Patent Publication Number: US-9840023-B2

Title: Sheet manufacturing apparatus and sheet manufacturing method

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a sheet manufacturing apparatus and a sheet manufacturing method. 
     2. Related Art 
     Sheet manufacturing apparatuses conventionally use a so-called wet method in which a raw material containing fibers is introduced into water and is repulped mainly by mechanical process. Such sheet manufacturing apparatuses need a large amount of water and energy for drying thereby leading to increase in the size of apparatus. JPA-2012-144819 proposes a sheet manufacturing apparatus which uses a dry method in order to reduce the size and energy. 
     However, this paper recycling apparatus has a problem that the grammage of produced sheet varies depending on the amount of raw material supplied at the upstream end. 
     SUMMARY 
     An advantage of some aspects of the invention is that a sheet manufacturing apparatus and a sheet manufacturing method which reduce variation in the grammage of sheet regardless of variation in the amount of raw material supplied at the upstream end are provided. 
     The present invention has been made to overcome at least part of the problem described above, and can be implemented in the following embodiments or application examples. 
     According to an aspect of the present invention, a sheet manufacturing apparatus includes a defibrating unit configured to defibrate a material containing fibers into a defibrated material, and a deposition unit configured to deposit a defibrated material defibrated by the defibrating unit, the deposition unit including a supply port through which the defibrated material from the defibrating unit is supplied, a plurality of opening ports through which the supplied defibrated material passes, and a dwell area disposed between the supply port and the opening ports so that the defibrated material temporarily dwells in the dwell area, wherein the dwell area allows the defibrated material to temporarily dwell in the dwell area so that a variation amount of the defibrated material that passes through the opening ports becomes smaller than a variation amount of the defibrated material supplied through the material supply port. 
     According to the above sheet manufacturing apparatus, since the defibrated material is allowed to temporarily dwell in the dwell area, variation in the supply amount of defibrated material can be absorbed, thereby reducing variation in the grammage of sheet to be manufactured. 
     In the sheet manufacturing apparatus according to the above aspect of the present invention, the dwell area may allow the defibrated material of an amount of 30% or more and 80% or less of a volume of the dwell area to dwell in the dwell area when an amount of the defibrated material supplied from the supply port per unit time is constant. 
     Accordingly the above sheet manufacturing apparatus, since the amount of deposition is set to be 30% or more and 80% or less of the dwell area, variation in the grammage of sheet to be manufactured can be reduced. 
     The sheet manufacturing apparatus according to the above aspect of the present invention, may further include a supplying unit configured to supply a material to be defibrated. The dwell area may allow the defibrated material having a mass of 10 times or more of that of the material supplied from the supplying unit per unit time when the amount of the defibrated material supplied from the supply port per unit time is constant. 
     Accordingly the above sheet manufacturing apparatus, since the dwell area allows the defibrated material having the mass of 10 times or more of that of the raw material to dwell in the dwell area, variation in the supply amount of raw material due to double feeding (multifeed) or feeding failure of a sheet can be absorbed, thereby reducing variation in the grammage of sheet to be manufactured. 
     In the sheet manufacturing apparatus according to the above aspect of the present invention, it is possible that the supply port is a second supply port, the opening port is a second opening port, the dwell area is a second dwell area, a first dwell area is further provided between the defibrating unit and the deposition unit so that the defibrated material temporarily dwells in the first dwell area, and the first dwell area is provided between a first supply port through which the defibrated material from the defibrating unit is supplied and a plurality of first opening ports through which the supplied defibrated material passes, and the first dwell area allows the defibrated material to temporarily dwells in the first dwell area so that a variation amount of the defibrated material that passes through the first opening ports becomes smaller than a variation amount of the defibrated material supplied through the first supply port. 
     According to the above sheet manufacturing apparatus, since two dwell areas are provided, variation in the supply amount can be absorbed in two steps, thereby reducing variation in the grammage of sheet to be manufactured compared with the case of one dwell area. 
     According to another aspect of the present invention, a sheet manufacturing method includes defibrating a material containing fibers into a defibrated material, and allowing the defibrated material to deposit through a plurality of opening ports to form a sheet, wherein the defibrated material temporarily dwells so that a variation amount of the defibrated material that passes through the opening ports becomes smaller than a variation amount of the supplied defibrated material. 
     According to the above sheet manufacturing method, since the defibrated material is allowed to temporarily dwell, variation in the supply amount of defibrated material can be absorbed, thereby reducing variation in the grammage of sheet to be manufactured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a schematic view of a sheet manufacturing apparatus according to the present embodiment. 
         FIG. 2  is a schematic view of a sieve. 
         FIG. 3  is a schematic view of the sieve and a detecting unit. 
         FIG. 4  is a chart simulating a relationship between time and flow rate at different positions in the case where a dwell area is not provided. 
         FIG. 5  is a chart simulating a relationship between time and flow rate at different positions in the case where a dwell area is provided. 
         FIG. 6  is a chart simulating a relationship among time, flow rate at different positions and sheet weight in the case where a dwell area is not provided. 
         FIG. 7  is a chart simulating a relationship among time, flow rate at different positions and sheet weight in the case where a dwell area is provided. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     With reference to the drawings, a preferred embodiment of the present invention will be described in detail. The embodiment described below is not intended to unreasonably limit the scope of the present invention defined in the appended claims. Further, all the configuration described below are not necessarily indispensable elements of the present invention. 
