Patent Grant
7873312
The present application claims priority from Japanese application serial No. 2007-148123, filed on Jun. 4, 2007, which claims priority from Japanese patent application serial No. 2006-348106, filed on Dec. 25, 2006, the contents of which are hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a transfer apparatus, a method of manufacturing the transfer apparatus and an image forming apparatus using the transfer apparatus.
2. Description of Related Art
An image forming apparatus using the electronic photographic method transfers a toner image formed on a toner image supporting body such as a photosensitive belt or an intermediate transfer body, to a recording medium, and melts and fixes the toner image on the surface of the recording medium by using a fixing device.
a) illustrates transfer to roughened surface paper as typical inexpensive paper or to a paper surface including concaves 17, such as a second surface deformed by heat generated during the fixing of a toner image to a first surface. The depth d of the concave 17 is 30 to 50 μm, and the width Wh of the concave is 8 to 10 mm. In this transfer, the negatively charged toner 21 needs to be attracted to the paper 16 by an electrostatic field acting between positive charges 20 supplied to the back of the paper 16 by the corona transfer unit 18 and the electrode layer 25b of the photosensitive belt. On the flat part of the paper 16, a toner image is brought into close contact with the surface of the paper 16 and thus a sufficient transfer electric field is applied to the toner 21a, so the toner 21a is efficiently transferred. For the toner 21b facing the concave 17 on the surface of the paper 16, there is a void with a depth of d between the concave 17 and the surface of the paper 16, so the transfer electric field acting on the toner 21b is weakened, lowering the toner transfer efficiency and thereby causing an image failure.
b) illustrates transfer of a color toner image formed on the intermediate transfer belt 19 to a surface of an embossed paper on which concaves and convexes are artificially formed by performing embossing on coated paper to form a embossed processing such as aventurine lacquer, the texture, the fine grain photoprint. Embossed paper is used to form tickets and front covers of catalogs and brochures. Although the depth d of the concave 17 varies with the type of embossing, the depth d falls within the range of 10 to 30 μm; the width Wh of the concave is 0.2 to 0.4 mm. In this transfer, the color toners of two or three layers formed on the intermediate transfer belt 19 need to be transferred together to the interior of the concave 17, which is narrower than the former concave 17. The transfer electric field is weak for the toner layer facing the concave 17 as in the transfer in
a) and 15(b) illustrate forces exerted on toner during electrostatic transfer. In
To overcome the resultant of the mirror image force FM and van der Waals's force Ff so as to transfer the toner 21 to the paper 16, the electrostatic force FE needs to be increased. A method for this is to increase the transfer electric field E by increasing a voltage/current applied to the corona transfer unit 18 so as to increase the corona charge amount of positive charges 20 supplied to the back of the paper 16. If the intensity of the transfer electric field E becomes too high, however, the electric field is locally concentrated and thereby the toner 21 scatters, lowering the image quality. A possible method of solving this problem is to reduce the force to attract the toner 21 to the toner image supporting body 38 (the sum of mirror image force FM and van der Waals's force Ff) and to supply another force to the toner 21 so as to direct the toner 21 toward the paper 16.
The mirror image force FM is electrostatic force acting between the charge on the toner 21 and a mirror image charge generated on the toner image supporting body 38; it depends on the particle diameter and charge of the toner 21 as well as the dielectric constant and thickness of the toner image supporting body 38. The van der Waals's force Ff, which is a non-electrostatic force, is derived from the following equation.
Ff=A×R/(6×D2) (1)
A is the Hamaker constant, which depends on the materials of the toner 21 and toner image supporting body 38. R is the radius of a toner particle. D is a distance between the toner 21 and the toner image supporting body 38. As seen from equation (1), Ff is proportional to the radius R and inversely proportional to the square of the distance D between the toner 21 and the surface of the toner image supporting body 38.
To reduce the force to attract the toner 21 to the surface of the photosensitive body, as shown in
As a means for applying vibration energy from the backside of the toner image supporting body 38 such as a photosensitive belt or an intermediate transfer belt, methods in which an electromagnetic oscillator or ultrasonic oscillator is used are proposed (Patent Document 1). Of these, only the method in which an ultrasonic oscillator is used is put into practical use.
In this method, as illustrated in
Patent Document 1: Japanese Patent Laid-open No. Sho 55(1980)-20231
Patent Document 2: Japanese Patent Laid-open No. Hei 04(1992)-234076
Patent Document 3: Japanese Patent Laid-open No. Hei 04(1992)-234082
Patent Document 4: Japanese Patent Laid-open No. Sho 62(1987)-248953
Patent Document 5: Japanese Patent Publication No. Hei 04(1992)-20276
Patent Document 6: Japanese Patent Laid-open No. 2005-303937
Wide printing is increasingly demanded for printers, copiers, and other image forming apparatuses. Cut-sheet printers are demanded to support the A3 size and wider, and continuous printers are demanded to support 20-inch width and wider. Accordingly, the toner image supporting body is also widened, and an area to which vibration energy is supplied by a vibrating source is 420 mm to 500 mm or more in width.
