Fixing device and image forming apparatus

A fixing device for heating a toner image born on a recording material and fixing to the recording material, the fixing device comprising:

• an endless belt member;
• a rotatable member configured to form a nip portion, between itself and the belt member, for nipping and conveying the recording material in contact with an outer peripheral surface of the belt member;
• a first stretching member in contact with an inner peripheral surface or the outer peripheral surface of the belt member and configured to stretch the belt member;
• a first heater configured to heat the first stretching member;
• a second stretching member in contact with the inner peripheral surface or the outer peripheral surface of the belt member and configured to stretch the belt member;
• and
• a second heater configured to heat the second stretching member,
• wherein when a speed at which a temperature of the belt member is raised by heating the first stretching member by the first heater is defined as a first temperature rising speed and a speed at which a temperature of the belt member is raised by heating the second stretching member by the second heater is defined as a second temperature rising speed, the first temperature rising speed is faster than the second rising temperature speed, and
• wherein at a start up of the fixing device, heating of the first heater is started prior to heating of the second heater, and the heating of the second heater is started after a temperature of the first stretching member reaches a target temperature.

An image forming apparatus comprising:

• an image forming portion configured to form a toner image on a recording material;
• a fixing device configured to heat the recording material on which the toner image is formed by the image forming portion and to fix the toner image onto the recording material;
• an electric power supplying portion;
• and
• a control portion configured to control the electric power supplying portion,
• wherein the fixing device comprising:
• an endless belt member;
• a rotatable member configured to form a nip portion, between itself and the belt member, for nipping and conveying the recording material in contact with an outer peripheral surface of the belt member;
• a first stretching member in contact with an inner peripheral surface or the outer peripheral surface of the belt member and configured to stretch the belt member;
• a first heater configured to heat the first stretching member;
• a second stretching member in contact with the inner peripheral surface or the outer peripheral surface of the belt member and configured to stretch the belt member;
• and
• a second heater configured to heat the second stretching member,
• wherein a heat capacity of the first stretching member is smaller than a heat capacity of the second stretching member;
• wherein the electric power supplying portion supplies the electric power to the first heater and the second heater, and
• wherein the control portion is capable of executing an operation in a mode in which a start up of the fixing device is performed by an electric power lower than a electric power at a time of heating the first heater and the second heater with the maximum amount of heat generation, respectively, and
• during executing the operation in the mode, controls the electric power supplying portion so as to start heating the first heater prior to heating the second heater, and to start heating the second heater after a temperature of the first stretching member reaches a target temperature.

An image forming apparatus comprising:

• an image forming portion configured to form a toner image on a recording material;
• a fixing device configured to heat the recording material on which the toner image is formed by the image forming portion and to fix the toner image onto the recording material;
• an electric power supplying portion;
• and
• a control portion configured to control the electric power supplying portion,
• wherein the fixing device comprising:
• an endless belt member;
• a rotatable member configured to form a nip portion, between itself and the belt member, for nipping and conveying the recording material in contact with an outer peripheral surface of the belt member;
• a first stretching member in contact with an inner peripheral surface or the outer peripheral surface of the belt member and configured to stretch the belt member;
• a first heater configured to heat the first stretching member;
• a second stretching member in contact with the inner peripheral surface or the outer peripheral surface of the belt member and configured to stretch the belt member;
• and
• a second heater configured to heat the second stretching member,
• wherein when a speed at which a temperature of the belt member is raised by heating the first stretching member by the first heater is defined as a first temperature rising speed and a speed at which a temperature of the belt member is raised by heating the second stretching member by the second heater is defined as a second temperature rising speed, the first temperature rising speed is faster than the second temperature rising speed;
• wherein the electric power supplying portion supplies the electric power to the first heater and the second heater, and
• wherein the control portion is capable of executing an operation in a mode in which a start up of the fixing device is performed by an electric power lower than a electric power at a time of heating the first heater and the second heater by the maximum amount of heat generation, respectively, and
• during executing the operation in the mode, controls the electric power supplying portion so as to start heating the first heater prior to heating the second heater, and to start heating the second heater after a temperature of the first stretching member reaches a target temperature.