PRESSURE ROLLER AND FIXATION DEVICE EMPLOYING SAME

Abstract: A pressure roller for a toner fixing device includes a core (10) made of metal or plastic, an elastic layer (11) made of foam material formed on the core and a layer (12) of porous tetrafluoroethylene resin on the outside circumferential surface of the elastic layer. The porous layer is impregnated with heat-resistant liquid rubber in its pores.

Description: BACKGROUND OF THE INVENTION The present invention relates to a pressure roller and more particularly to a pressure roller of the type used in a device for fixing toner on a piece of copying paper. In a pressure roller as disclosed in Japanese Patent Unexamined Application Publication No.9377/87 , a rubber layer is provided on a core of metal or plastic and a porous layer of a tetrafluoroethylene resin is provided on the outside circumferential surface of the rubber layer. The rubber layer may be composed of a foam material and the pores of the porous layer may be impregnated with a silicone oil. Since the hardness of the rubber is low, the width of the pinching area between the pressure roller and a fixation roller can be made large even under low pressure. For this reason the fixation of the toner is good and copying paper is less likely to be wrinkled. However, since the surface of the porous layer remains porous unless the porous layer is impregnated with the silicone oil, the toner clings to the surface and contaminates it over a period of time. But, if the porous layer is impregnated with the silicone oil and thin paper is used, the oil does not act as a separating agent for thin copying paper but causes the thin paper to cling to the pressure roller and wind thereon. For this reason, it is necessary to provide a separation claw for the copying paper. It is known from JP-A-60-179 970 to impregnate a roller having an outer layer of porous PTFE resin with heat-resistant liquid rubber. SUMMARY OF THE INVENTION It is an object of the present invention to provide a pressure roller that can be used with thin paper without having the paper cling to a porous layer thereof. It is another object of the present invention to provide a pressure roller which is essentially free from contamination by the toner. These and other objects of the present invention are achieved by a pressure roller as claimed herein. The pressure roller is used in a fixing device for thermally fixing transferred toner on copying paper between a fixation roller and the pressure roller. The roller includes an elastic layer made of a foam material on a core made of a metal or plastic, and a layer made of a porous tetrafluoroethylene resin formed on the outside circumferential surface of the elastic layer. The porous layer is impregnated with a heat-resistant liquid rubber in the pores of the resin so that the heat-resistant liquid rubber is set after the impregnation. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) is a front view of a pressure roller in accordance with the present invention. FIG. 1(b) is a sectional view of the pressure roller. FIG. 2 is a fixing device employing the pressure roller. FIG. 3 is a front view of an unfinished pressure roller. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the drawings, wherein like numerals represent like parts throughout, as shown in Figures 1(a) and 1(b) the pressure roller 4 provided in accordance with the present invention has an elastic layer 11 made of foam material, and a porous layer 12 made of tetrafluoroethylene resin on the outside circumferential surface of the elastic layer 11. A heat-resistant liquid rubber is impregnated in the pores of the porous layer 12 and set so that the surface of the pressure roller 4 is made unporous. The elastic layer 11 is disposed around a core 10 made of metal or plastic. The foam material for the elastic layer 11 is a heat-resistant rubber such as a silicone rubber or fluorine rubber. If the pressure roller 4 is brought into contact with the thermal fixation roller only at the time of fixation, the foam material may be EPDM (ethylene propylene diene terpolymer) or the like. The Asker hardness of the foamed material should preferably be 60 or less in order to make the width of the pinching area between the pressure roller 4 and the fixation roller 3 large even under low pressure so as to utilize the characteristics of the foamed material. The porous layer 12 of tetrafluoroethylene should preferably be a tube of 0.1 to 3 mm in thickness and 50 to 90% in porosity in order to fully utilize the low hardness of the foam material for the elastic layer 11. The inner diameter of the tube is slightly smaller than the outer diameter of the foam material before the tube is fitted thereon. After the tube is fitted on the foam material, the heat-resistant liquid rubber is impregnated in the pores of the tetrafluoroethylene resin and is then set so that the surface of the pressure roller 4 is made unporous. The hardness of the set heat-resistant rubber should preferably be as low as possible so that the foam material does not lose its softness. For that reason, a liquid silicone rubber of several thousands to scores of thousands in viscosity (cs) is normally used as the heat-resistant liquid rubber. From 0.5 to 30% by weight of carbon black, graphite and Indian red or the like are added to the heat-resistant liquid rubber to enhance the electric conductivity, thermal conductivity and heat resistance of the surface of the rubber, respectively, and