The width that the vibrating unit can cover is determined by the resonance characteristics of the ultrasonic oscillator 39b and horn 39a; the range of the width the vibrating unit can support is 2 to 3 inches. To support 20 inches or more, seven to ten or more resonators need to be aligned. This raises a problem that mutual interference (a phenomenon called cross coupling) is caused when a plurality of resonators are driven. Countermeasures, for example, for preventing adjacent horns from being brought into mutual contact (Patent Document 2) are needed. In this case, however, vibration energy cannot be supplied to the toner image supporting body between the adjacent horns.
When a plurality of resonators are disposed, the mutual interference impedes individual resonators from having uniform vibration characteristics (mainly, the vibration rate). Particularly, the vibration rate tends to be lowered at both sides. Since different driving voltages thereby need to be applied to the central part and both sides so that the vibration characteristics become uniform, countermeasures, for example, for driving different resonators with different voltages (Patent Document 3) are disposed.
In general, a Langevin oscillator tightened with bolts is used as the ultrasonic oscillator. Oscillators of this type are aligned. To drive a single Langevin oscillator, 70 to 140 W of electric power is needed, so hundreds of watts is needed to support the 20-inch width. Therefore, a power supply of high frequency and high voltage operating at a frequency of 20 kHz or higher is required, resulting in a high cost.
The present invention addresses the above problem involved in the prior art with the object of providing a transfer apparatus that enables transfer to embossed paper having concaves and convexes on its surface or to roughened surface paper and also enables superior toner transfer, without image failures, to concaves on a second surface of paper that are formed when the paper is deformed (for example, wrinkled) by heat generated during the fixing of toner image to a first surface, as well as an image forming apparatus that uses the transfer apparatus.
The present invention relates to a transfer apparatus having a corona transfer means, which faces a toner image supporting body such as a photosensitive belt or an intermediate transfer belt, and transfers a toner image formed on the toner image supporting body to a recording medium transferred to a transfer area disposed between the toner image supporting body and the corona transfer means, and also relates to an image forming apparatus using the transfer apparatus.
The present invention, which is a transfer apparatus, comprises a toner image supporting body; a corona transfer means, which is oppositely disposed to a toner image supporting body, wherein an electrostatic toner image formed on the toner image supporting body is transferred to a recording medium transported to a transfer area disposed between the toner image supporting body and the corona transfer means; and a vibrating unit that applies vibration energy to a back side of the toner image supporting body, the vibrating unit being disposed opposite to the corona transfer means with the toner image supporting body intervening therebetween, wherein: the vibrating unit has a cantilever structure for holding one end of a piezoelectric bimorph-type actuator having such a structure that a pair of piezoelectric bodies each having an electrodes on the surface thereof are bonded, and a protrusion portion is provided at an end of the cantilever opposite to a supporting and fixing part of the piezoelectric bimorph-type actuator, and reciprocal vibration, which is caused when a voltage is applied to the pair of piezoelectric bodies, is transmitted to the back side of the toner image supporting body through the protrusion portion. Better toner transfer with lower power consumption is possible without power supply of a high frequency and a high voltage hereby, in comparison with a conventional method using an ultrasound transducer.
The present invention, which is a transfer apparatus for electrostatically transferring a toner image formed on a toner image supporting body, includes a corona transfer means disposed opposite to the toner image supporting body and a vibrating means disposed opposite to the corona transfer means so as to supply vibration energy to the backside of the toner image supporting body; the vibrating means has a cantilever structure that supports an end of a piezoelectric bimorph element structured by attaching a pair of piezoelectric bodies, each of which has an electrode on its front surface, to both surfaces of a conductive elastic member (it is called a shim member in the following sentences), a protrusion portion being provided on the other end of the piezoelectric bimorph element; when a driving voltage is applied across the shim member and the electrode on the front surface of the piezoelectric body, no voltage is applied to an area, on the piezoelectric body area, in which the piezoelectric bimorph element is supported. This constitution makes a life of the piezoelectric bimorph element longer.
The piezoelectric bimorph element occupies areas in which the electrodes on the front surfaces of the piezoelectric bodies and the shim member are overlapped, the piezoelectric bodies and the shim member constituting the piezoelectric bimorph element; an area on the piezoelectric body in which the piezoelectric bimorph element is supported is not included on the piezoelectric body area, in which distortion occurs substantially due to a reverse piezoelectric effect.
The piezoelectric bimorph element occupies areas in which the electrodes on the front surfaces of the piezoelectric bodies and the shim member are overlapped, the piezoelectric bodies and the shim member constituting the piezoelectric bimorph element; only a vibration area including a free end of the piezoelectric bimorph element is included on the piezoelectric body area, in which distortion occurs substantially due to a reverse piezoelectric effect.
