Image forming apparatus employing fixing device and control method therefor

An image forming apparatus control method for controlling operation of an image forming apparatus, the image forming apparatus comprising:

• a controller;
• a database to register information on a plurality of types of recording media in the controller;
• an image carrier to carry a toner image;
• an exposure unit to expose the image carrier;
• a transfer unit to transfer the toner image from the image carrier to a recording medium;
• a fixing device to fix the toner image transferred on the recording medium at the transfer unit using heat and pressure, the fixing device comprising a temperature detector to detect a temperature of the fixing device and a first drive device to change a fixing nip angle of the fixing device, the fixing nip angle being an angle of departure of the recording medium from the fixing device;
• a speed changer disposed at an upstream side of the fixing device in a feed direction of the recording medium to change a feed speed of the recording medium;
• and
• an electric-current detector disposed at an upstream side of the transfer unit in the feed direction of the recording medium to measure an electric current flowing through the recording medium,
• the method comprising:
• calculating a toner adhesion amount of toner adhering to the toner image on the recording medium from an exposure amount of the exposure device;
• identifying a type of the recording medium based on the information on the plurality of types of recording media registered in the database when the recording medium is selected from the plurality of recording media by a user;
• detecting a temperature of the fixing device with the temperature detector;
• determining a moisture content of the recording medium from the electric current flowing through the recording medium measured with the electric-current detector;
• calculating a reaction force of the recording medium arising when the recording medium is fed out of the fixing device;
• calculating from the toner adhesion amount a toner adhesion force generated by the toner image fused with the heat of the fixing device;
• comparing the reaction force and the toner adhesion force;
• determining the fixing nip angle when the controller determines that the reaction force exceeds the toner adhesion force;
• driving the first drive device to set the fixing device to the fixing nip angle;
• determining a target speed of the feed speed of the recording medium;
• and
• driving the speed changer to change the feed speed of the recording medium to match the target speed.