Abstract:
In a method of cooling a tank molded from thermoplastic plastic material, a molded tank is received from a metallic die assembly at a temperature somewhat lower than a melting point of the plastic material, the molded tank is retained in a jig frame, and cooling water is sprayed onto an external surface of the molded tank. Preferably, the method further comprises the step of blowing cooling air into the tank substantially at the same time as spraying cooling liquid onto an external surface of the molded tank. Thus, the molded plastic tank can be removed from the metallic die assembly at a relatively high temperature, and the use efficiency of the metallic die assembly can be improved. Also, because the tank is not subjected to any pressure or buoyant force, it is prevented from undesirably distorting. In particular, by blowing cooling air into the tank, the tank can be cooled even more quickly.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a system and method for cooling a hollow molded plastic product for use as a fuel tank for vehicles and industrial applications.  
         BACKGROUND OF THE INVENTION  
         [0002]    It has become increasingly common to use hollow plastic tanks made by a blow molding process as fuel tanks for motor vehicles, watercraft and other applications. In a blow molding process, molten resin material that has been obtained from an extruder is held in a die assembly defining a desired outer profile of the product and blowing air into the molten resin material so as to force the plastic material onto the surface of the die cavity.  
           [0003]    Conventionally, the molded product is left in the die assembly for a certain period of time that is required for the molded product to become hard enough to retain its shape. If the molded product is not cooled to an adequately low temperature, it may deform during the subsequent handling. Therefore, conventionally, the product was kept in the die assembly for a significant period of time, and this prevented an efficient utilization of the die assembly.  
           [0004]    It has been proposed to immerse the molded tank in water for cooling the same. But the force required for keeping the tank immersed against the buoyant force is so great that it could deform the tank. It has also been proposed to introduce cooling water into the tank. But it was found that removing water out of the tank is time-consuming and not easy to accomplish.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In view of such problems of the prior art, a primary object of the present invention is to provide an improved method of cooling molded plastic fuel tank in a highly efficient manner.  
           [0006]    A second object of the present invention is to provide an improved method of cooling molded plastic fuel tank which would not cause deformation of the molded fuel tank.  
           [0007]    A third object of the present invention is to provide an improved method of cooling molded plastic fuel tank which can rapidly cool the fuel tank and release it for the subsequent process without any significant time delay.  
           [0008]    A fourth object of the present invention is to provide a plastic fuel tank cooling system which is suitable for implementing such a method.  
           [0009]    According to the present invention, such objects can be accomplished by providing a method of cooling a tank molded from thermoplastic plastic material, comprising the steps of: receiving a molded tank at a temperature somewhat lower than a melting point of the plastic material; retaining the molded tank in a jig frame; and spraying cooling liquid such as water onto an external surface of the molded tank. Preferably, the method further comprises the step of blowing cooling air into the tank substantially at the same time as spraying cooling liquid onto an external surface of the molded tank.  
           [0010]    Thus, the molded plastic tank can be removed from the metallic die assembly at a relatively high temperature, and the use efficiency of the metallic die assembly can be improved. Also, because the tank is not subjected to any pressure or buoyant-force, it is prevented from undesirably distorting. In particular, by blowing cooling air into the tank, the tank can be cooled even more quickly.  
           [0011]    A fuel tank is typically required to be provided with a relatively high positional precision at a few selected locations. Therefore, it is preferable for the jig frame to be provided with members for retaining such locations at a desired precision. The cooling liquid which may remain on the surface of the tank after the completion of the cooling process may be removed by blowing high pressure air.  
           [0012]    The tank which has been taken out of the metallic die assembly is relatively warm, and tends to shrink as it cools off. Therefore, it is preferable for the cooling air supplying unit for blowing cooling air into the tank to be able to accommodate the shrinking of the tank by using a moveable support such as a slide bearing or pivotal support. For an air tight connection of the cooling air supplying unit with a hole of the tank, the cooling air supplying unit may be provided with a suitable sealing arrangement.  
           [0013]    Such a method can be conveniently implemented by using a system for cooling a tank molded from thermoplastic plastic material, comprising: a jig frame for retaining a molded tank; and a cooling liquid spraying unit for spraying cooling liquid onto an external surface of the molded tank.  
