JP5873630B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  In an electrophotographic image forming apparatus such as a laser printer, a laser fax machine, or a digital copying machine, the photosensitive drum is charged almost uniformly, and the photosensitive drum is exposed to form an electrostatic latent image corresponding to the image signal. After formation, the toner charged by the developing device is supplied onto the photosensitive drum to be visualized, and the developer image is transferred to a printing medium such as transfer paper, and then fixed by the fixing device to form an image. Yes.

In an image forming apparatus, a method using a magnet roller is known as a method for supplying toner onto a photosensitive drum. For example, Patent Documents 1 and 2 describe an apparatus using a two-component developer composed of a toner and a carrier. Patent Document 1 discloses a configuration in which toner is preliminarily mixed with a developer and replenished.

Japanese Patent Laid-Open No. 10-142916 JP 2006-323238 A

Sato, Kimura, Journal of the Imaging Society of Japan, 2002, Vol. 41, No. 1, p. 34-p. 39

  In recent years, the processing capability of image forming apparatuses has increased, and the number of printable sheets per unit time has become very large. For this reason, in an apparatus using a two-component developer, it is necessary to continuously replenish consumed toner and to mix the replenished toner and the carrier in a short time.

  However, since the conventional apparatus does not assume such high-speed printing, there is a problem that the toner and the carrier are not sufficiently mixed when supplying and supplying the toner consumed by the high-speed printing. is there.

  In particular, in recent years, there has been an increasing demand for relatively small devices that print A4-size paper. In such a small apparatus, since the length of the toner conveyance path is shortened, the distance at which the toner and the carrier are agitated becomes short, and the mixing of the toner and the carrier becomes insufficient.

  When the mixing of the toner and the carrier becomes insufficient, the toner is frictionally charged with the carrier and cannot be controlled to a charge amount having an appropriate polarity. For this reason, there is a problem that the image quality is deteriorated by the toner having a reverse polarity and a low charge amount.

  In recent years, the amount of developer has been decreasing due to the downsizing of the image forming apparatus. In particular, the image density can be maintained even with a small amount of developer due to the high image quality and the low cost per page, and the high pigment addition of toner and the small particle size of carrier. However, in a system using a two-component developer, it is necessary to accurately measure the consumed toner, put it in a carrier, and stir and charge and disperse it. For this purpose, stirring time is required. When the stirring time is short, the toner is not sufficiently charged, and is transported to the development area in a state where it is not carried on the carrier surface.

  Further, as the developer, a powder having a resin coated on the surface thereof such as ferrite or magnetite is used, and a toner whose main component is the resin is mixed with the powder. Therefore, when the toner is mixed in the developer flow, there is a problem that the toner is not caught in the developer because of the difference in specific gravity, and thus the toner is not transported to the developing area with insufficient charging and dispersion. . The toner originally needs to be controlled to a charge amount having an appropriate polarity, but when the charge / dispersion is insufficient, the reverse polarity and the charge amount are lowered. For this reason, the toner adheres to an area on the photosensitive drum where the toner should not adhere, and image deterioration (image B / G increase) and image contamination occur. In addition, if charging is insufficient, there is a problem that internal contamination occurs. In particular, in the case of an A4 size high-speed developing device, the stirring time is reduced by about 25% compared to the A3 machine, so that the conventional developing device has a limit in speeding up.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to reliably mix toner and carrier in a minimum space when performing high-speed printing. Another object of the present invention is to provide a new and improved image forming apparatus.

In order to solve the above problems, according to an aspect of the present invention, a photosensitive drum that forms an image on a medium by transferring toner supplied according to an electrostatic latent image, and the electrostatic latent image carrier A developing roller that supplies toner to the body, and a developing unit that supplies a two-component developer in which toner and a carrier are mixed to the developing roller, and the developing unit has two in the axial direction of the developing roller. A conveying unit that conveys a component developer; and a preliminary agitating unit that is provided at an end of the conveying unit and includes a large-diameter agitating screw that mixes newly introduced toner and a carrier. The process speed for forming an image on the medium is 300 [mm / s] or more, and the width of the medium on which the photosensitive drum performs image formation is a letter size width of 216 [mm] or less. The pre-stirring section has a protruding amount of the stirring screw upward from the surface of the two-component developer [rmm], a pitch of the stirring screw P [mm], a rotation speed of the stirring screw R [rps], and mixed stirring An image forming apparatus satisfying the following formula (1) is provided when the time T [s] required for the above is set.

  According to the above configuration, by satisfying the above formula (1), even when the process speed is 300 [mm / s] or more, the pre-stirring unit completes the mixing of the toner and the carrier, and the toner is removed. A charge amount of a predetermined polarity can be obtained. When the stirring screw has a large diameter, it is possible to minimize the time T required for mixing and stirring in the preliminary stirring unit, and to minimize the length of the preliminary stirring unit in the transport direction. It becomes possible. Therefore, in an image forming apparatus with a high process speed, the toner and the carrier can be mixed in a short time in a minimum space.

