Image-forming apparatuses, e.g., copiers and printers, have in recent years been subjected to greater diversity in their intended uses and use environments as well as demands for greater speed, higher image quality, and greater stability. For example, printers, which in the past have been used mainly in the office, have also entered into use in severe environments, e.g., high temperatures, high humidities, and it is also critical in such instances that a stable image quality be provided.
Copiers and printers are also undergoing apparatus downsizing as well as advances in energy efficiency, and the use is preferred within this context of magnetic single-component developing systems that use a favorable magnetic toner.
In a magnetic single-component developing system, a magnetic toner layer is formed by a toner layer thickness control member (referred to herebelow as the developing blade) on a toner-bearing member (referred to herebelow as the developing sleeve) that is provided in its interior with a magnetic field-generating means such as a magnetic roll. Development is carried out by transporting this magnetic toner layer to the developing zone using the developing sleeve.
Charge is imparted to the magnetic toner by the friction generated when the developing blade and the developing sleeve come into contact in the contact region between the developing blade and the developing sleeve (referred to herebelow as the blade nip region).
Reducing the diameter of the developing sleeve is a critical technology for reducing the size of the apparatus. With such a reduced-diameter developing sleeve, the developing zone at the developing nip region is narrowed and fly over by the magnetic toner from the developing sleeve is then impaired and a portion of the magnetic toner will readily remain on the developing sleeve.
In this case, turn over of the magnetic toner in the magnetic toner layer within the blade nip deteriorates and the charging performance of the magnetic toner layer readily becomes nonuniform.
Moreover, when an extended durability test is carried out in such a state, the magnetic toner in the blade nip region is readily subjected to shear and deterioration phenomena then readily occur, for example, the external additive at the magnetic toner surface becomes buried. As a consequence, the flowability and the charging performance of the magnetic toner are prone to decline in the latter half of an extended durability test that uses a small-diameter sleeve, and the charging performance in particular readily becomes nonuniform.
In addition, these deterioration phenomena are particularly prone to occur with magnetic toners in high-temperature, high-humidity environments, and systems in which the process speed has been raised in support of the higher speeds of recent years will only continue to be more stringent with regard to charging performance uniformity.
In particular, with magnetic toners the dispersibility of the magnetic body readily exercises a substantial effect on charging performance uniformity, as compared to magnetic body-free nonmagnetic toners, and various image defects are readily produced when the magnetic toner has an inferior charging performance uniformity.
For example, the overcharged magnetic toner fraction remains on the developing sleeve, and as a result the image density is prone to decline and image defects, such as fogging in nonimage areas, can occur.
In addition, due to the influence of the curvature of a reduced-diameter sleeve, it is difficult to stir the magnetic toner at the back of the developing sleeve. When the flowability of the magnetic toner is unsatisfactory, the magnetic toner compacted at the back of the developing sleeve assumes a packed condition and a state may be assumed in which the magnetic toner cannot be satisfactorily fed to the developing sleeve.
In this case the magnetic toner in the vicinity of the developing sleeve becomes overcharged and the charging performance uniformity of the magnetic toner then readily becomes unsatisfactory due to the transport of the magnetic toner to the blade nip region in a state of nonuniform charge.
To respond to this problem, numerous methods have been proposed in which the dielectric properties, which are an index for the state of the dispersion of the magnetic body within a magnetic toner, are controlled in order to bring about a stabilization of the changes in the developing performance that accompany changes in the environment.
For example, in Patent Document 1 the dielectric loss tangent (tan δ) in a high-temperature range and the normal temperature range is controlled in an attempt to reduce the variations in toner charging performance associated with variations in the environment.
While certain effects are in fact obtained under certain prescribed conditions, in particular adequate consideration is not given to a high degree of starting material dispersity for the case of a high magnetic body content, and there is still room for improvement with regard to the charging performance uniformity of magnetic toners.
In order to suppress environmental variations by toners, Patent Document 2 discloses a toner for which the ratio between the saturation water content HL under low-temperature, low-humidity conditions and the saturation water content HH under high-temperature, high-humidity conditions is brought into a prescribed range.
This control of the water content does in fact provide certain effects for the image density reproducibility and transferability under certain prescribed conditions, but in particular no mention is made of the charging performance uniformity when the magnetic body is incorporated as a colorant in the reasonable amount, and this is inadequate for obtaining the effects of the present invention.
Patent Document 3 discloses an image-forming apparatus that contains toner particles as well as spherical particles that have a number-average particle diameter of from at least 50 nm to not more than 300 nm, wherein the free ratio of these spherical particles is from at least 5 volume % to not more than 40 volume %. This has a certain effect with regard to inhibiting, under a prescribed environment, contamination of the image carrier, scratching of the image carrier and intermediate transfer member, and image defects.
Patent Document 4, on the other hand, discloses a toner in which large-diameter particles are anchored and small-diameter particles are externally added. This supports an improvement in the fixing releasability and a stabilization of the toner flowability and makes it possible to obtain a pulverized toner with excellent charging, transport, and release properties.
Patent Document 5 discloses an art in which the coating state for the external additive is controlled and the dielectric properties of the toner are also controlled and that is effective mainly for the issue of streak prevention.
In these inventions, however, the free ratio of the spherical particles or large-diameter particles, as inferred from the anchoring conditions or free conditions of these particles, is relatively high, and control of the state of fixing of inorganic fine particles that are otherwise added is inadequate.
Due to this, the charging performance uniformity for magnetic toners is inadequate, for example, when an extended durability test is run in a high-temperature, high-humidity environment—where charging is already prone to become nonuniform, and the effects sought by the present invention are not obtained.
That is, there is still room for improvement to obtain, through the use of a magnetic toner that has a satisfactory charging performance uniformity, a high quality image even after an extended durability test in a system with a fast process speed in support of higher speeds and using a reduced-diameter sleeve in support of apparatus downsizing.