Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-08-04
2002-04-23
Grimley, Arthur T. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S299000
Reexamination Certificate
active
06377771
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to printers, in particular printers of the type including a moving image-carrying belt, and a plurality of toner image producing stations for sequentially depositing a plurality of toner images in register with each other on the surface of the image-carrying belt to form a composite toner image thereon.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,805,967 (De Cock et al./Xeikon NV) describes a single pass, multi-color electrostatographic printer. A plurality of toner images of different colors are electrostatically deposited sequentially in powder form in register with each other from a plurality of printing stations onto an image-carrying member which is in the form of a intermediate transfer belt. The so-formed composite toner image is then transferred to a silicone coated metal transfer belt which is brought into contact with a substrate. The composite toner image is thereby transferred to the substrate.
The intermediate transfer belt may have a toner image-carrying surface formed of an electrically non-conductive material selected from polyethylene terephthalate, silicone elastomer, polyimide (such as KAPTON—Trade Mark), and mixtures thereof.
It has been found that where several toner images are transferred electrostatically to the intermediate transfer belt, some distortion of the composite image may occur upon transfer to the transfer belt, revealing itself in badly transferred areas in the final printed product.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a printer of the type described in which these disadvantages can be overcome.
We have discovered that this objective, and other useful advantages, can be obtained when the image-carrying belt is formed of a non-conductive material having specified electrical properties.
According to the invention there is provided a printer including a moving image-carrying belt, and a plurality of toner image producing stations for sequentially electrostatically depositing a plurality of toner images in register with each other on the surface of the image-carrying belt to form a composite toner image thereon, wherein the image-carrying belt is formed of an electrically non-conductive material having a mean volume resistivity of from 10
8
to 10
11
ohm m.
In a preferred embodiment of the invention, the printer also includes a transfer member for transferring the composite image from the image-carrying belt to a substrate.
While not wishing to be bound by theory, we believe that in the case of a multi-color printer in which a plurality of toner images of different colors are deposited by electrostatics on the intermediate transfer belt in turn, the voltage which needs to be used to deposit the toner at a given printing station depends upon the voltage which is retained upon the belt surface and the toner deposited thereon at the preceding station or stations. Thus, as the belt progresses from one printing station to the next, the overall charge on the belt would tend to build up. If, as result of such a charge build-up, a discharge should take place between the belt and an imaging drum at one of these printing stations, or possibly between the belt and the transfer member, such a discharge would be liable to disturb the images on the belt. Thus, where several toner images are electrostatically deposited on the image carrying belt, charge on the belt can build up to the point where sparking or other uncontrolled discharge takes place, due to Paschen discharge where the image-carrying belt and the latent image carrier are being separated or are coming into contact with each other. By selecting the electrical properties of the image-carrying belt this excess charge is able to dissipate in a controlled manner between printing stations, the electrostatic voltages required for toner transfer at subsequent printing stations are thereby reduced and uncontrolled discharges are thereby avoided.
We are aware of U.S. Pat. No. 5,823,017 (Soga et al./Ricoh) which recommends the use of an intermediate transfer belt formed of a material having a volume resistivity of 10
8
&OHgr;cm to 10
10
&OHgr;cm (10
6
to 10
8
&OHgr;m), and a surface resistance of 10
7
to 10
13
&OHgr;, in a printer of a configuration different to that of the present invention. According to column 11 lines 37 et seq. of this patent, a device for discharging the intermediate transfer belt is not necessary because such a belt can be easily discharged by a ground roller. It is stated that volume resistivities lower than 10
8
&OHgr;cm (10
6
&OHgr;m) would prevent the toner image from being transferred to the belt while volume resistivities greater that 10
10
&OHgr;cm (10
8
&OHgr;m) would result in the need for an extra discharging device in addition to the ground roller.
We have found that where the resistivity is too low, such as a volume resistivity of about 10
4
ohm m or less, the image-carrying belt cannot sustain a charge sufficiently for efficient transfer of the toner image thereto. When the resistivity of the image carrying belt is too high, such as a volume resistivity of about 10
13
ohm m or more, excess charges are not able to dissipate in a controlled manner and some discharge may occur.
The image-carrying belt is ideally formed of a single layer of the electrically non-conductive material, the mean volume resistivity being from 10
8
to 10
11
ohm m volume throughout the thickness of the belt.
In the present context, volume resistivity is measured according to ASTM D257. Preferably, the non-uniformity of the resistivity of the image-carrying belt is not more than ±1 order of magnitude.
It is desirable that the material of which the image-carrying belt is formed is ozone resistant. In the present context, ozone resistance is measured by the standard NEN-ISO 1431/1-1990, in which the sample is subjected to a tensile stress of 5 MPa in an atmosphere containing 5 ppm ozone at 55% relative humidity.
It is also desirable that the material of which the image-carrying belt is formed has a low moisture adsorption. If moisture is adsorbed onto the surface of the image-carrying belt, the electrical properties thereof can change and in particular the mean volume resistivity can fall. We prefer that the material of which the image-carrying belt is formed has mean volume resistivity of at least 10
8
ohm m when measured in an atmosphere of 45% relative humidity. By using such a material, the need for extreme environmental conditioning measures to be taken in the printer is much reduced.
The surface of the image-carrying belt preferably has a hardness of at least 63 Shore D or equivalent. If the hardness is insufficient, the surface will be more susceptible to damage, e.g. by scratching.
The surface of the image-carrying belt, in contact with the image, preferably has a surface energy as low as possible to reduce the formation of toner filming and to reduce contamination from (sticky) components released by the printed substrates onto the surface of the transfer member. Most preferably, the image-carrying belt has a surface energy of less than 60 dynes/cm. Furthermore, a lower surface energy for the image-carrying belt is advantageous in maximizing the transfer efficiency and avoiding print defects in the transfer of the composite image to the transfer member.
Another important requirement with respect to the surface conditions is the roughness. The Ra-value for the surface of the image-carrying belt is preferably less than 0.5 &mgr;m to minimize the image noise and to maximize transfer efficiency.
The electrically non-conductive material from which the image-carrying belt is formed may be selected from within the classes of polyesters, polyethers, polyamides, polyimides and copolymers thereof. The material used can be homogeneous. Alternately, during the formation of the material, components can be added to modify the resistivity and/or the thermal properties and/or other properties of the material.
The image-carrying belt is preferably in the form of a non-woven continuous belt, espec
De Bock Jan
Verluyten Ludo
Grimley Arthur T.
Larson & Taylor PLC
Ngo Hoang
Xeikon NV
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