Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
2001-12-17
2003-10-28
Braun, Fred L. (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C399S176000
Reexamination Certificate
active
06640063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image forming apparatus such as printers, copying machines, facsimile machines, and so on.
More particularly, the invention relates to improvement in image forming apparatus of the indirect (transfer) method or the direct method permitting variation in process speed for formation of image and variation in pixel density for formation of image, in which a desired image is formed and supported on an image bearing member such as an electrophotographic, photosensitive member, an electrostatic recording dielectric member, or the like by suitable image-forming process devices of the electrophotographic method, the electrostatic recording method, or the like.
2. Description of the Related Art
Conventionally, for example, as a method of charging the surface of the image bearing member as a body to be charged, such as the photosensitive member, the dielectric member, or the like in the image forming apparatus such as the electrophotographic apparatus, the electrostatic recording apparatus, and so on, it was common practice to employ the corona charging method being a non-contact charging method, in which a high voltage was applied to a thin corona discharge wire to generate a corona and in which the corona was made to act on the surface of the image bearing member to charge it.
In recent years, a contact charging method of keeping a charging member of a roller type, a blade type, or the like in contact with the surface of the image bearing member as a body to be charged and applying a voltage to the charging member to charge the surface of the image bearing member is going mainstream for reasons of low voltage processes, small ozone evolving amounts, low cost, and so on. Particularly, the charging member of the roller type is able to implement stable charging over long periods of time.
Here the charging member does not always have to be in contact with the surface of the image bearing member being the body to be charged, but the charging member may be placed in no contact with and in proximity to the image bearing member (proximity charging), for example, with a clearance (gap) of several ten &mgr;m as long as a dischargeable area determined by a gap voltage and a corrected Paschen curve is ensured between the charging member and the image bearing member. In the present invention such proximity charging cases are also considered to be within the category of contact charging.
The voltage applied to the charging member may consist of only a dc voltage, but it is also possible to apply an oscillating voltage to the charging member to induce alternate, positive and negative discharges, thereby effecting even charging.
For example, it is known that when the oscillating voltage is applied in the form of superposition of a dc voltage (dc offset bias) and an ac voltage having a peak-to-peak voltage value not less than two times a discharge start threshold voltage (discharge start voltage or charging start voltage) of the charged object upon application of the dc voltage, the effect of averaging the charging of the charged body is achieved, so as to implement even charging.
The waveform of the oscillating voltage does not always have to be limited to only a sine wave, but may also be either of rectangular, triangular, and pulse waves. The oscillating voltage also embraces a voltage of the rectangular wave obtained by periodically switching the dc voltage on and off, and an output obtained by periodically changing values of the dc voltage so as to be equal to the superimposed voltage of the ac voltage and the dc voltage.
The contact charging method of charging the charging member by applying the oscillating voltage thereto as described above, will be referred to hereinafter as “AC charging method.” The contact charging method of charging the charging member by applying only the dc voltage thereto will be referred to hereinafter as “DC charging method.”
In the AC charging method, however, discharge amounts to the image bearing member (hereinafter referred to as a photosensitive drum) become larger than in the DC charging method. This was the cause of promoting deterioration of the photosensitive drum, e.g., shaving of the photosensitive drum or the like, and there were cases where an abnormal image such as image flow or the like was formed under a high temperature and high humidity environment because of discharge products.
In order to overcome this issue, it is necessary to minimize the alternate, positive and negative discharges, by applying the necessary and minimum voltage.
However, the relation between a voltage and a discharge amount is not always constant in practice, but varies depending upon the film thickness of the photosensitive drum, environmental variation of the charging member and/or air, and so on. Materials become dry under a low temperature and low humidity environment (L/L) to increase their resistances and resist discharge, so that the peak-to-peak voltage not less than a certain value becomes necessary for achievement of even charging. Even at the lowest voltage value to achieve even charging under the L/L environment, if the charging operation is carried out under the high temperature and high humidity environment (H/H), the materials will absorb moisture to decrease the resistances on the contrary and the charging member will cause more discharge than necessary. This will result in an increase in discharge amounts, which will pose problems of occurrence of image flow and blur, occurrence of toner fusion, shaving and life decrease of the photosensitive drum due to deterioration of the surface of the photosensitive drum, and so on.
In order to restrain this increase/decrease of discharge due to the environmental variation, the “AC constant current control method” of controlling the current value of an alternating current flowing upon application of the ac voltage to the charging member was also proposed, in addition to the “AC constant voltage control method” of always applying the fixed ac voltage as described above. According to this AC constant current control method, the peak-to-peak voltage value of the ac voltage can be increased under the low temperature and low humidity environment (L/L) where the resistances of the materials increase, whereas the peak-to-peak voltage value can be decreased under the high temperature and high humidity environment (H/H) where the resistances of the materials decrease. Therefore, it becomes feasible to restrain the increase/decrease of discharge, as compared with the AC constant voltage control method.
For aiming to further increase the life of the photosensitive drum, however, the AC constant current control method cannot be mentioned as perfect in order to suppress the increase/decrease of discharge amount due to variation of resistances caused by production dispersion and contamination of the charging member, capacitance variation of the photosensitive drum after long-term use, dispersion of high-voltage devices in the main body, and so on. In order to suppress this increase/decrease of discharge amount, it is necessary to employ means for decreasing the production variation of the charging member and the environmental variation and for canceling fluctuation of high voltage, which will increase the cost.
For stably providing high image quality and high quality over long periods of time, it is thus necessary to control the voltage and current applied so as not to cause over discharge and so as to implement even charging without a problem. As a method for realizing it, the inventors accomplished the invention of “discharge current amount control method” (Japanese Patent Applications No. 2000-11819 and No. 2000-11820), which is such a method that, where Vth stands for a discharge start voltage to the image bearing member upon application of the dc voltage to the charging member, during a non-image-forming period current values are measured upon application of at least one peak-to-peak voltage less than two times Vth and upon application of at least t
Adachi Motoki
Watanabe Yasunari
LandOfFree
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