JP2006208477A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for forming stable quality image during a life of a photoreceptor by accurately judging film the thickness of the photoreceptor, that is the life thereof. <P>SOLUTION: A control means 13 provided in an electrophotographic image forming apparatus 1, controls an action of a charging means 3 so that surface potential of the photoreceptor 2 becomes a predetermined prescribed potential and the film thickness of a photosensitive layer of the photoreceptor 2 can be obtained, based on data regarding the film thickness of the photosensitive layer read from a storing means 14 according to flowing-in current to the photoreceptor 2 detected by a current detecting means 11 and temperature of the photoreceptor 2 detected by a temperature detecting means 12 in a state in which the surface potential of the photoreceptor 2 is the prescribed potential. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine.

  In general, an image forming apparatus using an electrophotographic system includes a photosensitive member and a charging unit, an exposure unit, a developing unit, a transfer unit, a fixing unit, a cleaning unit, and a discharging unit disposed around the photosensitive member. The charging unit uniformly charges the surface of the photoreceptor. The exposure unit exposes the charged photoconductor surface with light according to image information to form an electrostatic latent image. The developing unit supplies toner to the electrostatic latent image formed on the surface of the photoreceptor to form a toner image. The transfer unit applies a charge having a polarity opposite to that of the toner to the recording material to transfer the toner image to the recording material. The fixing unit fixes the toner image transferred by means such as heating and pressing on the recording material. The cleaning unit removes and cleans the toner remaining on the surface of the photoreceptor without being transferred to the recording material. The neutralizing unit neutralizes the photoconductor after the toner image is transferred to the recording material. A desired image is formed on the recording material by the image forming apparatus using the electrophotographic system configured as described above.

  As a photoreceptor on which an electrostatic latent image is formed and a toner image is formed by supplying toner, those formed by laminating a photosensitive layer on a conductive substrate are widely used. As the photosensitive layer, those made of inorganic materials such as selenium alloy or zinc oxide have been used. However, in recent years, charge generating substances and charge transport in the resin due to low cost, no pollution and high sensitivity. A material in which an organic photoconductor (OPC) is dispersed as a material has been widely used. In such a photoreceptor having a photosensitive layer in which OPC is dispersed in a resin, the film thickness of the photosensitive layer decreases due to wear caused by long-term use. As the film thickness of the photoconductor decreases, it becomes difficult to maintain the quality stability of the formed image due to a decrease in charge retention capability, so the photoconductor has a service life. Therefore, in the image forming apparatus, stabilization of the image quality corresponding to the reduction in the thickness of the photosensitive layer of the photosensitive member, and the use life of the photosensitive member are accurately determined, and the useless member whose quality is deteriorated due to the photosensitive member having reached the end of its life. It is required not to form an image and not to damage the device around the photosensitive member.

  As a conventional technique for stabilizing the quality of a formed image, a charged potential of a photoconductor is detected, and an applied voltage required for setting the charged potential of the photoconductor to a target potential is determined according to the detected potential. By obtaining the difference between the voltage and the corrected applied voltage based on the detected temperature of the charging member that charges the photosensitive member, and further correcting the correction rule for the applied voltage based on the difference, the charging potential of the photosensitive member can be controlled stably. It has been proposed to stably form a good image (see, for example, Patent Document 1).

  However, Patent Document 1 merely discloses a technique for stabilizing the surface potential of the photoconductor, and the service life of the photoconductor cannot be determined. There is a problem that it is impossible to prevent image formation beyond the service life.

  Further, as a conventional technique for detecting the life of a photoconductor, there are n types of bias application conditions in which various combinations of DC bias and AC bias applied to charging means for charging the photoconductor are different, and each of the n types of bias application conditions. Is applied to the integrated value, and the damage value of the photoconductor is obtained according to an empirical formula based on an empirical rule using the integrated value. It has been proposed to obtain (see Patent Document 2).

  However, in Patent Document 2, the photoconductor lifetime is uniformly obtained according to an empirical formula based on an empirical rule. However, the actual film reduction amount, that is, the lifetime of the photoconductor mounted on the image forming apparatus is a mechanistic aspect between the individual devices. Therefore, it is difficult for the technique disclosed in Patent Document 2 to accurately determine the lifetime of each photoconductor mounted on the image forming apparatus.

