Abstract:
An image forming apparatus having a photoconductive drum to form an electrostatic latent image corresponding to a predetermined image by a laser beam scanned after being electrified to a predetermined electric potential, and a developing roller to rotate together with the photoconductive drum having a developing gap therebetween and to transfer a developer, which is a mixture of a toner and a carrier, to the photoconductive drum to form a toner image on the electrostatic latent image, wherein a linear velocity ratio of a linear velocity of the developing roller to a linear velocity of the photoconductive drum is provided at a predetermined ration, which provides maximum quality of the toner image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2003-70963, filed Oct. 13, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an image forming apparatus to develop an image using a two-component developer, and particularly, to an image forming apparatus capable of controlling a linear velocity ratio of a photoconductive drum and a developing roller.  
         [0004]     2. Description of the Related Art  
         [0005]     A general image forming apparatus, such as electrophotographic laser printer, forms an electrostatic latent image on a photoconductive medium (drum) using an exposure optical system, forms a toner image by developing the electrostatic latent image with a developing apparatus, and fuses the toner image transferred onto a recording paper.  
         [0006]     The developer used for the developing apparatus is divided into a one-component developer and a two-component developer. In a case of the one-component developer, the toner particles are electrified by friction among themselves or by friction with a proper electrification member.  
         [0007]     The two-component developer is a mixture of magnetic carrier particles and synthetic resin nonmagnetic toner particles, being mixed in a proper ratio. The toner particles are electrified while being mixed with the carrier particles. Thus, the electrified toner particles are transferred to a developing roller together with the carrier, and then adhere to an electrostatic latent image area on a surface of the photoconductive medium to form the toner image.  
         [0008]     Meanwhile, a developing method using the two-component developer has been developed. A developing apparatus using the two-component developer comprises a photoconductive drum, a developing roller (a magnet roller) which rotates while maintaining a predetermined developing gap with the photoconductive drum, a doctor blade for cutting the two-component developer attached to a surface of the developing roller in a certain thickness, and a mixer for mixing the two-component developer.  
         [0009]     In the above structure, the mixing ratio of the toner to the carrier (T/C) in the two-component developer is less than 5%. That is, if a high-speed printing is performed in a higher T/C toner ratio than 5%, the toner particles scatters and become afloat inside of the image forming apparatus, and therefore, peripheral parts beomces contaminated.  
         [0010]     If the T/C toner ratio is decreased, an image density is accordingly decreased. In order to compensate for this problem and to implement a desired image density, in general, the developing apparatus is designed such that a ratio of a linear velocity of the photoconductive drum to a linear velocity of the developing roller is not less than 1 to 2.5. That is, the linear velocity ratio of the developing roller to the photoconductive drum is increased so that the T/C toner ratio is controlled to remain low. Then, during the high-speed printing, the image density can be prevented from decreasing.  
         [0011]     However, in the above developing system, if the linear velocity ratio of the developing roller to the photoconductive drum is increased to much, a back portion of the toner image can be torn off with respect to a rotating direction of the photoconductive drum at a certain-thickness font, which is so-called a ‘brush mark’.  
         [0012]     More specifically, as shown in  FIG. 1 , a toner T adhering to a carrier C, which is electrified to a ‘+’ potential at the developing roller, is transferred to a toner image area  2   a  of the photoconductive drum  2 , which is electrified to a ‘−’ potential, to form the toner image. At this time, if a linear velocity of the developing roller  1  to the photoconductive drum  2  is increased, a ‘−’ potential of a non-image area  2   b  of the photoconductive drum  2  is increased by a heat increase of the photoconductive drum  2 . Furthermore, the ‘+’ potential of the carrier C on the developing roller  1  increases. Accordingly, the carrier C on the developing roller  1  is pulled to the non-image area  2   b  of the photoconductive drum  2 , which has an increased ‘−’ potential. In this process, the carrier C is mashed and scattered about a back portion of the toner T attached to the image area  2   a  (‘A’ area in  FIG. 1 ).  
       SUMMARY OF THE INVENTION  
       [0013]     In order to overcome the above-mentioned and/or other problems, it is an aspect of the present general inventive concept to provide an improved electrophotographic laser printer capable of controlling a linear velocity ratio of a developing roller and a photoconductive drum to improve quality of a toner image.  
