Abstract:
An object of the invention is to provide a granulation method and a granulation apparatus that can reduce the manufacturing costs of pellets. There is provided a granulation method which uses an underwater cutting (UWC) device  107  that cuts a medium to be processed extruded from holes of a die  106  by using cutter blades provided in a circulation box  109  and conveys the cut pellets from the circulation box  109  while cooling the cut pellets by pellet cooling/transport water (PCW). The granulation method includes circulating the PCW and stopping the circulation of the PCW after pushing the cutter blades against the die  106  while rotating the cutter blades, before the start of the granulation; storing a predetermined amount of PCW in the circulation box  109  by discharging the PCW; and heating the PCW, which is stored in the circulation box  109 , up to 69° C. or more.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a granulation method and a granulation apparatus that convey pellets formed by an UWC (underwater cutting) device while cooling the pellets. 
       BACKGROUND ART 
       [0002]      FIG. 5  is a side view showing an example of a plastic plasticizing-kneading-extrusion granulator  100 . The plastic plasticizing-kneading-extrusion granulator  100  shown in  FIG. 5  includes a plastic plasticizing-kneading machine  101 , a diverter valve  102 , a gear pump  103 , a molten polymer filtering device  104 , a die holder  105 , a die  106 , and a pelletization device (UWC device)  107 . 
         [0003]    The plastic plasticizing-kneading machine  101  is a device that plasticizes and kneads solid resin. The diverter valve  102  is a device that discharges molten polymer plasticized and kneaded by the plastic plasticizing-kneading machine  101 , and can switch a flow channel for molten polymer to a discharge side outside the system or the gear pump  103 . The gear pump  103  is a device that transports molten polymer, and has high boosting capability with respect to large pressure loss that is generated at a device as a transport destination. 
         [0004]    The molten polymer filtering device  104  is a device that filters out solid impurities contained in the molten polymer. The die holder  105  is a part that connects the molten polymer filtering device  104  with the die  106 . A flow channel, which guides the filtered molten polymer to a plurality of holes formed at the die  106 , is formed in the die holder  105 . The die  106  is a part that forms the filtered molten polymer into a spaghetti shape, and a plurality of holes are circumferentially arranged on the die  106 . 
         [0005]    The UWC device  107  is connected to the die  106 , and includes a circulation box to be described below. Pellet cooling/transport water (PCW) is circulated in the circulation box. The UWC device  107  forms the spaghetti-shaped molten polymer, which is continuously extruded from the holes of the die  106 , into fine grains (pellets) by cutter blades that are rotated in the circulation box. 
         [0006]    Next, the operation of the plastic plasticizing-kneading-extrusion granulator  100  will be described. In  FIG. 5 , solid resin is supplied to the plastic plasticizing-kneading machine  101  and is plasticized and melted by the thermal energy obtained from a barrel that can be heated and cooled and shear energy that is applied when a built-in screw  101   a  is rotated by a motor and a gear reducer. The plasticized and melted plastic is conveyed to the diverter valve  102 , which is a downstream device, by the conveying function of the screw  101   a  that is built in the plastic plasticizing-kneading machine  101 . The conveyed molten polymer is conveyed to the molten polymer filtering device  104  and the die holder  105  through the diverter valve  102  by the gear pump  103 , and is conveyed to the UWC device  107  through die holes to be described below. 
         [0007]      FIG. 6  is a cross-sectional view (inside view) of the UWC device  107  during the formation of pellets. The die holder  105 , the die  106  including die holes  108 , the UWC device  107 , pellets  116 , molten polymer  117 , and pellet cooling/transport water (PCW)  118  are shown in  FIG. 6 . 
         [0008]    The UWC device  107  includes a circulation box (water chamber)  109 , a movable carriage  110 , a motor (M)  111 , a cutter shaft  112 , a coupling  113 , a cutter holder  114 , cutter blades  115 , a forward pressure controller  119 , a backward pressure controller  120 , a gap measuring unit  121 , and a plate  122 . 
         [0009]    The cutter shaft  112  of the UWC device  107  is rotated by the start-up of the motor  111 . At the same time, the cutter blades  115 , which are fixed to the cutter shaft  112  through the cutter holder  114 , start to be rotationally moved in the circumferential direction. The cutter blades  115  are moved forward or backward by a hydraulic unit or a pneumatic unit (not shown) of the forward pressure controller  119  or the backward pressure controller  120 . It is possible to confirm a gap, which is formed between the die  106  and the cutter blades  115 , using the plate  122  indirectly fixed to the cutter shaft  112  and the gap measuring unit  121  fixed to a housing  122   a.    
         [0010]    The molten polymer  117 , which is extruded from the die holes  108 , is cut to the shape of pellets by the cutter blades  115  and is formed in the circulation box  109  in which the PCW  118  is circulated. Further, the die  106  is heated by a heating medium (steam, hot oil, an electrical heater, or the like). The UWC device  107  includes a hydraulic system (not shown), so that it is possible to fasten the circulation box  109  and the die  106 . 
         [0011]      FIG. 7  is a system diagram showing an example of a granulation apparatus  200  in the related art. The granulation apparatus  200  includes a die  106 , an UWC device  107 , a dehydration screen  123 , a centrifugal dehydrator  124 , a PCW tank  125 , a PCW pump  126 , a three-way valve  127 , a PCW flow rate detector  128 , a detector  129  for internal temperature of the PCW tank, a PCW temperature detector  130 , a PCW pressure detector  131 , and a manual drain valve  132 . 
