Toner charge control for image defect reduction

Details Toner charge control for image defect reduction Toner charge control for image defect reduction

2000-09-21

2002-04-30

Braun, Fred L. (Department: 2852)

Electrophotography

Control of electrophotography process

Control of developing

C399S285000

Reexamination Certificate

active

06381424

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a developing apparatus for developing latent images formed on a latent image carrier using electrostatic force by rendering toner particles cling as developing agents and to an image forming apparatus such as a printer or photocopier for forming images in use of this developing apparatus and, more particularly, to a developing apparatus and an image forming apparatus in which image defects such as fogs and the like are reduced.
2. Description of Related Art
With image forming apparatuses using electrophotographic technology such as photocopiers and printers, a non-magnetic single-component development process, so called, has been known in which non-magnetic materials make toner particles serving as developing agents and in which the apparatuses develop latent images formed on a latent image carrier such as a photosensitive drum.
FIG. 11
shows an example of a conventional image forming apparatus using such a non-magnetic single-component development process. Around the photosensitive drum
81
, provided are a charger
82
for charging the photosensitive drum
81
, a light emitting device exposing section
83
for exposing the surface of the toners
87
onto the photosensitive drum
81
, a transfer roller
86
for transferring the developed images on a sheet
90
, and a cleaning blade
88
for removing toners remaining on the photosensitive drum.
The toners
87
are supplied onto the developing roller according to rotation of a reset roller from the cartridge. The developing roller
84
at that time is biased to a voltage of about −320 V, and latent image portions on the photosensitive drum
81
are set to about −100 V where non-latent-image portions are set to about −700 V. The toners conveyed by the developing roller
84
are those of charged non-magnetic toners and designed to have a prescribed negative voltage from frictions made by the reset roller
89
. The toners
87
on the developing roller
84
are in contact with the blade to keep a prescribed thickness (e.g., 10 &mgr;m to 40 &mgr;m). The toners
87
are made to jump onto or come in contact with the latent image portions on the photosensitive drum
81
at a position where the developing drum
84
faces to the photosensitive drum, thereby developing electrostatic latent images.
The transfer roller
86
transfers the developed toners
87
on the latent image portions onto the sheet
90
, and the toners are fixed subsequently to the sheet. The cleaning blade
88
scrapes the toners passing through the transfer roller
86
and remaining on the photosensitive drum. In a meantime, the reset roller
89
scrapes the toners remaining on the developing roller
84
.
The printed images made by a printing system using such an electrophotographic process tend to be subject to image defects such as fogs and printing failures on fine lines. One of the reasons is that, because toners have distributed charge amounts, the toners having charges deviated from the center of the distribution do not properly contribute to image formation.
FIG. 12
is a diagram showing a charge amount distribution of the negatively charged non-magnetic single-component toners. These toners are to be used between the developing roller and the photosensitive drum, which are in the above voltage relationship; the toners are structured to be charged negatively; and the charge targeted amount Ct is a minus value. However, all the toners are not uniformly charged to have the targeted value Ct and form a Gaussian distribution as shown in
FIG. 12
in respect to the charge amounts. In
FIG. 12
, according to the charge amount distribution of the toners, there are a distribution Fn
1
located on a lower voltage side of a distribution Fp having the targeted charge amount Ct at the center of the distribution, and distributions Fn
2
, Fn
3
on a higher voltage side of the distribution Fp. Those distributions Fn
1
to Fn
3
may bring image failures such as fogs from the following mechanism.
That is, force exerted to the toners on the developing roller and the photosensitive drum during development of the electrostatic latent images can be thought as a summation of, generally, electrostatic force, mirror image force, and other forces. The electrostatic force is proportioned to the charge amount of the toners; the mirror image force is a negative force (i.e., in a direction attracted from the developing roller and the photosensitive drum) proportioned to a square of the charge amount of the toners; other forces are forces such as liquid crosslinking force, van der Waals attraction, etc. which are not depending on the charge amount of the toners.
The toners contained in the distribution Fn
1
, or high charged toners, have the mirror image force and other forces significantly larger than the electrostatic force, and as a general rule, the toners become strongly clinging to the developing roller. With the so-called jumping development method in which toners jump, however, most of toners move back and forth between the developing roller and the photosensitive drum, and therefore, the high charged toners contained in the distribution Fn
1
may jump to the side of the photosensitive drum from this influence. Because the toners thus jumping are easily made to cling to the photosensitive drum with strong mirror image force, the toners may cause image defects such as fogs. With the low charged toners contained in the distribution Fn
2
, the charge itself is so weak, and therefore, the electrostatic force is smaller than the summation of the mirror image force and other forces. Accordingly, such the low charged toners may cling to the non-latent-image portions on the photosensitive drum, thereby creating causes of fogs. The reverse charged toners contained in the distribution Fn
3
are charged with the reverse polarity to the targeted charge amount Ct, and the reverse charged toners tend to make developments on the non-latent-image portions by the electrostatic force.
As an approach to remove the low charged toners, an art has been known in which a conductive roller biased with a direct current voltage having the reverse polarity to the toners is placed to a position facing the developing roller. Another known art has a structure that a conductive roller biased with a direct current voltage having the same polarity to the toners is placed to a position facing to the developing roller. However, those arts are no more than removing the prescribed charged toners and do not operate for removing all the charged toners: the high charged (Fn
1
), the low charged (Fn
2
), and the reverse charged (Fn
3
). Those arts do not regulate the voltages of the photosensitive drum, the developing roller, and the conductive roller, so that the toner removal effect may not be obtained according to the voltage relations among the drum and the rollers.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a developing apparatus and an image forming apparatus for reducing at the same time high charged, low charged, and reverse charged developing agents, as described above, before the development at the latent image carrier.
In a developing apparatus as an embodiment of the invention, a developing apparatus for developing latent images includes a latent image carrier on which latent images are formed; a developing roller for developing latent images on the latent image carrier by forming a layer made of developing agents on the developing roller; and a charge amount controlling member to which a voltage having the same voltage polarity, with respect to the voltage of the developing roller, as the voltage polarity of non-latent-image portions on the latent image carrier is fed, the charge amount controlling member being disposed in facing to a surface of the developing roller on an upstream side, in a rotation direction of the developing roller, of a position at which the latent image carrier and the developing roller face to each other.
By disposing the charge amount controlling member on the u

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