Magnetic toner, and developing apparatus and image forming apparatus using it

Abstract

A magnetic toner contains, as a magnetic powder, magnetic iron oxide having an octahedral particle shape and an average particle diameter of 0.21 μm or larger and containing 18% or more by weight of FeO. The magnetic toner is so prepared as to have a bulk density A (g/cm 3 ) and a magnetic powder content B (% by weight) that fulfill the formulae 0.8≦A/B×100≦1.6 and 35≦B≦50. This alleviates the reddishness of images produced with the magnetic toner, and alleviates the disturbance of a thin toner layer on a developing sleeve and thus the fogging resulting therefrom.

What is claimed is:  
     
         1 . A magnetic toner containing a binder resin and a magnetic powder, 
 wherein the magnetic powder is magnetic iron oxide having an octahedral particle shape and an average particle diameter of 0.21 μm or larger and containing 18% or more by weight of FeO, and    the magnetic toner has a bulk density A (g/cm 3 ) and a magnetic powder content B (% by weight) that fulfill the following formulae (1) and (2):   0.8 ≦A/B× 100≦1.6  (1)35 ≦B≦ 50  (2)   
     
     
         2 . A magnetic toner as claimed in  claim 1 , 
 wherein the magnetic powder has an average particle diameter of 0.30 μm or smaller.    
     
     
         3 . A developing apparatus comprising a developer supporting member for supporting a developer and for transporting the developer to a developer section and a developing blade arranged at a predetermined distance from the developer supporting member so as to limit an amount of the developer transported to the developing section, 
 wherein the developing blade is fitted with a magnet having a magnetic force of 400 to 800 gausses, and the developer is a magnetic toner as claimed in  claim 1 .    
     
     
         4 . A developing apparatus as claimed in  claim 3 , 
 wherein the distance between the developer supporting member and the developing blade is in a range of from 0.2 to 0.4 mm.    
     
     
         5 . A developing apparatus as claimed in  claim 3 , 
 wherein the developer supporting member has a surface roughness in a range of from 3.0 to 5.5 μm.    
     
     
         6 . A developing apparatus as claimed in  claim 3 , 
 wherein the developer supporting member is formed out of stainless steel.    
     
     
         7 . An image forming apparatus comprising an image supporting member, charging means for charging a surface of the image supporting member with electric charge, exposing means for irradiating the charged surface of the image supporting member with light to form an electrostatic latent image, developing means for developing the electrostatic latent image by feeding a developer thereto, and transferring means for transferring the developer on the image supporting member developed by the developing means to a transferred-image member, 
 wherein the developing means is realized with a developing apparatus as claimed in one of claims  3  to 6.    
     
     
         8 . An image forming apparatus as claimed in  claim 7 , 
 wherein a distance between the image supporting member and the developer supporting member is greater than a thickness of a thin layer of the magnetic toner formed on the developer supporting member, and development of the electrostatic latent image is achieved by feeding the developer thereto with an alternating-current bias voltage applied between the image supporting member and the developer supporting member.    
     
     
         9 . An image forming apparatus as claimed in  claim 7 , 
 wherein, in the developing means, development of the electrostatic latent image is achieved by letting the magnetic toner charged with electric charge of a same polarity as an unexposed portion of the image supporting member attach to an exposed portion thereof.    
     
     
         10 . An image forming apparatus as claimed in  claim 7 , wherein the image supporting member is an amorphous silicon photoconductor.