A piezoelectric ceramic plate or piezoelectric film, which is part of the piezoelectric bimorph element, undergoes polarization over its surface in the thickness direction; an electrode for driving the piezoelectric bimorph element is formed only a particular area on the surface of the piezoelectric ceramic plate or piezoelectric film.
The transfer apparatus is a wide bimorph cell in which the piezoelectric body is a piezoelectric ceramic plate or piezoelectric film, the width of the shim member is equal to or more than the width of the transfer area from which a transfer occurs to the recording medium, a plurality of piezoelectric bodies are provided in the direction of the width of the transfer area width at fixed intervals, and expansion and contraction of each of the plurality of the piezoelectric bodies, which occur when a voltage is applied, are transferred to the transfer area by using the shim member as a common base.
The image forming apparatus in the invention comprises a toner image supporting body, such as an intermediate transfer belt or a photosensitive belt, which rotates and on the surface on which a toner image is formed, and a transfer unit, disposed opposite to the toner image supporting body, for transferring the toner image to a recording medium; the transfer apparatus described above is used as the transfer unit.
The present invention relates to a new transfer apparatus for an image forming apparatus comprising a bimorph type actuator, in which a plurality of piezoelectric bodies are bonded to both sides of a single shim member (an elastic reinforcing plate) and a cantilever structure holding an end of the actuator and a protrusion portion is provided on the other end and reciprocal vibration is applied to a toner supporting body. Advantages of the present invention is that a uniform vibration can be given to an overall width of the toner supporting body and that a cross coupling phenomenon does not occur and a single actuator can give the vibration to a wide printing (wider than 20 inches) since movement of the plural piezoelectric bodies appears as movement of a single shim member. As a driving voltage of the actuator in the present invention is 10 to 40 volts, it can decrease to ½ to ⅓ times voltage of a conventional resonator type actuator, and a driving frequency of the actuator in the present invention is equal to or less than 20 kHz. In addition, a power consumption of the actuator in the present invention can be reduced to a fraction (about ⅓ to 1/10).
In the piezoelectric bimorph element according to embodiments of the present invention, a reverse piezoelectric effect occurs only in the free end area during driving. Accordingly, the following advantage is obtained.
No stress occurs on a boundary between the piezoelectric bimorph element and a fixing member. When vibration occurs at a high frequency with a large displacement, piezoelectric ceramic plates (PZT), which are part of the piezoelectric bimorph element, are not damaged, making the vibrating means highly reliable.
Further, the same advantage can be obtained in the piezoelectric bimorph element for a width transfer because the element structure of the invention can be applied to the wide shim member.
The device structure according to the present invention can also be applied to a shim member, so the same advantage can be obtained from a piezoelectric bimorph element ready for wide transfer.
Embodiments of the present invention will be described in detail with reference to the drawings.
First, the main structure of a vibrating unit, which is the main part of the transfer apparatus in the invention, will be described.
As a means for vibrating a wide toner image supporting body at high speed, the vibrating unit in the present invention uses the bimorph-type actuator method in which transverse effect vibration (d31 mode) of a piezoelectric body is employed, rather than the resonator method in which longitudinal vibration (d33 mode), which causes mutual interference (cross coupling), is employed.
a) and 12(b) and
a) to 12(c) illustrate the operation of an actuator having a transverse effect, which causes expansion and contraction in a direction perpendicular to the thickness of the piezoelectric body, that is, in the plane direction, when a voltage is applied in the thickness direction. In
Reference numeral 131 indicates spontaneous polarization of the piezoelectric body 1.
c) illustrates another case in which a DC voltage Vd is applied to the piezoelectric body 1 so that an electric field is generated in the same direction as the direction of the spontaneous polarization 131. The piezoelectric body 1 contracts from both ends in the plane direction, by ΔL/2 each. The amount of expansion or contraction ΔL can be represented by using the piezoelectric distortion constant d31, length L, and thickness t of the piezoelectric body 1, as in equation (2).
ΔL=d31×L×Vd/t (2)
The value of the piezoelectric distortion constant d31 varies with the composition of the material of the piezoelectric body. Even materials comprehensively classified as PZT, the piezoelectric distortion constant of which varies within the range of 80×10−12 m/V to 375×10−12 C/N, are used in practical applications.
The reverse piezoelectric effect in
In
In
The displacement U and resonant frequency f of the piezoelectric bimorph element 7 are given by equations (3) and (4).
Displacement U(m)=3×d31×(L/tt)2×(1+ts/tt)×α×V (3)
Resonant frequency f(Hz)=0.162×(tt/L2)×√{square root over ((Y−ρ))} (4)
where tt is the total thickness of the piezoelectric bodies 1 and 5 as well as the shim member 4, ts is the thickness of the shim member 4, α is a nonlinear compensation constant, which is 2, Y is a Young's modulus as the piezoelectric bimorph element 7 (including the piezoelectric bodies 1, 5 and shim member 4), ρ is a density as the bimorph cell, d31 is the piezoelectric distortion constant, L is a vibration length, and V is an applied voltage.
