1. Field of the Invention
This invention relates to a transfer roller for an image forming apparatus such as a copier, a facsimile, a printer, and/or a hybrid machine having a plurality of functions including functions of an image forming apparatus, and in particular, relates to a transfer roller having an elastic layer of even resistance along a width thereof and capable of evenly applying a bias voltage when a transfer process is executed.
2. Discussion of the Related Art
In a prior image forming apparatus, a transfer roller of relatively low quality is generally employed to transfer a toner image obtained by developing a latent image formed on a surface of a photo-conductive member to a sheet.
The prior transfer roller generally includes a metal core axis and an elastic layer having an electrically conductive powder blended therein, which is engaged with a circumference of the metal core axis. The prior transfer roller generally transfers the toner image onto a sheet while a bias voltage having a prescribed amount of potential related to the toner image is applied to the transfer roller. Since the elastic layer has the conductive powder, it may have an amount of resistance of a middle range. The resistance of the elastic layer is generally about 10.sub.8 ohms, considering efficiency of a toner transfer process.
When measuring the resistance of the elastic layer, the inventor of the present invention discovered the phenomenon described below.
When bias voltage is applied, electrical current flowing from a point on the surface of the metal core axis does not reach the closest point on the outer surface of the elastic layer. This occurs because the electrical current may disperse into the elastic layer, thereby increasing the number of electrical current paths. The increase in the number of electrical current paths may not be suppressed around a middle position of the width of the transfer roller.
However, the number of electrical current paths is suppressed around both edge portions of the transfer roller since the increase may only be allowed in an inner side direction at around both the edge portions. As a result, it is more difficult for the bias current to flow around both edges more than it is around the middle position of the width of the transfer roller. Accordingly, high resistance exists around both the edge portions.
A graph showing the results of a test measuring the resistance of the elastic layer of the prior transfer roller is illustrated in FIG. 8. The vertical axis of the graph shows the amount of resistance. The horizontal axis shows the distance from the edge of the elastic layer of the transfer roller. As illustrated in the graph, the amount of current flowing from the middle portion of the distance is almost even. The amount of the current flowing from both side portions, with each side portion ranging from about three or four centimeters from each edge of the elastic layer, is sharply increased.
Thus, when bias voltage is applied to the prior transfer roller using a constant-current-applying controller and so on, it is difficult for the current to flow from each of the side edge portions due to its high impedance and it is easy for the current to flow largely from the middle portion due to its low impedance. As a result, toner transfer efficiency is lowered at both the sides and is increased at the middle portion of the transfer roller, thereby resulting in an unevenness of the density of the toner image on the sheet.