           [0014]    For the convenience of charging a tank into the cooling system, the jig frame preferably comprises a lower jig frame structure and an upper jig frame structure that can be selectively opened with respect to the lower jig frame structure. In particular, the lower jig frame structure may be provided with an open top for the convenience of charging and removing a tank into and from the jig frame, and the upper jig frame structure mat be adapted to be attached to and detached from the lower jig frame structure by using a suitable clamping arrangement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Now the present invention is described in the following with reference to the appended drawings, in which:  
         [0016]    [0016]FIG. 1 is an overall schematic view of a cooling system embodying the present invention;  
         [0017]    [0017]FIG. 2 is a simplified front view of the jig frame;  
         [0018]    [0018]FIG. 3 is a simplified front view of the cooling air supply unit;  
         [0019]    [0019]FIG. 4 is a simplified front view of the cooling air vent unit;  
         [0020]    [0020]FIG. 5 is a fragmentary view of the clamp/unclamp unit partly in section showing the clamp mechanism in the unclamped sate; and  
         [0021]    [0021]FIG. 6 is a view similar to FIG. 5 showing the clamp mechanism in the clamped sate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    [0022]FIG. 1 shows an overall view of a cooling system for plastic fuel tanks embodying the present invention. This cooling system comprises a load/unload station  2  for charging and removing a plastic tank  1  forwarded from a metallic die assembly (not shown in the drawing), a jig open/close station  5  for retaining and releasing the tank  1  in and out of a jig frame including an upper jig frame  3  and lower jig frame  4 , and a cooling station  6  for cooling the tank  1  while it is held between the upper and lower jig frames  3  and  4 . The tank  1 , once it is held between the upper and lower jig frames  3  and  4 , is conveyed by a carriage  8  traveling over a rail  7  extending between the load/unload station  2  and cooling station  6  and driven by a link chain or the like not shown in the drawing.  
         [0023]    The jig open/close station  5  comprises a lifting unit  9  for vertically moving the upper jig frame  4 , which is adapted to retain the tank  1  in cooperation with the lower jig frame  3 , by using an air cylinder, and a pair of clamp/unclamp units  10  for actuating clamp mechanisms (which will be described hereinafter) for joining the lower jig frame  3  with the upper jig frame  4  by using air cylinders. The lower end of the lifting unit  9  is provided with a gripping unit  11  for gripping the upper jig frame  4  by using an air cylinder.  
         [0024]    The cooling station  6  comprises upper shower nozzles  12  for spraying cooling water downward from above, lower shower nozzles  13  for spraying cooling water upward from beneath the rail  7 , a cooling water tank  14  for receiving and storing cooling water that has been sprayed from the shower nozzles  12  and  13 , and a circulating pump  15  for feeding the cooling water from the cooling water tank  14  to the shower nozzles  12  and  13  under pressure. The cooling station  6  preferable consists of a shower booth which is surrounded by a wall to prevent splashing of the water out of the station. Because the cooling water receives heat from the tank  1  and its temperature rises in time, it is preferable to cool the cooling water by using a suitable heat exchanger.  
         [0025]    Between the jig open/close station  5  and cooling station  6  is provided a water removing unit  16  including air nozzles for blowing away moisture from the tank  1  that is held between the upper and lower jig frames  3  and  4 .  
         [0026]    Referring to FIG. 2, the upper and lower jig frames  3  and  4  are each provided with a frame structure  21  and  22  made of light-weight steel shape metal or extruded aluminum and a plurality of shape retaining members  24  that abut appropriate parts of the tank  1 . The lower jig frame  3  is additionally provided with a base  23  made of steel or aluminum plate for supporting various components. The shape retaining members  24  are each adapted to be adjusted of its position by mounting it, via a slot or the like, onto a bracket  25  secured to the frame structure  21  and  23 , changing shims that are interposed between the shape retaining members  24  and base  23 , or by using a position adjusting unit  26  incorporated with a screw jack or air cylinder. Particularly in the case of the automotive fuel tank, certain parts of the tank such as the mounting surfaces for mounting the tank onto the vehicle body and attaching other auxiliary components such as a fuel pump and a vent valve onto the tank are required to be provided with a relatively high positional precision so that the shape retaining members  24  should be selectively arranged in such parts by taking into account the shrinkage of the tank that occurs during the cooling process.  