     A sheet manufacturing apparatus according to the present embodiment includes a defibrating unit that defibrates a material containing fibers into a defibrated material, and a deposition unit that deposits a defibrated material defibrated by the defibrating unit, the deposition unit including a supply port through which the defibrated material from the defibrating unit is supplied, a plurality of opening ports through which the supplied defibrated material passes, and a dwell area disposed between the supply port and the opening ports so that the defibrated material temporarily dwells in the dwell area, wherein the dwell area allows the defibrated material to temporarily dwell in the dwell area so that a variation amount of the defibrated material that passes through the opening ports becomes smaller than a variation amount of the defibrated material supplied through the material supply port. 
     1. Sheet Manufacturing Apparatus 
     1.1. Configuration 
     First, with reference to the drawings, a sheet manufacturing apparatus according to the present embodiment will be described.  FIG. 1  is a schematic view of a sheet manufacturing apparatus  100  according to the present embodiment. 
     As shown in  FIG. 1 , the sheet manufacturing apparatus  100  includes a supplying unit  10 , a manufacturing unit  102 , and a control unit  140 . The manufacturing unit  102  manufactures a sheet. The manufacturing unit  102  includes a crushing unit  12 , a defibrating unit  20 , a classifying unit  30 , a screening unit  40 , a first web-forming unit  45 , a mixing unit  50 , a deposition unit  60 , a second web-forming unit  70 , a sheet-forming unit  80  and a cutting unit  90 . 
     The supplying unit  10  supplies raw material to the crushing unit  12 . The supplying unit  10  is, for example, an automatic loading unit that is configured to continuously load the raw material into the crushing unit  12 . The raw material to be supplied by the supplying unit  10  is, for example, recycled paper or pulp sheet that contains fibers. 
     As the raw material is supplied by the supplying unit  10 , the crushing unit  12  cuts the raw material in air into small pieces. The small pieces are shaped and sized into, for example, a few centimeters square. In the illustrated example, the crushing unit  12  includes a crushing blade  14  so that the crushing blade  14  can cut the loaded raw material. For example, the crushing unit  12  may be a shredder. The raw material cut by the crushing unit  12  is received by a hopper  1  and is conveyed (transferred) to the defibrating unit  20  via a pipe  2 . 
     The defibrating unit  20  defibrates the raw material cut by the crushing unit  12 . The term “defibrate” as used herein means to untangle the raw material (defibration object) made up of a plurality of bonded fibers into individual fibers. The defibrating unit  20  also has a function of allowing materials such as resin particle, ink, toner and blur-preventing agent attached to the raw material to be separated from the fiber. 
     The raw material which has passed through the defibrating unit  20  is called “defibrated material.” The “defibrated material” may contain, in addition to the disentangled fibers of defibrated material, particles of resin (resin for bonding a plurality of fibers to each other), color agents such as ink and toner, and additive agents such as blur-preventing agent and strengthening agent which are separated from the fiber during untangling of fibers. The disentangled defibrated material is string-like or ribbon-like shape. The disentangled defibrated material may exist in the state of not being tangled with other untangled fiber (independent state), or alternatively, in the state of being tangled together with other disentangled defibrated material (the state of forming so-called “lumps”). 
     The defibrating unit  20  performs dry defibration in atmosphere (in air). For example, an impeller mill can be used as the defibrating unit  20 . The defibrating unit  20  has a function of generating an airflow so as to suction raw material and discharge defibrated material. Accordingly, the defibrating unit  20  can suction the raw material along with the airflow generated by the defibrating unit  20  through the inlet port  22 , perform a defibration process, and transfer the defibrated material to the outlet port  24 . The defibrated material which has passed through the defibrating unit  20  is conveyed to the classifying unit  30  via a pipe  3 . 
     The classifying unit  30  classifies the defibrated material which has passed through the defibrating unit  20 . Specifically, the classifying unit  30  isolates and removes the defibrated material having relatively small size or low density (such as resin particles, color agents and additive agents). As a result, the percentage of fibers having relatively large size or high density to the defibrated material can be increased. 
     The classifying unit  30  may be an airflow classifier. The airflow classifier generates a swirling airflow so as to separate the material depending on the centrifugal force applied to the material which are different depending on the size and density of the material to be classified. A classifying point can be adjusted by adjusting the rate of airflow and the magnitude of centrifugal force. Specifically, the classifying unit  30  may be cyclone classifier, elbow-jet classifier, Eddy classifier or the like. Particularly, since a cyclone classifier as shown in the figure has a simple configuration, it can be preferably used as the classifying unit  30 . 
     The classifying unit  30  includes, for example, an inlet port  31 , a cylindrical section  32  which is connected to the inlet port  31 , an inverted conical section  33  which is continuous from the cylindrical section  32  and is disposed under the cylindrical section  32 , a lower outlet port  34  provided at the center of a lower part of the inverted conical section  33 , and an upper outlet port  35  provided at the center of an upper part of the cylindrical section  32 . 
     In the classifying unit  30 , an airflow which involves the defibrated material introduced from the inlet port  31  moves in a circulating motion in the cylindrical section  32 . As a result, a centrifugal force is applied to the introduced defibrated material, and the classifying unit  30  can separate the defibrated material into fibers (first classified material) having a larger size and a higher density than those of resin particles and ink particles, and resin particles, color agents and additive agents (second classified material) having a smaller size and a lower density than those of fibers. The first classified material (fraction) is discharged from the lower outlet port  34  and introduced into the screening unit  40  via a pipe  4 . On the other hand, the second classified material (fraction) is discharged from the upper outlet port  35  into a receiving unit  36  via a pipe  5 . 