JP-A-9-101655 Japanese Patent No. 3565468

  An object of the present invention is to provide an image forming apparatus that accurately determines the film thickness, that is, the service life of a photoconductor, and enables image formation with stable quality within the service life of the photoconductor.

The present invention relates to an electrophotographic system comprising a photosensitive member having a photosensitive layer formed by being laminated on the surface of a conductive substrate and being exposed to light corresponding to image information to form an electrostatic latent image on the surface. In the image forming apparatus,
Charging means for charging the photoreceptor;
Surface potential detecting means for detecting the surface potential of the photoreceptor;
Exposure means for exposing the surface of the charged photoreceptor with light corresponding to image information;
Temperature detecting means for detecting the temperature of the photoreceptor;
Current detection means for detecting the inflow current of the photoreceptor;
Storage means for storing in advance data relating to the thickness of the photosensitive layer of the photosensitive member determined in accordance with the temperature of the photosensitive member and the inflow current;
The charging means controls the charging operation of the photosensitive member by the charging unit so that the surface potential of the photosensitive member detected by the surface potential detecting unit becomes a predetermined potential, and the current in a state where the surface potential of the photosensitive member is at the predetermined potential. In accordance with the inflow current of the photoconductor detected by the detecting means and the temperature of the photoconductor detected by the temperature detecting means, the film thickness of the photosensitive layer is determined based on the data relating to the film thickness of the photosensitive layer read from the storage means. And an image forming apparatus including a control unit for obtaining the image forming apparatus.

  The present invention is also characterized in that the surface potential detecting means for detecting the surface potential of the photosensitive member is disposed between the charging means and the exposure means.

In the present invention, the temperature detecting means for detecting the temperature of the photoreceptor is
It is characterized in that it is arranged on the upstream side of the charging means with respect to the rotation direction of the photoreceptor and in the vicinity of the charging means.

The present invention further includes display means for displaying information in response to a display operation command by the control means,
The storage means further stores a limit film thickness that is a film thickness of the photosensitive layer that is predetermined as the service life of the photosensitive member, and is determined corresponding to a film thickness difference between the film thickness of the photosensitive layer and the limit film thickness. Stores data related to the number of images that can be formed,
The control means
The film thickness difference between the film thickness of the photosensitive layer and the limit film thickness is calculated, and the operation of the display means is controlled to display the number of images that can be formed corresponding to the film thickness difference.

  Further, the present invention is characterized in that the control means controls the operation of the display means so as to display the end of the life of the photoconductor when the film thickness of the photosensitive layer becomes equal to the limit film thickness.

  According to the present invention, it is possible to accurately determine the film thickness of the photosensitive layer of the photoconductor based on the temperature of the photoconductor and the inflow current of the photoconductor. As a result, it is possible to always form an image within the service life of the photoreceptor and to form an image of stable quality.

  According to the present invention, the surface potential detecting means for detecting the surface potential of the photoconductor is disposed between the charging means and the exposure means. The surface potential can be accurately detected.

  Further, according to the present invention, the temperature detecting means for detecting the temperature of the photosensitive member is arranged upstream of the charging means and in the vicinity of the charging means, so that it is not affected by the temperature of a heat generating device such as a fixing means. By the way, the temperature of the photoconductor can be accurately detected.

  According to the present invention, the display means displays the number of images that can be formed corresponding to the film thickness difference between the film thickness of the photoconductor and the limit film thickness, so that it is possible to clearly know the replacement time of the photoconductor. .

  Further, according to the present invention, when the thickness of the photosensitive layer is equal to the limit thickness, the display means displays that the life of the photosensitive member has ended, so that it is prevented from being used beyond the lifetime. It becomes possible to suppress the breakdown of the apparatus main body and the damage of each apparatus provided around the photosensitive member.

  FIG. 1 is a diagram showing a simplified configuration of an image forming apparatus 1 according to an embodiment of the present invention, and FIG. 2 is a perspective view showing an overall appearance of the image forming apparatus 1 shown in FIG.

