Charge roller

Abstract

This charge roller includes a core material, a base material having a thickness L1 and covering the outer circumferential surface of the core material, and a surface layer having a thickness L2 and covering the outer circumferential surface of the base material. The outer peripheral surface of the surface layer is in contact with a facing member and has a contact area S, and the power difference ΔW between the power W1 of the base material and the power W2 of the surface layer is (3.4 × L2-0.004 × L1) / S [mW] or less.

Description

Charging Roll

[0001] The present invention relates to a charging roll used in electrophotographic image formation.

[0002] Electrophotographic image formation utilizes a charging roll that uniformly charges the surface of a photosensitive drum. For example, Patent Document 1 discloses a charging roll that includes a core material, a substrate covering the outer peripheral surface of the core material, and a surface layer covering the outer peripheral surface of the substrate.

[0003] Patent No. 7342136

[0004] The electrical resistance of the substrate and surface layer of a charging roll can increase over time, for example, in a low-temperature, low-humidity environment. When the electrical resistance of the substrate and surface layer increases, the amount of discharge from the charging roll becomes insufficient, which can result in a decrease in image quality. In consideration of the above circumstances, one aspect of the present disclosure aims to suppress the increase in electrical resistance of a charging roll over time.

[0005] In order to solve the above problems, a charging roll according to one aspect of the present disclosure includes a core material, a substrate having a thickness L1 that covers the outer peripheral surface of the core material, and a surface layer having a thickness L2 that covers the outer peripheral surface of the substrate, wherein the outer peripheral surface of the surface layer contacts an opposing member with a contact area S, and a power difference ΔW between a power W1 of the substrate and a power W2 of the surface layer is (3.4 × L2 − 0.004 × L1) / S [mW] or less.

[0006] FIG. 1 is a configuration diagram of an image forming apparatus in one embodiment; FIG. 2 is a cross-sectional view of a charging roll; FIG. 3 is an explanatory diagram of a test environment; FIG. 4 is a table showing the material composition of the surface layer in each comparative example and each example; FIG. 5 is a table showing the amount of conductive material added in each comparative example and each example; FIG. 6 is a table showing the power of each element of the charging roll in each comparative example and each example; FIG. 7 is a table showing the volume resistivity of each element of the charging roll in each comparative example and each example; and FIG. 8 is a table showing the volume resistance of each element of the charging roll in each comparative example and each example.

[0007] The embodiments for carrying out the present disclosure will be described with reference to the drawings. Note that the dimensions and scale of each element in each drawing may differ from those of the actual product. Furthermore, the embodiment described below is an exemplary embodiment that may be envisioned when carrying out the present disclosure. Therefore, the scope of the present disclosure is not limited to the embodiment exemplified below.

[0008] 1 is a configuration diagram of an image forming apparatus 100 according to one embodiment of the present disclosure. The image forming apparatus 100 is an electrophotographic printing apparatus (e.g., a multifunction printer) that forms an image on a sheet-like recording medium 200 such as printing paper, and includes a photosensitive drum 11, a charging roll 12, an exposure device 13, a supply roll 14, a developing roll 15, a transfer roll 16, a fixing roll 17, and a cleaning blade 18.

[0009] The photosensitive drum 11 is a cylindrical photosensitive body whose outer circumferential surface is made of a photosensitive material. The charging roll 12 is a conductive roll that uniformly charges the surface of the photosensitive drum 11. The exposure device 13 forms an electrostatic latent image by exposing the photosensitive drum 11 to light. The supply roll 14 supplies charged toner to the surface of the developing roll 15. The developing roll 15 adheres the toner to the electrostatic latent image on the surface of the photosensitive drum 11. The transfer roll 16 transfers the toner adhered to the surface of the photosensitive drum 11 to the recording medium 200. The fixing roll 17 fixes the toner transferred to the surface of the recording medium 200. The cleaning blade 18 wipes off any toner remaining on the surface of the photosensitive drum 11.

[0010] B: Charge Roll 12 Figure 2 is a cross-sectional view of the charge roll 12. As illustrated in Figure 2, the charge roll 12 is a cylindrical roll member including a core material 21, a substrate 22, and a surface layer 23. The core material 21 is a cylindrical member that forms the rotation shaft of the charge roll 12. The core material 21 is formed of, for example, a metal material such as a stainless steel alloy, or a resin material such as polyimide.

[0011] The substrate 22 is a cylindrical portion that covers the outer peripheral surface of the core material 21. The substrate 22 is formed on the outer peripheral surface of the core material 21 with a substantially constant thickness L1. The thickness L1 of the substrate 22 is, for example, 1 mm or more and 4 mm or less. The total length of the substrate 22 (i.e., the dimension in the axial direction) is 200 mm or more and 400 mm or less.