Further, the outer diameter of the photosensitive drum is 30 [mm] or less, the sectional area of the preliminary stirring unit is S1 [mm ² ], the sectional area of the photosensitive drum is S2 [mm ² ], and the developing roller When the sectional area is S3 [mm ² ] and the cross-sectional area of the conveying screw provided in the conveying unit is S4 [mm ² ], the following expression (2) is satisfied.
S1 <S2 + S3 + S4 (2)
According to this configuration, since the area S1 of the preliminary stirring unit can be minimized, the space occupied by the preliminary stirring unit can be minimized.

The outer diameter of the photosensitive drum is 30 [mm] or less, and a cross-sectional area in a cross section orthogonal to the axis of the photosensitive drum of a process unit including the photosensitive drum, the developing roller, and the developing unit. 3500 [mm ² ] or less. According to such a configuration, the cross-sectional area of the process unit can be minimized, so that further downsizing of the image forming apparatus can be achieved.

  The outer diameter of the photosensitive drum is 30 [mm] or less, and a horizontal direction in a cross section orthogonal to the axis of the photosensitive drum of a process unit including the photosensitive drum, the developing roller, and the developing unit. The width is 70 [mm] or less. According to such a configuration, the width of the process unit can be minimized, so that further downsizing of the image forming apparatus can be achieved.

  According to the present invention, it is possible to provide an image forming apparatus capable of reliably mixing toner and carrier in a minimum space when performing high-speed printing.

1 is a schematic diagram illustrating a schematic configuration of a general image forming apparatus. 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a schematic diagram illustrating a state where the developing unit is viewed from the upper part of FIG. 2. It is a schematic diagram which shows the structure of a preliminary stirring part. It is a schematic diagram which shows each condition which this embodiment assumes. FIG. 6 is a characteristic diagram illustrating a relationship between a toner density difference ΔCt on a mag roller (Mag Roller) and a developer conveyance amount W in Formula (1). FIG. 7 is a schematic diagram showing a developer conveyance amount W for setting a toner density difference ΔCt to 1% or less for each of condition 1, condition 2, and condition 3 in FIG. 6. It is a characteristic view which shows the relationship between the area S and the pitch P of a screw about each of condition 1, condition 2, and condition 3. FIG. It is a characteristic view which shows the relationship between the outer diameter D of a screw, and the pitch P about each of condition 1, condition 2, and condition 3. FIG. FIG. 6 is a schematic diagram showing a developer amount m, a time T1 during which the developer makes one round of the developing unit, and a mixing stirring time T2 for each of Condition 1, Condition 2, and Condition 3. It is a schematic diagram for demonstrating the time T of mixing and stirring. It is a characteristic view which shows the relationship between time T required for mixing stirring, and the value of r shown in FIG. It is a schematic diagram showing a configuration in which the photosensitive drum is φ30, the developing roller is φ20, and the first conveying screw is φ18. It is a schematic diagram which shows the example which minimized between process pitches using the large diameter preliminary stirring part. It is a characteristic view which shows the result of having calculated | required the relationship between r and T by experiment by setting rotation speed R = 500rpm and P = 15mm. It is a characteristic view which shows the result of having calculated | required the relationship of R and T by experiment, r = 3mm and P = 15mm. It is a characteristic view which shows the result of having calculated | required the relationship between P and T by experiment by setting rotation speed R = 500rpm and r = 3mm.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

[Configuration example of image forming apparatus]
FIG. 1 is a schematic diagram illustrating a schematic configuration of a general image forming apparatus. The image forming apparatus uses a two-component developer including a toner and a carrier. The developing roller 104 that carries the developer and supplies the toner to the photosensitive drum 102; and the toner particles by stirring the developer and friction The first and second stirring screws 106 and 108 for charging the toner and the developing case 110 for accommodating and holding them are provided.

  In a developing device using a two-component developer, a developer and a mixture of toner and carrier are used. In the development area, only the toner is consumed and the carrier is reused. In order to replenish consumed toner, the amount of toner printed in advance is estimated by measuring it with a bit count or the like, and the deficiency detected by the toner density sensor is replenished. In recent years, image forming apparatuses have been digitized and images are formed by a digital exposure apparatus such as a laser scanner or an LED head. Therefore, since the drawing amount can be digitally grasped and calculated, the toner consumption over time can be predicted and replenished. On the other hand, the actual toner consumption varies depending on the environment of use and the use of electrophotographic members. Further, since the toner replenishing device is a mechanical component, the amount of replenishment changes due to variations in mechanical dimensions. Therefore, it is desirable to check and replenish with a toner density sensor.