         [0012]    The PCW  118  is circulated in the granulation apparatus  200  by the PCW pump  126 . The pellets  116  formed by the UWC device  107  are cooled by the PCW  118  stored in the circulation box  109 , are transported to the dehydration screen  123 , and are separated from the PCW  118  by the dehydration screen  123  and the centrifugal dehydrator  124 . As a result, pellet-like products are obtained. 
         [0013]    The flow rate of the PCW  118  is detected by the PCW flow rate detector  128 , and is adjusted by a PCW flow rate adjusting valve (not shown). Further, the temperature of the PCW  118  stored in the PCW tank  125  is detected by the detector  129  for internal temperature of the PCW tank, and is adjusted and managed so as to become a setting temperature by a PCW temperature control system (not shown). The temperature of the circulating PCW  118  is detected by the PCW temperature detector  130 . The pressure of the PCW  118  during the pelletization is detected by the PCW pressure detector  131 . 
         [0014]    The following procedure is required when pelletization is started in this granulation apparatus  200 . 
         [0015]    (1) The UWC device  107  is separated from the die  106 , and the die  106  is sufficiently heated by a heating medium. 
         [0016]    (2) The plastic plasticizing-kneading machine  101  is started up, molten polymer  117  is discharged from the diverter valve  102  to the outside of the system, and cleaning (purging) is performed in the plastic plasticizing-kneading machine  101 . 
         [0017]    (3) The gear pump  103  is started up, the diverter valve  102  is switched to the gear pump  103 , and it is confirmed that molten polymer  117  is uniformly extruded from die holes  108 . 
         [0018]    (4) The diverter valve  102  is switched to the discharge side outside the system, and the gear pump  103  is stopped (this state is a state where the molten polymer  117  is discharged from the diverter valve  102 ). 
         [0019]    (5) The UWC device waits until a state where the molten polymer  117  is not discharged from the die holes  108 , the cutting surface of the die  106  is quickly cleaned, and the UWC device  107  is connected to the die  106 . 
         [0020]    (6) The motor  107  of the UWC device  107  starts up and rotates the cutter blades  115 . 
         [0021]    (7) The rotating cutter blades  115  come into contact with the die  106 . 
         [0022]    (8) The three-way valve  127  is switched from a state where the PCW  118  is circulated on the bypass side by the PCW pump  126  so that the PCW  118  is supplied to the circulation box  109  of the UWC device  107 . 
         [0023]    (9) The gear pump  103  is started up, the diverter valve  102  is switched to the gear pump  103 , and the molten polymer  117  is extruded from die holes  108  so that the pellets  116  are formed. 
         [0024]    (10) The formed pellets  116  are transported to the dehydration screen  123  or the centrifugal dehydrator  124  by the PCW  118 , and the PCW  118  attached to the pellets  116  is removed. 
         [0025]    In the operations of (7), (8), and (9) of the above-described procedure, the PCW  118  cools and solidifies the molten polymer  117  that is present in the die  106  and the die holes  108 . For this reason, there was a problem in that the molten polymer  117  is not extruded. Further, even though the molten polymer  117  is extruded from the die holes  108 , there was a problem in that the molten polymer  117  is caught by the cutter blades  115 . In order to avoid these problems, there is a method of making the cutter blades  115  come into contact with the die  106  before the molten polymer  117  is extruded from the die holes  108  and extruding the molten polymer  117  before the PCW  118  reaches the die holes  108  and cools the die holes  108 . 
         [0026]    However, in the above-described granulation apparatus  200 , the throughput of one series of devices was about 50 t/h. For this reason, the throughput where pellets  116  having good shape can be formed without the clogging of the die holes  108  is designed to be about 25 t/h. If throughput is lower than 25 t/h, the thermal energy transmitted to the die holes  108  from the molten polymer  117  is insufficient and the temperature of the die holes  108  is lowered due to the thermal energy that is absorbed from the surface of the die  106  by the PCW  118 . Accordingly, the molten polymer  117  is solidified in the die holes  108 . 
         [0027]    For this reason, it is preferable that granulation be started at 25 t/h as the throughput at the time of granulation start. However, since a large amount of molten polymer  117  is discharged from the diverter valve  102  in (4) and (5) of the granulation start procedure, many persons are required to perform the waste disposal of the molten polymer, and a large amount of plastic is scrapped. Further, many workers are concentrated in a small place, and work disorderedly. Furthermore, since feet are covered with a large amount of water and molten polymer  117 , footholds are poor. This is not preferable in terms of safety. 
         [0028]    In recent years, there has been a need for the throughput of one series of devices of 70 t/h or more. Considering the above-described contents, the throughput at the time of granulation start becomes 35 t/h or more. For this reason, waste disposal is more difficult and the amount of molten polymer  117  to be scrapped is large. Moreover, in order to pelletize a large amount of molten polymer  117 , the size of the UWC device  107  is increased and the diameter of the circulation box  109  is also increased. Accordingly, a time, which is required until the circulation box  109  is filled with PCW  118  from an inlet (bottom) of the circulation box  109  to the outlet (ceiling surface) thereof, is lengthened. For this reason, since it is difficult to adjust a timing where the molten polymer  117  is extruded from the die  106  and a timing where the PCW  118  reaches the lower portion of the die  106 , it is difficult to perform pelletization. 
         [0029]    For example, in the case where pelletization is performed using the PCW  118  that is heated up to 60° C. at a bypass line, the temperature of the PCW  118  cooled by a cold pipe is lowered to a temperature lower than 60° C. when the PCW  118  reaches the lower portion of the circulation box  109 . Further, if molten polymer  117  is extruded from the die  106  when the PCW  118  does not reach the vicinity of the lower end portion of the die holes  108  positioned at the lower portion of the die  106 , the molten polymer  117  extruded from the upper portion of the die  106  is caught by the cutter blades  115 . For this reason, a trouble where the pellets  116  cannot be formed occurs. 