         [0027]    The upper jig frame  4  is provided with a cooling air supplying unit  31  for supplying cooling air into the tank  1  and a cooling air vent unit  41  fore releasing the cooling air from the tank  1  although not shown in FIG. 2.  
         [0028]    Referring to FIG. 3, the cooling air supplying unit  31  comprises a linear air feed tube  34  provided with an air nozzle unit  32  in a free end (or lower end) thereof, a tank engaging member  33  integrally connected to an intermediate part of the air feed tube  34 , and a slide bearing  35  slidably supporting the base end (or upper end) of the feed tube  34 . The fixed part of the slide bearing  35  is attached to a part of the frame structure  22  of the upper jig frame  4  via a bracket  36  so that the tank engaging member  33  along with the air feed tube  34  can move vertically by a certain stroke relative to the upper jig frame  4 . The air nozzle unit  32  is adapted to be introduced into the tank  1  from a hole  37  provided in an upper central part of the fuel tank for mounting a fuel pump (not shown in the drawing), and comprises a plurality of orifices that are oriented in various directions so as to direct the cooling air in various directions within the tank  1 .  
         [0029]    The tank engaging member  33  is provided with an annular surface which is adapted to engage a substantially horizontal surface  39  surrounding the hole  37  via a resilient seal member  38  such as an O-ring. The horizontal surface  39  provides a mounting surface for the fuel pump. The assembly including the air nozzle unit  32 , tank engaging member  33  and air feed tube  34  is thus allowed to move linearly, and the weight of the assembly is selected so that a required sealing can be ensured for the cooling air without deforming the horizontal surface  39  of the tank  1 . The tank engaging member  33  can therefore follow the changes in the position of the horizontal surface  39 , that may occur due to the shrinking during the cooling process, under the weight of the assembly, and is thereby kept in close contact with the horizontal surface  39  so as to prevent the leakage of the cooling air that is conducted into the tank  1  via the air nozzle unit  32 .  
         [0030]    Referring to FIG. 4, the cooling air vent unit  41  comprises a tank engaging member  43  adapted to engage an inclined surface  42  formed in the tank  1  to provide a mounting surface for a filler pipe (not shown in the drawings), a vent passage member  44  connected to the tank engaging member  43  via a spherical joint  50 , a slide rod  45  having a free end fixedly attached to the vent passage member  44 , a slide bearing  46  slidably supporting the slide rod  45  relative to a bracket  48  which is fixedly secured to the frame structure  22 , and an actuator  47  typically consisting of an air cylinder for linearly actuating the slide rod  45  along the slide bearing  46 . The actuator  47  along with the fixed part of the slide bearing  46  is attached to the bracket  48 . Thus, the tank engaging member  43  along with the vent passage member  44  attached integrally thereto can move along an oblique axial line toward and away from the inclined surface  42  of the tank  1  by a prescribed stroke.  
         [0031]    The tank engaging member  43  is provided with an annular surface fitted with a resilient seal member  49  such as an O-ring which is adapted to engage the inclined surface  42  of the tank  1  in an air tight manner. The vent passage member  44  is connected to the tank engaging member  43  via the spherical joint  50  to accommodate a pivotal movement of the tank engaging member  43 . The vent passage member  44  is internally provided with a vent passage  51  that has an internal end having a substantially same diameter as a hole  52  provided in the inclined surface  42  and communicating with this hole  52 , and an external end fitted with an elbow or the like to avoid intrusion of cooling water into the tank  1  as will be described hereinafter.  
