JPH0594104A - Transfer roller of electrophotographic type printer and transfer method thereof - Google Patents

Abstract

PURPOSE: To increase a nip width and to prevent the occurrence of null characters by reducing a pressure per unit area between a transfer roller and a printing medium. CONSTITUTION: The transfer roller 10 is provided with an internal core member 12 functioning as the rotary shaft of the transfer roller 10 and a deformable cylinder 14 constituted of a member surrounding the core member 12 in a tight contact state and made of a prescribed material having prescribed thickness, volume resistivity and deformation ratio. The deformable cylinder 4 is coated with a thin insulating material 16, When it is in operation, the thin insulating material 16 comes into contact with the printing medium 26. That is, when the cylinder 14 formed of the selected material having the selected thickness, volume resistivity and deformation ratio is compressed inward in a direction of the core member 12 for regulating the rotary shaft of the transfer roller 10 and begins deforming, the nip width and a contact zone are increased between the transfer roller 10 and the printing medium 26, and a total pressure applied between the transfer roller 10 and the printing medium 26 is reduced. According to the operational characteristics, the occurrence of the null characters is prevented.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates generally to electrophotographic printing apparatus for transferring an image from a photoconductor to an adjacent medium, and more particularly to improving the printing quality of the transferred image.
It relates to a new and improved transfer roller that works closely with the media.

[0002]

BACKGROUND OF THE INVENTION In the field of electrophotographic printing, there are two major types of image transfer systems currently used to transfer latent images from organic photoconductors to adjacent printing media. There is. One of these systems is a corona discharge system in which the photoconductor is not in direct contact with the media, but operates electrostatically to transfer the image from the photoconductor to the print media. The corona system has excellent transfer efficiency and the transferred image quality is quite good. However, these corona systems need to operate at very high pressures,
There are some distinct drawbacks, including the corresponding production of ozone. In addition, these corona systems have been unable to produce edge sharpness for transfer characters, where sharpness is required for certain high print quality applications.

Another type of image transfer system in the field of electrophotographic printing is the so-called roller system, in which the photoconductor is in close contact with the adjacent media and the transfer efficiency is as high.
It is a type that creates an image without generating ozone.
These roller systems utilize a relatively low DC bias level of 1 to 3 kilovolts as compared to corona systems which require double DC operating voltage. The cost of these roller systems is significantly less than the cost of corona systems due to its less complex design and fewer mechanical parts. Moreover, the sharpness of the edges of the roller-transferred characters is superior to the corona system because the rollers provide a charged surface that is in intimate contact with the adjacent media.

The transfer force of these roller systems is a combination of both electrostatic and mechanical forces, with the mechanical force complimentarily accommodating the media and toner to enhance the image quality at the edges of the characters to be transferred. Is necessary for. However, this type of transfer roller system requires rigorous roller material and DC bias level selection to prevent air ionization and unacceptable toner perturbation over a wide range of media. is necessary. Of these, toner disturbance is difficult to control under all environmental conditions,
In addition, the roller system is subject to undesired contamination problems by the toner and / or the fibers of adjacent paper, as the paper is in constant contact with the photoconductor and the toner on it.

The area of the contact zone between the photoconductor and the medium is also very important to the performance of the roller system.
This contact zone, or so-called "nip width", affects both the electrostatic field strength and the mechanical force applied to the toner layer. Variations in the transfer roller size and / or its hardness also adversely affect the performance of the roller system.

One of the major problems with print quality in roller systems has been the "blank character" or blank of letters, for example, in the outline of black letters and numbers. The combination of mechanical and electrical forces exerted on the toner image is such that the pressure is sometimes too high in the area of mutual contact. This, on the other hand, means that the toner may be transferred from the photoconductor and stick to the surface of the photoconductor instead of being deposited on the medium. At the most basic level, this phenomenon can be compared to a "pull" between the photoconductor and the toner on the one hand and the toner and the medium on the other hand. In addition, small and narrow letters are
It has also been shown that the density of blank characters is higher than in larger print areas as a result of the pressure distribution (force applied per unit of area) acting on smaller lines and characters. Narrow lines or characters are subject to higher peak pressures over a narrower band than larger, wider ones.