         [0030]    On the other hand, if molten polymer  117  is extruded from the die when the circulation box  109  is filled with the PCW  118 , the molten polymer  117  present in the die holes  108  formed at the lower portion of the die  106  is solidified since the lower portion of the die  106  is particularly cold, so that clogging occur. If clogging occurs, the shapes of the pellets  116  become irregular and the extrusion speed of the molten polymer  117  to be extruded from the die  106  is increased. Accordingly, long pellets  116  are formed, so that yield is reduced. Further, if the incidence of clogging is increased, the pressure loss of the die  106  is increased and exceeds a design pressure. For this reason, a trouble where operation cannot be performed at a predetermined throughput also occurs. 
         [0031]    Accordingly, in order to solve the problems at the time of the pelletization, there has been proposed a granulation apparatus including: a detection unit that detects the temperature of PCW  118  stored in a circulation box  109 ; and a control unit that controls the temperature of the PCW  118  stored in the circulation box  109  so that the PCW  118  stored in the circulation box  109 , which is heated by the die  106 , does not boil (for example, see PTL 1). 
         [0032]    This apparatus stores the PCW  118  in the circulation box  109 , controls heating so that the PCW  118  does not boil in the circulation box  109 , and melts plastic present in die holes  108  by uniformly warming up the die  106 . The die  106  is heated to 200 to 250° C. in order to melt plastic. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: JP-A-11-179724 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0034]    However, it is not possible to avoid the partial boiling of the PCW  118  on the surface of the die  106 , and much heat is removed from the die  106  by the latent heat of vaporization of the PCW  118 . For this reason, it is difficult to make the temperature of the die  106  uniform. Further, in the circulation box  109 , a difference is apt to occur between the temperature of the PCW  118  in the vicinity of the die  106  and the temperature of the PCW  118  at a position distant from the die  106 . Furthermore, the PCW  118  stored in the circulation box  109  can be agitated only by the flow behavior of the PCW  118 , which is caused by a difference in the temperature of the PCW  118 , while the PCW  118  is heated. For this reason, much time was taken to uniformly warm up the die  106  by raising the temperature of the PCW  118 . Accordingly, there have been demands for the improvement of work efficiency, which is caused by the uniform warm-up of the die  106  in a short time, and finally the suppression of manufacturing costs. 
         [0035]    Moreover, after the circulation box  109  is filled with the PCW  118 , the supply of the PCW  118  is stopped and the PCW  118  stored in the circulation box  109  is heated. However, it is not possible to maintain the amount of the PCW  118 , which is to be heated, constant and the start time of pelletization varies. For this reason, work efficiency deteriorates, so that an increase in costs is caused. 
         [0036]    Further, the molten polymer  117 , which stays in the vicinity of the die  106  and uniformly warms up the die  106 , is discharged from the die  106  due to its own weight, and the discharged molten polymer  117  is caught by the cutter blades  115  or prevents the cutter blades  115  from coming into contact with the die  106 . Accordingly, it was difficult to perform pelletization. For this reason, time was taken to return the cutter blades  115  to a state where the cutter blades  115  can be used in the pelletization, so that the manufacturing costs of the pellets  116  were increased. 
         [0037]    Furthermore, in the above-described granulation apparatus  200 , the molten polymer  117  is required to be discharged until the start of granulation. For this reason, many persons were required to perform the waste disposal of the molten polymer  117 , a large amount of molten polymer  117  should be scrapped, and there were a safety problem and a problem in that manufacturing costs were increased. 
         [0038]    The invention has been made to solve the above-described problems and an object of the invention is to provide a granulation method and a granulation apparatus that can reduce the manufacturing costs of pellets. 
       Solution to Problem 
       [0039]    In order to solve the above problems, according to the invention, there is provided a granulation method using an underwater cutting (UWC) device, which is configured to granulate a medium to be processed extruded from holes of a die by cutting the medium by using cutter blades provided in a circulation box and to conveys the cut pellets from the circulation box while cooling the cut pellets by pellet cooling/transport water (PCW), the granulation method comprising: circulating the PCW and stopping the circulation of the PCW after pushing the cutter blades against the die while rotating the cutter blades, before the start of the granulation; storing a predetermined amount of PCW in the circulation box by discharging the PCW; and heating the PCW, which is stored in the circulation box, up to 69° C. or more. 
         [0040]    Further, at least one PCW heating temperature detector and at least two level detectors disposed above the PCW heating temperature detector are provided on an upper PCW pipe connected to the upper surface of the circulation box, an automatic drain valve is provided on a lower PCW pipe connected to the bottom of the circulation box, and at least one level detector of the at least two level detectors is disposed above the other level detector. And, the method, when storing the predetermined amount of PCW in the circulation box, further comprises: opening the automatic drain valve; adjusting the level of the PCW by the at least two level detectors; and managing the temperature of the PCW by the at least one PCW heating temperature detector. 
         [0041]    Further, a PCW pressure detector is provided on the lower PCW pipe, and a forward pressure controller and a backward pressure controller, which move the cutter blades forward and backward, are provided. And, the method, when storing the predetermined amount of PCW in the circulation box, further comprises: adjusting a contact force between the die and the cutter blades on the basis of the pressure, which is detected by the PCW pressure detector, by the forward pressure controller and the backward pressure controller or any one of the forward pressure controller and the backward pressure controller. 
