JP2005316205A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent toner scattering at a grounded stretching roller for an intermediate transfer belt with simple configuration. <P>SOLUTION: The intermediate transfer belt 11 is trained about a plurality of stretching rollers. Provided that a width of the intermediate transfer belt 11 in the driving direction (direction shown by arrow R11) at the contact position where the primary transfer downstream roller 6 as the grounded stretching roller arranged just on the downstream side of a primary transfer part comes in contact with the intermediate transfer belt 11 is expressed by W[m], the property time of the intermediate transfer belt 11 carrying no toner is expressed by τ[sec] and the rotating velocity of the intermediate transfer belt when receiving the rotational drive is expressed by V[m/sec], a relation of 0<W<V×τ×0.4 is established among those. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus, such as a printer, a copying machine, and a facsimile machine, provided with a belt-like intermediate transfer member.

  Patent Document 1 discloses an image forming apparatus using an intermediate transfer member as shown in FIG.

  The surface of the photosensitive drum 1 is uniformly charged by the charging roller 2 while rotating in the direction of the arrow R1. An electrostatic latent image is formed on the surface of the photosensitive drum 1 after being charged by the exposure device 3 being irradiated with a laser corresponding to the image information. The electrostatic latent image is developed as a toner image by electrostatically adhering charged toner to the developing device.

  The toner image thus formed on the photosensitive drum 1 is electrostatically primarily transferred onto the intermediate transfer belt 11 by the primary transfer roller 5 in the primary transfer portion T1. This intermediate transfer belt 11 is stretched over a primary transfer roller 5, a primary transfer downstream roller 6, a drive roller 7, a secondary transfer counter roller 8, and a driven roller 9 as a stretch roller, and is driven to rotate in the direction of arrow R11. It has come to be. The toner image t on the intermediate transfer belt 11 is transferred to the secondary transfer roller 13 on the transferred transfer material P in the secondary transfer portion T2 formed between the intermediate transfer belt 11 and the secondary transfer roller 8. Is electrostatically secondary transferred. The transfer material P after the transfer is heated and pressed by the fixing device 15 to fix the toner image on the surface. Reference numerals 12 and 14 in the figure denote a cleaning device for cleaning the surface of the photosensitive drum 1 and a belt cleaner for cleaning the surface of the intermediate transfer belt 11, respectively.

  In FIG. 7, the primary transfer downstream roller 6 is a roller disposed immediately downstream of the primary transfer portion T <b> 1 among a plurality of rollers that stretch an intermediate transfer belt 11 as an intermediate transfer member. The primary transfer downstream roller 6 is disposed so as to face the reference pattern detection unit 20 that reads a reference density pattern, a position information pattern, and the like, and is essential for improving the reading accuracy.

  In the primary transfer downstream roller 6 as described above, the toner image t carried on the intermediate transfer belt 11 exhibits a phenomenon in which the toner is scattered and disturbed when the back surface of the belt touches the primary transfer downstream roller 6. This is because, in the primary transfer portion T <b> 1, the toner carrying charge supplied from the primary transfer roller 5 to the intermediate transfer belt 11 causes rearrangement in the primary transfer downstream roller 6.

  As shown in FIG. 8, in the primary transfer portion T1, charge for carrying toner is injected from the primary transfer roller 5 to the intermediate transfer belt 11, and this charge is transferred from the back surface 11b of the intermediate transfer belt 11 to the toner carrying surface ( The toner particles a constituting the toner image t are carried on the intermediate transfer belt 11 by moving to the surface 11a. Here, the + mark in the figure schematically represents the toner carrying charge, and the polarity is opposite to that of the toner. In the figure, negative toner charged to a negative polarity is used, and the polarity of the toner carrying charge is positive. Further, the total amount of toner carrying charges is larger than the total charged amount of the toner image t, and therefore, an excess charge exists on the toner carrying surface 11a of the intermediate transfer belt 11 immediately after the primary transfer.