  The toner replenishing device is configured such that a replenishing roller of a shaft having a cut groove or a groove is passed through a narrow gap in order to quantitatively replenish toner. The replenishment toner is put into the developer in the developing device in a state where the secondary particles are aggregated. The charged toner particle group comes into contact with the carrier surface of the developer, and is charged and adheres to the surface of the carrier. In this process, the toner is appropriately charged and carried on the carrier surface. The charged toner particle group is in a developable state only in this state. In the mixing and stirring device, for example, toner is introduced into the developer by a circulation system using a screw as shown in FIG. 1, mixed and stirred, dispersed, and charged while being transported to a developing region such as the developing roller 104.

  In FIG. 1, the developing case 110 includes first and second stirring chambers 112 and 114 in which first and second stirring screws 106 and 108 are respectively disposed. The first stirring chamber 112 includes a developing roller. The developer is disposed in proximity to the developer 104 so as to contact the developer roller 104.

  In the developing case 110, the partition walls 116 are provided at both ends of the partition wall 116 that partitions both the stirring chambers 112 and 114 so that the developer is circulated and conveyed between the first and second stirring chambers 112 and 114. A passage for transferring the developer from the first stirring chamber 112 to the second stirring chamber 114 and a passage for transferring the developer from the second stirring chamber 114 to the first stirring chamber 112 are provided so as to penetrate. It has been.

  By the way, in image forming apparatuses such as printers, further speeding-up has progressed, and recently, speed-up apparatuses having a process speed exceeding 300 mm / s have appeared. In addition, with the increase in speed, a shift from an A3 size device to an A4 size (or Letter size) device, a transition from a triaxial configuration including a paddle to a biaxial spiral stirring screw, development used in the development unit Due to the demand for reducing the amount of the agent (from 400 g to 200 g which is halved), downsizing of the apparatus is also progressing.

  Due to such a demand, it is necessary to stir the toner at a higher speed in a more limited space, and the configuration of the biaxial development unit as shown in FIG. 1 is no longer limited. For this reason, in this embodiment, preliminary stirring is performed in order to facilitate the mixing and stirring of the toner. When pre-stirring is used, the outer diameter of the biaxial spiral stirring screw has a smaller influence on the layout configuration, and therefore can be significantly smaller than the outer diameter of the developing roller 104.

FIG. 2 is a schematic diagram illustrating a configuration of the image forming apparatus 10 according to an embodiment of the present disclosure, and illustrates a state viewed from the axial direction of the roller. The image forming apparatus 10 according to the present embodiment scans and exposes a photosensitive drum 14 with a laser beam 12 modulated according to an image signal, and performs printing on a printing medium 15 such as paper or a plastic sheet by a dry electrophotographic method. Is what you do. The image forming apparatus 10 can be used as a laser printer, a laser fax, a digital copying machine, or a part thereof. The image forming apparatus 10 performs printing or the like using a two-component developer composed of toner and carrier.

The image forming apparatus 10 includes a laser exposure unit (not shown) that emits a laser beam 12. The laser exposure unit scans the laser beam 12 imaged on the minute spot on a straight line parallel to the rotation axis of the photosensitive drum 14 in a fixed direction at the exposure position a on the photosensitive drum 14. When the laser beam 12 is scanned on the surface of the photosensitive drum 14, the exposed portion of the photosensitive drum 14 becomes a positive potential relative to the non-exposed portion.

A developing roller 16 is disposed downstream of the exposure position a on the photosensitive drum 14. A developing unit 18 is disposed adjacent to the developing roller 16. In the developing unit 18, a two-component developer C composed of toner and carrier is stored. The two-component developer C in the developing unit 18 moves in the axial direction by the first conveying screw 20 in the developing unit 18 and adheres to the surface of the developing roller 16 by the magnetic force of the developing roller 16. Of the two-component developer C supplied on the developing roller 16, the toner charged to a negative potential is supplied from the developing roller 16 to the surface of the photosensitive drum 14 due to a potential difference. On the other hand, the carrier is collected from the surface of the developing roller 16 into the developing unit 18.

In the developing unit 18, a two-component developer C composed of powder toner having a predetermined color and a metal powder carrier is stored. As an example, the toner includes a polyester particle having a diameter of about 7 to 8 μm and a pigment, CCA, PMMA, or the like. Further, as an example, the carrier is obtained by applying silicon coating to ferrite particles having a diameter of about 35 to 60 nm.

Further, the developing unit 18 is provided with a magnetic permeability sensor (not shown), and the toner is supplied to the developing unit 18 so that the weight percentage of the toner with respect to the total weight of the two-component developer C becomes a predetermined value. The amount is adjusted.

