1) Field of the Invention
The present invention relates to a technology for preventing noise caused by vibrational resonance produced in a latent image carrier due to a thin-walled structure of an image forming apparatus.
2) Description of the Related Art
In image forming apparatuses like a copying machine, a facsimile, a printer, and a printing machine, steps of charging, writing, developing, and transferring are carried out for a photoreceptor as a latent image carrier. In the step of transferring, a toner image that is transferred on a recording medium like a recording paper is fixed to give a copy or a printout.
A structure that employs a non-contact charging method with aerial discharge using a corona charger is used for charging of the photoreceptor. However, in the structure, discharge products like ozone and nitrogen oxide are generated during discharging which may result in deterioration of environment or deterioration of charging characteristics on the photoreceptor. Therefore, a contact charging method, which does not generate the problems and enables to apply low voltage, is proposed as a substitute for an aerial discharge. A structure that injects charge by applying voltage between the photoreceptor and any one of a brush, a roller, and a blade of a conductive material that is kept in contact with the photoreceptor is known as one of the contact charging methods.
In the contact charging method, it is possible to apply low voltage and there is no generation of discharge products. However, since any one of a brush, a roller, and a blade made of a conductive material is in direct contact with the photoreceptor, it is easy to carry out reverse transition of deposits of toner etc. that remain on the photoreceptor. The deposits that have undergone reverse transition hinder the injection of charge and may deteriorate the charging characteristics. Furthermore, when a charging member is left without being used in a charging process for hours, a portion of the charging member that is in contact with the photoreceptor, changes shape due to permanent deformation. As a result, when the charging process is carried out again, there is no uniform contact of the charging member with the surface of the photoreceptor, which may result in charging unevenness.
To solve this problem, a unit that forms a charging range between the photoreceptor and the charging member is proposed. The charging member is disposed so that a prescribed minute interval is maintained between the photoreceptor and the charging member. The unit is an intermediate structure of non-contact and contact charging methods. A charging method (proximity charging method) has been employed in recent years. In this method, a prescribed minute interval is provided between the photoreceptor and the charging member such as a brush, a roller, and a blade of a conductive material, and charging is carried out by applying either of only dc voltage and dc voltage superimposed by an ac voltage.
In a structure which employs the proximity charging method, in a case where the charging member is a roller, a film of prescribed thickness is wound on both ends of the charging roller in its axial direction for setting of a gap and size of the minute gap is prescribed by thickness of the film.
Maintaining the prescribed size of the minute gap is an important condition to have no variation in the charging characteristics. When it is assumed that the size of the minute gap is maintained, the uniform charging becomes possible by applying of dc voltage for which setting is comparatively easy. However, when the size of the gap varies considerably, there is a considerable variation in a charging electrical potential in proportion to the variation in the size of the gap. Therefore, conventionally, various ideas have been thought up to achieve uniform charging characteristics even in a case where the size of the gap is varied by superimposing ac voltage on dc voltage.
On the other hand, apart from the charging unit, a developing unit is there to set the charging characteristics, i.e. bias characteristics. In a case of the developing unit, a developing method that uses either of a one-component developer and a two-component developer, is known. In the developing unit which uses the two-component developer, a developer that includes a carrier made of a magnetic material (substance) for an insulating toner, is agitated by an agitator. The toner is then deposited by charging on the carrier and the developer is made to be in contact with the photoreceptor.
In a developer carrier used in the developing unit, a developing sleeve, which can carry the developer on its surface, is used and a magnetic roll with a plurality of south poles and north poles lined up alternately on it is provided inside the developing sleeve. In the developer carrier, the developer is drawn up by magnetic force of the magnetic roll and a magnetic brush is formed by making the developer erected in the form of a brush on the surface of the developer carrier.
When the magnetic brush carried on the surface of the magnetic sleeve comes in contact with the electrostatic latent image that is formed on the photoreceptor based on either of image information and a paper document image, a developing bias is applied between the photoreceptor and the magnetic sleeve as the developer carrier. Due to the developing bias, the toner in the magnetic brush undergoes electrostatic absorption by the electrostatic latent image thereby forming a toner image.