  At this time, as shown in FIG. 9, the charge for carrying the toner differs between where the toner is present and where it is not. This is because, as shown in FIG. 10, when a voltage is applied during primary transfer, the current value is different between where the toner is present and where it is not. In the absence of toner, since the surface layer of the photosensitive drum 1 is an insulating layer, charges can move to the intermediate transfer belt 11 only by discharge. For this reason, the electric charge does not move in the primary transfer portion T1 until a sufficient electric field can be applied to start discharge in the gap between the toner carrying surface 11a of the intermediate transfer belt 11 and the photosensitive drum. Therefore, no current flows. On the other hand, when toner is present, the charged toner can be moved from the photosensitive drum 1 to the intermediate transfer belt 11 only by applying an electric field. Charge can move.

  For the above reasons, when a voltage necessary for primary transfer of toner to the intermediate transfer belt 11 is applied, a current flows in the presence of toner, but only a small amount of current flows in the absence of toner. Therefore, since the amount of charge for carrying the toner moved to the intermediate transfer belt 11 is proportional to the current value, a large amount of charge exists in a place where the toner is present, and a small amount of charge exists where there is no toner. There is a difference in quantity.

  However, in some places where the toner is present, the charge amount equal to the total charge amount of the toner in the toner carrying charge cancels out with the toner, so that the macro has only a surplus charge. become.

  Further, it is known that the toner carrying charge distributed inside the belt is attenuated during driving. There are two types of attenuation, that is, attenuation due to natural disappearance and attenuation due to escape to a grounded stretch roller.

  As shown in FIG. 11, the intermediate transfer belt 11 that is a dielectric that retains electric charge is equivalent to an open-ended RC parallel circuit. At this time, in a state where a constant charge is held in the capacitor, a potential difference is generated in the RC parallel circuit, so that a phenomenon occurs in which a current flows through the R component and the capacitor charge is discharged. The spontaneous discharge phenomenon of the RC parallel circuit is a damping behavior that occurs in a float state in which the intermediate transfer belt 11 passes through the primary transfer portion T1 and is not in contact with any member such as a grounded member such as a stretching roller. . Further, this attenuation behavior is a phenomenon that depends on the conduction characteristics and dielectric characteristics of the dielectric itself, and is not affected by the presence of toner.

  On the other hand, when the intermediate transfer belt 11 touches the grounded stretching roller, the attenuation behavior in which the charge is lost to the roller is greatly influenced by the presence of the toner. As shown in FIG. 11, when the back surface 11 b touches the grounded stretching roller, one end of the RC parallel circuit equivalent to the intermediate transfer belt 11 is grounded. On the other hand, since the toner carried on the intermediate transfer belt 11 is insulated, it is regarded as a capacitor holding a certain electric charge, connected in series with the RC parallel circuit, and the other end is in an electrically floating state. The charge flow at this time is only the flow of the charge of the RC parallel circuit to the ground plane, but the attenuation behavior is not only the RC parallel circuit but also the capacitance of the capacitor equivalent to the toner layer connected in series. Under the influence of the components, there is a property that the characteristic time of relaxation becomes longer. That is, when the grounded roller touches the back surface 11b, the attenuation due to the escape of electric charge to the roller is longer in the characteristic time when the toner is present than when the toner is absent. There is.

  In summary, the behavior of the toner carrying charge injected into the intermediate transfer belt 11 in the primary transfer portion T1 and the behavior of the electric field inside the belt resulting from the distribution of the toner carrying charge are as follows. become.

  In the primary transfer portion T1, charged charges are injected both in the presence and absence of toner, and the amount of charge increases in the presence of toner, but the amount of charge equal to the total charge amount of toner is macroscopic. Therefore, only the surplus charge amount larger than the total toner charge amount is present. Therefore, the difference in effective charge amount is small where there is toner and where it is not. For this reason, the electric field in the belt from where the toner is present to where there is no toner is small.

  When both the toner carrying surface 11a and the back surface 11b of the intermediate transfer belt 11 are floated after passing through the primary transfer portion T1, the charge is naturally attenuated inside the belt regardless of the presence or absence of toner, and the attenuation characteristics. The time is the same. As a result, in the primary transfer portion T1, the difference in the amount of charge inside the belt caused by the presence or absence of toner is maintained. As a result, the electric field from the location where the toner is present to the location where there is no toner within the belt is the same size as the primary transfer portion T1.