FIG. 3 is a schematic diagram showing the developing unit 18 as viewed from the top of FIG. A first conveying screw 20 that conveys the two-component developer C in the axial direction (arrow A1 direction) is provided in the developing unit 18. A second conveying screw 22 is disposed in the developing unit 18 in parallel with the first conveying screw 20, and a partition is formed between the first conveying screw 20 and the second conveying screw 22. A wall 24 is provided. The two-component developer C conveyed by the first conveying screw 20 advances in the direction of the arrow A1 while being supplied to the developing roller 16, and the second conveying screw 22 side from the passage provided at the end of the partition wall 24. And is transported in the direction of arrow A2 by the second transport screw 22.

When the two-component developer C is stirred in the developing unit 18, the toner is negatively charged and the carrier is positively charged. At this time, the charge amount of the toner is, for example, about −25 to −15 [μC / g]. Thereby, the negatively charged toner adheres to the surface of the positively charged carrier. At this time, if the weight percentage of the toner stored in the developing unit 18 is about 6 to 10%, the toner adheres to about 60 to 80% of the surface area of the carrier.

The carrier to which the toner is attached is sent to the surface of the developing roller 16 while being transported in the direction of arrow A1 by the first transport screw 20. The carrier to which the toner is attached adheres to the surface of the developing roller 16 due to the magnetic force of the developing roller 16.

[Configuration of Pre-stirring Unit] Next, the pre-stirring unit 30 of the developing unit 18 will be described. As shown in FIG. 3, a preliminary stirring unit 30 is provided at the end of the developing unit 18. The preliminary stirring unit 30 is provided with a stirring screw 32. As shown in FIG. 2, the preliminary stirring unit 30 has a larger diameter than the photosensitive drum 14, the developing roller 16, the first transport screw 20, and the second transport screw 22. The preliminary stirring unit 30 is connected to a stirring unit in which the first transport screw 20 and the second transport screw 22 are arranged via a transport path. The two-component developer mixed in the preliminary stirring unit 30 is supplied from the conveyance path to the position of the end 20 a of the first conveyance screw 20 and is conveyed by the first conveyance screw 20.

As described above, the carrier to which the toner is attached is conveyed to the surface of the developing roller 16 while being conveyed in the direction of the arrow A1 by the first conveying screw 20, and thereafter, the arrow in FIG. It is conveyed in the A2 direction. For this reason, in the two-component developer C conveyed by the second conveying screw 22, the weight percentage of the toner with respect to the total weight is decreased. For this reason, the two-component developer C in which the weight percentage of the toner is reduced is sent to the preliminary stirring unit 30 by the second conveying screw 22, and the toner is newly charged in the preliminary stirring unit 30.

As shown in FIG. 3, the position where the toner is newly supplied is the end of the preliminary stirring unit 30. The two-component developer C supplied to the preliminary stirring unit 30 by the second conveying screw 22 is mixed with newly added toner by the rotation of the stirring screw 32, and the inside of the preliminary stirring unit 30 in the direction of the arrow A3. Sent. In the present embodiment, the newly added toner in the preliminary stirring unit 30 adheres to the carrier in the preliminary stirring unit 30 and is charged to a negative potential, so that the end of the first conveying screw 20 is charged. Sent to the position 20a. The two-component developer C sent to the position of the end 20a of the first conveying screw 20 is sent in the direction of the arrow A1 by the first conveying screw 20 and supplied to the developing roller 16.

As described above, in the present embodiment, the preliminary stirring unit 30 is configured to have a large diameter, so that newly added toner can be completely mixed with the carrier in the preliminary stirring unit 30 and charged to a predetermined potential. State.

Therefore, in the present embodiment, the time T (hereinafter, necessary for mixing and stirring) from when the toner is supplied at the toner supply position in FIG. 2 until the toner reaches the position of the end 20a of the first conveying screw 20 is described. (Referred to as time), the mixing of the toner and carrier can be completed and the toner can be at a negative potential.

Thus, when the two-component developer C is transported by the first transport screw 20, the toner is already charged at a negative potential, so that the toner charged at a positive potential with respect to the developing roller 16 is not charged. It is possible to reliably suppress supply.

In the present embodiment, a large-diameter preliminary stirring unit 30 is provided on the side of the main stirring unit. With such a configuration, as compared with the case where mixing is performed with the first and second stirring screws 106 and 108 having a small cross-sectional area as shown in FIG. Can be mixed. In addition, the toner potential can be reliably set to a negative potential only by stirring in the preliminary stirring unit 30.

In order to realize such agitation in the preliminary agitation unit 30, in this embodiment, the amount of the two-component developer C (including newly introduced toner) in the preliminary agitation unit 30 and the agitation screw 32 are used. The predetermined relationship is established among the pitch, the rotation speed R of the stirring screw, and the time T required for mixing and stirring.

