1. Field of the Invention
The present invention relates to a method for manufacturing an open cell rubber material from a continuous cell rubber material.
The present invention also relates to a method for manufacturing a rubber roller having a low rubber hardness.
Moreover, the present invention relates to a method for manufacturing a pressure roller used in a heat roll fixing device and in particular a pressure roller having a low rubber hardness and a method for manufacturing the same.
2. Description of the Related Art
In a copier or printer, a heat roll fixing device is used to fix a non-fixed toner image. The heat roll fixing device is comprised of a heat roller having therein a heater and a pressure roller having an elastic rubber layer. The heat roller and the pressure roller are in press-contact with each other through a press contact mechanism. The pressure roller is comprised of a metal shaft, an elastic rubber layer formed on the outer peripheral surface of the metal shaft and a fluororesin tube which surrounds the elastic rubber layer. As the pressure roller rotates in contact with the heat roller having a high temperature, the elastic rubber layer must be heat-resistant and, to this end, is made of a silicone rubber. In this heat roll fixing device, a non-fixed toner image transferred onto a recording paper is molten and fixed onto the recording paper during passage through a nip at which the heat roller and the pressure roller come into contact with each other. In order to enhance the fixing efficiency of the heat roll fixing device, it is preferable that the nip width, i.e., the contact width of the heat roller and the pressure roller be as large as possible. The nip width can be increased by increasing the pressing force of the press contact mechanism of the heat roller and the heat roller. However, an increase in the pressing force invites bending of the pressure roller, thus leading to failure to form a nip having a uniform width in the axial direction of the roller. If the rubber hardness of the elastic rubber layer of the pressure roller is reduced, a wide nip width can be obtained even by a relatively low pressing force. Therefore, there has been need of development of a pressure roller having a heat resistant elastic rubber layer of a low rubber hardness.
In a heat roll fixing device, an oil supply roller is used to supply silicone oil to the heat roller to thereby enhance the releasing efficiency of the heat roller. As the oil supply roller requires a high oil containability, an open cell urethane rubber roller having a large number of pores is used.
Furthermore, in an electro photographic copier or printer, a developing roller is provided adjacent to a photosensitive drum to develop an electrostatic latent image formed on the photosensitive drum. A toner is supplied to the developing roller through the toner supply roller. In the toner supply mechanism, if the hardness of the toner supply roller is high, an undesirable friction occurs between the toner supply roller and the developing roller, so that the toner cannot be stably supplied. Therefore, the toner supply roller requires an elastic rubber layer having a lower hardness to stably supply a predetermined amount of toner to the developing roller.
In order to reduce the hardness of the silicone rubber layer used for the pressure roller of a fixing device, it is possible to use a foaming silicone rubber material. The foaming silicone rubber has therein a large number of cells and, hence, the rubber hardness thereof can be made much smaller than a solid silicone rubber material. However, the foaming silicone rubber material is of a continuous cell type, and accordingly, if the pressure roller rotates in contact with the high temperature heat roller, the temperature of the pressure roller is increased. As a result, the gas in the cells is expanded, thus resulting in an increase in the roller diameter. The increase in the roller diameter causes an increase of the diameter of the fluororesin tube formed on the outer peripheral surface of the silicone rubber layer. If the temperature of the pressure roller is reduced to a room temperature after the fixing device is used, the volume of the gas in the cells of the pressure roller is reduced, so that the diameter of the pressure roller is reduced. Consequently, the fluororesin tube which constitutes the outermost layer is wrinkled. The wrinkled fluororesin layer or tube of the pressure roller considerably reduces the fixing efficiency of the fixing device and affects a smooth movement of the recording paper.
The known oil supply roller is made of an open cell urethane rubber having high porosity and, accordingly, the oil supply roller exhibits a high oil containability but is less heat-resistant, due to a low heat resistance of a urethane rubber. This problem can be solved if the oil supply roller is made of a foaming silicone rubber. However, as a foaming silicone rubber has a continuous cell structure in which the cells are independent from each other, it is impossible to externally fill each cell with silicone oil. Consequently, the amount of silicone oil which can be contained in the oil supply roller is restricted.
Moreover, as the known toner supply roller is made of EPDM rubber of a continuous cell sponge of silicone rubber, the roller hardness is limited approximately to 20° (ASCA C). In addition to the foregoing, if the toner supply roller is used for long time, the rubber hardness is undesirably increased. Under these circumstances, in order to stably supply a toner, a toner supply roller having a lower rubber hardness has been needed.