         [0014]     The above-described and/or other aspects of the present general inventive concept can be achieved by providing an electrophotographic laser printer that can include a photoconductive drum to form an electrostatic latent image corresponding to a predetermined image by a laser beam scanned after being electrified to a predetermined electric potential, and a developing roller to rotate together with the photoconductive drum having a developing gap therebetween and to transfer a developer, which is a mixture of a toner and a carrier, to the photoconductive drum to form a toner image on the electrostatic latent image, wherein a linear velocity ratio S of a linear velocity Vm of the developing roller to a linear velocity Vo of the photoconductive drum is 1.70 to 1.75.  
         [0015]     In an aspect of the general inventive concept, the developing gap can be 0.73 mm to 0.76 mm.  
         [0016]     In another aspect of the general inventive concept, the electrophotographic laser printer may further include a mixer to mix the toner and the carrier into the developer, a blade to cut the developer transferred to the developing roller into a predetermined thickness and to be mounted at a predetermined distance from the developing roller. It is another aspect a gap between the blade and the developing roller is 0.75 mm to 0.80 mm.  
         [0017]     In another aspect of the general inventive concept, an outer circumference of the photoconductive drum may not be more than 30 mm, and an outer circumference of the developing roller may not be more than 25 mm.  
         [0018]     In another aspect of the general inventive concept, the developer which is a mixture of the carrier and the toner, may have a 5% to 8% mixing ratio of the toner with respect to the carrier.  
         [0019]     In another aspect of the general inventive concept, the photoconductive drum and the developing roller can rotate in an opposite direction to each other.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0021]      FIG. 1  schematically shows a state that a toner image is damaged in a conventional electrophotographic printer;  
         [0022]      FIG. 2  is a schematic view showing an electrophotographic laser printer according to an embodiment of the present general inventive concept; and  
         [0023]      FIG. 3  is a view showing the main parts of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
         [0025]      FIG. 2  is a schematic structure view explaining an image forming apparatus, such as an electrophotographic laser printer, according to an embodiment of the present general inventive concept. In  FIG. 2 , a photoconductive drum  10  may be mounted as a photoconductive medium to rotate clockwise. An electrifying roller  12  and a cleaning roller  13  to clean the electrifying roller  12  can be mounted to rotate while contacting each other. A laser-scanning unit  14  can scan a laser beam on a surface of the photoconductive drum  10  electrified to a predetermined electric potential by the electrifying roller  12  and forms an electrostatic latent image.  
         [0026]     At a lower part of the printer with respect to the photoconductive drum  10 , a developing unit  20  can be provided to develop a two-component toner on the electrostatic latent image. At an upper part of the printer, a waste toner collecting unit  30  can be provided to remove and collect the toner remaining on the photoconductive drum  10 . Here, the waste toner collecting unit  30  may include a waste toner cartridge  31 , a cleaning blade  32 , a collecting roller  33 , and an auger  34 . A structure of the waste toner collecting unit  30  is generally known, and therefore a detailed description thereof will be omitted.  
         [0027]     The developing unit  20  may include a developing cartridge  21  to receive the two-component developer D, a supplying roller  22 , a mixer  23 , a developing roller  24 , and a blade  25 .  
         [0028]     The two-component developer D can be made by mixing magnetic carrier particles and nonmagnetic toner particles in a predetermined mixing ratio. According to this embodiment of the present general inventive concept, a mixing ratio T/C of the toner T with respect to the carrier C may be 5% to 8%. Thus, if the ratio T/C of the toner T to the carrier C is controlled to be lowered, the toner can be prevented from scattering and becoming afloat in an inside of the printer even during a high-speed printing. In addition, according to a characteristic structure of this embodiment, a desired image density can be obtained even if the T/C ratio is lowered.  