         [0032]    A compression coil spring  54  is interposed between the piston rod  53  of the actuator  47  and an annular shoulder of the slide rod  45 , and the spring force of this coil spring  54  is selected in such a manner that a required sealing performance can be achieved without deforming the inclined surface  42  of the tank  1 . Thus, the tank engaging member  43  is pressed against the inclined surface  42  of the tank  1  by the thrust force of the actuator  47  while compressing this compression coil spring  54  so that the tank engaging member  43  can follow the displacement of the inclined surface  42  due to the shrinking of the tank  1  that may occur during the cooling process and is kept in close contact with the inclined surface  42 . Therefore, the cooling water that is sprayed onto the tank  1  is prevented from getting into the tank  1 .  
         [0033]    A clamp mechanism  61  is provided in each of the front and rear parts of the upper and lower jig frames  3  and  4 . Each of the clamp mechanisms  61  comprises a slide plate  64  laterally slidably supported by a slide support member  63  which is fixedly attached to a part of a lateral member  62  forming a part of the frame structure  22  of the upper jig frame  4 , a pair of moveable clamp members  66  (only one of them is shown in FIG. 5) attached to either lateral end of the slide plate  64  each via a pair of parallel links  65  each having a lower end pivotally attached to the slide plate  64 , and a pair of fixed clamp members  68  fixed attached to either lateral end of a lateral member  67  forming a part of the frame structure  21  of the lower jig frame  3  so as to be selectably engaged by the corresponding moveable clamp members  66 . The upper ends of each pair of parallel links  65  are slidably guided by a guide member  76  fixedly attached to the lateral member  62  and extending in parallel with the lateral member  62 .  
         [0034]    The clamp/unclamp mechanism  10  for actuating each of these clamp mechanisms  61  comprises a slide plate drive actuator  70  consisting of an air cylinder having an end pivotally attached to a fixed bracket  69  for driving the slide plate  64 , and a shift actuator  71  likewise consisting of an air cylinder for pivotally actuating the free end of the drive actuator  70  so as to cause a substantially vertical shifting movement of the free end of the drive actuator  70 . The free end of the piston rod  72  of the slide plate drive actuator  70  is provided with a claw  74  for selectively engaging a striker  73  provided at an end of the slide plate  64 . The two clamp/unclamp mechanisms  10  are provided in diagonally opposing two of the four columns provided in the four corners of the jig frame open/close station  5 , and the two slide plates provided in the front and rear faces of the upper jig frame  4  are adapted to be actuated in mutually opposite directions.  
         [0035]    The mode of operation of the cooling system for cooling plastic fuel tanks according to the present invention is described in the following.  
         [0036]    Figure of all, the tank  1  which has been taken out of the metallic die assembly (not shown in the drawing) and is still warm is placed on the lower jig frame  3  which is secured to the carriage  8  and located in the load/unload station  2 , and is transported by the carriage  8  to the jig frame open/close station  5 . As the load/unload station  2  is located before the jig frame open/close station  5 , and has an open top, the charging and removing the tank  1  into and out of the lower jig frame  3  can be carried out without any difficulty.  
         [0037]    When the carriage  8  which has traveled from the load/unload station  2  has reached and come to a stop at a prescribed position in the jig frame open/close station  5 , the upper jig frame  4  which has been retained by the gripping unit  11  provided in the lower end of the lifting unit  9  is lowered onto the tank  1  placed on the lower jig frame  3 . At this time, the clamp mechanisms  61  of the upper and lower jig frames  3  and  4  are still in the unclamped state, and the slide plate drive actuators  70  of the clamp/unclamp units  10  are still in the upwardly shifted position that does not interfere with the clamp mechanisms  61 . In the following description, only one of the clamp mechanisms  61  is described as they are identical in both structure and operation.  
         [0038]    Then, the shift actuator  71  is extended while the slide plate drive actuator  70  is still in the retracted state to shift the slide plate drive actuator  70  downward until the claw  74  at the free end thereof engages the striker  73  at the corresponding end of the slide plate  64  in the unclamped state (see FIG. 5). As the slide plate drive actuator  70  is extended from this state, the slide plate  64  moves rightward until the moveable clamp member  66  engages a stopper surface  81  of the slide support member  63 . As a result, a lateral projection  75  provided in the lower end of the moveable clamp member  66  is allowed to move into a location immediately under the fixed clamp member  68  fixedly attached to the lower jig frame  4 . As the slide plate  64  is pushed laterally even further, the lower ends of the parallel links  65  are moved rightward while the moveable clamp member  66  along with the upper ends of the parallel links  65  are prevented from moving rightward any further. This causes the parallel links  65  to rise into upright positions. The terminal position of the slide plate  64  is defined by a lateral projection  80  of the slide plate  64  abutting the moveable clamp member  66 . The rising of the parallel links  65  causes the moveable clamp member  66  to be lifted upward, and the lateral projection  75  to engage the fixed clamp member  68  (FIG. 6). The lower and upper jig frames  3  and  4  are thus joined to each other.  