One conventional method for eliminating or minimizing the occurrence of such blank characters is the pressure applied between the photoconductor and the medium during transfer of the characters to the medium. That is, a method of weakening the force per unit of area is included. An example of this method is described in US Pat.
No. 6,047. This method employs a medium transport structure that utilizes multiple medium rollers,
The media wraps over a larger than normal surface area of the transfer roller,
It is operable to be compatible with this. This mechanism, on the one hand, increases the area of the roller / media contact zone, ie the nip width, and thus reduces the pressure in the area where toner is transferred from the photoconductor to the media. However, the use of multiple rollers in the above techniques adds to the mechanical complexity and cost of the transfer roller system, and also allows the multiple rollers to move precisely relative to each other and to the photoconductor and the media. In order to do so, a high degree of control is required.
Further, such multiple rollers are precisely positioned and precise to both the media and transfer rollers to maintain a precise and constant contact zone, that is, the nip width between the roller, media and adjacent photoconductive drums. Must be held in orientation and position.

[0008]

The basic objects and problems of the present invention are, for example, the above-mentioned US Pat.
Novel for increasing the contact zone, i.e. the nip width, between the surface of the transfer roller, the media and the photoconductor surface, without resorting to complex and costly mechanical drive mechanisms as disclosed in US Pat. And to provide an improved method.

[0009]

To this end, and in accordance with the present invention, a deformable deformable drive for driving an adjacent medium to, and beyond, an adjacent surface of a photoconductor. The transfer roller was discovered and developed. The deformable transfer roller, along with other components, defines an inner core member that defines the axis of rotation of the roller and a specific material, thickness, volume resistivity that surrounds and is in close contact with the inner core member. And a deformation cylinder with a deformation rate.

The deformable cylinder is preferably coated with a thin absolute material, which in operation is in direct contact with the adjacent print medium. When the cylinder of the selected material, thickness, volume resistivity and deformation rate is compressed inward in the direction of the inner core member that defines the rotation axis of the roller and begins to deform, the nip width and the transfer roller and medium The contact zone therebetween is increased, thus reducing the overall pressure applied between the transfer roller and the print medium. This operational characteristic in turn eliminates the previously mentioned blank characters that occur with conventional transfer rollers.

Another object of the present invention is to provide a new and improved transfer roller of the aforesaid type which is relatively economical to manufacture and which can utilize a plurality of materials in making the aforementioned deformable cylinder member. It is to be. The above materials can be selected from the group of insulators, resistors, lossy dielectrics and conductors depending on the print quality and print speed needs of the particular transfer roller system.

Another object of the present invention is generally about 10 ⁶ ohm centimeters for high conductivity or wet paper and high resistance or dry paper, respectively, for a wide range of office environments. To provide a new and improved transfer roller of the above type which operates to eliminate blank characters using a wide variety of media with volume resistivities varying from 1 to 10 ¹² ohm centimeters. ..

A novel feature of the present invention is to provide a method comprising the following steps for minimizing or eliminating blank characters during electrostatic printing while at the same time maintaining a sufficiently high electrostatic toner transfer pressure. That is. That is, (a) the transfer roller is provided close to the medium located between the surface of the transfer roller and the photoconductor, and (b) the surface contact area between the contact surface of the transfer roller and the medium adjacent thereto, that is, The surface area of the transfer roller is deformed to increase the nip width.
_The nip width equal to _e = [E _A + σ _net / 2 ε _O ] · σ _net , ie, the controlled non-linear electrostatic pressure corresponding to the contact area versus the dwell time (dwell t) of the nip width.
ime) Each stage of establishing characteristics. Where | P _e
│ is the electrostatic voltage acting on the toner, E _A is the electrostatic field on the toner that depends on time, ε _O is the tolerance of free space,
Also, σ _net is the net charge on the toner. The ideal condition for effective toner transfer is to create maximum negative pressure on the toner. The above-mentioned objects and problems of the present invention and the advantages and novel features related thereto will be further clarified by the following description with reference to the accompanying drawings.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1A, a cylindrical deformable transfer roller 10 comprises an internal core member 12 which constitutes its rotational axis. The inner core member 12 is surrounded by a deformable cylinder 14 having a predetermined thickness, volume resistivity and deformation rate. The deformable cylinder 14 is made of a conductive material such as polyurethane having a Shore hardness of 15 durometer and a thickness of 2 cm in the preferred embodiment. The deformable cylinder 14 is in intimate contact with the core member 12 and its outer surface is surrounded by a thin outer protective covering material 16 of a selected insulating material such as Teflon 25-50 microns. Generally, the inner core member 12 is connected by a conductor 18 to a DC power source 20 as shown, from which it is adapted to receive a predetermined DC bias voltage, typically about 1-3 kilovolts.