Hereinafter, these relationships will be described in detail. FIG. 4 is a schematic diagram illustrating the configuration of the preliminary stirring unit 30, and schematically illustrates a state in which the preliminary stirring unit 30 is viewed from the axial direction of the stirring screw 32. As shown in FIG. 4, the preliminary stirring unit 30 is provided with a stirring screw 32. As shown in FIG. 4, in this embodiment, in the preliminary stirring unit 30, the stirring screw 32 is placed outside the stirring screw 32 so that the two-component developer C (including newly added toner) is not completely buried. A distance r is secured between the uppermost part of the diameter and the surface of the two-component developer C in the preliminary stirring unit 30.

  In Non-Patent Document 1, in the study of image density uniformity in two-component magnetic brush screw development using a biaxial spiral stirring screw (Auger) as shown in FIG. 1, a mag roller (Mag Roller; book The relationship between the toner density difference ΔCt of the developer at both ends on the upper and lower ends (corresponding to the developing roller 18 of the embodiment) and the developer transport amount W (the following expression (1)) is shown. Here, the developer transport amount W is the amount (g / s) of developer transported by the developing unit per unit time. The expression (2) is an expression showing the relationship between the developer conveyance amount W, the conveyance (developer) cross-sectional area S, the screw pitch P, and the screw rotation speed R. Further, the expression (3) is an expression for calculating the cross-sectional area S of the developer (the area of the region marked with dots in FIG. 4) in the state shown in FIG.

ΔCt = (M / A) * V * L / W (1)
W = η * ρ * S * P * R (2)
^{S = π * (D / 2} + G) 2/2 + (D + 2 * G) * d-π * (d / 2) 2 ··· (3)

In the above formula,
P: Screw pitch (Pitch)
R: Number of rotations of screw D: Screw outer diameter d: Screw shaft diameter G: Gap between screw and casing (see FIG. 4)
ΔCt: Toner density difference M / A: Amount of toner adhered on the photoconductor (photosensitive drum 14) V: Peripheral speed of the photoconductor (photosensitive drum 14) L: Development width (effectiveness of developing roller 16 and photoconductive drum 14) Long)
W: developer transport amount η: transport efficiency ρ: developer bulk density S: transport (developer) cross-sectional area.

  From equation (1), it can be seen that the toner density difference ΔCt can be reduced as the developer transport amount W increases. FIG. 5 is a schematic diagram showing each condition assumed in the present embodiment. In FIG. 5, Condition 1 indicates a conventional condition that does not assume a high speed of about 300 mm / s, and the process speed (photoconductor peripheral speed V) is about 161 mm / s. On the other hand, conditions 2 and 3 are conditions assumed by the image forming apparatus 10 according to the present embodiment, and a process speed of 300 mm / s or more is assumed. Condition 1 assumes an A3 size printing medium 15 and the development width is a relatively large value of 294 mm. On the other hand, Condition 2 and Condition 3 assume an A4 size (or Letter size) printing medium 15, and the development width is as small as 220 mm.

  As the process speed increases, it is necessary to mix newly introduced toner and carrier at a higher speed. As can be seen from FIG. 1, when the developing width is reduced, the distance agitated by the first and second agitating screws 106 and 108 is reduced, which is disadvantageous from the viewpoint of mixing the toner and the carrier. As shown in FIG. 5, in any of the conditions 1 to 3, the toner density difference ΔCt is assumed to be 1% or less. This is because when the toner density difference ΔCt of the developer at both ends of the mag roller exceeds 1%, the density difference at both ends of the paper becomes conspicuous. In the present embodiment, these factors are solved by providing a large-diameter preliminary stirring unit 30.

  From the equation (1), the relationship between the toner density difference ΔCt and the developer transport amount W of the developing unit 18 can be obtained. FIG. 6 is a characteristic diagram showing the relationship between the toner density difference ΔCt on the mag roller (Mag Roller) and the developer transport amount W in the equation (1). If ΔCt exceeds 1.0%, a density difference on the print medium 15 is recognized, so 1.0% or less is set as a target value. FIG. 6 shows the relationship between the toner density difference ΔCt and the developer transport amount W for each of the conditions 1, 2 and 3 in FIG. As shown in FIG. 6, as the developer conveyance amount W increases, the toner and the carrier can be mixed efficiently, so the toner density difference ΔCt decreases.

  FIG. 7 is a schematic diagram showing the developer conveyance amount W for making the toner density difference ΔCt 1% or less for each of the conditions 1, 2 and 3 in FIG. According to FIG. 6, in order to make the toner density difference ΔCt 1% or less, as shown in FIG. 7, the developer transport amount W is 28 g / s or less in the condition 1, and the developer in the condition 2 as shown in FIG. In the case of Condition 3 where the transport amount W is 47 g / s or less, the developer transport amount W needs to be 37 g / s or less.