     The screening unit  40  allows the first classified material (defibrated material defibrated by the defibrating unit  20 ) which has passed through the classifying unit  30  to be introduced through the inlet port  42  so as to screen the defibrated material depending on the length of fibers. For example, a sieve can be used as the screening unit  40 . The screening unit  40  may include a mesh (filter, screen) so as to separate the material contained in the first classified material into the fibers or particles having a size smaller than the size of mesh opening (first screened material, which pass through the mesh), and the fibers, undefibrated piece or lumps having a size larger than the size of mesh opening (second screened material, which does not pass through the mesh). For example, the first screened material is received by the hopper  6  and then conveyed to the mixing unit  50  via a pipe  7 . The second screened material is returned to the defibrating unit  20  from the outlet port  44  via a pipe  8 . Specifically, the screening unit  40  is a cylindrical sieve that can rotate by means of a motor. The mesh of the screening unit  40  may be, for example, a wire mesh, an expand metal formed by expanding a metal sheet having notches or a punching metal formed by punching a metal sheet by using a press machine or the like. 
     The first web-forming unit  45  allows the first screened material which has passed through the screening unit  40  to be transferred to the mixing unit  50 . The first web-forming unit  45  includes a mesh belt  46 , stretching rollers  47  and a suctioning unit (suction mechanism)  48 . 
     As the first screened material passes through the opening port (mesh openings) of the screening unit  40  and is dispersed in air, the suctioning unit  48  can suction the first screened material onto the mesh belt  46 . The first screened material is deposited on the moving mesh belt  46  to form a web V. Basic configurations of the mesh belt  46 , the stretching rollers  47  and suctioning unit  48  are similar to those of a mesh belt  72 , stretching rollers  74  and a suction mechanism  76  of the second web-forming unit  70 , which will be described later. 
     The web V is formed softly bulky containing abundant air while it is fed through the screening unit  40  and the first web-forming unit  45 . The web V deposited on the mesh belt  46  is introduced into the pipe  7  and is transferred to the mixing unit  50 . 
     The mixing unit  50  mixes the first screened material which has passed through the screening unit  40  (the first screened material transferred by the first web-forming unit  45 ) and an additive agent which contains resin. The mixing unit  50  includes an additive agent supply unit  52  that supplies an additive agent, a pipe  54  that transfers the screened material and the additive agent, and a blower  56 . In the illustrated example, the additive agent is supplied from the additive agent supply unit  52  to the pipe  54  via the hopper  9 . The pipe  54  is connected to the pipe  7 . 
     The mixing unit  50  generates an airflow by the blower  56  so as to mix and transfer the first screened material and the additive agent in the pipe  54 . The mechanism for mixing the first screened material and the additive agent is not specifically limited, and may have a blade that rotates in high speed for stirring, or alternatively, may be a V-type mixer that uses rotation of the container. 
     The additive agent supply unit  52  may be a screw feeder as shown in  FIG. 1  or a disk feeder, which is not shown in the figure. The additive agent supplied by the additive agent supply unit  52  includes resin for bonding a plurality of fibers. At the time when the resin is supplied, a plurality of fibers are not bonded. The resin melts while it passes through the sheet-forming unit  80  and bonds a plurality of fibers to each other. 
     The resin supplied by the additive agent supply unit  52  is thermoplastic resin or heat-curable resin, and may be, for example, AS resin, ABS resin, polypropylene, polyethylene, polyvinyl chloride, polystyrene, acryl resin, polyester resin, polyethylene terephthalate, polyphenylene ether, polybutylene terephthalate, nylon, polyamide, polycarbonate, polyacetal, polyphenylene sulfide, or polyether ether ketone. Those resins may be used alone or in combination thereof as appropriate. The additive agent supplied by the additive agent supply unit  52  may be in the form of fiber or powder. 
     Further, the additive agent supplied by the additive agent supply unit  52  may include, in addition to the resin that bonds fibers to each other, coloring agent for coloring fibers, anti-aggregation agent for preventing aggregation of fibers, or flame retardant agent for retarding flaming of fibers depending on the type of sheet to be manufactured. The mixture (mixture of the first classified material and the additive agent) which has passed through the mixing unit  50  is conveyed to the deposition unit  60  via the pipe  54 . 
     The deposition unit  60  allows the mixture which has passed through the mixing unit  50  to be introduced through an inlet port  62 , and allows the entangled defibrated material (fibers) to be disentangled so that they are dispersed in air and deposited in the deposition unit  60 . Further, when the resin of the additive agent supplied by the additive agent supply unit  52  is in the form of fiber, the deposition unit  60  allows the entangled resin to be disentangled. Accordingly, the deposition unit  60  allows the mixture to be uniformly deposited in the second web-forming unit  70 . 
     The deposition unit  60  may be a cylindrical sieve that rotates. The deposition unit  60  includes a mesh and allows fibers or particles contained in the mixture which has passed through the mixing unit  50  and having a size smaller than the size of mesh opening (fibers or particles which pass through the mesh) to be precipitated. The deposition unit  60  has the same configuration as that of, for example, the screening unit  40 . 
     The “sieve” of the deposition unit  60  may not have a function of screening a specific target. That is, the “sieve” used as the deposition unit  60  may be any device having a mesh, and the deposition unit  60  may precipitate all of the mixture introduced into the deposition unit  60 . 
