1. Technical Field
The present invention relates to a density control technique for an image forming apparatus which forms a toner image using toner contained in a developer which is attachable to and detachable from an apparatus main body.
2. Related Art
An image forming apparatus which forms an image using toner, such as a copier machine, a printer and a facsimile machine, executes a density control operation for forming a predetermined patch image using toner, detecting a density of the patch image, controlling an image forming condition based on the density detection result, to thereby obtain a stable image quality. According to a technique described in JP-A-2004-177928 for instance, in an apparatus which is structured to use with a developer which stores toner attached to an apparatus main body, the timing of executing the density control operation is determined based on the use history of the developer. To be more precise, the density control operation is executed when one of the operation time of a developing roller provided in the developer and the remaining toner amount in the developer reaches a predetermined threshold value. The purpose of this is to maintain a stable image density regardless of a change of the property of toner inside the developer with time by means of execution of the density control operation at such timing which corresponds to the use history of the developer.
The timing of executing the density control operation is determined based only on the use history of the developer irrespectively of the type and the like of contained toner according to the conventional technique described above. However, the property of toner may change differently with time depending upon the difference of the manufacturing method of the toner or the difference of the production batches. For example, between toner manufactured by a crushing method (crushed toner) and toner manufactured by a polymerization method (polymerized toner), the speed of the toner getting degraded is greatly different due to different degrees of sphericity, different particle diameter distributions. Hence, the timing of executing the density control operation should preferably determined considering the property of toner held inside a developer to use as well, in which respect the conventional technique described above leaves room for improvement.
Meanwhile, according to a technique described in JP-A-2004-78062 for example, an apparatus which is used with a developer which stores toner attached to an apparatus main body deals with different toner properties among developers in the following manner. That is, toner properties are classified in advance into a several ranks, memories of the developers store the ranking of the toner inside the developers, and the apparatus main body executes a density control operation using values which correspond to the ranking of the toner chosen from among plural sets of control parameters prepared for the respective ranks.
In accordance with the conventional technique described above, variation of the properties of the toner to be used in the apparatus is tolerated only within a range predicted in advance. However, for reduction of the manufacturing cost of the toner or a running cost of the apparatus, a more enhanced tolerable range for the toner properties is desirable. In this respect, the conventional technique described above leaves room for improvement.
Further, in light of the fact that a density detected on an intermediate transfer belt, which is an intermediate image carrier, does not necessarily match with the optical density of an image finally fixed on a recording material, in a technique described in JP-A-2005-316242 for example, the density detected on the intermediate transfer belt is converted into an image density on the recording material with reference to a conversion table and the tone correction characteristic of the apparatus is determined using thus obtained conversion result.
According to the conventional technique described above, one-to-one correspondence between densities detected on the intermediate transfer belt and optical densities of images on the recording material is made and the conversion processing is performed. When the types of toner to use are limited, the properties of the toner are known, and therefore, this kind of method causes no problem. However, the correlation between the densities detection result on the intermediate transfer belt and image densities on the recording material is slightly different depending upon the types of toner (types of pigments and external additives, manufacturing methods, and the like) as well. The conventional technique described above therefore fails to meet the demand to form images using various types of toner and leaves room for improvement in this respect.