may be also added to the tetrafluoroethylene resin. Because the porous layer 12 made of the tetrafluoroethylene resin is on the outside circumferential surface of the elastic layer 11, which is made of the foam material, and the heat-resistant rubber is impregnated in the pores of the porous layer 12, the softness of the elastic layer 11 is not lost. Therefore, the width of the pinching area between the pressure roller 4 and the thermal fixation roller 3 is made large enough (about 2 to 5 mm) even under low pressure. For that reason, the toner 2 is fixed well on the copying paper 1 to make a sharp image thereon, and the copying paper 1 is not wrinkled even when copying is performed on both the sides of the paper or the thickness of the paper is small. Since the surface of the pressure roller 4 is made unporous by impregnating the heat-resistant rubber in the pores of the porous layer 12, the toner 2 does not cling to the surface of the pressure roller 4 to contaminate the reverse side of the copying paper 1. EXAMPLE A porous tube 12 was made of PTFE (polytetrafluoroethylene) in a method disclosed in Japanese Patent Examined Application Publication No.13560/67 or No.15295/83, having an inner diameter of 25 mm, a thickness of 0.3 mm, a porosity of 80 % and a pore diameter of 2µm. An elastic roller 11, which was the unfinished pressure roller 4,was manufactured and had dimensions L₁=370 mm, L₂=330 mm, L₃= 20mm, D₁=26 mm, D₂=15 mm, and D₃=8 mm, as shown in FIG. 3. The core 10 of the pressure roller 4 was made of aluminum. The elastic layer 11 of the pressure roller 4 was made of a foamed silicone rubber of 20 in Asker hardness. The porous tube 12 of PTFE was fitted on the elastic roller. A liquid silicone rubber, which was the CY52-005 produced by Toray silicone Co., Ltd. and was 4,500 cs in viscosity and 45 in hardness, was impregnated in the pores of the porous tube 12 and then set at a temperature of 150°C for 2 hours. Subsequently, the surface of the porous tube 12 was polished to obtain a smooth surface. The pressure roller 4, as shown in FIG.1, was thus manufactured. Since the Asker hardness of the surface of the pressure roller was 30, the surface had a high degree of softness. A fixing device 20 was composed of a thermal fixation roller 3 and the pressure roller 4 as shown in FIG. 2. Also shown in Fig. 2 are a separation claw 5, a blade 6, a heater and a paper separating agent applicator 8. The thermal fixation roller 3 was a roller coated with a fluorine resin. The pressing load upon these rollers was 0.5 kg. At that time, the width of the pinching area between both rollers was 3 to 4 mm. Copying paper 1 onto which toner 2 was transferred was passed between thermal fixation roller 3 and pressure roller 4 so that toner 2 was fixed on copying paper 1 at such speed that twenty sheets of A4-size copying paper 1 were passed between both rollers per minute. At that time, the toner 2 did not cling to pressure roller 4 to contaminate the reverse side of the copying paper 1. Therefore, the quality of the fixed toner image on the copying paper 1 was good and more than 100,000 sheets of the copying paper 1 were used. In addition, none of the sheets of the copying paper 1 were wrinkled. Because,in the present invention as claimed, a heat-resistant liquid rubber is impregnated in a porous layer 12 made of a tetrafluoroethylene resin so that the softness of the surface of the porous layer 12 is maintained, a toner 2 or the like is prevented from clinging in the pores of the porous layer 12 and contaminating it, thereby preventing contamination of the reverse side of copying paper 1. Therefore, the reverse side of copying paper 1 is prevented from being contaminated. If carbon black or/and graphite are added to at least one of the tetrafluoroethylene resin and the heat-resistant liquid rubber, the electric conductivity or/and thermal conductivity thereof are enhanced, respectively, and the toner 2 is less likely to cling to the surface of porous layer 12. If Indian red is added to the heat-resistant liquid rubber, the heat resistance thereof is enhanced.

Claims: A pressure roller for a toner fixing device, comprising: a core (10) made of metal or plastic; an elastic layer (11) made of a foam material disposed on said core; and a layer (12) of a porous tetrafluoroethylene resin disposed on the outside circumferential surface of said elastic layer, characterized in that said layer (12) made of porous tetrafluoroethylene resin is impregnated with a heat-resistant liquid rubber in the pores of said resin so that said rubber is set after said impregnation, whereby the heat-resistant liquid rubber has a viscosity of 100,000 cs or less and an after-setting Asker hardness of 70 or less. A pressure roller according to claim 1, wherein the elastic layer (11) made of foamed material has a thickness of 2 to 10 mm and said material is a silicone rubber having an Asker hardness of 60 or less. A pressure roller according to claim 1, wherein the layer (12) made of porous tetrafluoroethylene resin is a porous tube of 0.1 to 3 mm in thickness and 50 to 90 % in porosity. A pressure roller according to claim 1, wherein 0.5 to 30% by weight of an additive selected from the group consisting of carbon black, graphite and Indian red is added to at least one of the porous tetrafluoroethylene resin and the heat-resistant liquid rubber.

Inventors: KATO, CHIAKI C/O KUMATORI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: G03G15
IPC codes: G03G15