[0012] The base material 22 is formed mainly from various rubber materials, such as polyurethane rubber (PUR), epichlorohydrin rubber (ECO), nitrile rubber (NBR), styrene rubber (SBR), and chloroprene rubber (CR).

[0013] A conductive additive, such as a carbon conductive material or an ionic conductive material, is added to the rubber material constituting the substrate 22. The electrical resistance of the substrate 22 is controlled by adding the additive. The carbon conductive material added to the substrate 22 is, for example, carbon black, such as acetylene black. The ionic conductive material added to the substrate 22 is, for example, an organic salt, such as sodium trifluoroacetate, or an inorganic salt, such as lithium perchlorate. Note that another layer, such as an adhesion layer, may be formed between the outer circumferential surface of the core material 21 and the inner circumferential surface of the substrate 22 to improve adhesion therebetween.

[0014] The surface layer 23 is a film that covers the outer peripheral surface of the substrate 22. The surface layer 23 is formed on the outer peripheral surface of the substrate 22 with a substantially constant thickness L2. The thickness L2 of the surface layer 23 is sufficiently smaller than the thickness L1 of the substrate 22. Specifically, the thickness L2 of the surface layer 23 is, for example, 4 μm or more and 25 μm or less.

[0015] The surface layer 23 is formed primarily from a resin material such as urethane resin. A conductive additive, such as a carbon conductive material or an ionic conductive material, is added to the resin material constituting the surface layer 23. The electrical resistance of the surface layer 23 is controlled by adding the additive. The carbon conductive material added to the surface layer 23 is, for example, carbon black such as acetylene black. The ionic conductive material added to the surface layer 23 is, for example, an organic salt such as sodium trifluoroacetate or an inorganic salt such as lithium perchlorate.

[0016] Because the volume of the surface layer 23 is sufficiently small compared to the volume of the base material 22, it is difficult to add a sufficient amount of conductive material, such as an ionic conductive material, to the surface layer 23. In this embodiment, the ionic conductive material added to the base material 22 migrates to the surface layer 23. Therefore, even in a situation where it is difficult to add a sufficient amount of conductive material to the surface layer 23, the electrical resistance of the surface layer 23 can be reduced by the migration of the ionic conductive material from the base material 22 to the surface layer 23.

[0017] C: Study on the Characteristics of the Substrate 22 and the Surface Layer 23 The characteristics of the substrate 22 and the surface layer 23 are studied from the perspective of suppressing an increase in the electrical resistance of the charging roll 12 over time. In studying the characteristics, the test environment illustrated in FIG. 3 is assumed. The test environment is a low-temperature, low-humidity environment with a temperature of 10°C and a humidity of 20% (relative humidity (RH)). The test was also conducted after 24 hours of seasoning.

[0018] As shown in FIG. 3 , in the test environment, the charge roll 12 and the metal roll 30 are placed parallel to each other. The outer peripheral surface of the charge roll 12 (the outer peripheral surface of the surface layer 23) contacts the outer peripheral surface of the metal roll 30. The metal roll 30 is a cylindrical body made of a metal material with a diameter of 30 mm, and corresponds to the photosensitive drum 11 in the image forming apparatus 100. A total load of 10 N (5 N on each side) is applied to both ends of the core material 21 of the charge roll 12, thereby pressing the charge roll 12 against the metal roll 30. In this state, the metal roll 30 rotates at a rotation speed of 97 rpm (rotations per minute). The charge roll 12 also rotates in conjunction with the rotation of the metal roll 30. The metal roll 30 is an example of an "opposing member."

[0019] The measuring device 40 is a measuring instrument for measuring the electrical characteristics of the charging roll 12. The measuring device 40 is equipped with a measuring terminal 41 and a measuring terminal 42. The measuring terminal 41 is connected to an end Ea of the charging roll 12 in one direction along the rotation axis. The measuring terminal 42 is connected to an end Eb of the metal roll 30 in the other direction along the rotation axis.

[0020] The measuring device 40 is capable of passing a measurement current (hereinafter referred to as "measurement current I") between measuring terminals 41 and 42. Specifically, the measurement current I supplied from the measuring terminal 41 to the end Ea passes through the core material 21 and substrate 22 of the charging roll 12, reaches the surface layer 23, is further supplied to the metal roll 30 via a region of the surface layer 23 that contacts the metal roll 30, and returns to the measuring device 40 from the end Eb of the metal roll 30 via the measuring terminal 42. Passing the measurement current I makes it possible to measure the electrical resistance and applied voltage of the charging roll 12. In the test environment, the current value of the measurement current I is set to, for example, 100 μA.