  The image forming apparatus 1 is, for example, a copying machine using an electrophotographic system, and is arranged around a photoconductor 2 that is exposed to light corresponding to image information to form an electrostatic latent image on the surface, and the photoconductor 2. The following means: charging means 3, surface potential detecting means 4, exposure means 5, developing means 6, transfer means 7, fixing means 8, static eliminating means 9, cleaning means 10, current detection The unit 11 includes a temperature detection unit 12, a control unit 13 that controls the overall operation of the image forming apparatus 1, a storage unit 14, and a display unit 15.

  In the overall configuration of the image forming apparatus 1, the image forming unit 21 including the main part of the apparatus, the image information reading unit 23 including the document placing unit 22 on which the image document is placed, and the image forming unit 21 are roughly included. On the other hand, it includes a paper supply unit 24 that supplies recording paper 16 as a recording medium, and a paper discharge unit 25 that discharges the image-formed recording paper 16.

  The photoreceptor 2 is a roller-like member that is supported by a driving unit (not shown) so as to be rotatable around an axis. The photoreceptor 2 has a cylindrical conductive substrate and a photosensitive layer formed by being laminated on the surface of the conductive substrate. As the conductive substrate, for example, an aluminum base tube that is commonly used in this field can be used. The photosensitive layer may be a two-layer type formed by laminating a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material. It may be a single layer type included in one layer. Various types such as those having an undercoat layer between the photosensitive layer and the conductive substrate can be used.

  The charging unit 3 charges the surface of the photoreceptor 2 to a uniform potential. In the present embodiment, the charging unit 3 includes a charging roller 31 provided in contact with the photoconductor 2 and a voltage applying unit 32 that applies a voltage to the charging roller 31. The voltage application unit 32 is connected to the control unit 13, applies a voltage to the charging roller 31 in accordance with an operation command from the control unit 13, and applies a surface potential to the photoreceptor 2 via the charging roller 31.

  The surface potential detection unit 4 is disposed between the charging unit 3 and the exposure unit 5 and detects the surface potential of the photoconductor 2 based on, for example, the strength of the magnetic field near the surface of the photoconductor 2. The exposure means 5 includes a light emitting light source and an optical scanning system, and the light corresponding to the image information read and image-processed by the image information reading unit 23 is applied to the surface of the photoreceptor 2 as indicated by an arrow 33. Irradiate and scan to expose the photosensitive layer and form an electrostatic latent image.

  The developing unit 6 includes a toner container 34 that stores toner as a developer, a developing roller 35 that is rotatably provided so as to face the photosensitive member 2, and is connected to the developing roller 35 to apply a developing bias to the developing roller 35. Development bias applying means 36 to be applied. The toner container 34 has an opening facing the photoreceptor 2, and a developing roller 35 is provided facing the photoreceptor 2 in a state where a part is exposed from the opening. A toner agitation roller (not shown) is provided in the toner container 34, and the toner is frictionally charged by the agitation of the toner agitation roller and supplied to the developing roller 35. The toner supplied to the developing roller 35 is supplied to the surface of the photoreceptor 2 by the developing bias having the same polarity as the toner applied to the developing roller 35 by the developing bias applying means 36 to develop the electrostatic latent image, and the toner Form an image.

  In this embodiment, the transfer unit 7 includes a transfer roller 37 and a transfer voltage applying unit 38 that applies a transfer voltage to the transfer roller 37. The transfer roller 37 is in contact with the photoreceptor 2 and is rotatably provided. The transfer unit 7 conveys the recording paper 16 supplied from the paper supply unit 24 so as to pass through the contact portion between the transfer roller 37 and the photosensitive member 2, and applies a transfer voltage to the transfer roller 37 in the conveyance process. A voltage having a polarity opposite to that of the toner is applied from the means 38 to transfer the toner image onto the recording paper 16. The recording paper 16 is supplied to the transfer means 7 in synchronization with the exposure of the exposure means 5 by a conveyance means (not shown).

  The fixing unit 8 is a thermocompression fixing unit including a heating roller 8a and a pressure roller 8b, and is provided so that the pressure roller 8b is in pressure contact with the heating roller 8a. The heating roller 8a includes heating means (not shown) and is heated to a predetermined temperature. The recording paper 16 to which the toner image has been transferred is conveyed to the fixing means 8 and is heated and pressed to fix the toner image when passing through the pressure contact portion between the heating roller 8a and the pressure roller 8b. The recording paper 16 on which the toner image has been fixed is transported by the transport means and discharged to the paper discharge unit 25.