     The second web-forming unit  70  allows the passed material which has passed through the deposition unit  60  to be deposited thereon so as to form a web W. The second web-forming unit  70  includes, for example, the mesh belt  72 , the stretching rollers  74  and the suction mechanism  76 . 
     While the mesh belt  72  moves, it allows the passed material which has passed through the opening port of the deposition unit  60  (mesh openings) to be deposited thereon. The mesh belt  72 , which is hung on the stretching rollers  74 , is formed not to easily permit the passing of the passed material but permit the passing of air. The mesh belt  72  moves by rotation of the stretching rollers  74 . While the mesh belt  72  continuously moves, the passed material which has passed through the deposition unit  60  is continuously deposited on the mesh belt  72  to form the web W on the mesh belt  72 . The mesh belt  72  is made of, for example, metal, resin, cloth or non-woven fabric. 
     The suction mechanism  76  is disposed under the mesh belt  72  (opposite to the deposition unit  60 ). The suction mechanism  76  can generate a downward airflow (airflow directed from the deposition unit  60  to the mesh belt  72 ). The suction mechanism  76  allows the mixture which has been dispersed in air by the deposition unit  60  to be suctioned onto the mesh belt  72 . As a result, a discharge rate from the deposition unit  60  can be increased. Further, the suction mechanism  76  can generate a downflow in a falling path of the mixture, thereby preventing the defibrated material and additive agent from being entangled. 
     As described above, as the material passes through the deposition unit  60  and the second web-forming unit  70  (web-forming process), the web W is formed softly bulky containing abundant air. The web W deposited on the mesh belt  72  is transferred to the sheet-forming unit  80 . 
     In the illustrated example, a moisture-adjusting unit  78  that adjusts moisture of the web W is provided. The moisture-adjusting unit  78  can adjust the ratio of the amount of the web W and water by adding water or water vapor to the web W. 
     The sheet-forming unit  80  forms a sheet S by applying heat and pressure on the web W deposited on the mesh belt  72 . The sheet-forming unit  80  can bond a plurality of fibers in the mixture to each other via the additive agent (resin) by applying heat on the mixture of the defibrated material and additive agent mixed in the web W. 
     The sheet-forming unit  80  may be, for example, heater roller, hot press forming machine, hot plate, heated air blower, infrared heater or flash fixing device. In the illustrated example, the sheet-forming unit  80  includes a first bonding unit  82  and a second bonding unit  84 , and the bonding units  82  and  84  each includes a pair of heating rollers  86 . Since the bonding units  82  and  84  are provided as the heating rollers  86 , the sheet S can be formed while the web W is continuously transferred unlike the case where the bonding units  82  and  84  are provided as a plate-shaped press machine (plate press machine). The number of the heating rollers  86  is not specifically limited. 
     The cutting unit  90  cuts the sheet S which has been formed by the sheet-forming unit  80 . In the illustrated example, the cutting unit  90  includes a first cutting unit  92  that cuts the sheet S in a direction crossing a transfer direction of the sheet S and a second cutting unit  94  that cuts the sheet S in a direction parallel to the transfer direction. For example, the second cutting unit  94  cuts the sheet S which has passed the first cutting unit  92 . 
     As described above, the sheet S in the form of a cut sheet having a predetermined size is formed. The sheet S in the form of a cut sheet is discharged into the discharge unit  96 . 
     1.2. Dwell Area 
     Referring to  FIGS. 2 and 3 , the dwell area  320  will be described.  FIG. 2  is a schematic view of a drum  300  of the sieve  800 , and  FIG. 3  is a schematic view of the sieve  800  and a detecting unit  700 . In  FIGS. 2 and 3 , the configuration other than the sieve  800  and the detecting unit  700  is not shown. 
     Although the sieve  800  shown in  FIGS. 2 and 3  is the sieve of the above described deposition unit  60 , it may be used as the sieve of the screening unit  40 . 
     The sieve  800  of the deposition unit  60  includes a material supply port  560  which is a supply port through which the mixture containing the defibrated material from the defibrating unit  20  is supplied, a plurality of opening ports  311  through which the mixture containing the supplied defibrated material passes, and a dwell area (staying portion)  320  disposed between the material supply port  560  and the opening ports  311  so as to allow the mixture containing the defibrated material to temporarily dwell therein. 
     The configuration of the sieve  800  will be further described in detail. The sieve  800  includes two side portions  500 ,  500  which do not rotate, a drum  300  which is a rotating body disposed between the side portions  500 ,  500 , and a fixation member  600  disposed in the drum  300 . 
     The side portions  500 ,  500  rotatably support the drum  300  by a support section which is not shown in the figure. At least one of the side portions  500  includes an introduction unit  540 , and the introduction unit  540  includes the material supply port  560 . The material supply port  560  is disposed in a center area which is the same area as the rotation axis R of the drum  300 , or alternatively, vertically above the rotation axis R. The defibrated raw material is introduced into the drum  300  via the material supply port  560  of the introduction unit  540 . 