  On the other hand, the number of rotations of the screw conveying the developer is preferably around 500 rpm in consideration of factors such as a rise in the temperature of the bearing portion and stress on the developer. Here, the number of rotations of the screw is set to 500 rpm. Since the value of W is obtained under each of the conditions 1 to 3 and R = 500 rpm, the relationship between the required conveyance (developer) cross-sectional area S and the pitch P is obtained from equation (2). Can do.

  FIG. 8 shows the relationship between the area S and the pitch P of the screw for each of condition 1 (W = 28 g / s), condition 2 (W = 47 g / s), and condition 3 (W = 37 g / s). FIG. The rotation speed of the screw is 500 rpm. Thus, the developer conveyance amount W can be increased as the screw area S is increased and the pitch P is increased.

  By the way, the inner diameter of the screw needs to have a lower limit of 6 mm from the viewpoint of strength. For this reason, the inner diameter of the screw is set to a minimum of 6 mm. Further, the gap G between the screw and the housing needs to be a gap where the two do not contact within the tolerance, and G = 1. For this reason, the relationship between the outer diameter D of the screw and the pitch P can be obtained by introducing the expression (3) with respect to the result shown in FIG. FIG. 9 shows the relationship between the outer diameter D of the screw and the pitch P for each of condition 1 (W = 28 g / s), condition 2 (W = 47 g / s), and condition 3 (W = 37 g / s). FIG.

  As shown in FIG. 9, the developer conveyance amount W can be increased as the outer diameter D of the stirring screw 32 is increased and the pitch P is increased. Therefore, as in this embodiment, the developer conveying amount W can be increased by increasing the diameter of the stirring screw 32 of the preliminary stirring unit 30, and as a result, the toner density difference ΔCt can be reduced. . As an example, according to FIG. 9, in order to secure the developer conveyance amount W = 37 g / s, it is possible to achieve by setting the outer diameter D of the stirring screw 32 to 400 mm and the pitch P to 3 mm.

  On the other hand, as described above, the developer amount m present in the developing unit 18 tends to be reduced due to the downsizing of the developing unit 18 and was conventionally about 350 g. About 200 g. Further, in the configuration in which the developer unit circulates in the developing unit with two screws from the developer amount existing in the developing unit 18 and the developer transport amount W, the time required for the developer to make one round in the developing unit is taken. The time required for the mixing and stirring of the biaxial system can be obtained. FIG. 10 shows the developer amount m and the developer in the developing unit for each of condition 1 (W = 28 g / s), condition 2 (W = 47 g / s), and condition 3 (W = 37 g / s). It is a schematic diagram which shows time T1 which makes one round, and time T2 of mixing and stirring. As shown in FIG. 10, in this embodiment, the developer amount m in the developing unit 18 under the conditions 2 and 3 is 200 g. On the other hand, the developer amount m under conventional condition 1 is 350 g. The time T1 for the developer to make a round in the developing unit can be calculated by dividing the developer amount m (g) by the developer transport amount W (g / s). In addition, the mixing stirring time T2 is a time when stirring is performed only in the forward path in a configuration having a biaxial screw, and is a value that is 1/2 of the time T1 during which the developer makes one round of the developing unit. .

  As shown in FIG. 10, in this embodiment (condition 2 and condition 3), it can be seen that the mixing stirring time T2 is about 1/2 to 1/3 of the conventional mixing stirring (condition 1). Therefore, when the preliminary stirring unit 30 using a large-diameter screw is provided, the size of the developing unit 18 itself remains the same as before, the toner concentration difference ΔCt is minimized, and the mixing stirring time is also reduced. It needs to be shortened.

  Here, FIG. 11 is a schematic view for explaining the mixing stirring time T described in FIG. 3 in more detail. 11 schematically illustrates how the toner 110 supplied at the toner supply position shown in FIG. 11 is mixed with the two-component developer C when the two-component developer C is stirred while being conveyed in the direction of arrow A4 by the stirring screw 100. Is shown.

As shown in FIG. 11, when the toner 101 is newly supplied at the toner supply position, the specific gravity of the toner 101 is smaller than that of the two-component developer C, so that the toner 101 is mixed with the two-component developer C immediately after being supplied. In other words, the toner 101 is laminated on the two-component developer C. Thereafter, when the toner 101 and the two-component developer C are conveyed in the direction of the arrow A4, the toner 101 is mixed with the two-component developer C as time passes, and the toner 110 is a carrier of the two-component developer C. Will adhere to. Accordingly, as the toner 101 advances to the left in FIG. 11, the thickness of the toner 110 on the two-component developer C becomes thinner, and when reaching the point B, the toner 101 is completely mixed with the two-component developer C. It becomes a state.

