A density adjusting method that adjusts image formation density by an image forming unit in an image forming apparatus including a memory unit, the image forming unit including a photoconductive drum and a developing unit that supplies toner to the photoconductive drum and that forms a toner image on a circumferential surface of the photoconductive drum by applying a developing bias voltage to the developing unit, an intermediate transfer member to which the toner image formed on the circumferential surface of the photoconductive drum is transferred and from which the toner image is secondarily transferred to a recording medium, and a density detecting unit that detects the density of a surface of the intermediate transfer member, comprising:
- a background density measuring step of storing background density, which is density of the intermediate transfer member surface detected by the density detecting unit in a state where the toner image is not transferred, in the memory unit in correspondence with position information on the intermediate transfer member surface before an execution timing of a calibration process that adjusts the image formation density separately from the calibration process;
- a test patch formation controlling step of causing the image forming unit to form a test patch, which is a toner image for density adjustment, on the intermediate transfer member surface while causing the intermediate transfer member to drive at a predetermined driving speed;
- a density correcting step of executing the calibration process, after the background density has been stored in the memory unit in correspondence with the position information by the background density measuring step and after a predetermined number of sheets are printed by the image forming apparatus;
- a member position detecting step of detecting the passage of a position detecting member provided on the intermediate transfer member surface;
- and
- a time measuring step of measuring a driving time of the intermediate transfer member after the detection of the passage of the position detecting member by the member position detecting step;
- wherein the density correcting step includes adjusting the image formation density by the image forming unit according to a difference between the background density already stored in the memory unit and corresponding to the position information indicating the formation position of the test patch on the intermediate transfer member surface and the density of the test patch formed on the intermediate transfer member surface in the test patch formation controlling step and detected by the density detecting unit at the time of executing the calibration process,
- the density correcting step includes setting a developing bias voltage value that is a value of the developing bias voltage according to the difference to adjust the image formation density,
- the density correcting step of executing the calibration process uses the background density stored in the memory unit without measuring the background density by the background density measuring step every time the calibration process is executed
- the background density measuring step includes storing a multiplication result of the driving time of the intermediate transfer member measured by the time measuring step after the detection of the passage of the position detecting member and the predetermined driving speed of the intermediate transfer member as position information indicating a position on the intermediate transfer member surface in a moving direction of the intermediate transfer member surface in the memory unit in correspondence with the background density,
- the test patch formation controlling step includes causing the image forming unit to form a plurality of the test patches on the intermediate transfer member surface,
- the density correcting step includes calculating the position information indicating the position on the intermediate transfer member surface for the respective plurality of test patches formed by the image forming unit by multiplying the driving time of the intermediate transfer member measured by the time measuring step after the detection of the passage of the position detecting member and the predetermined driving speed of the intermediate transfer member,
- the density correcting step includes plotting points corresponding to combinations of the developing bias voltage values at the time of forming the respective plurality of test patches and the difference values between densities of the plurality of test patches and the background densities in a two-dimensional coordinate system, and calculates an approximation line representing a relation between the developing bias voltage values and toner densities by using the points, and
- the density correcting step includes obtaining a target developing bias voltage value corresponding to a target density set beforehand based on the approximation line, and sets the target developing bias voltage value as the developing bias voltage value at the time of an image forming operation by the developing unit.