The vibration frequency of the piezoelectric bimorph element 7 is several kilohertz or less, which is lower than the vibration frequency of an ultrasonic oscillator. However, its displacement U is hundreds of micrometers to several millimeters. By comparison, the displacement of the ultrasonic oscillator is 10 μm or less; the piezoelectric bimorph element is greater in the displacement U than the ultrasonic oscillator by a few orders of magnitude. Other features of the piezoelectric bimorph element are low driving power and absence of electromagnetic noise.
PZT piezoelectric ceramics and a PVDF piezoelectric film will be described. To form PZT piezoelectric ceramics, powder of PbO, TiO2, ZrO2, and the like are mixed and crashed, and then tentatively fired at 700° C. to 800° C., after which a binder, PVA, or another organic substance is added and the resulting mixture is kneaded. The mixture is then heated at 300° C. to 500° C. to remove the binder, and finally fired at 1100° C. to 1300° C. The resulting substance is machined to prescribed dimensions, after which an electrode is formed on its surface by plating, baking, vapor deposition, or the like.
To complete polarization, a DC voltage of 2 to 3 kV/mm is applied across the electrode in insulating oil heated at about 100° C., for several tens of minutes.
A PVDF piezoelectric film is formed by performing polarization on a uniaxially oriented film of vinylidene fluoride resin at a high voltage. The PVDF piezoelectric film has a low piezoelectric distortion constant, which is one-fifth or less the piezoelectric distortion constant of PZT piezoelectric ceramics, but can have a large area and can be thinned.
Patent Document 4 discloses that an AC voltage is applied to a cantilever piezoelectric bimorph element formed with a piezoelectric film so that resonance is mechanically caused and a free end of the piezoelectric bimorph element is vibrated so as to cause an air flow, which is used as a source of an air flow to a thermistor and the like. An apparatus disclosed in Patent Document 5 has a plurality of cantilever piezoelectric bimorph elements, each of which has a wire at its end and performs printing independently by pressing an ink ribbon against a recording medium according to print signals. Patent Document 6 discloses a structure in which both ends are supported to enable a piezoelectric bimorph element to be used in a touch panel.
The structure in which a plurality of bimorph actuators are disposed in the width direction has the same problem as in the conventional structure which uses a resonator formed by combining an ultrasonic piezoelectric cell and a horn; vibration cannot be applied to a toner image supporting body between adjacent actuators. To address this problem, in the present invention, a vibrating unit that uses an actuator adaptable to a wide width is devised.
The width of the electrodes 3 and 6 is (Lc1−Lt). A protrusion portion 12 made of metal or resin with a length of Lt, a width of Wt and a height of H is bonded and fixed to an area at one end of the piezoelectric body 1, 5, with a width of Lt, in which no electrode is formed, by using an isolative adhesive 8. The width Wt of the protrusion portion 12 is equal to or more than the printing width Wp and equal to or less than the width Ws of the shim member 4. A plurality of electrodes 3a to 3f and another plurality of electrodes 6a to 6f formed on the surfaces of the piezoelectric bodies 1a to 1f and 5a to 5f connected collectively to an electrode terminal. Thus, a piezoelectric bimorph element 7, which is wide and is formed as an integrated type, is composed. Its structure is illustrated in
A key point in this structure is that areas with a width of Lt on the piezoelectric bodies 1, 5, to one of which the protrusion portion 12 is bonded and fixed, do not include the electrodes 3 and 6. This is because if the electrodes 3 and 6 are included in these areas, the areas also become active areas that expand and contract and thereby would otherwise cause interfacial peeling due to a shearing stress exerted on the bonded interface between the piezoelectric body 1 and the protrusion portion 12. These areas free from electrodes will be referred to below as inactive areas (dummy areas).
A method of making an area inactive is to prevent the electrode 3 or 6 from being included in the area. In another method, the area is excluded from polarization in the polarization process.
As seen from equations (3) and (4), the displacement U and resonant frequency f depend on the length L and thickness tt of the piezoelectric element. The contraction and expansion when a voltage is applied to the piezoelectric elements bonded to both sides of the shim member 4 exhibit a function of the piezoelectric bimorph element 7, causing the protrusion portion 12 disposed at the end of the piezoelectric body 1 to vibrate up and down. The piezoelectric bimorph element 7 having this structure is ready for a wide width and does not raise the mutual interference problem involved in the use of the conventional ultrasonic oscillator.
a) is a plan view illustrating a vibrating unit that uses the piezoelectric bimorph element in the present invention. The piezoelectric bimorph element 7 used is structured as illustrated in
b) is a side view of the vibrating unit. The electrode terminals of the actuator are connected to an AC power supply 13. A driving wave Vr was an AC sine wave with a peak value of ±30 V. The vibration displacement at the top of the protrusion portion 12 was measured with a laser displacement meter at different driving frequencies. According to measurement results, the vibration amplitude is maximized at a resonant frequency f of 3 kHz, and the displacement U is 4 μm. These values approximately match the values derived from equations (3) and (4) (resonant frequency f=3.5 kHz and displacement U=4.6 μm) The driving wave was then changed to an AC rectangular wave with a peak value of ±30 V. It was found that the resonant frequency f remained unchanged, but the displacement was increased to 5 μm. As a factor for this, it can be estimated that the AC rectangular wave has a larger voltage leading edge dV/dt and uses a larger energy supplied than the AC sine wave. In
a) and 9(b) illustrate another vibrating unit that uses the piezoelectric bimorph element in the present invention. The piezoelectric bimorph element 7 used is structured as illustrated in
a) to 10(c) illustrate characteristics of vibration applied by the vibrating unit to the toner image supporting belt 19.
