An image forming apparatus that makes use of an electrophotographing system has been used as a high speed image forming apparatus. In the image forming apparatus, a toner image formed on an image forming member such as a photosensitive drum and the like is transferred onto a print medium. In the high speed image forming apparatus, since the print medium is transported at a high speed, a transfer mechanism is necessary to maintain a transfer performance.
FIG. 10 is a structural view of a transfer mechanism of a conventional image forming apparatus. A toner image is formed on a photosensitive drum 104 rotating in the direction of an arrow by a known electrophotographing system. A continuous sheet 100 acting as a print medium is transported by upper and lower tractors not shown. A pair of sheet guide members (referred to as transfer guides) 102 facing each other and each having a predetermined distance from the photosensitive drum 104 are disposed in the vicinity of a corona transfer unit 106 of a transfer section.
The continuous sheet 100 to be transferred is guided by the transfer guides 102 in the transfer section and bent by the curvature of the sheet guide members 102. At the time, the bent portion of the sheet 100 comes into intimate contact (nipped by) with the photosensitive drum 100 by the stiffness of thereof, and the toner image on the photosensitive drum 104 can be transferred by the charging executed by the corona electric charger 106.
The transfer characteristics of the sheet 100 greatly depend on the intimate contact property thereof with the photosensitive drum 104 which is determined by the stiffness of the sheet at that time. Further, when sheet is loaded or when print is stopped, the transfer guides 102 are retracted to the positions shown by dotted lines in the figure to prevent the pollution of the sheet 100 by preventing the bent portion of the sheet 100 from coming into contact with the photosensitive drum 104.
In contrast, recently, the image forming apparatus is required to handle various types of print mediums, and it is required to make print on such mediums as, for example, a thin sheet having an extremely small ream weight and a stepped medium having a health insurance card and the like attached on a surface thereof. Further, an increase in a print speed reduces a margin to the stiffness of a sheet for obtaining desired transfer characteristics.
Accordingly, in a sheet having weak stiffness (for example, a thin sheet having a ream weight less than 45 kg) has an insufficient press force (intimate contact force) to the photosensitive drum 104, thereby the transfer performance is deteriorated. For example, transfer omission is caused in perforated tear lines of a sheet. On the contrary, in a sheet having strong stiffness (for example, a thick sheet having a ream weight of 135 kg or more), since the press force (intimate contact force) to the photosensitive drum 104 is excessively large, a load between the photosensitive drum and the sheet is increased, thereby a trouble occurs in the transportation of the sheet, from which a secondary trouble such as the detachment of the sheet from the tractors, and the like is caused.
Heretofore, the following methods have been proposed as a method of preventing the variation of the transfer characteristics resulting from the diversification of the print mediums.
A first method is to use a transfer roller as a transfer unit and to change the nip width of a sheet by the press force of the transfer roller. That is, the press force of the transfer roller is increased to a thin sheet so that an amount of bite is increased by increasing the nip width, whereas the press force of the transfer roller is reduced to a thick sheet so that the amount of bite is reduced by decreasing the nip width.
A second method is to change the position at which a sheet begins to come into contact with a photosensitive drum so that a thin sheet is caused to come into contact with the photosensitive drum more upstream of a transfer section (for example, Japanese Patent Application Laid-open No. 6-348152).
In the conventional arts described above, the press force of a sheet to the photosensitive drum is optimized by increasing or decreasing an amount of bite of nip, by which the nip width of the sheet is changed.
It seems that, in the relationship between the circumferential speed of a photosensitive drum and a sheet transportation speed, a constant speed difference is maintained at all times when it is examined from a macroscopic viewpoint. However, when this relationship is examined from a microscopic viewpoint, it generally has an error component due to the jitters of a drum motor and a sheet transportation motor and to the fluctuation of the rotation number of the drum.
Accordingly, when the nip width is increased, that is, when the portion where the photosensitive drum is in contact with the sheet is increased, the error component is increased in proportion to the increase in the contact portion, which may result in the promotion of transfer displacement.
FIGS. 11 and 12 are views illustrating the result of measurement of a nip width and an amount of transfer displacement. First, as shown in FIG. 11, as to the nip width, after the sheet 100 is loaded on the sheet guide members 102, a carbon sheet 108 is inserted between the photosensitive drum 104 and the transfer section in the state that the sheet 100 is fixed, and only the photosensitive drum 104 is rotated. Next, the rotation of the photosensitive drum 104 is stopped, and the width of the carbon transferred onto the sheet 100, that is, the nip width is measured. With the above operation, the nip width can be measured at the positions of the transfer guides.
Next, the sheet guide members 102 are raised using spacers and the like and moved in the directions of arrows in FIG. 11, and the nip width is measured by inserting a carbon sheet 108 at the positions likewise. With the above operations, the positions of the transfer guides corresponding to respective nip widths are obtained.
Then, a toner image of one-dot line is formed on the photosensitive drum 104 in an auxiliary scan direction at each of the positions of the transfer guides, and the toner image is transferred onto the sheet by transporting the sheet. The thickness of the one-dot line on the sheet, onto which the toner image has been transferred, is measured with a dot analyzer, and an amount of transfer displacement is measured.
FIG. 12 is a graph showing the result of measurement of an amount of transfer displacement with reference to a nip width, in which the lateral axis shows the nip width (mm), and the vertical axis shows the amount of transfer displacement (mm). As shown in FIG. 12, it can be found that the amount of transfer displacement is increased by the increase of the nip width. For example, although a nip width of 5 mm results in a line width (0.3 mm), a nip width of 15 mm results in a line width (0.8 mm) that is about 2.6 times as large as the above line width, by which print quality is deteriorated.