         [0042]    Further, the method, when heating the PCW up to the temperature of 69° C. or more so as to circulate the PCW, further comprises: adjusting a contact force between the die and the cutter blades on the basis of the pressure, which is detected by the PCW pressure detector, by the forward pressure controller and the backward pressure controller or any one of the forward pressure controller and the backward pressure controller. 
         [0043]    Further, a distance between the at least two level detectors is 1 m or less. 
         [0044]    Further, the automatic drain valve and a manual drain valve are disposed in series in this order from the side of the lower PCW pipe close to the circulation box. 
         [0045]    Further, the method, after conveying the pellets from the circulation box, further comprises: continuing the PCW to be circulated so as to lower the temperature of the PCW. 
         [0046]    Further, the method, after conveying the pellets from the circulation box, further comprises: making a heating medium heating the die not flow into the die and cooling the die with PCW; and making the UWC device wait without separating the UWC device from the die. 
         [0047]    Further, according to the invention, there is provided a granulation apparatus comprising an underwater cutting (UWC) device, which is configured to granulate a medium to be processed extruded from holes of a die by cutting the medium by using cutter blades provided in a circulation box and to convey the granulated pellets from the circulation box while cooling the granulated pellets by pellet cooling/transport water (PCW), the granulation apparatus comprising: a three-way valve configured to circulate the PCW and to switch the circulation of the PCW to a bypass side after pushing the cutter blades against the die while rotating the cutter blades, before the start of the granulation; the circulation box configured to store a predetermined amount of PCW by discharging the PCW; and the die configured to heat the PCW, which is stored in the circulation box, up to 69° C. or more. 
         [0048]    Further, the granulation apparatus further comprises: at least one PCW heating temperature detector and at least two level detectors that are provided on an upper PCW pipe connected to the upper surface of the circulation box; an automatic drain valve connected to a lower PCW pipe that is connected to the bottom of the circulation box; and a first control unit configured to: open the automatic drain valve; adjust the level of the PCW by the at least two level detectors; and manage the temperature of the PCW by the at least one PCW heating temperature detector, when storing the predetermined amount of PCW in the circulation box, wherein the at least two level detectors are disposed above the at least one PCW heating temperature detector, and at least one level detector is disposed above the other level detector. 
         [0049]    Further, the granulation apparatus further comprises: a PCW pressure detector provided on the lower PCW pipe; a forward pressure controller and a backward pressure controller configured to move the cutter blades forward and backward; and a second control unit configured to control a contact force between the die and the cutter blades on the basis of the pressure, which is detected by the PCW pressure detector, by the forward pressure controller and the backward pressure controller or any one of the forward pressure controller and the backward pressure controller, when storing the predetermined amount of PCW in the circulation box. 
         [0050]    Further, when heating the PCW up to a temperature of 69° C. or more so as to circulate the PCW, the second control unit is configured to: control a contact force between the die and the cutter blades on the basis of the pressure, which is detected by the PCW pressure detector, by the forward pressure controller and the backward pressure controller or any one of the forward pressure controller and the backward pressure controller. 
         [0051]    Further, a distance between the at least two level detectors is 1 m or less. 
         [0052]    Further, the automatic drain valve and a manual drain valve are disposed in series in this order from the side of the lower PCW pipe close to the circulation box. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0053]      FIG. 1  is a view showing an example of a granulation apparatus according to the present embodiment. 
           [0054]      FIG. 2  is a table showing the results of Examples 1 to 4 and Comparative Examples 1 and 2. 
           [0055]      FIG. 3  is a table showing the results of Examples 5 and 6 and Comparative Examples 3 and 4. 
           [0056]      FIG. 4  is a view showing the dependency of a die hole-aperture ratio on PCW heating temperature. 
           [0057]      FIG. 5  is a side view showing an example of a plastic plasticizing-kneading-extrusion granulator. 
           [0058]      FIG. 6  is a cross-sectional view (inside view) of an underwater cutting (UWC) device during the formation of pellets. 
           [0059]      FIG. 7  is a view showing an example of a granulation apparatus according to a related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0060]    An embodiment of the invention will be described below with reference to the drawings. Incidentally, the description of the same reference numerals as those, which have been described in the background art, will be omitted. 
         [0061]      FIG. 1  is a system diagram showing an example of a granulation apparatus  1  according to this embodiment. As shown in  FIG. 1 , the granulation apparatus  1  includes a PCW heating temperature detector  3 , an upper level detector  4 , a lower level detector  5 , an automatic drain valve  6 , a control system (first control unit)  7 , a control system (second control unit)  8 , a die  106 , an UWC device  107 , a dehydration screen  123 , a centrifugal dehydrator  124 , a PCW tank  125 , a PCW pump  126 , a three-way valve  127 , a PCW flow rate detector  128 , a detector  129  for internal temperature of the PCW tank, a PCW temperature detector  130 , a PCW pressure detector  131 , and a manual drain valve  132 . 
         [0062]    The PCW heating temperature detector  3 , the upper level detector  4 , and the lower level detector  5  are provided on a pipe (hereinafter, referred to as an upper PCW pipe) that is connected to the upper surface of the UWC device  107 . These devices are provided in the order of the PCW heating temperature detector  3 , the lower level detector  5 , and the upper level detector  4  from the side close to the UWC device  107 . 
         [0063]    The PCW heating temperature detector  3  is configured to detect the temperature of PCW  118  stored in a circulation box  109  that is heated by the die  106 . Incidentally, the die  106  is heated up to 200 to 300° C. by a heating medium (not shown) as described above. 