In this embodiment, in FIG. 11, after the toner 101 is newly supplied at the toner supply position, the time until the supplied toner reaches the point B and is completely mixed with the carrier is the time T required for mixing and stirring. It is defined as As described with reference to FIG. 10, when the developer amount in the developing unit 18 is reduced to 200 g as in the conditions 2 and 3 of the present embodiment, the time T2 becomes smaller. It is necessary to make it sufficiently small. In the case of condition 2, it is necessary to reduce the time to 2.1 seconds, and in the case of condition 3, it must be reduced to about 2.7 seconds. Therefore, in order to achieve conditions 2 and 3 that are high-speed processes, it is necessary to mix the toner with the carrier in a shorter time, and it is necessary to reduce the time T required for mixing and stirring as much as possible.

Therefore, from the results of FIG. 9, it is necessary to increase the diameter of the stirring screw 32 and increase the pitch P to ensure the desired developer conveyance amount W and to minimize the time T required for mixing and stirring. . In this regard, the present inventor found that the time T required for mixing and stirring is expressed by the following equation (4) between the value of r described in FIG. 4, the rotational speed R of the screw, and the pitch P of the screw. I found that there is a relationship.
T> α · P / (r · R) (4)

  In other words, the time T required for mixing and stirring decreases as r increases. Further, the value of time T decreases as R increases. Furthermore, the value of T decreases as the pitch P decreases. Here, based on FIGS. 15 to 17, the basis on which the equation (4) is established will be described. First, using an actual apparatus, the relationship between r and T was determined by experiment with the rotation speed R = 500 rpm and P = 15 mm, and the result shown in FIG. 15 was obtained. From the results of FIG. 15, it was found that the time T decreases as r increases. Similarly, when r = 3 mm and P = 15 mm, the relationship between R and T was obtained by experiment, and the result shown in FIG. 16 was obtained. From the results of FIG. 16, it was found that when R is increased, the value of T is decreased. Moreover, when the rotation speed R = 500 rpm and r = 3 mm, the relationship between P and T was obtained by experiment, the result shown in FIG. 17 was obtained. From the results of FIG. 17, it was found that increasing P increases the value of T. Therefore, it has been found that the relationship of Expression (4) is established with α as a coefficient.

  According to FIG. 9, in the preliminary stirring unit 30, when a large stirring screw 32 having an outer diameter D of about 40 mm is used, the pitch P of the stirring screw 32 is about 3 mm (when W = 37 g / s). FIG. 12 is a characteristic diagram showing the relationship between the time T required for mixing and stirring and the value of r shown in FIG. 4. The stirring screw 32 has an outer diameter D of 40 mm, and the screw pitch P is 14 mm. It is a characteristic view which shows the theoretical value and experimental value in case the theoretical value and experimental value (actually measured value) in case, and the pitch P shall be 3 mm. Here, the theoretical value represents the formula (4), and is a characteristic curve showing the relationship between r and T when the inequality sign in the formula (4) is an equal sign. The experimental value is a value obtained by actually measuring the stirring screw 32 having an outer diameter D of 40 mm and measuring the time T while changing the value of r. At time T, a sensor (charge amount distribution measuring device) that measures the toner charge distribution is arranged at an arbitrary position in the transport path, and is at a point closest to the toner supply position, and there is a positive potential toner. It can be obtained by measuring the time until the toner reaches the point where it does not (point B in FIG. 11). As described above, when the relationship between r and T is obtained, the theoretical value and the experimental value shown in FIG. 12 are obtained, and when the constant α in the equation (4) is obtained from this result, it is found that α = 20.

  From the results of FIG. 12, it is understood that when the pitch P is 3 mm, the value of r should be secured to 3 mm or more in order to reduce the time T required for mixing and stirring up to about 2 seconds described in FIG. .

In this way, in the preliminary stirring unit 30, when the relationship between r and T satisfies the formula (4), the conditions for mixing and stirring are satisfied and conveyed from the preliminary stirring unit 30 in the direction of arrow A3 in FIG. The toner that reaches the end 20a of the conveying screw 20 is all negatively charged. Therefore, by modifying the equation (4), the required relationship between r and T when the large-diameter preliminary stirring unit 30 is provided can be expressed by the following equation (5).
r> α · P / (R · T) (5)

  As an example, when the size of the outer diameter D of the preliminary stirring unit 30 is 40 mm, the inner diameter is 6 mm, and the rotation speed R is 500 rpm (8.3 rps), the pitch P is about 3 mm (condition 3; W = 37 g / s). At this time, if the value of r is 7 mm, the time T required for mixing and stirring is about 1 second from the equations (4) and (5).

  Therefore, when the large-diameter stirring screw 32 is provided, the preliminary stirring unit 30 and the stirring screw 32 are configured so as to satisfy the formula (5), and the necessary value of r is ensured, so that it is necessary for mixing and stirring. It is possible to set the time T to a desired time.