[0021] In the following description, comparative examples 1 to 3 and examples 1 to 5 are assumed. The charging roll 12 in each comparative example and example has the configuration exemplified in FIG.

[0022] FIG. 4 is a diagram showing the material composition of the surface layer 23 in each comparative example and each example. As illustrated in FIG. 4, the surface layer 23 is formed from a mixed material that uses a urethane resin as a base material and blends a carbon conductive material, an ionic conductive material, acrylic silicone, and urethane particles. FIG. 4 lists the parts by weight of each constituent material. The parts by weight are the parts by weight of each constituent material when the weight of the base material (specifically, urethane resin) of the surface layer 23 is taken as 100.

[0023] FIG. 5 is a chart showing the amounts of conductive materials added in each comparative example and each example. A carbon conductive material (trade name: Denka Black (registered trademark)) manufactured by Denka Corporation and an ionic conductive material (tetraethylammonium p-sulfonate) manufactured by Kanto Chemical Co., Ltd. were used as conductive additives for the substrate 22. A carbon conductive material (trade name: MHI-BK) manufactured by Mikuni Shikiso Co., Ltd. and an ionic conductive material (trade name: Sankonol (registered trademark)) manufactured by Sanko Chemical Industry Co., Ltd. were used as conductive additives for the surface layer 23. As can be seen from FIGS. 3 and 4 , the weight parts of the conductive materials (carbon conductive material and ionic conductive material) added to the substrate 22 or the surface layer 23 differ between the comparative examples and examples.

[0024] 6 is a table showing the power W of each element of the charging roll 12 in each comparative example and each example. Generally, the power W when a measurement current I is applied to a conductive member is defined by the following formula (1): W = I × V = I ２ ×ρ×L / S =α×L / S (1)

[0025] In formula (1), the symbol ρ is the resistivity of the conductive member, and the symbol V is the voltage applied to the conductive member. The symbol L is the overall length of the conductive member as a resistor. Focusing on the charge roll 12, the overall length L in formula (1) corresponds to the thickness L1 of the substrate 22 and the thickness L2 of the surface layer 23. The symbol S is the cross-sectional area of ​​the conductive member as a resistor. Focusing on the charge roll 12, the cross-sectional area S in formula (1) corresponds to the area of ​​the region of the charge roll 12 that contacts the metal roll 30 (hereinafter referred to as the "contact area").

[0026] The contact area S of the charge roll 12 is expressed, for example, by the following mathematical formula (2): S = N x D x σ (2) In mathematical formula (2), the symbol N is the width of the region of the charge roll 12 that contacts the metal roll 30 (the nip width in the circumferential direction). The symbol D is the total length of the region of the charge roll 12 that contacts the metal roll 30 (for example, the total length of the substrate 22). The symbol σ is the contact area ratio between the charge roll 12 and the metal roll 30 (the ratio of the real contact area to the apparent contact area).

[0027] The dimensions of each element in each comparative example and each example are as follows: L1 = 2.0 × 10 －３ [m] L2=1.0×10 －５ [m] S=5.8×10 －６ [m ２ ] σ=3.0[%] N=6.0×10 －４ [m] D=0.32 [m]

[0028] 6 shows the power W1 of the substrate 22, the power W2 of the surface layer 23, and the power difference ΔW between the power W1 and the power W2 in the form of a mathematical expression using the dimensions (L1, L2, S) of each element as variables, and the measured value obtained by the measuring device 40. The power difference ΔW is the difference between the power W1 and the power W2 (ΔW=W2−W1).

[0029] 7 and 8 are tables relating to the electrical resistance of each element of the charge roll 12 in each comparative example and each example. Fig. 7 shows the volume resistivity ρ1 of the substrate 22 and the volume resistivity ρ2 of the surface layer 23, the overall volume resistivity (ρa, ρb) of the charge roll 12 in the initial state and after energization, and the increase Δρ in the volume resistivity ρ of the charge roll 12. The volume resistivity ρ is the electrical resistance value per unit volume.

[0030] The initial volume resistivity ρa is the volume resistivity ρ of the charge roll 12 (substrate 22 and surface layer 23) in an initial state before the measurement current I has been passed. The post-current volume resistivity ρb is the volume resistivity ρ of the charge roll 12 (substrate 22 and surface layer 23) after the measurement current I has been passed for 16 hours. The electrical resistance of the charge roll 12 increases when the measurement current I is passed through the charge roll 12. The increase Δρ in the volume resistivity ρ is the difference (Δρ = ρb - ρa) between the volume resistivity ρb after the current has been passed and the volume resistivity ρa in the initial state.