     The drum  300  has a generally cylindrical shape, and includes cylindrical sections  315 ,  315  disposed on both ends, and an opening port section  310  interposed between the cylindrical sections  315 ,  315  and having a plurality of opening ports  311  (mesh openings of the sieve). An inner space of the drum  300  is the dwell area  320 . The opening port section  310  allows at least the defibrated material (defibrated fiber) to pass therethrough in air. The opening port section  310  and the cylindrical sections  315  rotate together. The opening port section  310  may be a punching metal having holes as a plurality of opening ports  311 . The size, forming area and the like of the opening ports  311  may be appropriately defined depending on the size, type and the like of the fiber. A plurality of opening ports  311  have the same size (opening port area) and are arranged with an equal interval. Further, the opening port section  310  is not limited to a punching metal, and may be a wire mesh material. 
     The fixation member  600  is a plate-shape member which is disposed in the drum  300  and at a position vertically above the rotation axis R. The fixation member  600  is disposed in the longitudinal direction of the drum  300  with the both ends being fixed to the side portions  500 ,  500 . The fixation member  600  has a width larger than that of the opening port section  310 . As the drum  300  rotates relative to the side portions  500 ,  500 , it comes into contact with at least defibrated material which moves along with the opening port section  310 . 
     As the drum  300  rotates about the rotation axis R which extends in the horizontal direction, the rotation causes the defibrated material to rotate in the rotation direction of the drum  300 . Further, the defibrated material is urged against the inner peripheral surface of the opening port section  310  by a centrifugal force, and the fibers having a size smaller than the size of mesh openings of the opening ports  311  pass through the opening ports  311 . The defibrated materials are disentangled in the sieve  800  of the deposition unit  60  so that all the defibrated material introduced into the sieve  800  is essentially allowed to pass through the opening ports  311 . Further, when the sieve  800  is used as a sieve of the screening unit  40 , the defibrated material is sieved into those allowed to pass through the opening ports  311  and those not allowed to pass through the opening ports  311  depending on the size of the defibrated material. 
     Further, as the defibrated material affixed on the inner peripheral surface of the opening port section  310  comes into contact (collides) with the fixation member  600 , the defibrated material is peeled off from the inner peripheral surface of the opening port section  310  and disentangled. This facilitates the defibrated material to pass through the opening ports  311 . 
     The drum  300  rotates about the rotation axis R by an electric motor which is not shown in the figure. The electric motor is electrically connected to a control unit  140  so as to rotate the drum  300  by a command from the control unit  140  in a direction indicated by the arrow with a predetermined rotation rate. 
     The dwell area  320  allows the defibrated material to temporarily dwell therein so that the variation amount of the defibrated material that passes through the opening ports  311  becomes smaller than the variation amount of the defibrated material supplied from the material supply port  560 . Since the defibrated material is allowed to temporarily dwell in the dwell area  320 , variation in the supply amount of defibrated material can be absorbed, thereby reducing variation in the grammage of sheet to be manufactured. 
     The term “dwell” as used herein refers to a state in which the mixture is retained in the sieve  800  for a period of time longer than the minimum time from when the mixture is supplied into the sieve  800  to when it passes through a plurality of opening ports  311 , which are the mesh openings of the sieve. 
     The dwell area  320  may retain the mixture (defibrated material) of the amount of 30% or more and 80% or less of the volume of the dwell area when the amount of the mixture (defibrated material) supplied from the material supply port  560  is constant per unit time (e.g., per second). Accordingly, variation in the supply amount of mixture can be absorbed by setting the dwell amount to be 30% or more and 80% or less of the dwell area  320 , thereby reducing variation in the grammage of sheet to be manufactured. 
     In variation in the supply amount of mixture, as the supply amount to the material supply port  560  decreases, the mixture sieved through the opening port section  310  decreases, thereby effecting on the grammage of sheet. Accordingly, it is advantageous to increase the dwell amount in the dwell area  320  as possible in order to reduce the variation in the grammage of sheet. Further, it has been revealed that clogging occurs when the dwell amount exceeds 80% of the dwell area  320 , thereby reducing the mixture sieved through the opening port section  310 . Therefore, variation in the supply amount of mixture can be absorbed by setting the dwell amount to be 30% or more and 80% or less of the dwell area  320 , more preferably, 50% or more and 70% or less, thereby reducing variation in the grammage of sheet to be manufactured. 
     As shown in  FIG. 3 , the dwell amount in the dwell area  320  can be measured by the detecting unit  700 .  FIG. 3  is a schematic view of the sieve  800  as shown in the transfer direction of the web W, in which the drum  300  is shown in a vertical cross sectional view so as to show the inside thereof. 
     The detecting unit  700  is, for example, an optical sensor, and includes a light emitting section  702  and a light receiving section  704  disposed so as to oppose each other with the sieve  800  interposed therebetween. The light emitting section  702  and the light receiving section  704  each extend at least in the length which is the same as the height of the dwell area  320 . Light emitted from the light emitting section  702  is incident into the drum  300  through a transparent window, which is not shown in the figure, disposed on the side portion  500 . Light is not transmitted through a portion of the dwell area  320  in the drum  300  in which the mixture F is present, which is shown by the hatching in the figure, while light is transmitted through the remaining portion in which the mixture F is not present. The dwell amount of the mixture in the dwell area  320  can be measured by an output from a portion of the light receiving section  704  which receives the transmitted light. 
     The detecting unit  700  may output the detection result to the control unit  140  shown in  FIG. 1 , and the control unit  140  may calculate the percentage of the dwell amount to the volume of the dwell area  320  on the basis of the detection result and display it to a display unit or the like. Further, the control unit  140  may control a paper feeding rate (g/sec) from the supplying unit  10  to the crushing unit  12  based on the calculated percentage of the dwell amount so that the dwell amount becomes 30% or more and 80% or less of the dwell area  320 . 