         [0064]    The lower level detector  5  is installed at a position higher than the uppermost portion of the circulation box  109  so that the circulation box  109  is filled with the PCW  118 . It is preferable that the upper level detector  4  be provided at a position higher than the lower level detector  5  within 1 m so that the amount of stored PCW  118  is maintained substantially constant on the basis of the PCW discharge rate of the automatic drain valve  6  and the manual drain valve  132 . If the upper level detector  4  is provided at a position that is higher than the lower level detector  5  by 1 m or more, a distance between the respective level detectors  4  and  5  is great, so that the amount of stored PCW  118  varies. Thus, it may be difficult to warm up the PCW  118  in a predetermined time. 
         [0065]    The automatic drain valve  6  is provided between a pipe (hereinafter, referred to as a lower PCW pipe) connected to the bottom of the circulation box  109  and the manual drain valve  132 , and is connected to the manual drain valve  132  in series. The opening and closing of the automatic drain valve  6  is controlled by the control system  7 . Incidentally, the manual drain valve  132  is usually opened from the preparation before the operation until the end of operation. However, when safety is regarded as an important factor, it is preferable that the manual drain valve  132  be closed except while PCW is discharged. In this case, it is necessary to open the manual drain valve  132  PCW is discharged. 
         [0066]    Each of the control systems  7  and  8  includes a CPU (Central Processing Unit) (not shown) and a memory (not shown). The control system  7  is connected to the upper level detector  4 , the lower level detector  5 , and the automatic drain valve  6 . The control system  7  is configured to control the opening and closing of the automatic drain valve  6  on the basis of information from the respective level detectors  4  and  5 . 
         [0067]    The control system  8  is connected to the PCW pressure detector  131  and a forward pressure controller  119  and a backward pressure controller  120  shown in  FIG. 6  or one of the forward pressure controller  119  and the backward pressure controller  120 . The control system  8  is configured to control the forward pressure controller  119  and the backward pressure controller  120  or one of the forward pressure controller  119  and the backward pressure controller  120  on the basis of pressure obtained from the PCW pressure detector  131 , and to adjust a contact force between the die  106  and cutter blades  115  shown in  FIG. 6 . 
         [0068]    Next, a method of manufacturing pellets  116  of this embodiment will be described with reference to  FIG. 1 . Incidentally, members, which form the plastic plasticizing-kneading-extrusion granulator  100  shown in  FIG. 5  and the UWC device  107  shown in  FIG. 6 , may be used in the following description. 
         [0069]    First, the UWC device  107  is separated from the die  106 , and the die  106  is sufficiently heated by a heating medium such as hot oil. Next, a plastic plasticizing-kneading machine  101  shown in  FIG. 5  is started up, molten polymer  117  is discharged to the outside of the system from a diverter valve  102 , and cleaning (purging) is performed in the plastic plasticizing-kneading machine  101 . 
         [0070]    Next, a gear pump  103  shown in  FIG. 5  is started up, the diverter valve  102  is switched to the gear pump  103 , and it is confirmed that molten polymer  117  is uniformly extruded from die holes  108  shown in  FIG. 6 . 
         [0071]    Then, the diverter valve  102  shown in  FIG. 5  is switched to the discharge side outside the system, and the plastic plasticizing-kneading machine  101  and the gear pump  103  are stopped. Incidentally, this state is a state where the molten polymer  117  is not discharged from the diverter valve  102 . Further, the UWC device waits until a state where the molten polymer  117  is not discharged from the die holes  108  shown in  FIG. 6 , the cutting surface of the die  106  is quickly cleaned, a carriage  110  on which the UWC device  107  is mounted is moved toward the die  106 , and the circulation box  109  is connected to the die  106 . 
         [0072]    Subsequently, (1) after a motor  111  of the UWC device  107  is started up to rotate the cutter blades  115 , (2) the rotating cutter blades  115  come into contact with the die  106 . (3) The three-way valve  127  is switched from a state where the PCW  118  is circulated on the bypass side by the PCW pump  126  shown in  FIG. 1  so that the PCW  118  is supplied to the circulation box  109  of the UWC device  107 . Incidentally, (3) of this order may be performed first and (1) and (2) may be subsequently performed in this order. 
         [0073]    After the PCW  118  starts to be circulated, the three-way valve  127  is switched to the bypass side to stop the circulation of the PCW  118 . Further, the control system  7  opens the automatic drain valve  6  and discharges the PCW  118  while acquiring a detection result, where the circulation box is filled with the PCW  118 , from the upper level detector  4 . 
         [0074]    When the control system  7  receives a detection result, where the circulation box is not filled with the PCW  118  and is empty, from the upper level detector  4 , the control system  7  closes the automatic drain valve  6  and stops discharging the PCW  118 , so that a substantially constant amount of PCW  118  is stored in the circulation box  109 . Incidentally, when the control system  7  closes the automatic drain valve  6 , it is necessary that the control system  7  acquires a detection result, where the circulation box is filled with the PCW  118 , from the lower level detector  5 . The reason for this is that a substantially constant amount of PCW  118  is stored in the circulation box  109  when the level of the PCW  118  is higher than the lower level detector  5  and lower than the upper level detector  4 . 