As a developing bias, a bias as follows is used. The bias superimposes the ac voltage on the dc voltage to improve the developing capability, carry out the developing to the electrostatic latent image identically with utmost clarity, and improve uniformity of dots. Moreover, the bias has a first peak value V1 for transferring the toner from the developer carrier to the photoreceptor and a second peak value V2 for transferring the toner from the photoreceptor to the developer carrier. A method using the bias as a developer bias, in which a vibrating electric field is created in a developing area between the developer carrier and the photoreceptor and charged toner is applied on the photoreceptor, is known.
For the ac voltage which is superimposed on dc voltage, a rectangular waveform as in FIG. 44, a sine waveform as in FIG. 45, a triangular waveform as in FIG. 46, or a duty bias as shown in FIG. 47 is used.
In a case of using the duty bias shown in FIG. 47, a ½ value of the waveform differs from an average value of time integral. In FIG. 47, such a bias as follows is used. That is, the bias includes a time required for application of the first peak value V1 and a time required for application of the second peak value V2. At the first peak value V1, an electric field is created such that the electric field is biased in a direction in which the toner is transferred from the developing sleeve to the photoreceptor. At the second peak value V2, an electric field is created such that the electric field is biased in a direction in which the toner is transferred from the photoreceptor to the developing sleeve.
In a case of using the duty bias, by optimizing a frequency of ac voltage, a duty ratio (=t1/(t1+t2)×100% in FIG. 47), and a peak-to-peak value i.e. a difference between a maximum value of ac voltage (V1) and a minimum value of ac voltage (V2), it is possible to deposit the toner efficiently on an image area of the photoreceptor or not to deposit the toner on non-image area of the photoreceptor. Moreover, the optimization also enables the adjustment for increasing the density of an image while improving the uniformity of toner dots identical to the latent image.
Among methods which use the other developer i.e. the one-component developer, a jumping developing is a known method. In the jumping developing, an electrostatic latent image on the photoreceptor is developed while the developing sleeve as the developer carrier of the developing unit and the photoreceptor are maintained in the non-contact state. In the jumping developing, a layer of the one-component developer is formed on the developing sleeve. More specifically, a magnetic roll having a plurality of south poles and north poles lined up alternately on it and facing the electrostatic latent image carrier, is fixed on the developing sleeve. Furthermore, a toner brush is formed in a developing area and the developer (toner) is splashed and applied on the photoreceptor by applying a developing bias obtained by superimposing an ac component on a dc component, to the developing sleeve. Fogged toner is then returned in the direction of the developing sleeve and the latent image is visualized as a toner image.
When the developer is a one-component developer, in the same manner as the two-component developer, the developing bias method is used. In the developing bias method, by varying the peak-to-peak value, frequency, and duty ratio, it is possible to deposit the toner efficiently on the image area of the photoreceptor or not to deposit the toner on the non-image area of the photoreceptor. Moreover, the image density is increased while improving the uniformity of toner dots.
Depending on a setting of the bias characteristics that is carried out in the charging unit and the developing unit, noise is caused by applying ac voltage during shifting of the photoreceptor. Following is a reason for the generation of noise. A lighter weight conductive material in cylindrical form is used for the photoreceptor. Concretely, an aluminum cylinder having thin walls is used for the photoreceptor with a structure that resonates easily. Besides, not only units used in charging and developing processes are disposed facing the photoreceptor, but units for carrying out writing, transferring, and cleaning processes are also disposed facing the photoreceptor. In particular, the unit that carries out the cleaning process is disposed close to the photoreceptor, other than the units for charging process and developing process. Therefore, the photoreceptor can resonate easily due to vibrating electric field created when ac voltage is applied. Furthermore, due to a cleaning blade of the cleaning unit that is in contact with the photoreceptor, the vibrations are generated in the thin-walled cylinder due to repetition of deformation and restoration of shape of the cleaning blade when the cleaning blade scrapes the photoreceptor, and resonance in the photoreceptor produces noise.