     Further, the detecting unit  700  may not be provided in the sieve  800 , and another detection unit such as an optical sensor or a sheet thickness measuring sensor may be provided in the supplying unit  10  shown in  FIG. 1  so that the number of sheets or the weight of sheet of the raw material which is supplied per unit time from the supplying unit  10  to the crushing unit  12  is constantly monitored to calculate (estimate) the dwell amount in the dwell area  320  on the basis of the detection result. 
     When the amount of the mixture (defibrated material) supplied per unit time (e.g., per second) from the material supply port  560  is constant, the dwell area  320  may retain the defibrated material having the mass of 10 times or more, more preferably, 30 times or more of that of the raw material supplied per unit time from the supplying unit  10 . Accordingly, when the dwell area  320  allows the defibrated material having the mass of 10 times or more of that of the raw material to dwell therein, variation in the supply amount of raw material due to double feeding (multifeed) or feeding failure of a sheet can be absorbed, thereby reducing variation in the grammage of sheet to be manufactured. Double feeding of the raw material means that two sheets or more are supplied at one time from the supplying unit  10  shown in  FIG. 1 , although they should have been supplied one by one. Feeding failure of the raw material means that a sheet fails to be supplied from the supplying unit  10  for one time or more, although they should have been supplied one by one. 
     The inner space of the drum  300  as the dwell area  320  can be achieved, for example, by any of decreasing the size of mesh openings of the opening ports  311 , selecting the volume of drum (surface area of the opening port section  310 ) having an appropriate (small) size relative to the processing ability (g/min), increasing the rotation speed of the drum  300 , providing the fixation member  600  having an appropriate size, decreasing a flow rate of the suction mechanism  76  (suctioning unit  48 ), or combination thereof as appropriate. Those conditions can be appropriately selected depending on the type of raw material, the supply rate of raw material, the productivity of the sheet, the size of apparatus or the like. For example, when used paper of copy sheet in a typical A4 size is provided as the raw material, the mesh openings of the opening ports  311  may be sized in 1 mm, the drum  300  may have a diameter of 220 mm, a width of 210 mm and a rotation speed in the range of 150 rpm to 250 rpm. 
     Although the above described dwell area  320  is provided only in the deposition unit  60 , a first dwell area that allows the defibrated material to temporarily dwell therein may be further provided at a position between the defibrating unit  20  and the deposition unit  60 . The first dwell area may have the same configuration as that of the dwell area  320  and may be used for the sieve of the screening unit  40 . In this case, the material supply port  560  of the sieve  800  in the deposition unit  60  shown in  FIGS. 2 and 3  is provided as a second supply port, the opening ports  311  are provided as second opening ports, and the dwell area  320  is provided as a second dwell area. 
     The first dwell area of the screening unit  40  will be described in association with the sieve  800  shown in  FIGS. 2 and 3 . The first dwell area  320  is disposed between the first supply port  560  in which the defibrated material from the defibrating unit  20  is supplied and a plurality of first opening ports  311  which the supplied defibrated material pass through. The first dwell area  320  allows the defibrated material to temporarily dwell therein so that the variation amount of the defibrated material that passes through the first opening ports  311  becomes smaller than the variation amount of the defibrated material supplied from the first supply port  560 . Accordingly, since variation in the supply amount is absorbed in two steps by providing two dwell areas, variation in the grammage of sheet to be manufactured can be reduced compared with the case of one dwell area. 
     1.3. Simulation 
     With reference to  FIGS. 1, 4 and 5 , pulsation of the flow of defibrated material depending on the presence or absence of the dwell area will be described.  FIG. 4  is a chart simulating a relationship between time and flow rate at different positions in the case where a dwell area is not provided, and  FIG. 5  is a chart simulating a relationship between time and flow rate at different positions in the case where a dwell area is used. 
     As shown in  FIG. 4 , a simulation was performed for a sheet flow rate Mg supplied from the supplying unit  10  ( FIG. 1 ) having an average of approximately 100 (g/min) which varied by ±50% in sine-wave of 50 second cycle. A flow rate Vg of the defibrated material sieved by a first sieve of the screening unit  40  ( FIG. 1 ) and a flow rate Wg of the mixture sieved by a second sieve of the deposition unit  60  ( FIG. 1 ) were the same, and they varied by a significant amount slightly after the variation of the sheet flow rate Mg. The variation of the flow rate Vg led to the variation of the grammage of the web V ( FIG. 1 ), and the variation of the flow rate Wg led to the variation of the grammage of the web W ( FIG. 1 ), which appeared as change in thickness of the sheet S ( FIG. 1 ). This was because the first sieve and the second sieve did not have the dwell area. Further, the flow rate Vg and the flow rate Wg were slightly smaller than the sheet flow rate Mg due to dwelling in the defibrating unit  20  ( FIG. 1 ). 
     As shown in  FIG. 5 , a simulation was performed under the same conditions as those of  FIG. 4  except for providing a dwell area for each of the first sieve and the second sieve. The flow rate Vg of the defibrated material sieved by the first sieve of the screening unit  40  ( FIG. 1 ) in  FIG. 5  had a variation range smaller than that of  FIG. 4 , and the flow rate Wg of the mixture sieved by the second sieve of the deposition unit  60  ( FIG. 1 ) had a variation range further smaller than the flow rate Vg. The difference between the variation ranges in  FIG. 4  and  FIG. 5  was due to the fact that the variation of the sheet flow rate Mg was absorbed by the first dwell area of the first sieve and the variation of the sheet flow rate Vg was absorbed by the second dwell area of the second sieve. As a result, providing two dwell areas can reduce the variation of the flow rate Wg (variation in the grammage of web W ( FIG. 1 )), thereby reducing variation in the grammage (thickness) of the sheet S ( FIG. 1 ). 
