Fixing device, image forming apparatus, and fixing method

An image forming apparatus comprising:

• an image bearer to bear an image;
• and
• a fixing device to fix the image on at least one recording medium, the fixing device including:
• a fixing belt that is endless;
• a heater to heat the fixing belt;
• a support roller supporting the fixing belt rotatably;
• a fixing roller supporting the fixing belt rotatably;
• a pressure roller to press against the fixing roller via the fixing belt to form a nip portion between the fixing belt and the pressure roller, the nip portion through which the at least one recording medium is conveyed;
• a mover to move the pressure roller with respect to the fixing roller in an approach direction in which the pressure roller approaches the fixing belt and a separation direction in which the pressure roller separates from the fixing belt;
• a linear speed sensor to detect a linear speed of the fixing belt in a state in which the pressure roller separates from the fixing belt;
• and
• processing circuitry configured to:
• calculate an amount of thermal expansion of the fixing roller in a radial direction of the fixing roller based on the linear speed of the fixing belt;
• and
• control the mover to move the pressure roller based on the amount of thermal expansion of the fixing roller to adjust an amount of pressure applied by the pressure roller to the fixing roller.

A fixing method comprising:

• receiving a print job;
• determining that a recording medium specified in the print job is an envelope;
• determining a nip width of a nip portion formed between a fixing belt and a pressure roller to be set based on a type of the envelope, the nip width being in a circumferential direction of the pressure roller;
• obtaining a current linear speed of the fixing belt;
• calculating an amount of pressure applied by the pressure roller based on the nip width preset for the type of the envelope and the current linear speed of the fixing belt;
• controlling the amount of pressure applied by the pressure roller;
• and
• starting conveyance of the envelope.