That is, the image forming apparatuses like a copying machine, a printer, a facsimile, or a multifunction machine including any functions of these have been known widely. The image carrier drum includes either of a photoreceptor drum on surface of which a toner image is formed by charging, exposing, and developing and an intermediate transfer drum on surface of which a toner image is transferred from the photoreceptor and formed. The image carrier drum vibrates due to an external force that imparts vibrations, thereby resulting in generation of noise from the image carrier drum. For example, image forming units like a charging unit and a cleaning unit are provided around the photoreceptor drum. The charging member vibrates due to effect of ac voltage applied to the charging unit. The charging member vibrates due to stick-slip which is caused by the cleaning blade that is in pressed contact with the surface of the image carrier drum. The stick-slip starts as the image carrier drum rotates. The vibrations are transmitted to the image carrier drum to make the image carrier drum vibrate, and to thereby generate noise. A user may feel unpleasant because of noise. Therefore, measures have been taken in conventional techniques by providing the damper inside the image carrier drum to minimize vibrations of the image carrier drum to reduce the noise.
On the other hand, an image forming apparatus explained below has been in practical use to enable conservation of energy. The image forming apparatus uses a toner having a low melting point, and is structured such that a transferred toner image can be fixed on the recording medium at comparatively low temperature. However, a case of using the toner having a low melting point tend to generate noise easily as compared to a case of using a toner having a high melting point. Therefore, it is found that the conventional damper is unable to reduce the noise sufficiently. It is not sure that the use of the toner having a low melting point increases the noise. However, additives like wax or the like contained in the toner tend to stick to the surface of the image carrier drum. Since the amount of the additive that is deposited becomes non-uniform depending on an image pattern, a component like the cleaning blade in contact with the surface of the image carrier drum does not move uniformly. It is considered that loud noise that is generated in the image carrier drum is due to vibrations caused by non-uniform movement of the cleaning blade.
A structure in which the photoreceptor is made solid i.e. a solid cylinder has been disclosed, for example, in Japanese Patent Application Laid Open Publication (“JPA”) No. HEI 07-72641, as the conventional structures to reduce the noise. Furthermore, a structure in which at least two of an elastic body and a cylinder member are fitted inside the photoreceptor and resonance in peripheral wall of the thin-walled cylinder is reduced has been disclosed in JPA No. HEI 11-184308, for the same purpose.
Moreover, there is another structure made by using a cylinder unit in which the damper is inserted inside the cylinder to reduce vibrations of the cylinder and therefore the noise is minimized. This type of structure has been disclosed in JPA No. HEI 11-35167 and JPA No. HEI 10-97158.
In recent years, products which can be recycled are promoted with an object of protection of environment and saving of resources. Same thing is expected about the cylinder unit. To have better recycling of a product formed by a plurality of components, it is necessary that the product be structured in such a way that each component of the product can be dismantled easily after the product is used and the dismantled component can be reused or can be reprocessed. However, in the conventional cylinder unit, the damper inserted inside the cylinder is fixed to the cylinder and therefore it is difficult to remove the damper from the cylinder. Thus, the conventional cylinder unit is found difficult to be recycled.
However, in the structure for prevention of noise, increase in cost of the photoreceptor and complications in structures are matters of concern. When the photoreceptor is structured using a solid body, it not only raises the cost but also increases weight. Due to increase in the weight, there is an increase in driving force required for rotation, which results in increase in inertial force. The increase in weight of the photoreceptor affects its portability, which may result in damaging the surface of the photoreceptor or causing an injury to a person due to heavy weight on dropping of the photoreceptor during replacement job. If a plurality of damping structures are provided inside the photoreceptor, there is a rise in cost due to the increased number of components and assembling processes.
Moreover, the image forming apparatus that uses the toner having a low melting point tends to generate noise easily as compared to the case of using the toner having a high melting point. Therefore, the conventional damper is unable to reduce the noise sufficiently.