[0031] 8 also shows the volume resistance R1 of the substrate 22 and the volume resistance R2 of the surface layer 23, the volume resistances (Ra, Rb) of the charging roll 12 in the initial state and after energization, and the increase ΔR in volume resistance R. The increase ΔR in volume resistance R is the difference (ΔR=Rb−Ra) between the volume resistance Rb after energization and the volume resistance Ra in the initial state.

[0032] 7 and 8 show the results of evaluating the increase in electrical resistance for each comparative example and each example. Note that "good" in FIGS. 7 and 8 means that the increase in electrical resistance was well suppressed, and "poor" means that the increase in electrical resistance was not sufficiently suppressed. "Intermediate" in FIGS. 7 and 8 means an evaluation between "good" and "poor."

[0033] 7, in Examples 1 to 5, the increase Δρ of the volume resistivity ρ is suppressed compared to Comparative Examples 1 to 3. Specifically, in Examples 1 to 5, the increase Δρ of the volume resistivity ρ is suppressed by 8.9×10 ２8, in Examples 1 to 5, the increase ΔR in volume resistivity R is suppressed compared to Comparative Examples 1 to 3. Specifically, in Examples 1 to 5, the increase ΔR in volume resistivity R is suppressed to less than 0.3 [log Ω].

[0034] From the results of the study in FIG. 7, as in Examples 1 to 5, the volume resistivity ρ1 of the substrate 22 is 3.6×10 ２ [Ω m] or less, and the volume resistivity ρ2 of the surface layer 23 is 3.4 × 10 ５ It is preferable that the resistance be [Ω·m] or less.

[0035] 6 in consideration of the results of FIGS. 7 and 8, it is clear that, as in Examples 1 to 5, the increase in the electrical resistance of the charging roll 12 over time can be suppressed by setting the power difference ΔW to (3.4×L2−0.004×L1) / S [mW] or less. Furthermore, when attention is focused on the measured values ​​of the power W (W1, W2), it is possible to suppress the increase in the electrical resistance of the charging roll 12 over time by setting the power difference ΔW to 4.7 [mW] or less.

[0036] Referring to FIG. 5 in consideration of the results of FIGS. 7 and 8 , as in Examples 1 to 5, from the viewpoint of suppressing an increase in the electrical resistance of the charging roll 12, it is preferable that the amount of ionic conductive material added to the substrate 22 be 0.4 parts by weight or more.

[0037] On the other hand, if an excessive amount of ionic conductive material is added to the base material 22, problems such as bleeding caused by the ionic conductive material may occur on the surface of the base material 22. From the viewpoint of suppressing the above problems, it is preferable that the amount of ionic conductive material added to the base material 22 be less than 2.5 parts by weight.

[0038] Furthermore, when the results of FIGS. 7 and 8 are taken into consideration and FIG. 5 is referred to, as in Examples 1 to 5, from the viewpoint of suppressing an increase in the electrical resistance of the charging roll 12, it is preferable that the amount of carbon conductive material added to the surface layer 23 be 28 parts by weight or more.

[0039] On the other hand, if an excessive amount of carbon conductive material is added to the surface layer 23, for example, the mechanical strength of the surface layer 23 may not be sufficiently ensured, which may result in defects such as cracks in the surface layer 23. From the viewpoint of suppressing the above-mentioned defects, it is preferable that the amount of carbon conductive material added to the surface layer 23 be less than 62 parts by weight.

[0040] D: Supplementary Notes From the above-described exemplary embodiments, the following configurations can be understood, for example.

[0041] A charging roll according to one aspect (aspect 1) of the present disclosure includes a core material, a substrate having a thickness L1 that covers the outer peripheral surface of the core material, and a surface layer having a thickness L2 that covers the outer peripheral surface of the substrate, the outer peripheral surface of the surface layer contacts an opposing member with a contact area S, and a power difference ΔW between a power W1 of the substrate and a power W2 of the surface layer is (3.4 × L2 − 0.004 × L1) / S [mW] or less.

[0042] According to the above aspect, the power difference ΔW between the base power W1 and the surface layer power W2 is suppressed to (3.4×L2−0.004×L1) / S [mW] or less, thereby reducing the possibility of an excessive voltage being applied to only one of the base material and the surface layer (for example, only the surface layer).As a result, it is possible to suppress an increase in the electrical resistance of the charging roll over time.