The actuator illustrated in
As seen from equations (3) and (4), this problem can be solved from the viewpoint of design by selecting a piezoelectric material having a large piezoelectric distortion constant d31 and a large Young's modulus Y and by making the dimensions of constituents appropriate. Another solution is derived from the structure of the vibrating unit.
a) illustrates the structure of a vibrating unit using two bimorph actuators. In this drawing, two piezoelectric bimorph elements 7a and 7b are disposed so that their protrusion potions 12a and 12b are brought close to each other. The piezoelectric bimorph elements 7a and 7b are respectively driven by AC power supplies Vr1 and Vr2. As seen from
When the piezoelectric bimorph element 7 is used in the image forming apparatus, it is important to assure life and reliability necessary for a device as well as its vibration characteristics (vibration amplitude and vibration frequency). Since the piezoelectric bimorph element 7 is used in a cantilever structure, it is necessary to prevent mechanical stress from being applied to the supporting and fixing part and the joint of the protrusion potion 12a, 12b, which provides a contact with the toner image supporting belt 19. Therefore, areas with which the supporting and fixing part and the joint of the protrusion portion 12a, 12b are brought into contact are preferably inactive; the structures illustrated in
Next, problems when a piezoelectric bimorph element having a conventional structure is used as a vibrating mechanism for providing vibration energy, which is on object of the present invention, will be described.
Since a voltage is applied over the entire piezoelectric body 1, 5, the areas 35 and 36 also undergo distortion (stress) proportional to the strength of the electric field due to the reverse piezoelectric effect. Although the purpose of the fixing member is to hold the area 35, which vibrates up and down, areas, on the piezoelectric body 1, 5, near both sides 37a and 37b of the fixing member repeatedly undergo a large stress at the driving frequency. The area 36 also vibrates up and down, the frequency being smaller than in the area 34. The displacement U and resonant frequency f2 at that time are derived from equations (3) and (4) by substituting Ld for L. Since Lf is larger than Ld, f1 becomes smaller than f2. Therefore, vibration at a high frequency (f2) is superimposed on vibration (f1) in the free end area 34.
The piezoelectric bimorph element 7 illustrated in
FB=4π2f2·U·m(N) (5)
where f is the vibration frequency of the piezoelectric bimorph element 7, and m is the weight of a single toner particle.
As seen from equation (5), FB is proportional to the square of the vibration frequency f and the displacement U. Accordingly, the conventional piezoelectric bimorph element has problems described below.
(a) Since the vibration of the area 36 (power supply terminal connecting part) is superimposed to the up and down vibration in the area 34 (free end area), the vibration characteristics in the area 34 is in a non-uniform vibration mode, worsening the vibration characteristics of the vibrating means used as a vibration source. (b) To increase the inertia force FB, both the vibration frequency f and the displacement U need to be increased. However, the stress is then increased, causing a reliability problem such as damage to the piezoelectric body.
To address these problems, the electrodes 3, 6 bonded to the surfaces of the piezoelectric bodies 1 and 5 and the shim member 4 are shaped so that when a voltage is applied to the piezoelectric bimorph element 7, the reverse piezoelectric effect is exerted only on the area 34, in which free vibration occurs.
a-1) to 19(d) illustrate the structure of the piezoelectric bimorph element in the present invention.
The polarization 2 is performed over the entire area, including parts lacking electrodes, in the thickness direction. The electrodes 3a and 3b were initially formed on both sides of the piezoelectric body 1 and polarization processing was performed. In this method, it was found that large distortion occurred between a polarized area and a non-polarized area and a crack was generated. So, the method was changed to the above method.
c-1) and 19(c-2) also illustrate the shape of the lower piezoelectric body 5 in the piezoelectric bimorph element and the shapes of the electrodes 6a, 6b formed on the surfaces of the piezoelectric body 5; the electrode 6a in 4C-1 is rectangular and is formed on the front surface, and the electrode 6b in
d) illustrates the shape of the piezoelectric bimorph element 7 formed by laminating the piezoelectric body 1, shim member 4, and piezoelectric body 5 by bonding them in that order with an adhesive. A conductive adhesive 8 is used on both sides of the area 4a on the T-shaped shim member 4 so that an electrical continuity is established between the electrode 3b on the piezoelectric body 1 and the electrode 6a on the piezoelectric body 5. An isolative adhesive 9 is used on the rest 4b of the shim member 4 (areas excluding the electrode 3b on the backside of the piezoelectric body 1 and the electrode 6a on the front surface of the piezoelectric body 5). The area 3aL is the power supply terminal connecting part connected to the electrode 3a on the piezoelectric body 1, and the area 6bL is the power supply terminal connecting part connected to the electrode 6b on the piezoelectric body 5. The area 4b is a power supply terminal connecting part connected to the shim member 4.