         [0075]    After that, the UWC device  107  waits until the PCW  118  stored in the circulation box  109  is heated up to 69° C. or more by absorbing heat from the die  106 . Here, when the manual drain valve  132  is closed, the amount of PCW  118  stored in the circulation box  109  can be kept substantially constant even if the automatic drain valve  6  is opened due to the occurrence of an abnormality. Accordingly, it is safer that the manual drain valve  132  is closed. Further, while the UWC device  107  waits, the cutter blades  115  continue to rotate. For this reason, the PCW  118  stored in the circulation box  109  is agitated by the cutter blades  115  and uniformly warmed up. Since the surface of the die  106  can be uniformly warmed up in this embodiment, the PCW  118  may boil. Further, since the cutter blades  115  continuously rotate while always coming into contact with the die  106 , it is possible to prevent the molten polymer  117 , which oozes from the die holes  108  with time, from entangling the cutter blades  115 . 
         [0076]    Incidentally, while the PCW  118  is heated, the temperature of the PCW  118  stored in the circulation box  109  is managed by the PCW heating temperature detector  3  that is provided at the position lower than the lower level detector  5 . 
         [0077]    Further, when a predetermined amount of PCW  118  is discharged, the PCW pressure detector  131  installed on the lower PCW pipe of the UWC device  107  detects the pressure of the PCW  118  at a position where the head of the PCW  118  is present at the position between the upper level detector  4  and the lower level detector  5 . 
         [0078]    Here, when the PCW  118  is discharged, the level of the PCW  118  stored in the granulation apparatus  1  is lowered and pressure in the circulation box  109  is reduced. Accordingly, a force for separating the cutter blades  115  from the surface of the die  106  is reduced. As a result, a contact force between the die  106  and the cutter blades  115  is increased, so that the wear rate of the cutter blades  115 , which are worn out by the frictional force between the cutter blades  115  and the die  106 , is increased. Accordingly, in order to suppress this increase, the control system  8  feeds back the pressure detected from the PCW pressure detector  131  and automatically adjusts an adequate contact force between the die  106  and the cutter blades  115  by controlling the forward pressure controller  119  and the backward pressure controller  120  or one of the forward pressure controller  119  and the backward pressure controller  120 . Further, for this adjustment, a spring force or a magnetic force may be used other than a pressure medium. Only the case of the adjustment using a pressure medium will be described below. 
         [0079]    At the time of automatic adjustment, by the same method as the method in the related art, the control system  8  reduces forward pressure when automatically controlling forward pressure and increases backward pressure when automatically controlling backward pressure. A contact force between the die  106  and the cutter blades  115  is reduced by this adjustment, so that it is possible to suppress the excessive wear of the cutter blades  115 . 
         [0080]    Specifically, the UWC device  107  shown in  FIG. 6  includes a sleeve (not shown) that is held rotatably and concentrically with a cutter shaft  112 , and a housing  122   a  in which the cutter shaft  112  is built. Further, voids (not shown) in which a pressure medium is supplied are provided in gaps between the sleeve and the housing  122   a . The control system  8  can adjust a contact force between the cutter blades  115  and the die  106  by detecting the pressure of the respective voids with P 2  and P 3  shown in  FIG. 1  and controlling the pressure of the voids with the forward pressure controller  119  and the backward pressure controller  120 . 
         [0081]    Further, after recognizing that the temperature of the PCW  118  rises up to 69° C. or more from the detection result acquired from the PCW heating temperature detector  3 , the control system  8  notifies a system (not shown) that the temperature of the PCW  118  rises up to 69° C. or more. The system (not shown) outputs a signal indicating that a granulation start condition is satisfied. 
         [0082]    After that, the plastic plasticizing-kneading machine  101  is started up, and the molten polymer  117  is discharged to the outside of the system. Moreover, the three-way valve  127  is switched to the UWC device  107  from the bypass side to supply PCW  118  to the circulation box  109  of the UWC device  107 . 
         [0083]    Here, when the PCW  118  starts to be circulated in the granulation apparatus  1 , the level of the PCW  118  rises, the pressure of the PCW  118  stored in the circulation box  109  is increased, and a contact force between the die  106  and the cutter blades  115  is reduced. Accordingly, the control system  8  feeds back the pressure of the PCW  118  stored in the circulation box  109 , which is detected by the PCW pressure detector  131 , and automatically adjusts the cutter blades  115  shown in  FIG. 6  by the forward pressure controller  119  and the backward pressure controller  120  or one of the forward pressure controller  119  and the backward pressure controller  120 . 
         [0084]    At the time of automatic adjustment, the control system  8  increases forward pressure when automatically controlling forward pressure and reduces backward pressure when automatically controlling backward pressure. Since a contact force between the die  106  and the cutter blades  115  is kept at a predetermined value, it is possible to prevent a trouble that pelletization is unable to be performed due to the backward movement of the cutter blades  115 . 
         [0085]    Finally, the gear pump  103  shown in  FIG. 5  is started up and the diverter valve  102  is switched to the gear pump  103 , the molten polymer  117  is uniformly extruded from the die holes  108  shown in  FIG. 6 , and pelletization is started using the cutter blades  115 . Formed pellets  116  are conveyed to the dehydration screen  123  or the centrifugal dehydrator  124  by the PCW  118 , and the PCW  118  and the pellets  116  are separated from each other, so that the pellets  116  are manufactured. 
         [0086]    As described above, in the granulation method according to this embodiment, the plastic plasticizing-kneading machine  101  is stopped while the PCW  118  is stored in the circulation box  109  and warmed up. Accordingly, it is possible to reduce waste disposal work by significantly reducing the amount of wasted molten polymer  117 . Accordingly, it is possible to reduce the manufacturing costs of pellets  116 . 