  When the intermediate transfer belt 11 is conveyed and touches the grounded stretching roller, attenuation due to escape of toner-carrying charges to the roller occurs. At this time, the characteristic time of attenuation varies depending on the presence / absence of toner, and the characteristic time becomes longer when the toner is present, so that the charge is less likely to attenuate. As a result, the electric field in the belt from the place where the toner is present to the place where the toner is not present is maximized when the roller is in contact with the grounded roller.

  In this way, when the roller that is grounded is touched, the electric field in the belt from where the toner is present to where there is no toner increases, so as shown in FIG. A part of the existing toner carrying charge moves to a place where there is no toner. Due to the rearrangement of the toner carrying charge, the toner that can no longer be electrostatically held on the intermediate transfer belt 11 is repelled by the repulsion of the polarity between the toners, and attracted to the rearranged toner carrying charge and moves. As a result, a toner scattering phenomenon occurs.

  The above is the explanation of the cause of toner scattering in the ground roller.

  Further, the above-mentioned scattering phenomenon is remarkable when a substantially spherical toner such as a toner synthesized by a polymerization method such as a suspension polymerization method is used. This is because as the sphericity increases, the contact area between the toners or other members decreases, and the adhesion force decreases.

  For the toner scattering phenomenon, Patent Document 1 discloses a countermeasure by grounding a roller via a resistor instead of directly grounding. By grounding the roller through a resistor, the characteristic time of attenuation when the roller is touched can be extended, and the amount of attenuation of charge for toner carrying is reduced, so that charge relocation is unlikely to occur. Therefore, the toner scattering phenomenon can be alleviated.

JP 2000-298408 A

  However, since the roller is not directly grounded, the configuration becomes complicated and the cost may increase.

  The present invention has been made in view of the above circumstances, and provides an intermediate transfer type image forming apparatus capable of avoiding scattering of toner on a grounded tension member without taking a complicated configuration. It is intended to provide.

In the present invention, a toner image is formed on an image carrier, the toner image on the image carrier is transferred onto an intermediate transfer member in a primary transfer portion, and then the toner image on the intermediate transfer member is used as a transfer material. In the image forming apparatus to be transferred, the intermediate transfer member is configured by an endless belt that is stretched and driven to rotate by a plurality of stretching members, and is immediately downstream of the primary transfer unit among the plurality of stretching members. The intermediate transfer member in a state where the width of the intermediate transfer member in the direction of receiving rotational driving is W [m] at the contact portion between the tension member disposed on the intermediate transfer member and the intermediate transfer member, and the toner is not carried. When the characteristic time is τ [sec] and the rotational speed of the intermediate transfer member when receiving rotational driving is V [m / sec],
0 <W <V × τ × 0.4
It is characterized in that the relationship is established.

  According to the present invention, it is possible to effectively avoid the scattering of the toner image by preventing the rearrangement of the charges generated when contacting the stretching member. Accordingly, it is possible to maintain a stable high image quality without causing scattering.

  An image forming apparatus to which the present invention can be applied will be described below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent. Further, the materials, shapes, etc. of the members once described in the following description are the same as those in the first description unless otherwise described.

<Embodiment 1>
With reference to FIG. 1, a schematic configuration of an electrophotographic full-color laser printer (hereinafter referred to as “image forming apparatus”) will be described as an image forming apparatus to which the present invention can be applied.

  The image forming apparatus shown in the figure is an image forming apparatus that forms a full-color image by superposing toner images of a total of four colors, yellow (Y), magenta (M), cyan (C), and black (K).

  The image forming apparatus shown in the figure includes a drum-type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) as an image carrier. The photosensitive drum 1 is configured by applying an organic photoreceptor (OPC) to an aluminum cylinder having an outer diameter of 84 mm. After the surface is uniformly charged by a charging roller (charging means) 2, exposure is performed. The apparatus (exposure means) 3 is irradiated with laser light corresponding to image information, and an electrostatic latent image corresponding to the yellow component is formed on the surface thereof.

