1. Field of the Invention
This invention relates to a non-contact developing method for forming a toner image on a latent image element such as a photosensitive drum, and to that developing device and image formation device, and more particularly to a non-contact developing method, image developing device and image formation device which applies a superimposed DC and AC voltage as a developing bias voltage, to fly the toner and to form a latent image on the latent image element.
2. Description of the Related Art
In recent years, electronic photographic equipment, and particularly electronic photograph printers are desired that are fast and capable of high image quality. Therefore, a non-contact developing method has been proposed. A non-contact developing method maintains a non-contact state between the photosensitive drum and the developing roller of the developing unit, while causing the toner on the developing roller to fly to the photosensitive drum to develop the static-electrical latent image on the photosensitive drum.
In order to develop an image in a non-contact state, it is necessary to make the toner on the developing roller to fly to the photosensitive drum. In order to do this, a bias voltage is applied between the developing roller and the photosensitive drum. The applied voltage is a voltage having an AC voltage superimposed on the DC offset voltage. By applying this voltage, the toner vibrates between photosensitive drum and developing roller and adheres to the latent image on the photosensitive drum to develop the image.
This method is a non-contact method, so it is possible to speed up the image-formation process, and it is possible to prevent the developing agent on the developing roller from damaging the toner image on the photosensitive drum, thus high-speed printing with high image quality is possible. This method of applying a AC electric field has been disclosed in Japanese Unexamined Published patents No. H6-19213, H7-72699, and H7-114223.
However, there is a problem that when an AC electric field having a frequency as proposed by the prior art (for example 6000 Hz) is applied, the image quality is good during the first printing, however the image quality decreases as printing continues. In other words, the particle size of the toner of the developer is not uniform and is somewhat distributed. Generally, for toner having a large particle size, the amount of charge is low, and for toner having a small particle size, the amount of charge is high.
The Coulomb force F for the electric field E in the space between the developing roller and the photosensitive drum is F=qE, so it becomes easier for highly charged small toner particles to fly to the photosensitive drum. Therefore, the smaller the particle size of the toner is, the faster the toner of the developer is consumed, and the large particle sized toner remains. Therefore, as the number of prints to be made increases, the particle size of the toner used for developing becomes larger. This is called the selective developing phenomenon. Since the resolution drops as the quantity of printing increases, there is the problem that the image quality decreases.
In order to prevent the selective developing phenomenon, it is necessary to make sure the particle size of the toner that is put into the developing unit is uniform. However, it is difficult to find toner with a uniform particle size. Especially, with the recent improvement of image quality, the average particle size is 10 microns or less. It is nearly impossible to make uniform toner with this kind of minute particle size.
The objective of this invention is to provide a non-contact developing method, a non-contact developing device and image-formation device that is capable of effectively preventing the selective developing phenomenon from occurring.
Another objective of this invention is to provide a non-contact developing method, a non-contact developing device and image-formation device that utilizes the flying characteristics of the toner to prevent the selective developing phenomenon from occurring.
A further objective of this invention is to provide a non-contact developing method, a non-contact developing device and image-formation device which pays attention to the fact that large particle sized toner does not lose its high speed due to air resistance, and effectively utilizes the flying characteristics of large particle sized toner to prevent the selective developing phenomenon from occurring.
Yet another objective of this invention is to provide a non-contact developing method, a non-contact developing device and image-formation device that prevents the selective developing phenomenon from occurring by setting the frequency of the AC component.
In order to achieve this objective, the non-contact developing method of this invention is a method of flying toner from the developer carrying unit to the latent image bearing element by applying a developing bias voltage with an AC component superimposed on a DC component to develop the static-electric latent image on the latent image bearing element. In addition, the frequency f of the AC component is set with in a range such that the selective developing of the toner does not occur.
The non-contact developing device of this invention comprises a developer carrying unit for carrying the toner, and means for applying a developing bias with an AC component superimposed on a DC component to the developer carrying unit, in order to fly the toner from the developer carrying unit to the latent-image bearing element to develop the static-electric latent image on the latent-image bearing element. In addition, the frequency f of the AC component is set with in a range such that the selective developing of the toner does not occur.
The image-formation device of this invention comprises a latent-image bearing element, a developer carrying unit for carrying the toner, and means for applying a developing bias with an AC component superimposed on a DC component to the developer carrying unit, to fly the toner from the developer carrying unit to the latent-image bearing element to develop the static-electric latent image on the latent-image element. In addition, the frequency f of the AC component is set with in a range such that the selective developing of the toner does not occur.
The inventors of this invention studied the relationship between the frequency of the AC component and the flying speed of the toner, and found that the flying speed of the toner varied according to the particle size of the toner. It was found that when the flying speed of toner with a large particle size becomes faster than the flying speed of toner with a small particle size, it becomes easier for the large article sized toner with the small charged amount to fly and thus it is possible to prevent selective developing from occurring.
It was found that the ratio between the flying speed of toner having large-sized particles and that of toner having small-sized particles changes according to the frequency of the AC voltage, so by setting the frequency xe2x80x98fxe2x80x99 of the AC component with in a range such that selective developing of the toner does not occur, it is possible to effectively prevent the selective developing phenomenon from occurring. Only the frequency xe2x80x98fxe2x80x99 of the AC component needs to be set, so the selective developing phenomenon can be prevented without having to adjust the particle size of the developing agent.
In another form of the non-contact developing method of this invention, the frequency range is 5xc3x9710*v less than =f less than =1.36*xcex7/(r2* xcex4). Here, v is the process velocity of the latent image bearing element, r is the radius of the toner particles, n is the viscous resistance of air, and xcex4 is the density of the toner particles.
In this form of the invention, the upper limit of the frequency is set such that the velocity ratio between the flying speed of toner with large-sized particles and the flying speed of toner with small-sized particles is a fixed value or greater, and the lower limit is set in a range such that no variation in print density occurs. Therefore, no variation in print density occurs and it is possible to prevent selective developing from occurring.
In the non-contact developing method of another form of the invention, the frequency xe2x80x98fxe2x80x99 of the AC component is set such that the velocity ratio of toner with a particle size that is 1.5 times that of the average particle size, is 1.1 times or more with respect to toner with an average particle size.
In this form of the invention, the particle size distribution of normal toner is within the range of about 1.5 times that of the average particle size, so with a velocity ratio of this toner having 1.5 times particle size with the average particle size 1.1 times or more, it is possible to effectively prevent selective developing from occurring.
In the non-contact developing method of yet another form of the invention, the frequency xe2x80x98fxe2x80x99 of the AC component is set such that the amount of movement of latent-image bearing element during one cycle of the AC component is 0.2 mm or less.
In this form of the invention, when one cycle of the AC component is long (in other words, when the frequency is low), variation in print density occurs, so the lower limit is set such that no variation in print density occurs. Therefore, it is possible to prevent selective developing from occurring without the occurrence of variation in density.