         [0087]    Further, the cutter blades  115  of the UWC device  107  are rotated immediately after the die  106  and the UWC device  107  are connected to each other. Accordingly, even though a small amount of molten polymer  117  is discharged from the die  106 , the molten polymer  117  on the surface of the die  106  is cleaned and removed by the cutter blades  115 . For this reason, it is possible to shorten a time that is required until the start of pelletization while the molten polymer  117  at the time of pelletization is not caught by the cutter blades  115  or the cutter blades  115  are not hindered from coming into contact with the die  106 . Accordingly, since manufacturing time is shortened, it is possible to reduce the manufacturing costs of the pellets  116 . 
         [0088]    Further, when the PCW  118  stored in the circulation box  109  absorbs heat from the surface of the die  106 , it is possible to forcibly agitate the PCW  118  boiling on the surface of the die  106  by the rotation of the cutter blades  115 . Accordingly, it is possible to uniformly warm up the surface of the die  106  in a short time. Therefore, since manufacturing time is shortened, it is possible to reduce the manufacturing costs of the pellets  116 . 
         [0089]    Furthermore, since it is possible to keep the amount of the PCW  118 , which is stored in the circulation box  109  and is heated, substantially constant by using the upper level detector  4  and the lower level detector  5 , it is possible to make the start time of pelletization uniform. Accordingly, work efficiency is improved and costs can be reduced. 
         [0090]    Moreover, by discharging only an appropriate amount of PCW  118  from the granulation apparatus  1 , the pressure of the PCW  118  stored in the circulation box  109  is reduced, so that the molten polymer  117  is easily and uniformly extruded from the die holes  108 . Accordingly, it is possible to form pellets  116  having uniform length. Therefore, the yield of the pellets  116  is improved, so that it is possible to reduce the manufacturing costs of the pellets  116 . 
         [0091]    In addition, since a control unit, which controls the PCW  118  so that the PCW  118  stored in the circulation box  109  does not boil, is not necessary unlike in the granulation apparatus in the related art, it is possible to perform granulation by a simple system. Further, since it is possible to form pellets  116  by a small number of persons, it is possible to reduce overhead costs and to reduce the manufacturing costs of the pellets  116 . 
         [0092]    In the granulation method according to the embodiment, the circulation of the PCW  118  and cutting using the cutter blades  115  have been performed at different timings. Alternatively, these operations may be performed at the same time. Moreover, the control systems  7  and  8  have been shown as separate systems in  FIG. 1 , but the control of the control systems  7  and  8  may be performed by one control system. The reason for this is to perform granulation by simpler processes and devices. 
         [0093]    Further, the automatic pressure control of the cutter blades  115  may be adjusted so that the pressure of the PCW  118  gradually increasing together with the circulation of the PCW  118  is always fed back to the control system  8 . Furthermore, when the pressure of the PCW  118  at the time of the circulation of the PCW  118  is known in advance, it may be performed such that the three-way valve  127  is switched to the UWC device  107 , the PCW  118  is supplied to the circulation box  109 , and the pressure of the PCW  118  is adjusted to a predetermined pressure by the control system  8  so that the cutter blades  115  are not moved backward. 
         [0094]    Moreover, when granulation is ended and pelletization is stopped, the PCW  118  may continue to be circulated, and a heating medium heating the die  106  may be made not to flow into the die  106 . Since the die  106  is cooled through the circulation of the PCW  118 , the molten polymer  117  filling the die holes  108  is cooled and the outflow of the molten polymer  117  is suppressed. Accordingly, since the UWC device  107  can resume granulation without being separated from the die  106 , it is possible to save time and effort for connecting the UWC device  107  to the die  106 . 
         [0095]    Further, in this embodiment, an aspect where two level detectors are provided has been described. Alternatively, three or more level detectors may be provided. Furthermore, in this embodiment, an aspect where one PCW heating temperature detector  3  is provided has been described. Alternatively, two or more PCW heating temperature detectors may be provided. Since it is possible to detect the change of the level of the PCW  118  or the change of temperature in detail, it is possible to further stabilize the state of the PCW  118  that is stored in the circulation box  109 . 
         [0096]    Furthermore, a granulation method, which uses molten polymer  117  as a medium to be processed, has been described in this embodiment, but the invention may be applied to the granulation of synthetic rubber or the like. 
       EXAMPLES 
       [0097]    1. Various resins having different melt flow rates (MFR) were granulated using various granulation methods, and a die hole-aperture ratio was examined. 
       Example 1 
       [0098]    The aperture ratio of the die holes was measured using the granulation apparatus according to this embodiment by the granulation method according to this embodiment. The raw material of molten polymer, the UWC device, the die, die heating temperature, the number of the cutter blades, initial PCW temperature, throughput, the rotational speed of the cutter blades, and the temperature of a resin on the upstream side immediately ahead of the die were used as the following conditions and evaluation was performed. 
         [0099]    Raw material of molten polymer: polypropylene (MFR=0.25 (230° C., 2.16 kg load)) 
         [0100]    UWC device: ADC-10 model manufactured by Japan Steel Works, Ltd. 
         [0101]    Die: heat channel die (diameter of hole: φ2.5 mm, the number of holes: 24) 
         [0102]    Die heating temperature: 300° C. (oil heating, setting value of heat medium heating device) 
         [0103]    The number of cutter blades: 6 
         [0104]    Initial PCW temperature: 60° C. 
         [0105]    Throughput: 380 kWh 
         [0106]    The rotational speed of cutter blades: 2200 rpm 
         [0107]    The temperature of a resin on the upstream side immediately ahead of the die: 213° C. 
         [0108]    After the temperature of PCW rose to 85° C. from 60° C., granulation was started. Incidentally, a die hole-aperture ratio in this example was calculated using the following expression. 