  The formed electrostatic latent image is made into a yellow toner image by attaching the yellow toner by the yellow developing device 4Y containing yellow toner. The developing device (developing means) 4 includes a rotatable developing rotary 4a and four color developing devices mounted thereon, that is, yellow, magenta, cyan, and black developing devices 4Y, 4M, 4C, and 4K. Yes. Among the four developing devices, a developing device used for developing the electrostatic latent image on the photosensitive drum 1 is arranged at a developing position facing the surface of the photosensitive drum 1 by the rotation of the developing rotary 4a. Yes. In the above description, in order to develop the yellow electrostatic latent image on the photosensitive drum 1, the yellow developing device 4Y is disposed at the developing position by the rotation of the developing rotary 4a.

  The yellow toner image formed on the photosensitive drum 1 is primarily transferred onto a belt-like intermediate transfer belt 11 as an intermediate transfer member. The intermediate transfer belt 11 is stretched around a plurality of stretching rollers, that is, a primary transfer roller 5, a primary transfer downstream roller 6, a driving roller 7, a secondary transfer counter roller 8, a driven roller 9, and a primary transfer upstream roller 10. The drive roller 11 is rotationally driven in the direction of arrow R11 by the rotation in the direction of the arrow. The toner image on the photosensitive drum 1 is electrostatically primarily transferred onto the intermediate transfer belt 11 by the primary transfer roller 5 in the primary transfer portion T1. The toner (residual toner) remaining on the surface of the photosensitive drum 1 having the yellow toner image transferred to the intermediate transfer belt 11 is removed by the cleaning blade 12a of the cleaning device 12 and used for the next and subsequent image forming operations.

  The remaining three colors, magenta, cyan, and black, are subjected to the same image forming process as that for yellow described above, that is, the image forming process of charging, exposure, development, primary transfer, and cleaning, on the intermediate transfer belt 11. Primary transfer is performed sequentially. As a result, the four color toner images are superimposed on the intermediate transfer belt 11.

  The four color toner images superimposed on the intermediate transfer belt 11 are conveyed to a secondary transfer portion T2 constituted by the secondary transfer counter roller 8 and the secondary transfer roller 13, and are supplied by a feeding / conveying means (not shown). Are secondarily transferred at once to the transfer material P conveyed at the timing.

  After the toner image is transferred, the transfer material P is conveyed to the fixing device 15 where it is heated and pressurized to fix the toner image on the surface. Thereby, the image formation on one surface (front surface) of one transfer material P is completed. The surface of the intermediate transfer belt 11 after the transfer of the toner image is removed by a belt cleaner 14 and used for image formation from the next time.

  In the following, the configuration of each member and the image forming conditions in the image forming unit that forms a visible image of yellow toner will be described.

  The yellow developing device 4Y conveys yellow toner to the developing sleeve by a yellow toner conveying mechanism (not shown) in the developing device, and yellow on the outer periphery of the developing sleeve (not shown) by a regulating blade pressed against the outer periphery of the developing sleeve. After applying a thin layer of toner and applying a charge to the yellow toner, a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing sleeve, whereby an electrostatic latent image formed on the photosensitive drum 1 is applied. Develop. The developing sleeve is disposed at a position facing the photosensitive drum 1 with a minute interval (300 μm).

  In the present embodiment, the potential of the photosensitive drum 1, the potential of the developing sleeve, and the potential applied to the primary transfer roller 5 are set as described below.

  Under an environment of a temperature of 23 ° C. and a humidity of 50% Rh, the surface potential of the photosensitive drum 1 is −by applying an AC bias in which an AC bias having a peak-to-peak voltage Vpp of 900 V is superimposed on a DC bias of −450 V to the charging roller 2. Control is performed so as to be 450V.

  On the other hand, an AC bias in which an AC component having a peak-to-peak voltage Vpp of 1.2 kV is superimposed on a DC component of −300 V is applied to the developing sleeve. The waveform of the AC component at this time is a blank pulse waveform, and a waveform combining a 9 kHz AC waveform and a 4.5 kHz blank is applied as a developing bias.

  When subjected to laser exposure, the photosensitive drum has a bright portion potential of −200 V at a portion where an electrostatic latent image that is a maximum density image is formed.

  At this time, a potential difference (primary transfer contrast) between the primary transfer roller 5 and the bright portion of the photosensitive drum 1 becomes 600 V by applying a potential of 400 V as the primary transfer bias to the primary transfer roller 5. Due to this primary transfer contrast, the negatively charged toner is primarily transferred onto the intermediate transfer belt 11.