A protrusion portion 12 is bonded to the front surface of the free end of the piezoelectric body 1 so that the vibration energy of the piezoelectric bimorph element 7 is applied to the backside of the toner image supporting body. Since a voltage is applied to the power supply terminals of the piezoelectric bimorph element 7 from the AC power supply 13, the protrusion portion 12 vibrates up and down. The AC power supply 13 supplies a voltage of ± tens of volts at several kilohertz or less, and its power consumption is several watts or less.
To improve corona transfer performance by using the vibrating means in which the piezoelectric bimorph element 7 is employed, it is important to obtain stable vibration (a uniform vibration amplitude and uniform vibration frequency) and reliability including the life of the piezoelectric bimorph element 7.
When the piezoelectric bodies 1 and 5 are bonded to the shim member 4 in a laminated structure in
When an isolative adhesive is used, the voltage applied to the piezoelectric bodies 1 and 5 is reduced substantially because part of the applied voltage is shared by a bonding layer 8 in the area 34 (free end area). As seen from equation (3), the displacement U is then reduced. When a conductive adhesive is used, the shim member 4 is spread over the entire surface substantially, so the above problem at the area 34 is solved; however, a voltage is applied to the piezoelectric bodies 1 and 5 at the power supply terminal connecting parts, and thus distortion occurs due to the reverse piezoelectric effect. Accordingly, there is a risk that the piezoelectric bodies 1 and 5 may be damaged around the notches 11 during vibration.
In the best mode of the present invention, an isolative adhesive is used in the area 35 fixed with the fixing member, and a conductive adhesive is used in the area 34 (vibrating area). It is preferable to select adhesive compositions and adhesive application conditions so that after curing, both adhesive layers 8 have the same thickness and their glass transition temperatures and hardness are approximately the same.
As described above, the transfer apparatus in the invention comprises a corona transfer means 18, which faces a toner image supporting body 19, and a vibrating means 25 for applying vibration energy to the backside of the toner image supporting body 19 at a position opposite to the corona transfer means 18; the transfer apparatus electrostatically transfers a toner image on the intermediate transfer belt 19 to a recording medium. The vibrating means 25 has a cantilever structure, that is, it holds one end of a piezoelectric bimorph element 7 structured so that paired piezoelectric bodies 1 and 5, on the surfaces on which electrodes 3 and 6 are formed, are bonded to both sides of a conductive elastic body 4, a protrusion portion 12 being provided on the other end. When a voltage is applied across the electrode 3 on the piezoelectric body 1 and the shim member 4 and across the electrode 6 on the piezoelectric body 5 and the shim member 4, no voltage is applied to areas, on the piezoelectric bodies 1 and 5, in which the piezoelectric bimorph element 7 is supported. That is, the electrodes 3, 6 are not installed on the piezoelectric bodies 1 and 5, in which the piezoelectric bimorph element 7 is supported.
Further, the transfer apparatus in the invention uses the mechanical vibration of a bimorph element that employs the transverse vibration (d31 mode) of a piezoelectric body, and can thereby transfer a toner image uniformly over a large area. The vibrating unit of the transfer apparatus can be made compact and consume less power when compared with a method in which a horn and an ultrasonic oscillator that uses the longitudinal vibration (d33 mode) of a conventional piezoelectric body are combined. Accordingly, the image forming apparatus that uses the transfer apparatus in the invention can perform high-quality printing on various types of paper and wide paper that has not been able to be handled by the conventional electrophotographic method, and can deal with printing on roughened surface paper, double-sided printing, and printing on embossed paper.
