1. Field of the Invention
The present invention relates to rolls to be used in copying machines or printers, and particularly to conductive rolls to be provided around a photoconductor drum.
2. Description of the Related Art
In a copying machine or a printer, an electrostatic latent image formed on a photoconductor drum is developed by toner (developer) fed from a developing roll. The toner image on the photoconductor drum is transferred to a transfer roll. The toner image is then transferred to a recording paper. Then, the recording paper is heated and pressed so that a picture image is formed on the recording paper. Such a roll as used in a copying machine or printer is generally has a structure in which an outer circumference of pipe-like core made of metal such as aluminum or iron is coated with rubber or resin material.
With reduction in hardness of such rolls, particularly, such conductive rolls including a charge roll, a developing roll, and a transfer roll which are used around a photoconductor drum, the roll is required to have high releasability to prevent the toner (developer) from adhering, for the purpose of ensuring nipping property between the roll and the photoconductor drum. Generally, each of the rolls such as conductive rolls adopts a structure to have a base material layer made of low hardness rubber material and provided on an outer circumference of a metal core, and a front surface layer made of resin material and provided on the base material layer.
As the rubber material constituting the base material layer of such a roll, low hardness rubber in which a softening agent such as oil is added to general-purpose rubber such as NBR or silicon rubber, or foamed rubber of polyurethane has been used. Especially silicon rubber has been used in view of stability in resistance value. As the resin material constituting the front surface layer, resin material have been used which is high in releasability, such as fluororesin, acrylate resin, urethane resin, or silicon resin.
The conductive rolls to be provided around a photoconductor drum are generally used with a difference in peripheral velocity between each conductive roller and the photoconductor drum due to the function of the conductive rollers. Especially, in the developing roller, taking the carrying quantity and developing property of toner into consideration, it is general to provide a peripheral velocity difference so that the peripheral velocity of the developing roll is about 1.1 times-1.5 times as large as the peripheral velocity of the photoconductor drum. Further, although it is general to particularly provide no peripheral velocity difference between the charge roll or the transfer roll and the photoconductor drum, there occurs actually a peripheral velocity difference in some degree due to variations in outer diameter of each roll per se.
Further, in the rolls other than the conductive rolls used around the photoconductor drum, for example, in the carrying rolls for carrying recording paper or in the fixing rolls, each roll is rotating in such a condition that a pair of rolls opposed to each other, that is, a roll in question and another roll which is a matter to be opposed to each other, are disposed to be in contact with each other. Accordingly, even if any peripheral velocity difference is not particularly provided between the rolls opposed to each other, it can be said that there exists, in fact, a certain amount of peripheral velocity difference between the thus opposed rolls.
If any peripheral velocity difference exists between a roll and a matter opposed to the roll, for example, in a case where a photoconductor drum is disposed as the matter opposed to the roll in question, it is apt to generate ripple-like deformation called wrinkles in the surface of the roll in question in the rotation driving. It is considered that such wrinkles are generated as follows. That is, as stated above, the roll is constituted by a base material layer of low hardness rubber material provided on the outer circumference of a metal core and a front surface layer of resin material provided on the base material layer. Accordingly, the soft base material layer is deformed by a peripheral velocity difference between the roll and a matter opposed to the roll in question, so that the deformation causes plastic deformation on the upper front surface layer.
As a method of preventing such ripple-like wrinkles from generating, it is considered that the front surface layer is made of a soft material, or the base material layer is made hard. However, when the front surface layer is made of a soft material, the lowering of the releasability is caused so that the function to be required originally cannot be satisfied. Further, the lowering of the releasability causes a disadvantage called filming due to adhesion of toner in the case where the roll in question is a developing roll. If the base material layer is made hard, on the contrary, it becomes difficult to ensure a sufficient nip width between the roll and an opposed matter such as a photoconductor drum. Accordingly, it is necessary to provide a countermeasure to heighten the accuracy of size of the roll in question per se so that a predetermined nip width can be obtained stably.
Further, in a case of a transfer roll which requires a certain nip width, generally, the roll in question could not help taking a soft roll structure at the sacrifice of the releasability. In addition to the problem of generation of ripple-like wrinkles, it is the existing condition that the mechanism has to be complex because a cleaning member is required to be provided to remove surplus toner adhering onto the roll surface.
An object of the present invention is to provide a roll which can secure necessary nip properties and releasability, and at the same time, the roll in question does not generate ripple-like wrinkles in the front surface layer even if there exists a peripheral velocity difference between the roll in question and any other opposed matter such as a photoconductor drum.
In order to achieve the object, according to the invention, there is provided a roll including: a base material layer made of low hardness rubber material and provided on an outer circumference of a metal core; and a front surface layer made of resin material and provided on the base material layer; wherein a stress relaxation layer is provided between the base material layer and the front surface layer, the stress relaxation layer being made of material which is at least higher in hardness than the base material layer and larger in stretch than the front surface layer.
In the roll of the invention, specifically, the base material layer is made of rubber material with low hardness not higher than 25 degrees in JIS-A hardness, the front surface layer is made of resin material not larger than 30% in stretch, and the stress relaxation layer is made of material having stress not lower than 5 MPa at 10% stretch.
In the roll of the invention, the stress relaxation layer is made of rubber material added with at least one kind of self-crosslinking resin such as blocked type self-crosslinking isocyanate resin, blocked type self-crosslinking epoxy resin, and blocked type self-crosslinking phenolic resin. The loading of the self-crosslinking resin to the rubber material is 20-150 parts by weight relative to 100 parts by weight of the rubber material.
In the roll according to the invention, the stress relaxation layer has a thickness in a range of from 5 to 50 xcexcm. In the roll according to the invention, preferably, the base material layer has a thickness not smaller than 3 mm, and the roll according to the invention is suitable as a conductive roll having roll hardness not higher than 60 degrees in Asker-C hardness at a load of 1 kg.