[0043] The base material power W1 is the power value when a predetermined current (e.g., 100 μA) is passed through the base material, and the surface layer power W2 is the power value when this current is passed through the base material. The power difference ΔW is the difference between the power W1 and the power W2 (ΔW = W2 - W1). The base material thickness L1 is the average thickness of the base material. Similarly, the surface layer thickness L2 is the average thickness of the surface layer.

[0044] In a specific example (Aspect 2) of Aspect 1, the volume resistivity of the substrate is 3.6×10 ２ [Ω m] or less, and the volume resistivity of the surface layer is 3.4 × 10 ５ [Ω m] or less. According to the above-described embodiment, the volume resistivity of both the substrate and the surface layer is suppressed, thereby reducing the possibility of excessive voltage being applied to the substrate and the surface layer. Therefore, the aforementioned effect of being able to suppress an increase in the electrical resistance of the charging roll over time is particularly remarkable.

[0045] In a specific example (Aspect 3) of Aspect 1 or Aspect 2, an ionic conductive material is added to the substrate in an amount of 0.4 parts by weight or more and less than 2.5 parts by weight. In the above aspect, by adding 0.4 parts by weight or more of the ionic conductive material to the substrate, the volume resistivity of the substrate can be effectively reduced. Furthermore, by limiting the amount of the ionic conductive material to less than 2.5 parts by weight, the possibility of bleeding caused by the ionic conductive material occurring on the surface of the substrate or the surface layer can be reduced.

[0046] The unit of "parts by weight" for the amount of additive (e.g., ion conductive material) added to the substrate is the weight part of the additive when the weight of the base rubber of the substrate is 100 (PHR: Part per Hundred parts of Rubber).

[0047] In a specific example (Aspect 4) of any one of Aspects 1 to 3, the surface layer contains a carbon conductive material in an amount of 28 parts by weight or more and less than 62 parts by weight. In the above aspects, by adding 28 parts by weight or more of the carbon conductive material to the surface layer, the volume resistivity of the surface layer can be effectively reduced. Furthermore, if an excessive amount of carbon conductive material is added to the surface layer, the mechanical strength of the surface layer cannot be sufficiently ensured, which may result in cracks occurring in the surface layer. By limiting the amount of carbon conductive material to less than 62 parts by weight, the mechanical strength of the surface layer is ensured, thereby reducing the possibility of cracks occurring in the surface layer.

[0048] The unit of "parts by weight" for the amount of additive (e.g., carbon conductive material) added to the surface layer is the weight part of the additive when the weight of the resin component (e.g., urethane resin) that makes up the surface layer (PHR: Part per Hundred parts of Resin) is taken as 100.

[0049] In a specific example (Aspect 5) of any of Aspects 1 to 4, the thickness L1 of the substrate is 1 mm or more and 4 mm or less. In a specific example (Aspect 6) of any of Aspects 1 to 5, the thickness L2 of the surface layer is 4 μm or more and 25 μm or less. Furthermore, in a specific example (Aspect 7) of Aspects 1 to 6, the total length of the substrate is 200 mm or more and 400 mm or less.

[0050] 100...image forming apparatus, 200...recording medium, 11...photosensitive drum, 12...charging roll, 13...exposure device, 14...supply roll, 15...developing roll, 16...transfer roll, 17...fixing roll, 18...cleaning blade, 21...core material, 22...substrate, 23...surface layer, 30...metal roll, 40...measuring device, 41, 42...measuring terminals

Claims

1. A charging roll comprising a core material, a base material with a thickness L1 covering the outer peripheral surface of the core material, and a surface layer with a thickness L2 covering the outer peripheral surface of the base material, wherein the outer peripheral surface of the surface layer contacts a counter member with a contact area S, and the power difference ΔW between the power W1 of the base material and the power W2 of the surface layer is (3.4×L2 - 0.004×L1) / S [mW] or less.

2. The volume resistivity of the base material is 3.6 × 10 ２ [Ω·m] or less, and the volume resistivity of the surface layer is 3.4 × 10 ５ [Ω·m] or less. The charged roll according to claim 1.

3. The charging roll according to claim 1, wherein an ion conductive material is added to the base material in an amount of 0.4 parts by weight or more and less than 2.5 parts by weight.

4. The charging roll according to claim 1, wherein a carbon conductive material is added to the surface layer in an amount of 28 parts by weight or more and less than 62 parts by weight.

5. The charging roll according to claim 1, wherein the thickness L1 of the base material is 1 mm or more and 4 mm or less.

6. The charging roll according to claim 1, wherein the thickness L2 of the surface layer is 4 μm or more and 25 μm or less.

7. The charging roll according to claim 1, wherein the total length of the base material is 200 mm or more and 400 mm or less.

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