong rubbing by the carrier. It is considered difficult to adopt.
Thirdly, the results of studying the DC developing bias will be described with reference to FIG. Developer de-1 is used at a toner concentration of 15%, the development gap is 0.5 mm, and the peripheral speed ratio is 1.5.

  As shown in FIG. 5, when the DC developing bias is increased, the amount of increase in the line drawing and solid adhesion amount is substantially the same. As in the case of toner density, increasing the developing bias has the effect of bringing the amount of adhesion between the line image and the solid closer.

  Fourth, the results of examining the peripheral speed ratio will be described with reference to FIG. The developer de-1 is used at a toner concentration of 15%, the development gap is 0.4 mm, and the development bias is DC 350V.

  When the peripheral speed ratio is increased, the solid adhesion amount increases and the adhesion amount of the line drawing tends to decrease. Increasing the peripheral speed ratio has the effect of bringing the line drawing and the solid amount of adhesion closer.

Fifth, the result of studying the effect of superposing an AC voltage on the developing bias will be described with reference to FIG. Developer de-1 is used at a toner concentration of 15%, the development gap is 0.4 mm, the DC development bias is 350 V, the peripheral speed ratio is 1.8, and the AC frequency is 7 kHz. As shown in FIG. 7, when the AC voltage is 0.6 kVpp, the adhesion amount is almost the same as when only the DC bias is applied. However, when the AC voltage is increased to 0.9 kVpp, the solid adhesion amount increases. It was found that (L1-2, L2-4, L8-16) decreased. In particular, the adhesion amount of the 8-dot line (L8-16), which is a problem, is slight but below the solid, which means that there are conditions that can make the adhesion amount the same within the range of conditions studied so far. ing. In this study, the AC frequency was set to 7 kHz due to the limitation of the maximum current of the AC power supply, but the same effect can be obtained if the frequency is 4 kHz or more. Here, Vpp is a peak-to-peak voltage.
As described above, by adjusting the toner density, DC development bias voltage, peripheral speed ratio, AC development bias voltage amplitude to be higher or larger and the development gap to be narrower, the amount of adhesion between the line drawing and the solid image is adjusted. I found out that

  Under these conditions, items other than the toner density are items that can be freely set under conditions relating to the adjustment of the developing device, and problems due to the setting do not occur in principle. However, the toner density must be appropriately charged by a combination of carrier and toner and cannot be freely adjusted. The conditions achieved in the present invention were those achieved with the developer de-1 shown in FIG. With the developers de-2 and de-3, when the toner density is set to 15%, the problem that the image background portion becomes dirty has occurred.

  The essential point of the present invention is to obtain the toner density characteristic of the toner charge amount like the developer de-1.

  The developer de-2 has a toner density of 8% or less and the highest toner charge amount Q / M, but has a characteristic of rapidly decreasing as the toner density increases. In the developer de-2, the background portion was conspicuous when the toner concentration was about 10%, and the background portion was unusable because the background portion was very dirty. The cause of background stains is when the toner is not sufficiently charged. It is considered that such a problem is likely to occur in a developer in which the toner charge amount greatly changes depending on the toner concentration because the carrier has a low ability to charge the toner.

  Further, the developer de-3 is a developer whose change rate with respect to the toner concentration is almost the same as the developer de-1, but has a slightly lower charge amount, and could be used even at a toner concentration of 15%. The background was slightly noticeable.

  Further, even in the developer de-1, when the toner concentration is 16%, the background portion is slightly stained.

  For this reason, as a developer usable in the present invention, when the toner density Tc characteristic of the toner charge amount Q / M is applied to Equation 3, the value of B is 0.035 or less (developer de-1). , De-3), and it is necessary to make the toner density satisfying B × Tc <0.54. Further, as a condition that the fog in the background portion is not conspicuous, the average of the respective charge amounts at the toner concentration of 15% is 14.8 μC / g based on the results of the developers de-1 and de-3, and is about 15 μC / g. The above (developer de-1) is desirable.

  Using the developer having such characteristics, the conditions for actually matching the adhesion amount between the line drawing and the solid are almost the same. The development gap is 0.4 mm or less, the DC development bias is 350 V or more, and the peripheral speed ratio is 1. 8 or more, AC developing bias voltage is 900 Vpp or more.

  In Non-Patent Document 1, an appropriate toner concentration is defined from the particle diameters of toner and carrier. However, it is defined by the theory of Non-Patent Document 1 by using the toner concentration characteristic of the charge amount as in the present invention. It could be used even when the toner density was higher.

6 is a graph showing toner density characteristics of toner charge amount. 1 is a schematic diagram of an image forming apparatus. 6 is a graph showing the relationship between toner density and toner adhesion amount. 5 is a graph showing a relationship between a development gap and a toner adhesion amount. 6 is a graph showing a relationship between a DC developing bias voltage and a toner adhesion amount. 6 is a graph showing a relationship between a peripheral speed ratio and a toner adhesion amount. 6 is a graph showing a relationship between an AC developing bias and a toner adhesion amount.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Photoconductor, 2 ... Charging device, 3 ... Exposure device, 4 ... Developer, 5 ... Transfer device, 6 ... Cleaning device, 7 ... Paper, 8 ... Developer, 11 ... Toner transfer characteristic, 41 ... Developing roller 42, a toner hopper, 43, a toner supply roller, 44, a toner density sensor, 45, a stirring screw, 47, a developer regulating member, 48, a developing bias power supply, 53, a transfer power supply.

Claims (6)

In an image forming method for visualizing an image on a sheet or the like using a two-component developer comprising a toner and a carrier,
When A and B in which the relationship between the charge amount Q / M per unit toner weight and the toner concentration Tc (%) in the developer applies to the equation (1) are obtained, the value of B is 0.035 or less. An image forming method comprising using a developer and using the developer under a condition of toner concentration Tc satisfying a condition of B × Tc <0.54.
Q / M = A × exp (−B × Tc) (1)   2. The image forming method according to claim 1, wherein an absolute value of Q / M is 15 μC / g or more. In an image forming apparatus that visualizes an image on a sheet or the like using a two-component developer composed of a toner and a carrier,
When A and B in which the relationship between the charge amount Q / M per unit weight of toner and the toner concentration Tc (%) in the developer applies to the equation (1) are obtained, the value of B is 0.035 or less. An image forming apparatus using a developer having an absolute value of Q / M of toner of 15 μC / g or more at a toner concentration Tc satisfying a condition of B × Tc <0.54.
Q / M = A × exp (−B × Tc) (1) In an image forming apparatus having a developing machine that holds a two-component developer including a toner and a carrier and includes a developing roll, a photoconductor facing the developing roll, and a developing bias power source that applies a superimposed bias voltage to the developing roll.
When A and B in which the relationship between the charge amount Q / M per unit weight of toner and the toner concentration Tc (%) in the developer applies to the equation (1) are obtained, the value of B is 0.035 or less. The absolute value of Q / M of the toner at Tc = 15% is 15 μC / g or more, the development gap between the developing roll and the photoconductor is 0.4 mm or less, the peripheral speed ratio is 1.8 or more, and the development An image forming apparatus having an image forming condition in which a DC developing bias is 350 V or more and an AC developing bias voltage is 900 Vpp or more in a superposed bias voltage by a bias power source.
Q / M = A × exp (−B × Tc) (1)   The image forming apparatus according to claim 4, wherein a value of the development gap is 0.2 mm or more and 0.4 mm or less. 5. The image forming apparatus according to claim 4, wherein the frequency of the AC voltage is 4 kHz or more.

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