1 ) . The photosensitive drum fall

The new drum has a dark surface potential decay slowly fades out and fast , but there will be a period of time that the phenomenon of decline photosensitivity , so that the effect of exposure after the poor .

2 ) . Exposure light degradation

After prolonged use of the exposure lamp , there is more surface adhesion of dust , aging, partial or total reduction in luminance phenomenon also occurs , resulting in underexposure , which makes part of the white area corresponds to the document are not strong illumination , bright bad enough, the formation of post-exposure potential is too small , there will be part of the gray and white print , which is often a "bottom ash ."

3 ) , the optical system contamination

When exposed , the original optical image through the platen glass , mirrors , lenses ( or optical fiber ) , as well as some machines , such as dust and glass refraction , transmission to the drum surface. Thus, the surface of these components tainted dust that will affect reflective, translucent effect , the same will be " bottom ash " phenomenon.