     1.4. Other Simulations 
     With reference to  FIGS. 1, 6 and 7 , other simulations of pulsation of the flow of defibrated material depending on the presence or absence of the dwell area will be described.  FIG. 6  is a chart simulating a relationship among time, flow rate at different positions and sheet weight in the case where a dwell area is not provided, and  FIG. 7  is a chart simulating a relationship among time, flow rate at different positions and sheet weight in the case where a dwell area is provided. 
     As shown in  FIG. 6 , a simulation was performed for double feeding of two sheets which occurred around the time of 720 seconds under the condition that the 4 g sheets were supplied from the supplying unit  10  ( FIG. 1 ) one by one in every 2.5 seconds. The sheet weight Ma, Mb were 0.0 g and 4.0 g, respectively. The sheet weight Mc at the time around 720 seconds was 8.0 g, indicating that double feeding of the sheets occurred. The flow rate Vg of the defibrated material sieved by the first sieve of the screening unit  40  ( FIG. 1 ) and the flow rate Wg of the mixture sieved by the second sieve of the deposition unit  60  ( FIG. 1 ) were the same, and they varied by a significant amount slightly after the sheet weight Mc and were then returned to the original values with the elapse of time. Those large variation of the flow rates Vg, Wg occurred since the first sieve and the second sieve did not have the dwell area. 
     As shown in  FIG. 7 , a simulation was performed under the same conditions as those of  FIG. 6  except for providing a dwell area for each of the first sieve and the second sieve. The flow rate Vg of the defibrated material sieved by the first sieve of the screening unit  40  ( FIG. 1 ) in  FIG. 7  had a variation range smaller than that of  FIG. 6 , and the flow rate Wg of the mixture sieved by the second sieve of the deposition unit  60  ( FIG. 1 ) had a variation range further smaller than the flow rate Vg. The difference between the variation ranges was due to the fact that the variation from the sheet weight Ma to the sheet weight Mc was absorbed by the first dwell area of the first sieve and the variation of the sheet flow rate Vg was absorbed by the second dwell area of the second sieve. As a result, providing two dwell areas can reduce the variation of the flow rate Wg (variation in the grammage of web W ( FIG. 1 )) regardless of double feeding of the sheets, thereby reducing variation in the grammage (thickness) of the sheet S ( FIG. 1 ). 
     2. Sheet Manufacturing Method 
     A sheet manufacturing method according to the present embodiment includes defibrating a material containing fibers into a defibrated material, and allowing the defibrated material to deposit through a plurality of opening ports to form a sheet, wherein the defibrated material temporarily dwells so that a variation amount of the defibrated material that passes through the opening ports becomes smaller than a variation amount of the supplied defibrated material. 
     The sheet manufacturing method can be implemented by the sheet manufacturing apparatus  100  which is shown in  FIGS. 1 and 2 . A specific example will be described with reference to  FIGS. 1 and 2 , but the invention is not limited thereto. 
     First, when a user requests a process for manufacturing the sheet S via an operation device, which is not shown in the figure, in the control unit  140 , the control unit  140  starts processing for the respective processing units. 
     (A) The supplying unit  10  supplies sheets of paper as raw material containing fibers to the defibrating unit  20  via the crushing unit  12  one by one with a predetermined interval. 
     (B) The defibrating unit  20  defibrates the material containing fibers into defibrated material. The defibrated material defibrated by the defibrating unit  20  is transferred to the classifying unit  30  via the pipe  3 . 
     (C) The classifying unit  30  classifies the defibrated material, for example, by density. The defibrated material classified by the classifying unit  30  is transferred to the screening unit  40  via the pipe  4 . 
     (D) The screening unit  40  sieves the defibrated material by the first sieve depending on the length of the fiber. The first sieve includes the drum  300  shown in  FIGS. 2 and 3 , and the defibrated material temporarily dwells in the dwell area  320 , and after that, the defibrated material passes through a plurality of opening ports  311 . Since the defibrated material dwells in the dwell area  320 , the variation amount of the defibrated material which passes through the opening port  311  becomes smaller than the variation amount of the defibrated material supplied to the first sieve. The first web-forming unit  45  allows the defibrated material which has passed through the opening port  311  to be deposited to form the web V. 
     (E) The mixing unit  50  mixes an additive agent such as resin with the web V. The mixture obtained by the mixing unit  50  is transferred to the deposition unit  60 . 
     (F) The deposition unit  60  introduces the mixture containing the defibrated material into the second sieve so that the mixture is deposited on the second web-forming unit  70  to form the web W. The second sieve includes the drum  300  shown in  FIGS. 2 and 3 , and the mixture containing the defibrated material temporarily dwells therein, and after that, the mixture passes through a plurality of opening ports  311 . Since the defibrated material dwells in the dwell area  320 , the variation amount of the defibrated material which passes through the opening ports  311  becomes smaller than the variation amount of the supplied defibrated material. 
     (G) The web W is transferred from the second web-forming unit  70  to the sheet-forming unit  80  so as to manufacture the sheet S. The sheet-forming unit  80  applies heat and pressure on the web W, and cuts into a predetermined size to discharge the sheet S into the discharge unit  96 . 