The intermediate transfer belt 11 uses a polyimide resin film having a thickness of 85 μm as a base material, and carbon black is dispersed to obtain a surface resistivity of 1 × 10 ¹² Ω / □ and a volume resistivity of 1 × 10 ^9.5 Ω · The resistance was adjusted to be cm. The peripheral length of the intermediate transfer belt 11 was 527.5 mm, and the driving speed (process speed) was 130 mm / sec.

The secondary transfer outer roller 13 is a sponge roller in which a foamed rubber layer based on foamed NBR (nitrile butadiene rubber) is provided on a steel core with an outer diameter of 12 mm, and includes an NBR rubber layer. The outer diameter was 24 mm. The resistance of the roller was adjusted by dispersing an ion conductive resistance adjusting agent so as to be 10 ^7.5 Ω (when 2 kV was applied) in an environment of 23 ° C. and 50% Rh.

  The primary transfer downstream roller 6 is a SUS roller having an outer diameter of 16 mm, and is grounded as shown in FIG. Since the primary transfer downstream roller 6 carries the intermediate transfer belt 11 charged with the charge from the primary transfer roller 5 at the primary transfer portion T1 from the back surface, there is a possibility that the primary transfer downstream roller 6 may be charged up when not grounded. It is preferable to ground the image forming apparatus since it may leak and cause electrical stress to the image forming apparatus main body.

  In the present embodiment, the tension member of the intermediate transfer belt 11 that contacts the toner image carried on the intermediate transfer belt 11 first after passing through the primary transfer portion T1 is the primary transfer downstream roller 6. It is. In this embodiment, as shown in FIG. 2, the radius of the primary transfer downstream roller 6 is set to 8 mm, the belt winding angle θ is set to 59 degrees or less, and the intermediate between the primary transfer downstream roller 6 and the intermediate transfer belt 11 is set. By setting the contact width of the transfer belt 11 in the drive direction (arrow R11 direction) to 2.6 mm or less in this embodiment, the image forming apparatus can maintain high image quality without causing toner images to scatter. I was able to.

  The scattering of the toner image is due to the difference between the excess charge amount where the toner is present and the portion where the toner is present, excluding the charge amount corresponding to the total charge amount of the toner among the toner carrying charges existing in the intermediate transfer belt 11. to cause. That is, if there is no difference in surplus charge regardless of the presence or absence of toner, scattering does not occur.

  The largest difference in the surplus charge occurs when the surplus charge dissipates with respect to a grounded stretching member such as the primary transfer downstream roller 6 where the toner is present and where there is no toner. This is because there is a difference.

The excessive charge decay behavior can be observed as the surface potential decay behavior of the intermediate transfer belt 11. As shown in FIG. 3, a surface potential meter (reference pattern detection means) 20 is disposed at a position facing the primary transfer downstream roller 6 via the intermediate transfer belt 11, and the toner image t is transferred to the primary transfer downstream roller 6. When the position was reached, the operation was forcibly stopped, and the decay behavior of the surface potential of the toner image and the decay behavior of the surface potential of the intermediate transfer belt 11 in the portion without toner were measured. The results are shown in FIG. By fitting these attenuation curves to V = V ₀ × exp (−t / τ), the characteristic times, τ, τ _T of the respective relaxation behaviors when the toner is present and where the toner is absent are derived. Can do. The characteristic time of the relaxation behavior in the present embodiment can be derived as τ = 48 [msec] and τ _T = 178 [msec].

When the characteristic time τ where the toner is present and the characteristic time τ _T where the toner is not present are multiplied by 130 [mm / sec] as the driving speed (rotational speed) V of the intermediate transfer belt 11, 6.2 [mm] and 23, respectively. 1 [mm]. This is because, for example, the surplus charge in the absence of toner is 48 [msec] to attenuate to 1 / e (that is, about 36.8%), and the intermediate transfer belt 11 is temporarily driven. This indicates that it is necessary to keep in contact with a grounding member having a width of 6.2 mm.