         [0000]      Die hole-aperture ratio (%)=Throughput (g/min)×100/(the number of cutter blades×the rotational speed of cutter (rpm)×the number of die hole×average pellet weight (g/piece))  [Expression 1]
 
         [0109]    Pellet weight for the expression was obtained by randomly collecting fifty pellets, which had been formed, and calculating the average weight per pellet through the measurement of the weights of the fifty pellets. Further, this measurement was performed two times, and the average value of the first and second calculation results was used as the pellet weight. This result is shown in  FIG. 2 . Incidentally, in  FIG. 2 , a granulation method A shows the granulation method according to this embodiment. 
       Examples 2 to 4 
       [0110]    Granulation was performed under the same conditions as Example 1 except that the MFR value of polypropylene and the temperature of a resin on the upstream side immediately ahead of the die were used as conditions shown in  FIG. 2 , and a die hole-aperture ratio was evaluated. Incidentally, in Example 3, the extrusion of polypropylene was stopped without the separation of the UWC device from the die after the end of the evaluation of Example 2, PCW was discharged up to a defined value, and pellets were evaluated again under the same conditions as Example 2. These results are shown in  FIG. 2 . 
       Comparative Examples 1 and 2 
       [0111]    Granulation was performed under the same conditions as Example 1 except that PCW was not heated by the circulation box and the conditions shown in  FIG. 2  were used while PCW of 60° C. was circulated in the granulation apparatus at a flow rate of 10 m 3 /h, and a die hole-aperture ratio was evaluated. These results are shown in  FIG. 2 . Incidentally, in  FIG. 2 , a granulation method B shows the granulation method in the related art under the above-described conditions. 
         [0112]    In  FIG. 2 , it was possible to granulate polypropylene having a MFR in the range of 0.25 to 8 at an aperture ratio of 100% without the clogging of the die holes by using the granulation method according to the invention. In addition, when Example 1 was compared with Comparative Example 1, it was possible to granulate a resin having a low MFR at a die hole-aperture ratio of 100% even under the condition where the temperature of a resin on the upstream side immediately ahead of the die was low. 
         [0113]    2. A relationship between a die hole-aperture ratio and the temperature of PCW stored in the circulation box immediately before the start of granulation was examined. 
       Example 5 
       [0114]    Granulation was performed by the same method as Example 1 except that the raw material of molten polymer, the UWC device, the die, die heating temperature, the number of the cutter blades, initial PCW temperature, PCW heating temperature, throughput, the rotational speed of the cutter blades, and the temperature of a resin on the upstream side immediately ahead of the die were set to the following conditions; and an aperture ratio of the die holes was measured. This result is shown in  FIG. 3 . 
         [0115]    Raw material of molten polymer: polypropylene (MFR=5 (230° C., 2.16 kg load)) 
         [0116]    UWC device: ADC-10 model manufactured by The Japan Steel Works, Ltd. 
         [0117]    Die: heat channel die (diameter of hole: φ2.5 mm, the number of holes: 56) 
         [0118]    Die heating temperature: 300° C. (oil heating, setting value of heat medium heating device) 
         [0119]    The number of cutter blades: 4 
         [0120]    Initial PCW temperature: 60° C. 
         [0121]    PCW heating temperature: 69° C. 
         [0122]    Throughput: 570 kg/h 
         [0123]    The rotational speed of cutter blades: 2100 rpm 
         [0124]    The temperature of a resin on the upstream side immediately ahead of the die: 194° C. 
       Example 6 and Comparative Examples 3 and 4 
       [0125]    Granulation was performed under the same conditions as Example 5 except that PCW heating temperature was set to values shown in  FIG. 3 , and a die hole-aperture ratio was evaluated. These results are shown in  FIGS. 3 and 4 . 
         [0126]    In  FIGS. 3 and 4 , a die hole-aperture ratio was increased to 89% or more when PCW heating temperature was set to 69° C. or more. Here, since a machine is usually designed so as to have an allowance of about 10% of required capacity, required capacity can be achieved if a die hole-aperture ratio is about 90% or more. Accordingly, in the case of polypropylene of which an MFR is 5, the capacity of the machine is achieved when PCW heating temperature is set to 69° C. or more. 
         [0127]    The invention is not limited to the above-described embodiment, and may appropriately have modifications, improvements, and the like. In addition, since the materials, the shapes, the dimensions, the numerical values, the forms, and the number of the respective components of the above-described embodiment, the places where the respective components of the above-described embodiment are disposed, and the like are arbitrary, they are not limited as long as the invention is achieved. 
         [0128]    The invention has been described in detail with reference to a specific embodiment, but it is apparent to those skilled in the art that various changes or alterations can be added to the invention without departing from the spirit and scope of the invention. 
         [0129]    This application is based on Japanese Patent Application (Japanese Patent Application No. 2010-111657) filed May 14, 2010, the content of which is incorporated herein by reference. 
       INDUSTRIAL APPLICABILITY 
       [0130]    According to the invention, it is possible to reduce the manufacturing costs of pellets. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 : Granulation apparatus 
               3 : PCW heating temperature detector 
               4 : Upper level detector 
               5 : Lower level detector 
               6 : Automatic drain valve 
               7 : Control system (first control unit) 
               8 : Control system (second control unit) 
               106 : Die 
               107 : Underwater cutting (UWC) device 
               108 : Die holes 
               109 : Circulation box 
               115 : Cutter blade 
               119 : Forward pressure controller 
               120 : Backward pressure controller 
               131 : PCW pressure detector 
               132 : Manual drain valve