Toner-image fixing device with roller-temperature limitation

A toner image fixing device for use in an image forming device, which comprises

• a heating roller with a heater disposed therein for heating the roller surface to a desired temperature,
• a pressure roller for pressing a toner-image carrying medium against the heating roller and which is capable of maintaining the heating roller surface at a temperature suitable for fixing an unfixed toner-developed image onto the medium by supplying electric power to the heater, and which is provided with
• a temperature control system that controls a first preset temperature at which a covering layer of the heating roller cannot peel off and controls a second preset temperature, being higher than the first preset temperature and suitable for fixing a toner-developed image onto the toner-image carrying medium, and that includes
• a temperature sensor for detecting a surface temperature of the heating roller,
• a comparator for comparing a temperature detected by the temperature sensor with the first preset temperature and the second preset temperature and
• a control means that continuously operates the heater immediately after turning on the electric power supply, drives the heating roller and the pressure roller into rotation, operates the heater at a first preset ON-OFF ratio to heat the heating roller surface to the second preset temperature while the temperature detected by the temperature sensor and compared by the comparator is equal to or higher than the first preset temperature and stops the rotation of the heating roller and the pressure roller when the heating roller reaches the second preset temperature and that also operates the heater at a second preset ON-OFF ratio, said second ratio having a longer ON-duration in comparison with the first ON-OFF ratio, in response to the operation of the image forming device to maintain the heating roller surface temperature at the second preset temperature.

A toner image fixing device, comprising

• a pair of rollers through which a recording medium, carrying an unfixed toner image formed thereon, passes under a contact pressure,
• an internal heater disposed in at least one of said rollers and an external heater disposed opposite to a surface of the recording medium whereon the toner image formed, and which is capable of fixing the toner image onto the recording medium by heat by using the internal heater and the external heater, wherein
• both heaters are operated to heat the toner to a first present temperature lower than the melting point of the unfixed toner and only the external heater is operated thereafter to heat and maintain toner at a second present temperature for fixing the toner image onto the recording medium,
• wherein only the internal heater is operated for a period of rising temperature and only the external heater is operated after temperature of the toner has reached the first preset temperature.