Durability of the developer is an important subject with regard to decreasing customer service costs as well as reducing environmental load, such as resource saving and waste reduction. Konica Minolta has focused on the coating layer of the carrier of the developer to increase the durability of the developer.
Fig.1: Two-component developing process The two-component developer consists of a toner and a carrier. The toner is image forming particulate and is constantly being consumed. The carrier's purposes are to impart proper electric charge to the toner by frictional electrification when it is mixed with the toner in the developing unit, to transport the toner within the developing area opposite the photoconductor, and to create a development field so that the toner can properly develop an image that is identical to the latent image on the photoconductor.
Generally, the carrier is made up of magnetized ferrite particles which have been coated by resin that can impart electric charge. To consistently charge the toner, it is ideal not to change the carrier surface. However, since the carrier surface repetitively comes in contact with the toner within the developing unit, and is also subject to mechanical and thermal stress, the current situation is that the carrier surface gradually becomes contaminated to change the surface condition. Specifically, a high-speed machine requires the toner to become quickly charged and the developing property to be of the highest qualities in addition to extending the developer life.
Fig.2: The capacitor model at the developing region and an equality expression
To increase developing property, it is necessary to increase development field intensity. According to the above model, to increase the intensity of development field E, it is effective to choose material so that both the core's permittivity ε1 and the coating layer's permittivity ε2 are high. Furthermore, technology, which was developed to slightly wear the carrier surface to stabilize charging performance, resulted in a decrease in thickness d2 of the coating layer in the above equation which indicates that development field E increases as the use continues and development property changes. To stabilize development property, it is considered appropriate to design the coating layer so as to decrease both the thickness d2 and permittivity ε2 of the coating layer.
Fig.3: The carrier structural model To increase permittivity of the coating layer, we researched adding high-permittivity material to the conventional acrylic resin coating layer. This time, we have used high-permittivity material which has a relative permittivity about 40 times higher than that of acrylic resin.
Fig.6 is a model drawing of the studied carrier.
The coating method is an environment-friendly dry method.
The dry method uses a complete solventless coating process when forming a coating layer. Therefore, compared to the solvent coating, it can reduce carbon dioxide (CO2) emission by about 60 percent, thereby contributing to the reduction of environmental load.
Fig.4: Relationship between development field index and coating layer thickness index
Changes of the field intensity due to wear of the film of the coating layer significantly change development property.
To stabilize the development property, we decided to control the concentration of the high-permittivity material thereby inhibiting the changes of the field intensity. Fig.7 shows the comparison of the development field intensity of three types of carriers (A, B, C) which was the result of experiments in which the method of concentration control was changed.
When comparing the intensity of development field of C to that of A and B, the development field intensity of C is extremely stable.
By adopting this technology, it has become possible to maintain an extremely stable ability to impart the electric charge currently being used and also maintain high intensity of the development field. As a result, it has become possible to obtain a stable and high development property for a long period of time.