Image forming apparatus that can adjust the timing of performing image density correction

An image forming apparatus comprising:

• an image forming portion including:
• an image carrier that has a photosensitive layer formed on a surface thereof;
• a charging device that charges the surface of the image carrier;
• and
• a development device that includes a developer carrier which carries developer including toner, the development device developing an electrostatic latent image formed on the image carrier into a toner image,
• the image forming portion performing image formation using the toner;
• an exposure device that forms the electrostatic latent image with an attenuated electric charge by exposing to light the surface of the image carrier charged by the charging device;
• an image density sensor that detects a density of the toner image formed in the image forming portion and transferred to an intermediate transfer belt;
• and
• a control portion that controls the image forming portion and the exposure device,
• wherein
• the control portion can, by detecting with the image density sensor an image density of a reference image for image density correction formed in the image forming portion and adjusting a development voltage fed to the developer carrier and an amount of exposure from the exposure device based on a result of the detection, perform image density correction to adjust the image density in the image forming portion,
• when performing the image density correction, the control portion
• predicts a toner charge amount based on a formation condition of the reference image and the image density of the reference image detected by the image density sensor and
• determines a timing of performing the image density correction next time based on a rate of change of the toner charge amount calculated using:
• a first toner charge amount predicted when the image density correction was performed last time;
• a second toner charge amount predicted when the image density correction is performed this time;
• and
• an interval between the image density correction performed last time and this time,
• the image forming apparatus further includes a temperature and humidity detection device that senses a temperature and a humidity near the development device,
• the control portion corrects the timing of performing the image density correction next time if, after the image density correction was performed last time, before the timing of performing the image density correction next time, a change equal to or more than a predetermined value occurs in at least one of
• the temperature and the humidity sensed by the temperature and humidity detection device, and
• a rate of printing of the image formed in the image density portion,
• a correction coefficient corresponding to the rate of change of the toner charge amount is set for each of the temperature and the humidity sensed by the temperature and humidity detection device, and the rate of printing, and
• the control portion corrects the timing of performing the image density correction by multiplying by the correction coefficient the timing of performing the image density correction next time.