  The neutralizing unit 9 is constituted by a neutralizing lamp or the like, and after transferring the toner image to the recording paper 16, removes the charge on the surface of the photoreceptor 2. The cleaning unit 10 is a member that removes and cleans the toner by scraping off the toner remaining on the surface of the photoreceptor 2. The cleaning unit 10 includes a cleaning blade 10 a provided in contact with the surface of the photoconductor 2, and a recovery container 10 b that is mounted with the cleaning blade 10 a and stores residual toner scraped off from the surface of the photoconductor 2. including.

  The temperature detection means 12 for detecting the temperature of the photoconductor 2 is arranged on the upstream side of the charging means 3 in the rotation direction indicated by the arrow 39 of the photoconductor 2 and in the vicinity of the charging means 3. For example, a contact-type temperature sensor using a thermocouple or a non-contact temperature sensor that measures radiant heat using a laser or infrared rays can be used as the temperature detection / knowledge means 12. The temperature of the photoreceptor 2 detected by the temperature detection unit 12 is input to the control unit 13.

  As the photosensitive member 2 is charged by the charging unit 3, current flows from the surface of the photosensitive member 2 toward the conductive substrate of the photosensitive member 2. The current detection unit 11 detects an inflow current (sometimes called a drum current) of the photosensitive member 2. The current detection means 11 is an ammeter provided in a circuit connecting the conductive base and the ground, and is preferably a digital ammeter from the viewpoint of data processing in the control means 13. The inflow current detected by the current detection unit 11 is input to the control unit 13.

  The control means 13 is a processing circuit realized by a central processing unit (abbreviated as CPU). The storage unit 14 is a memory attached to the control unit 13 and realized by, for example, a read only memory (ROM) and a random access memory (RAM) or a hard disk drive (HDD).

  The storage unit 14 stores in advance a program for controlling the operation of the entire image forming apparatus 1, data on operation conditions, and the like, and corresponds to the temperature and inflow current of the photoconductor 2, which will be described in detail later. Data regarding the thickness of the photosensitive layer of the photosensitive member 2, the limit thickness predetermined as the service life of the photosensitive member 2, and an image determined corresponding to the difference in thickness between the thickness of the photosensitive member 2 and the limiting thickness Data relating to the number of sheets that can be formed is stored. Here, when the difference in film thickness between the film thickness of the photosensitive member 2 and the limit film thickness is obtained, the number of images that can be formed is a standard image such as Nihon Kogyo using the photosensitive member 2 having the film thickness difference. It means the number of images that can be formed after reaching the limit film thickness when converted to an image formed at a printing ratio of 5% or less on A4 size recording paper defined in the standard (JIS) P0138. To do.

  FIG. 3 is a block diagram showing an electrical configuration relating to the control operation of the control means 13. The control means 13 outputs an operation command to each part of the apparatus corresponding to the input information and executes various processes such as calculation. In other words, the control unit 13 controls the charging operation of the photosensitive member 2 by the charging unit 3 so that the surface potential of the photosensitive member 2 detected by the surface potential detecting unit 4 becomes a predetermined potential, and the surface potential of the photosensitive member 2. Is read from the storage means 14 in accordance with the inflow current of the photosensitive member 2 detected by the current detecting means 11 and the temperature of the photosensitive member 2 detected by the temperature detecting means 6 in a state where the voltage is at the predetermined potential. The film thickness of the photosensitive layer is determined based on data relating to the film thickness of the photosensitive layer. Further, the control means 13 calculates the film thickness difference between the film thickness of the photoreceptor 2 obtained as described above and the limit film thickness stored in the storage means 14, and image formation corresponding to the film thickness difference is possible. The operation of the display unit 15 is controlled so as to display the number of sheets, and when the film thickness of the photosensitive layer becomes equal to the limit film thickness, the operation of the display unit 15 is displayed so as to display the end of the life of the photosensitive member 2. Control.

  The display means 15 is composed of, for example, a liquid crystal display (LCD), and an operator who operates the image forming apparatus 1 inputs a condition such as the power operation of the apparatus, the number of images formed in an image forming (printing) job, and an image forming job start command. It is provided in the vicinity of the operation unit 26 that performs input operations such as input.

