The present invention relates to an electrophotographic apparatus represented by, for example, a laser printer and, in particular, to a charging device for electrophotographic which electrically charges a recording sheet.
In the electrophotographic apparatus, a reversal development method has been widely adopted as well known in the art. This reversal development method has steps of electrically charging a surface of a photosensitive drum, selectively canceling the charge on the surface of the photosensitive drum by a laser beam to form an electrostatic latent image (negative latent image), and supplying toner particles to the surface of the photosensitive drum to develop the latent image into a toner image. The toner particles are charged with the same polarity as that of the photosensitive drum on a development roller and supplied to the surface of the photosensitive drum. The toner image is formed of toner particles adhered to no charge or less charge area of the charged surface. The development roller is directly contacted with the photosensitive drum to increase the strength of an electric field between the development roller and the no charge or less charge area of the charged surface of the photosensitive drum.
In addition, to electrically charge the surface of the photosensitive drum, a method of bringing a charging brush into contact with the surface of the photosensitive drum is widely used.
A typical electrophotographic apparatus comprises, as shown for example in FIG. 1, a photosensitive drum 31, charging brush 34, cleaning blade 35, and developing section 38. The developing section 38 includes a supply roller 42 and a developing roller 43. The charging brush 34 is arranged parallel to the shaft of the photosensitive drum 31 and set to be in contact with the surface of the photosensitive drum 31. The charging brush 34 is formed in a roller-shape by attaching many conductive fibers 52 to one surface of a belt-like cloth 53 and helically winding the cloth 53 around a rotary shaft 51 of a metal such as iron with a double-sided adhesive tape interposed. The fibers 52 are attached as piles to the cloth 53. In the case where the surface of the photosensitive drum 31 is electrically charged, the charging brush 34 is rotated together with the photosensitive drum 31, for example, in a direction opposite to the rotational direction of the photosensitive drum 31 at the same circumferential speed as that of the photosensitive drum 31.
However, the contact charging system using the charging brush 34 poses a problem as will be set out below. That is, since the charging brush 34 is so formed that the belt-like cloth with many fibers 52 attached thereto as shown in FIG. 2 is helically wound around the rotation shaft 51, a gap is created between each turn of the cloth 53. Further, even if the cloth 53 is so wound as to leave no gap between each turn, a helical gap (helical turn) is formed along a boundary of each turn because the fibers 52 are not attached to a full length from end to end in a width direction. When, as shown in FIG. 1, charging is made using the charging brush 34 of the above-mentioned structure, the fibers 52 is set in contact with part of the surface of the photosensitive drum 31 and not with the remaining part. Since charging is made in this manner, the amount of charge in the surface of the photosensitive drum 31 is not constant.
In the case where, therefore, a half-tone image is recorded on a recording sheet, there is a tendency that the density becomes higher at an area corresponding to the part not contacted with the fibers of the charging brush 34 and lower at the other area. This uneven density imparts a greater influence to an image quality and causes the deterioration of the image. Further, less area of the fibers 52 of the charging brush 34 is contacted with the surface of the photosensitive drum 31, thus leaving no contact between the surface of the photosensitive drum 31 and the fibers 52. This makes it difficult to uniformly charge the surface of the photosensitive drum 31, causing the surface potential to be irregular.
With the charging section using the conventional charging brush 34, it is not possible to constantly obtain a high quality image, thus presenting a problem to be solved.