Magnetic developing process and toner containing high coercive force magnetic powder

Claim 1

In a process for developing latent electrostatic images with a single component toner consisting essentially of a resin which is solid at ambient temperature and rendered molten under heating, coloring material, and a ferromagnetic powder, the toner being used in a developing apparatus having a latent image-bearing material for carrying the latent images and a magnet roll which includes a cylindrical rotatable shell and a rotatable permanent magnet member positioned coaxially within the shell, the permanent magnet member having a plurality of adjacent axially extending magnetic poles, the process including the steps of (a) attracting the toner to the surface of the magnet roll shell; (b) conveying the single component magnetic toner on the shell surface to the latent image-bearing material having the electrostatic latent images previously disposed thereon by rotating at least one of the shell and the permanent magnet member to form a magnet brush of the single component magnetic toner; (c) inducing an electric charge on the magnetic toner in the magnet brush on the shell surface adjacent the latent image-bearing material by subjecting the brush to the electric field due to the electric charge of the electrostatic latent images; and (d) rubbing the image-bearing material by the magnet brush to adhere the charged magnetic toner onto the electrostatic latent images on the image-bearing material by the attraction force between the charge induced in the toner and that of the electrostatic latent images, the improvement comprising the initial step of selecting the magnetic toner and the permanent magnet member in respect to one another according to the following relation: (i) the permanent magnet member to produce a magnetic field strength of at least about 400 gauss at its peak value on the shell surface; (ii) the magnetic toner to comprise at most about 55%, by weight, of ferromagnetic powder; and (iii) the magnetic toner to have saturated magnetic flux density 4πIs of a value between about 300 and 1,000 gauss and a magnetic property, in the relation between the saturated magnetic flux density 4πIs and the coercive force iHc of the magnetic toner, lying within the region on a graph of coercive force iHc, in oersteds, vs. saturation flux density 4πIs, in gauss, above a line connecting a point where iHc is 1,000 oersted for 4πIs at 300 gauss and a point where iHc is 200 oersted for 4πIs at 300 gauss and below a line connecting a point where iHc is 400 oersted for 4πIs at 300 gauss and a point where iHc is 400 oersted for 4πIs at 1,000 gauss.