         [0029]     The developer D of the above mixing ratio can be supplied from the supplying roller  22  to the mixer  23 . The mixer  23  can rotate to mix the toner T and the carrier C. In this process, the carrier C and the toner T can be electrified by a friction of the carrier C and the toner T For example, the carrier C can be electrified to a ‘+’ potential, and the toner T can be electrified to a ‘−’ potential. In an aspect of the general inventive concept, the carrier C and the toner T can be electrified alternatively, for example, by changing a material thereof. In this embodiment, the carrier C can be electrified to the ‘+’ potential, and the toner T can be electrified to the ‘−’ potential. Meanwhile, the toner T particles electrified to the ‘−’ potential can adhere to a surface of the carrier C particles which are electrified to the ‘+’ potential. Therefore, the developer D can maintain a regular mixing ratio. The mixed developer D can adhere to an outer circumference of the developing roller  24 .  
         [0030]     The developing roller  24  may be a magnet roller including a magnet  27  therein, and made of an electrically conductive metal. The developing roller  24  can be directed toward the photoconductive drum  10  with a predetermined developing gap G 1 , as shown in  FIG. 3 , and can rotate in an opposite direction to the photoconductive drum  10 , that is, counterclockwise. Accordingly, the developer D including the magnetic carrier C can adhere to the surface of the developing roller  24  by a magnetic force of the magnet  27 . In an aspect of the general inventice concept, the developing roller  24  and the photoconductive drum  10  can be electrified to the ‘−’ potential of a predetermined level.  
         [0031]     The blade  25  can cut the developer D adhered to the developing roller  24  into a predetermined thickness. Therefore, the blade  25  can be mounted to have a predetermined gap G 2 , and the predetermined gap G 2  can range from 0.75 mm to 0.80 mm. Accordingly, on a surface of the developing roller  25  that has passed the blade  25 , the developer D-can adhere to the developing roller  25  to form a layer having a thickness of approximately 0.75 mm to 0.80 mm.  
         [0032]     Additionally, the developing gap G 1  between the developing roller  24  and the photoconductive drum  10  may be 0.73 mm to 0.76 mm. That is, the developing gap G 1  can be smaller than the gap G 2  such that the thickness of the cut developer D is larger than the developing gap G 1  to enable a normal development.  
         [0033]     Furthermore, in the above structure, the photoconductive drum  10  and the developing roller  24  may have small outer diameters as much as possible to realize a compact-sized developing unit  20  and a printer. It has been determined by experiments by the inventor herein that the photoconductive drum  10  should have an outer diameter of not more than 30 mm, and that the developing roller  24  should have an outer diameter of not more than 25 mm according to an embodiment of the general inventive concept.  
         [0034]     In addition, in order to overcome a ‘brush mark’ which is a problem of a conventional printer, a photoconductive drum linear velocity Vo and a developing roller linear velocity Vm should have a ratio of 1:1.70˜1.75. In other words, when the linear velocity ratio S, which is Vm/Vo, is 1.70/1 to 1.75/1, the ‘brush mark’ can be prevented and the desired image density can be obtained. Especially, in a case that an outer circumference of the photoconductive drum  10  is 30 mm, an outer circumference of the developing roller  24  is 25 mm, the developing gap G 1  is 0.73 mm to 0.76 mm, and the gap G 2  is 0.75 mm˜0.80 mm, the best image quality can be obtained.  
         [0035]     Table 1 below is a quality evaluation of the toner image which is developed on the photoconductive drum  10  according to the linear velocity S.  
                           TABLE 1                                   velocity ratio (S)   Image evaluation (brush mark)                           1:1.5   X           1:1.7   □            1:1.75   ◯           1:1.9   X           1:2.0   X            1:2.25   XX                      
 
         [0036]     In the above Table 1, the image evaluation was accomplished by measuring an image area through an optical microscope. 
        O: The brush mark of the image is not found.     □: The brush mark of the image partially occurs.     X: The brush mark of the image apparently occurs.        
 
         [0040]     According to the Table 1, when the velocity S is controlled to be 1.70 to 1.75, a high-quality image can be obtained without the brush mark occurred.  
         [0041]     As described above, according to an embodiment of the electrophotographic laser printer, an image density deterioration and a brush mark appearance can be prevented by controlling a linear velocity ratio of a photoconductive drum and a developing roller to a predetermined value without increasing the ratio T/C.  
         [0042]     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.