The AC power supply 13, which is an AC power supply for supplying rectangular waves, is used to apply an AC voltage across the piezoelectric body 1 on the electrode 3 and the shim member 4 and across the piezoelectric body 5 on the electrode 6 and the shim member 4. A corona transfer unit 18 connected to a DC high-voltage power supply 113 is disposed on the backside of the paper 16. Positive corona charges are applied to the back of the paper 16 and an electrostatic force FE acts on the toner in an area facing the corona transfer unit 18. A mirror image force FM and van der Waals's force Ff act across the toner and the photosensitive belt 19 as an adherence force. The strength of the mirror force FM varies with the amount of charge on the toner. The strength of the van der Waals's force Ff varies with the state of the surface of the toner. Silica adheres to the surface of the toner as an external additive. When the coverage is 25%, which is the value of the coverage of ordinary external additives, FF is about 10 nN. Since the mirror force FM is an electrostatic adherence force, when the electrostatic force FE produced by the corona transfer unit 18 is enlarged, it becomes possible to overcome the mirror image force FM. The van der Waals's force Ff is a non-electrostatic adherence force. When a rectangular wave AC voltage with a frequency of 5 kHz and a peak value of ±40 V was applied to the piezoelectric bimorph element 7 during printing, the protrusion portion 12 vibrated up and down with amplitude of about 18 μm. An inertia force FB of 11 nN then acted on the toner 115 on the belt. This value is greater than the van der Waals's force FF. It was then found that the toner 115 is released from a constraint by the mirror image force and the van der Waals's force due to the electrostatic force FE and inertia force FB, and flies in the concave 17 as a toner 117, and that the toner 117 can be thereby transferred to the paper 16 with large concaves and convexes on the front surface. This embodiment has been described for a case in which continuous form is used as the paper 16, but it should be understood that the same advantage can be obtained for cut sheets. Although the piezoelectric bimorph element 7 structured shown in
The structure of the transfer unit in this embodiment is as described in the second embodiment, so the explanation of the structure will be omitted. The toner 20, 21, 22 and 115 transferred to the paper 16 is fused and fixed to the paper by a heat roll 30a fixing unit comprising a heat roll 30a and a backup roll 30b. In this case, since the vibrating unit is mounted in the image forming apparatus, the vibrating unit must be disposed in the spacing surrounded by the rotating intermediate transfer belt 19. This embodiment differs from the second embodiment in that the vibrating unit is disposed vertically diametrically with respect to the belt. Accordingly, when protrusion portions 12a and 12b are displaced downwardly, the bimorph actuators 7a and 7b are brought into contact with the back surface of the belt 19 and cause it to vibrate, applying the inertia force FB to the toner. The protrusion portion 12 is preferably made of a material superior in wear resistance and low in specific gravity, such as aluminum or polycarbonate.
The driving of the vibrating unit comprising the bimorph actuators 7a and 7b can be selected according to the type of paper 16. When coated paper or woodfree paper, the surface which is relatively flat, is used, only corona transfer may be performed. The vibrating unit may be operated only for embossed paper and other types of paper 16 having concaves and convexes on the surface. It should be understood that when the vibrating unit is operated regardless of the type of paper, transfer performance is increased to the extent by which an inertia force is applied to the toner.
The bimorph vibrating source device may use any of the structures shown in
a) to 16(c) illustrate another structure of a wide piezoelectric bimorph element 7, which is the main device of the vibrating means used in the present invention to improve the efficiency of transfer.
Three PZT plates 1 are bonded to one surface of the shim member 4 with an adhesive, and another three PZT plates 5 are similarly bonded to the other surface. These plates have the same size; 300 μm in thickness, 140 μm in width (Lw), and 30 mm in length (Lc). Their polarization 2 is oriented in the same direction (in
A piezoelectric distortion constant d31 of the PZT used is 330×10−12(C/N), a Young's modulus of it is 5.9×1010(N/m2) and a density of it is 7.75×103(kg/m3), differing from the PZT property used in the
c) illustrates a structure of the vibrating apparatus in which the wide piezoelectric bimorph element 7 in
The power feeding lines 14 and 15 are connected together to the high-voltage side of the AC driving power supply 13, and the power feeding line to the shim member 4 is connected to the ground side of the AC driving power supply 13. When a voltage is supplied from the AC driving power supply 13 to the piezoelectric bimorph element 7, the protrusion portion 12 disposed on the surface of the free end area 34 (with a length of Lf) vibrates up and down.
Vibration energy is applied to the backside of the intermediate transfer belt 19 so that an inertia force acts on the toners 22, 23, 24, and 21. The toners 22, 23, 24, and 21 thereby fly and are transferred from the intermediate transfer belt 19 to the concave 17 in the front surface of the paper 16. The diameter of a particle of the toners 22, 23, 24, and 21 is 9 μm. If Lf is 10 mm, then the resonant frequency is 1.6 kHz. When a rectangular wave AC voltage with a frequency of 1.6 kHz and a peak value of ±40 V was applied to the piezoelectric bimorph element 7 as the driving voltage, the protrusion portion 12 vibrated up and down with amplitude of about ±200 μm. An inertia force FB of about 12 nN then acts on the toners 22, 23, 24, and 21 on the intermediate transfer belt 19.
As a result, the toners 21b, 22b, 23b, and 24b receive not only the electrostatic force FE due to the positive charge 20 applied on the back of the paper 16 by the corona transfer means 18 but also the above inertia force FB, so these toners are released from the constraint by the van der Waals's force and fly to the concave 17 and a flat part on the paper 16, indicating that superior transfer to the embossed paper is possible. Although an embossed cut sheet was used as the paper 16 in this embodiment, it should be understood that the embodiment could be applied to all types of paper including paper having concaves and convexes on the front surface, flat paper, and continuous paper.
