Image fixing apparatus with variable fixing modes
A fixing apparatus is provided, in which a problem such as the melting of a heater retaining member does not occur when a fixing nip portion performs a fixing processing in a reduced pressure state. In a fixing apparatus of an image forming apparatus capable of feeding a paper by applied pressures of two types or more, the fixing apparatus is changed to a mode in which the applied force is low, and is allowed to perform an operation for cooling the fixing apparatus before starting a current supply to a heater.
1. An image fixing apparatus for fixing an image formed on a recording material, comprising:
a heater;
a sleeve having an inner peripheral surface contacted with said heater;
a backup member for forming a fixing nip portion with said heater through said sleeve;
a first temperature detection element for detecting a temperature of said sleeve;
a second temperature detection element for detecting a temperature of said heater;
a current supply control portion for controlling a current supply to said heater so that the detection temperature of said first temperature detection element maintains at a set temperature; and
a pressure adjustment mechanism for adjusting a pressure applied to the fixing nip portion, said pressure adjustment mechanism being capable of setting the pressure applied to the fixing nip portion to a first pressure and a second pressure lower than the first pressure;
wherein said fixing apparatus is capable of setting a first fixing mode for executing a fixing process under the first pressure and a second fixing mode for executing the fixing process under the second pressure, and
wherein in case the detection temperature of said second temperature detection element before starting the fixing process under the second fixing mode is higher than a reference temperature, a time period from upon receipt of the print signal until starting the fixing process under the second fixing mode is longer than the case where the detection temperature is lower than the reference temperature.
2. The image fixing apparatus according to claim 1 , wherein in case the detection temperature of said second temperature detection element before starting the fixing process under the second fixing mode is higher than the reference temperature, a cooling operation for rotating said sleeve is performed without proving the current supply to said heater during the time period from upon receipt of the print signal until starting the fixing process under the second fixing mode.
3. The image fixing apparatus according to claim 1 , wherein in case the detection temperature of said second temperature detection element before starting the fixing process under the second fixing mode is higher than the reference temperature, a cooling operation for cooling said heater is performed by activating a cooling fan during the time period from upon receipt of the print signal until starting the fixing process under the second fixing mode.
4. The image fixing apparatus according to claim 2 or claim 3 , wherein the cooling operation is executed before the pressure applied to the fixing nip portion is set to the second pressure.
5. The image fixing apparatus according to claim 1 , wherein a fixing process speed of the second fixing mode is slower than the first fixing mode.
6. The image fixing apparatus according to claim 1 , wherein the current supply to said heater is started in order to execute the second fixing mode, and when the detected temperature of the first temperature detection element reaches a temperature lower than the set temperature, an input power per unit period to said heater is reduced.
7. The image fixing apparatus according to claim 6 , wherein after the input power to said heater is reduced, the input power is increased immediately before the recording material enters into the fixing nip portion.
8. The image fixing apparatus according to claim 7 , wherein a timing for increasing the input power is set when a leading end of the recording material reaches a position where a distance until the fixing nip portion is equivalent to approximately one round of said sleeve.