  The operation for determining the film thickness of the photosensitive layer of the photoreceptor 2 by the control means 13 will be described below. FIG. 4 is a diagram showing the relationship between the temperature of the photoconductor 2 and the inflow current of the photoconductor 2. FIG. 4 qualitatively shows the relationship between temperature and inflow current when the surface potential of the photoreceptor 2 is kept constant.

  When the photosensitive layer of the photosensitive member 2 has a certain film thickness value, the inflow current of the photosensitive member 2 decreases as the temperature increases and the electrical characteristics change. The inflow current of the photoconductor 2 increases as the film thickness of the photoconductive layer of the photoconductor 2 decreases when the temperature is constant at a certain value. Therefore, for each type of photoconductor mounted on the image forming apparatus, a relationship among the temperature, the inflow current, and the film thickness of the photosensitive layer as shown in FIG. 4 is obtained in advance, and the relationship is stored as table data. 14, the film thickness of the photosensitive layer of the photosensitive member 2 at an arbitrary time can be obtained by the following procedure.

  FIG. 5 is a flowchart for explaining the operation for obtaining the film thickness of the photosensitive layer of the photoreceptor 2 by the control means 13. In step a1, for example, when an image forming (printing) job is executed, the surface potential of the photosensitive member 2 is detected by the surface potential detecting means 4. Usually, as the image forming operation is repeated, the surface potential of the photosensitive member 2 is lowered from a predetermined potential (V0) set in advance. Therefore, in step a2, the control means 13 detects the surface potential detection means 4. The operation of the charging unit 3 is controlled in accordance with the surface potential, and the surface potential of the photoconductor 2 is adjusted so as to become a predetermined potential V0. In step a3, the inflow current is detected by the current detection means 11 in a state where the surface potential of the photoreceptor 2 is the predetermined potential V0. In step a4, the temperature of the photosensitive member 2 is detected by the temperature detecting means 12. In step a5, the control means 13 reads out table data about the relationship between the inflow current and the film thickness of the photosensitive layer from the memory 14 as the storage means, and in accordance with the inflow current and the temperature of the photoconductor 2, Determine the thickness of the layer.

  The image forming apparatus 1 according to the present embodiment is not limited to obtaining the film thickness of the photosensitive layer of the photoreceptor 2 and can execute processing described below. FIG. 6 is a flowchart for explaining the display operation of the display unit 15 based on the operation command of the control unit 13.

  Step b1 is a step executed after step a5 in the flow shown in FIG. 5, and the control means 13 reads from the memory 14 the limit film thickness of the photosensitive layer that is predetermined as the service life of the photoreceptor 2. . In step b2, the control means 13 calculates a film thickness difference (= photosensitive layer film thickness-limit film thickness) which is a difference between the film thickness obtained in step a5 and the limit film thickness. In step b3, the control means 13 determines whether or not the film thickness difference is zero (0) or less. That is, the control means 13 determines whether or not the film thickness of the photosensitive layer is equal to or less than the limit film thickness. When the film thickness of the photosensitive layer is less than or equal to the limit film thickness, the process proceeds to step b4, and when the film thickness of the photosensitive layer exceeds the limit film thickness, the process proceeds to step b5.

  In step b4, since the film thickness of the photosensitive layer is equal to or less than the limit film thickness, the control means 13 outputs an operation command to the display means 15, and the photoconductor 2 has reached the service life, for example, the photoconductor 2 is replaced. A display that indicates what should be done. When the process reaches step b4, the photoconductor 2 reaches the service life, and if the image forming job is executed as it is, the image quality may be deteriorated or the peripheral device may be damaged. Then, the image forming job is started.

  On the other hand, in step b5, since the film thickness of the photosensitive layer has not yet reached the limit film thickness, the number of images that can be formed corresponding to the film thickness difference is read from the memory. In step b6, the control means 13 outputs an operation command to the display means 15 to display the read image formable number of sheets. Then, it returns to step a1 shown in FIG. 5, and progresses to the subsequent steps. Note that while the current image forming job is being executed, the operation relating to the film thickness detection of the photosensitive member 2 may be terminated by temporarily proceeding to the end without returning to step a1.