In the case of the transfer roller of the typical size shown in FIG. 1A, the conductive inner roller, that is, the core member 1 is used.
2 has a diameter of about 8 mm, while the outer coating 1
The total diameter of 6 is about 32 millimeters. Thereby, between the edge of the roller 12 and the outer surface of the outer coating 16 approximately 12
Millimeter spacing is left. In operation, the deformable transfer roller 10 is directed to a larger driven organic photoconductive cylinder 22 that has been processed using known electrophotographic processes to form a toner layer 24 thereon as shown. It is arranged to rotate.

The medium 26 is located between the outer surface of the thin coating material 16 on the transfer roller 10 and the thin toner layer 24 on the organic photoconductor 22, as shown. Thus, in certain toner layer areas, i.e. areas within layer 24 developed, for example by controlled laser radiation, the inner core member 1 is
The DC voltage 20 applied to 2 electrostatically draws it into the medium 26, which results in the writing of letters, numbers and other images on the print medium 26.

Before using the non-deformable transfer roller before the present invention, the transfer roller had a relatively steep dwell time characteristic with respect to the nip width, and a bell-shaped curve having a small area was formed. It was drawn, which meant that undesirably high mechanical pressure and force per unit area were applied to the particular characters printed. In turn, this caused the toner to be transferred from the surface of the photoconductor and not to be deposited on the medium but to adhere to the surface of the photoconductor. This phenomenon, in turn, created "blank" characters, resulting in white, unprinted areas in the characters corresponding to toner that was not properly transferred from the photoconductive drum to the media.

However, in accordance with the present invention, the deformable transfer roller 10 is provided with a significantly increased nip width, that is, an increased contact zone representing the actual contact area between the medium 26 and the transfer roller 10. This increased contact area between the transfer roller and the adjacent media is represented by the dotted line in FIGS. 2A and 2B, described below, where the increased contact area reduces the pressure on the contact zone, ie per unit area. Of the toner, which promotes the transfer of toner from the photoconductor 22 to the print medium. This feature, in turn, eliminates the aforementioned blank characters previously due to this nip width, i.e., excessive mechanical force on the contact zone between the conventional non-deformable transfer roller and the adjacent print media. However, those skilled in the art will appreciate that the desired rotational speed of the transfer roller of FIG. 1A above can be optimized for best print quality when considered in conjunction with the desired media transport speed and rotational speed of the photoconductive drum 24. Be understood.

It is not sufficient to simply reduce the nip width, ie the mechanical force and pressure applied to the contact zone, by making the transfer roller deformable as shown in FIGS. 1A and 2A. In addition, the electrostatic force acting on the toner must now be optimized, taking into account the following parameters along with the now reduced mechanical force on the nip.

First of all, consider that the absolute value of the electrostatic voltage | P _e | is equal to the product of the net charge σ _net on the toner and the average electrostatic field E _AVG on the toner according to equation 1 below.

Equation 1: P _e = σ _net · E _AVG Then, since the average electrostatic field E _AVG is equal to the sum of the electric field E _A above the toner and the electric field E _B ÷ 2 below the toner, | P _e | It can be expressed as:

Equation 2: P _e = σ _net · [E _A + E _B / 2] The absolute value of this electrostatic voltage | P _e | can be further converted and expressed according to the following equation.

Equation 3: P _e = [E _A + σ _net / 2ε _O ] · σ _net where ε _o is the admissibility of free space.

Now, the above equation for static voltage | P _e | ranges from 10 ⁶ ohm centimeters for wet paper, ie high conductivity, to 10 ¹² ohm centimeters for dry paper, ie high resistance. It must be considered in view of the possible changes in paper resistivity. The electrostatic voltage transfer characteristic as shown in FIG. 2B reaches 98% of the maximum value in a very short time in the case of a wet medium, that is, in the case of a medium having a high conductivity, while in the case of a dry paper (or Since the worst case case) only reaches 55% to 60% of the maximum value, the maximum value of | P _e | is calculated for many dwell times t _{nip in} both psi and Pascal, and the paper resistivity is Was assumed to be in the range of 10 ⁴ to 10 ¹² and the roller core resistivity of the conductive core 14 was 10 ¹² ohm centimeters.

Referring now to FIG. 2C, the results of these calculations are tabulated, where the maximum value of | P _e | is 0.1.
Corresponding t _nip varying from 00 seconds to 0.500 seconds
About 0.11 to 0.20p depending on dwell time
It is the range of si. Here, paper resistivity = 10 ¹² OHM: CM, roller core resistivity = 10 ⁴ -10 ⁸ OHM-CM, P _a = N /
M ^2, E _A = electric field on the toner, sigma _{net Non} = toner net _{charge, P e = (E A +} σ net / 2E O) · σ net exactly these results nip the nip width, i.e. the contact zone illustrated Results in the | P _e | absolute value vs. t _nip dwell time measured by dividing by the rotational speed of the medium 26 passing through. In this same series of measurements, the static voltage | P _e | maximum value for high resistivity, ie, transparent paper is about 0.07 psi in psi.
While the wetted or most conductive paper 26 has a static voltage of about 0.27 psi, FIG.
It was observed to be slightly outside the boundary of the maximum range of | P _e | shown in C.

