Curl straightening method and image forming apparatus

An image forming apparatus, comprising:

• a fixing apparatus including a nip portion formed by a heating unit which heats one face side of a sheet and a pressurizing unit which presses the sheet to the heating unit side;
• a curl straightening apparatus which is arranged on a conveying path of the sheet from the fixing apparatus to a paper-discharging outlet to straighten a curl of the sheet;
• a sheet moisture content calculating unit which calculates moisture content of the sheet before being supplied to the fixing apparatus;
• a heating unit side temperature sensor which measures a temperature of the heating unit;
• a pressurizing unit side temperature sensor which measures a temperature of the pressurizing unit;
• sheet temperature/moisture content calculating unit which calculates a temperature and moisture content of the sheet in the respective conveying sections of multiple conveying sections into which are divided a conveying path from the nip portion to the paper-discharging outlet such that a curvature rate of the conveying path in the respective conveying sections is constant;
• a conveying time calculating unit which calculates a conveying time of the sheet in the respective conveying sections;
• a conveying section strain calculating unit which calculates a residual strain of the sheet in the respective conveying sections from the temperature and the moisture content of the sheet in the respective conveying sections that are calculated in the sheet temperature/moisture content calculating unit, the conveying time calculated in the conveying time calculating unit, and the stress relaxation characteristics of the sheet;
• a final curl amount calculating unit which calculates a final curl amount using the residual strain of the sheet in the respective conveying sections calculated in the conveying section strain calculating unit;
• a curl straightening apparatus control value determining unit which determines a control value of the curl straightening apparatus based on a final curl amount calculated by the final curl amount calculating unit;
• and a curl straightening apparatus control unit which controls the curl straightening apparatus based on a control value determined by the curl straightening apparatus control value determining unit.