  Such an operation for determining the film thickness of the photosensitive layer of the photoconductor 2 by the control means 13, determination of whether or not the photoconductor 2 has reached the end of life, display of the determination result, and film thickness of the photosensitive layer and the limit film thickness The operation for obtaining and displaying the number of images that can be formed corresponding to the thickness difference may be executed whenever the image forming job is being executed, or for each image forming job instructed by the operator. Alternatively, it is possible to count the cumulative number of image formations every time the photosensitive member 2 is updated, and to execute the process every time the cumulative number reaches a predetermined value, for example, every 1,000 or 2,000. good.

1 is a diagram illustrating a simplified configuration of an image forming apparatus 1 according to an embodiment of the present invention. FIG. 2 is a perspective view showing an overall appearance of the image forming apparatus 1 shown in FIG. 1. 3 is a block diagram showing an electrical configuration relating to a control operation of the control means 13. FIG. FIG. 4 is a diagram illustrating a relationship between the temperature of the photosensitive member 2 and the inflow current of the photosensitive member 2. 4 is a flowchart for explaining an operation for obtaining the film thickness of the photosensitive layer of the photoreceptor 2 by the control means 13; 4 is a flowchart illustrating a display operation of the display unit 15 based on an operation command of the control unit 13.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Photoconductor 3 Charging means 4 Surface potential detection means 5 Exposure means 6 Development means 7 Transfer means 8 Fixing means 9 Static elimination means 10 Cleaning means 11 Current detection means 12 Temperature detection means 13 Control means 14 Storage means 15 Display means 16 Recording Paper 21 Image Forming Unit 22 Document Placement Unit 23 Image Information Reading Unit 24 Paper Supply Unit 25 Paper Discharge Unit 26 Operation Unit

Claims (5)

An electrophotographic image forming apparatus comprising a photosensitive member having a photosensitive layer formed by being laminated on the surface of a conductive substrate and being exposed to light corresponding to image information to form an electrostatic latent image on the surface. In
Charging means for charging the photoreceptor;
Surface potential detecting means for detecting the surface potential of the photoreceptor;
Exposure means for exposing the surface of the charged photoreceptor with light corresponding to image information;
Temperature detecting means for detecting the temperature of the photoreceptor;
Current detection means for detecting the inflow current of the photoreceptor;
Storage means for storing in advance data relating to the thickness of the photosensitive layer of the photosensitive member determined in accordance with the temperature of the photosensitive member and the inflow current;
The charging means controls the charging operation of the photosensitive member by the charging unit so that the surface potential of the photosensitive member detected by the surface potential detecting unit becomes a predetermined potential, and the current in a state where the surface potential of the photosensitive member is at the predetermined potential. In accordance with the inflow current of the photoconductor detected by the detecting means and the temperature of the photoconductor detected by the temperature detecting means, the film thickness of the photosensitive layer is determined based on the data relating to the film thickness of the photosensitive layer read from the storage means. An image forming apparatus comprising: a control unit for obtaining the image forming apparatus; Surface potential detection means for detecting the surface potential of the photoconductor,
The image forming apparatus according to claim 1, wherein the image forming apparatus is disposed between the charging unit and the exposure unit. A temperature detecting means for detecting the temperature of the photoreceptor,
3. The image forming apparatus according to claim 1, wherein the image forming apparatus is disposed on the upstream side of the charging unit and in the vicinity of the charging unit with respect to the rotation direction of the photosensitive member. Further including display means for displaying information in response to a display operation command by the control means,
The storage means further stores a limit film thickness that is a film thickness of the photosensitive layer that is predetermined as the service life of the photosensitive member, and is determined corresponding to a film thickness difference between the film thickness of the photosensitive layer and the limit film thickness. Stores data related to the number of images that can be formed,
The control means
The film thickness difference between the film thickness of the photosensitive layer and the limit film thickness is calculated, and the operation of the display means is controlled so as to display the number of images that can be formed corresponding to the film thickness difference. 4. The image forming apparatus according to any one of items 3. The control means
5. The image forming apparatus according to claim 4, wherein the operation of the display means is controlled so as to display that the life of the photosensitive member has expired when the film thickness of the photosensitive layer becomes equal to the limit film thickness.

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