  FIG. 5 shows the difference in time obtained by subtracting the potential of the toner without the toner from the potential of the toner with respect to the surface potential measured as described above, with the start time being 100%. As described above, since the time scale of relaxation of the surplus charge at the location where the toner is present is slow, the potential difference between where the toner is present and where the toner is absent is large.

  FIG. 6 shows the result of plotting the variation of time against the time variation of the potential difference of FIG. In the figure, it can be seen that the time variation of the potential difference tends to increase until approximately 20 [msec], but tends to decrease after approximately 20 [msec]. This indicates a time scale in which the charge is rearranged from where the toner is present to where it is not, and it is predicted that the charge rearrangement will occur in approximately 20 [msec], and the mobility of the excess charge inside the belt is predicted. It is considered that the time scale is due to

From the above consideration, the surplus charge existing in the intermediate transfer belt 11 where there is no toner is attenuated to the primary transfer downstream roller 6 earlier, and the surplus charge is rearranged after about 20 [msec]. It can be seen that splattering starts. The characteristic time for charge decay where there is no toner, that is, the time required for the excess charge to decay to 1 / e (about 36.8%) is 48 [msec]. Even if the surplus charge without toner is reduced to approximately (1 / e) ^20/48 = 67% at 20 [msec], charge rearrangement does not occur, and therefore toner image scattering occurs. I understand that I do not.

  As described above, by setting the winding amount W between the primary transfer downstream roller 6 and the intermediate transfer belt 11 to 130 [mm / sec] × 20 [msec] = 2.6 [mm] or less, scattering can be effectively avoided. It becomes composition.

1 is a diagram schematically illustrating a schematic configuration of an image forming apparatus to which the present invention can be applied. FIG. 6 is a diagram illustrating a contact state between an intermediate transfer belt and a primary transfer downstream roller. It is a figure for demonstrating the measurement system of a surface potential. It is a figure for demonstrating the attenuation | damping behavior of a surplus charge. FIG. 6 is a diagram for explaining a difference in surplus charge amount where toner is present and where toner is not present. It is a figure for demonstrating the time scale of the rearrangement of an electric charge. It is a figure which shows typically schematic structure of the conventional image forming apparatus. FIG. 5 is a schematic diagram for explaining toner-carrying charges inside a belt. It is a schematic diagram for demonstrating the IV characteristic of a primary transfer. It is a figure for demonstrating the natural attenuation | damping of the electric charge inside a belt. It is a figure for demonstrating attenuation | damping by dissipation of the electric charge from a belt. It is a schematic diagram for demonstrating rearrangement of an excess charge.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (charging means)
3 Exposure equipment (exposure means)
4 Developing device (Developing means)
5 Primary transfer roller 6 Primary transfer downstream roller (stretching member disposed immediately downstream of the primary transfer portion)
11 Intermediate transfer belt (intermediate transfer member)
13 Secondary transfer roller 20 Surface potential meter (reference pattern detection means)
P Transfer material R11 Intermediate transfer belt rotational drive direction T1 Primary transfer portion T2 Secondary transfer portion V Intermediate transfer belt drive speed (intermediate transfer member rotational speed)
W Wrapping amount (width in the direction of receiving the rotational drive of the intermediate transfer belt at the contact portion)
τ Characteristic time of intermediate transfer belt (intermediate transfer member)

Claims (2)

Image formation in which a toner image is formed on an image carrier, the toner image on the image carrier is transferred to an intermediate transfer member in a primary transfer portion, and then the toner image on the intermediate transfer member is transferred to a transfer material In the device
The intermediate transfer member is composed of an endless belt that is stretched and driven by a plurality of stretching members.
The width in the direction of receiving the rotational drive of the intermediate transfer body at a contact portion between the tension member disposed immediately downstream of the primary transfer portion and the intermediate transfer body among the plurality of stretch members is W [ m], when the characteristic time of the intermediate transfer member in a state where no toner is carried is τ [sec], and the rotational speed of the intermediate transfer member when subjected to rotational driving is V [m / sec], Between these,
0 <W <V × τ × 0.4
Configured so that the relationship of
An image forming apparatus. The stretch member disposed immediately downstream of the primary transfer portion is grounded.
The image forming apparatus according to claim 1.

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