Next, a specific application example of the preliminary stirring unit 30 according to the present embodiment will be described. FIG. 13 shows a configuration in which the photosensitive drum 14 is φ30, the developing roller 16 is φ20, the first conveying screw 20 is φ18, and the stirring function of the toner and the carrier in the developing unit 18 is performed only by the preliminary stirring unit 30. Yes. The outer diameter of the stirring screw 32 is 400 mm. By satisfying the formula (5), the toner and the carrier are completely mixed by the preliminary stirring unit 30 and supplied to the position of the end 20 a of the first conveying screw 20. As a result, the first conveying screw 20 and the second conveying screw 22 can have only the function of conveying the two-component developer C that has been mixed. Therefore, the first conveying screw 20 and the second conveying screw 22 do not need a stirring function and need only have a conveying function. Therefore, the outer diameters of the first conveying screw 20 and the second conveying screw 22 are the same. Can be greatly reduced, and the process unit shown in FIG. 13 can be significantly downsized. In the arrangement according to the example of FIG. 13, the horizontal width can be reduced to about 60 mm to 70 mm and the height can be reduced to about 40 mm to 50 mm from the drawing. Accordingly, the cross-sectional area of the process unit configured as shown in FIG. 13 can be greatly reduced (in the example of FIG. 13, 3500 mm ² or less), and in the case of a color printer or the like capable of printing a plurality of colors, adjacent other colors The inter-process pitch between the process units can be reduced. In the example of FIG. 13, since the left and right widths are 70 mm at the maximum, the inter-process pitch can be suppressed to 70 mm or less.

In FIG. 13, when the outer diameter of the photosensitive drum 14 is 30 [mm] or less, the cross-sectional area of the preliminary stirring unit 30 is S1 [mm ² ], and the cross-sectional area of the photosensitive drum 14 is S2 [mm ^2]. ], When the cross-sectional area of the developing roller 16 is S3 [mm ² ] and the cross-sectional area of the first conveying screw 20 is S4 [mm ² ], the following expression (6) is established.
S1 < S2 + S3 + S4 (6)

  FIG. 13 shows an example in which the developer delivery position from the preliminary stirring unit 30 to the first stirring screw 20 is set in consideration of the developer flow depending on the rotation direction of the stirring screw 32 of the preliminary stirring unit 30. It is a schematic diagram shown. As shown in FIG. 13, when the stirring screw 32 of the preliminary stirring unit 30 rotates in the direction of the arrow A <b> 10, the two-component developer C indicated by dots in FIG. Thus, the surface in the preliminary stirring unit 30 is inclined. For this reason, a developer passage is provided in the region indicated by a one-dot chain line B in FIG. 13 so that the developer is placed in the carrier passage by connecting the preliminary agitating unit 30 and the carrier chamber provided with the first carrier screw 20. Complete filling can be avoided and a gap can be secured in the conveyance path, so that the two-component developer C can be efficiently moved to the first stirring screw 20.

  FIG. 14 is a schematic diagram showing an example in which the gap between the process pitches is minimized by using the large-diameter preliminary stirring unit 30. As shown in FIG. 14, by disposing the first conveying screw 20 below the second conveying screw 22, the inter-process pitch can be made smaller than that of the configuration of FIG. In this case, the inter-process pitch can be reduced to about 50 mm to 60 mm.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 14 Photosensitive drum 16 Developing roller 18 Developing unit 30 Pre-stirring part 32 Stirring screw 20 1st conveying screw 22 2nd conveying screw

Claims (2)

A photosensitive drum that forms an image on a medium by transferring toner supplied according to an electrostatic latent image; and
A developing roller for supplying toner to the photosensitive drum;
A developing unit for supplying a two-component developer in which a toner and a carrier are mixed to the developing roller,
The developing unit is
A transport unit that transports a two-component developer along the axial direction of the developing roller;
A preliminary agitation unit including an agitation screw provided at an end of the conveyance unit and mixing a newly charged toner and a carrier;
The process speed at which the photosensitive drum forms an image on the medium is 300 [mm / s] or more, and the width of the medium on which the photosensitive drum forms an image is a letter size width of 216 [mm] or less. ,
The pre-stirring unit has a protruding amount of the stirring screw in the upward direction from the surface of the two-component developer as r [mm], a pitch of the stirring screw as P [mm], and a rotation speed of the stirring screw as R [rps], An image forming apparatus satisfying the following formula (1) when the time T [s] for passing through the stirring screw is satisfied.
The outer diameter of the photoconductive drum is 30 [mm] or less, the area of the end surface of the pre-stirring portion in the rotation axis direction of the stirring screw is S1 [mm ² ], and the cross-sectional area of the photoconductive drum is S2 [mm] ² ], where S3 [mm ² ] is the cross-sectional area of the developing roller, and S4 [mm ² ] is the cross-sectional area of the transport screw provided in the transport unit, the following equation (2) is satisfied: The image forming apparatus according to claim 1.
S1 < S2 + S3 + S4 (2)