Without departing from the spirit and scope of the present invention,
Many modifications can be made to the embodiment described above and in addition. For example, the invention is not limited to the particular materials shown in FIGS. 1A and 1B above, and the mechanical positioning and biasing configurations shown in FIG. 1A. In particular,
The deformable flexible core member 14 of FIG. 1A can be formed of another conductive foam instead of polyurethane foam, and loss that is considered more conductive depending on the particular needs of the user. Dielectrics, and materials with even higher resistivity can be suitably used. For example,
Higher conductivity allows the use of thicker core materials at lower DC bias levels and faster processing speeds. The lossy dielectric material limits the thickness that can be used as the core material and increases the required DC bias level.

Criteria for the range of resistivity of the core material are also important and must be controlled according to appropriate environmental conditions. The ionization of the bursting air can be controlled in the areas in front of the nip of the transfer roller, and the toner transfer electric field in these areas in front of the nip is the nip for changes in the resistivity of the media
It is less sensitive than the contact zone. On the other hand, increasing the resistivity of the core material limits the variability of its thickness,
Significantly higher DC bias levels are required for proper operation of the transfer roller. However, such high resistivity core materials generally eliminate the need for a thin protective coating and reduce changes in the transfer field due to changes in the physical and electrical parameters of the medium.

In embodiments where a thin outer cover layer 16 is required, similar insulating materials other than Teflon may be used to form it. Further, the novel transfer roller method and apparatus described above can be utilized with many different types of photoconductive drum and toner layers 22 and 24, respectively.

[0029]

As described above, according to the present invention, the pressure per unit area between the transfer roller and the print medium is reduced to prevent the toner from adhering to the photoconductor, and thus the occurrence of blank characters. To prevent. 2. The manufacturing cost can be made more economical than the conventional type. 3 The white space can be prevented even if a wide variety of media are used. And so on.

[Brief description of drawings]

FIG. 1A is a schematic side view showing an embodiment of the present invention.

1B is a cross-sectional view taken along the line BB of FIG.

FIG. 2 is a diagram showing a static voltage applied to a transfer roller and its positional relationship.

FIG. 3 is a chart showing the relationship between the static voltage and the dwell time of the nip width based on the measurement results.

[Explanation of symbols]

 10: Transfer roller 12: Inner core member 14: Deformable cylinder 16: Outer coating 20: DC power supply 22: Photoconductor 24: Toner layer 26: Printing medium

Claims (3)

[Claims] 1. A transfer roller for image transfer of an electrophotographic printer, comprising: (a) an inner core member, which is a rotation axis of the transfer roller, and (b) a close contact with the inner core member, surrounding the inner core member. And a deformable cylinder made of a member having a predetermined thickness, volume resistivity and deformation ratio. By the deformation of the cylinder, the contact area or the nip width between the transfer roller and the print medium adjacent to the transfer roller is expanded. A transfer roller characterized by the following. 2. (a) Positioning the print medium between the transfer roller and the photoconductor by providing the transfer roller in contact with the print medium, and (b) the surface of the transfer roller and the transfer roller. Deforming the surface of the transfer roller to increase the contact area or nip width formed by the surface of the print medium in contact, (c) the absolute electrostatic voltage acting on the toner | P _e |, above the toner electrostatic field and E _a, the tolerance epsilon _O of free space, the net charge on the toner when the sigma _net, P _e = [E _a + σ _net / 2ε _O] · sigma as relationship _net is established And a step of setting a dwell time characteristic of a predetermined non-linear static voltage and a nip width, and an image transfer method in an electrophotographic printer. 3. A step of (a) forming a relatively large contact area or a nip width between a photoconductor drum and a transfer roller, and (b) an absolute electrostatic voltage acting on the toner | R _e |, Letting E _{A be} the electrostatic field on the toner, ε _O be the admissibility of the free space, and σ _{net be} the net charge on the toner, the relationship of P _e = [E _A + σ _net / 2 ε _O ] ・ σ _net An image transfer method in an electrophotographic printer, comprising the step of varying the static voltage so that