Although the width L2 of the wide piezoelectric bimorph element 7 in this embodiment is 422 mm, it is also possible to use another wide piezoelectric bimorph element 7 with a width of 20 inches (508 mm) or more by widening the width of the shim member 4 and using more piezoelectric bodies 1 and 5. PVDF films, which are piezoelectric films, can also be used as the piezoelectric bodies 1 and 5.
The OPC photosensitive belt 228 is charged by the charger 226a, after which an optical pattern corresponding to a K toner image is exposed by the exposing part 227a including a laser optics and LED so as to form an electrostatic latent image. The developing unit 229a then forms a K toner image on the OPC photosensitive belt 228. The surface of the OPC photosensitive belt 228 is then charged by the charger 226b so as to restore the potential of an area in which potential reduction was caused by light illumination by the exposing part 227a. Next, an optical pattern corresponding to a C toner image is exposed by the exposing part 227b so as to form an electrostatic latent image, and the developing unit 229b forms a C toner image on the OPC photosensitive belt 228.
The developing rolls of the developing units 229b, 229c, and 229d are disposed so that their surfaces do not touch the photosensitive belt 228, preventing the toner image formed on the photosensitive belt 228 from being scratched by the developing rolls. When an M toner image and Y toner image are then formed in succession in this way, a color image comprising the K toner 21a, C toner 22a, M toner 23a, and Y toner 24a is formed on the photosensitive belt 228.
A corona transfer unit 18 is disposed outside the rotating OPC photosensitive belt 228, and a vibrating means 25, which uses the piezoelectric bimorph element 7, is disposed inside. A driving power supply 13 is connected to the vibrating means 325. The driving power supply 13 is a rectangular wave or sine wave AC power supply. When the protrusion portion 12 of the piezoelectric bimorph element 7 is displaced downward, it touches the backside of the photosensitive belt 228 and applies an inertia force FB to the toners 21a, 22a, 23a, and 24a.
The paper 16, which is a cut form, is moved by the resisting rollers 31 to the transfer part. The toners 21a, 22a, 23a, and 24a transferred to the paper 16 are fused and fixed to the paper 16 by a heat roll fixing unit comprising a heat roll 30a and a backup roll 30b. This completes printing.
The driving of the vibrating means 325 can be selected according to the type of paper 16. When coated paper or woodfree paper, the surface on which is relatively flat, is used, only corona transfer is performed; the vibrating unit 325 is operated only for paper 16 having concaves and convexes, such as embossed paper. It should be understood that when the vibrating unit 325 is operated, transfer performance is increased to the extent by which an inertia force is applied to the toner 21a, 22a, 23a and 24a, regardless of the type of paper. In the description so far, a cut form has been used. If the system for moving the paper 16 is modified so as to adapt to continuous form, the image forming apparatus can handle continuous paper.
The OPC photosensitive belt 228 in the third embodiment servers as a toner image supporting body as in the case of the intermediate transfer belt 19 in the fifth embodiment. Accordingly, when a toner image is transferred from this type of flexible toner image supporting body to a recording medium, the transfer apparatus in the invention can be used. In the fifth and sixth embodiments, color printing using toners 21a, 22a, 23a, and 24a in four colors has been described; the transfer apparatus can of course also be applied to monochrome images.
Electrophotographic printers can print variable information on a recording medium such as paper at high speed, so they have been used in a wide range of fields from business printing to personal printing. As these printers are spread, printing on many types of paper and wide paper, which conventional electrophotographic printers could not handle, is being demanded. Specifically, printing on inexpensive, roughened surface paper, double-sided printing for use paper resources effectively, and color printing on embossed paper to produce tickets and brochures are demanded. Demands for wide paper ranges from A3 cut sheets (420 mm or 16.54 inches) to continuous paper 20.5 inches wide. An object of the present invention to meet these demands for the transfer mechanism is to develop a compact transfer apparatus with a low power consumption that can uniformly transfer a toner image over a wide area even when the paper has large concaves and convexes on the surface and there is no sufficient contact between the paper and the toner image supporting body such as a photo sensitive body or intermediate transfer body.
The transfer apparatus in the invention uses the mechanical vibration of a bimorph element that employs the transverse vibration (d31 mode) of a piezoelectric body and it is possible to transfer the toner image uniformly in a broad area. Further, the vibrating unit of the transfer apparatus can be made to be compact and consume less power when compared with a method in which a horn and an ultrasonic oscillator that uses the longitudinal vibration (d33 mode) of a conventional piezoelectric body are combined. Accordingly, when the transfer apparatus in the invention is applied to an image forming apparatus, the image forming apparatus can print an image of high quality on a variety of paper and a wide paper to which a conventional electrophotographic method cannot be applied and it can deal with printing on roughened surface paper, double-sided printing, and printing on embossed paper.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-348106 | Dec 2006 | JP | national |
| 2007-148123 | Jun 2007 | JP | national |
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| Number | Date | Country | |
|---|---|---|---|
| 20080152399 A1 | Jun 2008 | US |