     In this sheet manufacturing method, since the defibrated material is allowed to temporarily dwell, variation in the supply amount of defibrated material can be absorbed, thereby reducing variation in the grammage of sheet to be manufactured. 
     In the above process (A), sheet feeding may not be limited to intermittent supply as long as the supply amount of sheet per unit time is constant. For example, other sheet feeding method such as continuous feeding that feeds sheets without interval may be used. 
     Further, the above process (C) may be performed at the same time with the screening process in the first sieve of the screening unit  40  and the first web-forming unit  45 . That is, since the defibrated material having relatively small size or low density (which corresponds to the second classified material) passes through the mesh belt  46  and is not deposited on the mesh belt  46 , the classifying unit  30  and the above process (C) may be omitted. 
     The above process (D) may not form the web V, and may transfer the mixture which has passed the opening port  311  to the mixing unit  50  or the deposition unit  60 . Further, although the example has been described that the defibrated material dwells in the process (D), the invention is not limited thereto, and the dwell area may be provided only in the process (F). 
     3. Modification 1 
     As a modification 1, an operation during the initial operation of the sheet manufacturing apparatus  100  shown in  FIGS. 1 and 2  will be described. 
     In the initial operation of the sheet manufacturing apparatus  100  in which it is first operated after the installation, the defibrated material and the mixture are not present in any of the units. Accordingly, after the operation starts, at least the drum  300  of the deposition unit  60  is not rotated for a certain period of time. The drum  300  starts to rotate when a predetermined amount of mixture is accumulated in the dwell area  320 . Since the drum  300  starts to rotate when a predetermined amount of mixture is accumulated in the dwell area  320 , the sheet can be manufactured with stable grammage in a relatively short period of time after the operation starts even during the initial operation. 
     In the case where the dwell area  320  is provided in the screening unit  40 , the above operation in the deposition unit  60  can also be applied to the screening unit  40 . Specifically, after the operation starts, the drum  300  of the deposition unit  40  is not rotated for a certain period of time. The drum  300  starts to rotate when a predetermined amount of defibrated material is accumulated in the dwell area  320  of the screening unit  40 . Accordingly, the web V can be manufactured with stable grammage in a relatively short period of time even during the initial operation. The drum  300  of the deposition unit  60  does not start to rotate for a certain period of time after the drum  300  of the screening unit  40  starts to rotate until a predetermined amount of defibrated material of the web V is accumulated in the dwell area  320  of the deposition unit  60 . Since the grammage of the web V is stable, the grammage of the web W and the sheet S also becomes stable. 
     The above initial operation can be performed by an initial operation mode which is preset in the control unit  140  of the sheet manufacturing apparatus  100 . Further, this initial operation mode can be selected for the first operation after the maintenance of the sheet manufacturing apparatus  100 , not only as the initial operation of the sheet manufacturing apparatus  100 . 
     4. Modification 2 
     As a modification 2, an operation during the termination of operation of the sheet manufacturing apparatus  100  shown in  FIGS. 1 and 2  will be described. 
     During the termination of operation of the sheet manufacturing apparatus  100 , the drum  300  stops to rotate when a predetermined amount of mixture is accumulated in the dwell area  320  of the drum  300  in the deposition unit  60 . The second web-forming unit  70  and the sheet-forming unit  80  continue to operate even after the drum  300  stops to rotate, and stop to operate after the sheet S is discharged. Since the drum  300  stops to rotate when a predetermined amount of mixture is accumulated in the dwell area  320 , the sheet can be manufactured with stable grammage immediately after the operation starts since a predetermined amount of mixture is accumulated in the dwell area  320  at the start of the next operation. 
     In the case where the dwell area  320  is provided in the screening unit  40 , the drum  300  of the screening unit  40  is operated in the same manner as the drum  300  of the deposition unit  60 . Accordingly, the grammage of the web V at the start of the next operation can be stabilized. 
     The above termination operation can be performed by a termination operation mode which is preset in the control unit  140  of the sheet manufacturing apparatus  100 . 
     Examples of the sheet described herein include a thin sheet shaped material made of raw material such as pulp and used paper, for example, recording paper used for handwriting or printing, wall paper, wrapping paper, autograph board, drawing paper and kent paper. The non-woven fabric described herein is a material having a larger thickness or a lower strength than that of paper sheet and includes common non-woven fabric, fiber board, tissue paper (tissue paper for cleaning), kitchen paper, cleaner, filter, liquid (waste ink or oil) absorption material, acoustic absorption material, heat insulation material, shock absorbing material, mat and the like. The raw material may be plant fiber such as cellulose, chemical fiber such as PET (polyethylene terephthalate) and polyester or animal fiber such as wool and silk. 
     The present invention may be partially omitted or the embodiments and modifications of the invention can be combined without departing from the features and effects described in the invention. 
     The present invention includes a configuration which is substantially the same as those described in the above embodiment (a configuration having the same function, method and result, or a configuration having the same purpose and effect). Further, the present invention includes a configuration having a non-essential part described in the above embodiment being replaced. Further, the present invention includes a configuration that achieves the same operation and effect as described in the above embodiment or a configuration that achieves the same objective. Further, the present invention includes a configuration described in the above embodiment with a known technique added thereto. 
     The entire disclosure of Japanese Patent Application No. 2015-018189, filed Feb. 2, 2015 is expressly incorporated by reference herein.