         [0039]    Thereafter, the shift actuator  71  is retracted while the slide plate drive actuator  70  is kept extended so as to upwardly shift the slide plate drive actuator  70  and release the claw  74  from the striker  73  at the end of the slide plate  64 . At the same time, the gripping unit  11  at the lower end of the lifting unit  9  is removed from the upper jig frame  4 , and only the gripping unit  11  is moved upward to a position that does not interfere with the upper jig frame  4 .  
         [0040]    Concurrently with this clamping process, the cooling air supplying unit  31  and cooling air vent unit  41  are brought into contact with the corresponding parts of the tank  1 .  
         [0041]    Cooling air is then supplied into the tank to cool the tank  1  and to maintain the internal pressure of the tank  1  slightly higher than the atmospheric pressure so that the contact between the shape retaining members  24  of the upper and lower jig frames  3  and  4  and the tank  1  may be ensured. The internal pressure of the tank  1  can be selected as required by adjusting the flow rate of the admitted cooling air and the resistance at the cooling air vent unit  41 . The sealing of the contact surfaces of the cooling air supplying unit  31  and cooling air vent unit  41  is ensured at all times irrespective of the deformation of the tank  1  owing to the moveable supports for the cooling air supplying unit  31  and cooling air vent unit  41  and the provision of the seal members  38  and  49 . The tank  1  along with the upper and lower jig frames  3  and  4  is then transported to the cooling station  6 .  
         [0042]    When the tank  1  carried by the carriage  8  has advanced into the cooling station  6  from the jig frame open/close station  5 , cooling water is sprayed from the upper and lower shower nozzles  12  and  13  onto the outer surface of the tank  1  for a prescribed period of time. The timing of this water spraying process may be synchronized with the movement of the carriage  8 . To avoid an excessive splashing of the cooling water that may occur depending on the shape of the tank  1  and jig frames  3  and  4 , the spraying of the cooling water from the shower nozzles  12  and  13  may be started only after the carriage  8  has come to a stop in the cooling booth and a water shut-off door has been closed.  
         [0043]    Because the tank  1  is cooled by the cooling water that is sprayed onto the outer surface of the tank  1  as opposed to the conventional arrangement where the tank is immersed in water, the tank is not subjected to any buoyant force or water pressure, and is thereby protected from any undesired distortions. By controlling the number and positions of the upper and lower shower nozzles  12  and  13  and/or the flow rate of the cooling water, the tank  1  can be cooled uniformly and at a controlled rate depending on the shape and wall-thickness of the particular tank  1 . Also, at the same time as cooling the tank from outside with the cooling water, the tank  1  is cooled from inside by the fresh cooling air that is introduced into the tank. This allows the tank  1  to be cooled both rapidly and evenly. An uneven cooling of the tank could cause undesired residual stress and/or distortion. Also, rapid cooling is beneficial in improving the production efficiency.  
         [0044]    When the tank  1  has cooled to a prescribed degree, the carriage  8  is returned to the jig frame open/close station  5 . During this process, high pressure air from a water removing unit  16  provided around the exit of the cooling station  6  and rail  7  is impinged upon the tank  1 , and expels the water from the surface of the tank  1  and upper and lower jig frames  3  and  4 .  
         [0045]    When the carriage  8  has returned from the cooling station  6  and come to a stop in the jig frame open/close station  5 , the high pressure air is shut off and the clamp mechanisms  61  of the upper and lower jig frames  3  and  4  are unclamped by reversing the clamping process described above. The lifting unit  9  is lowered to grip the upper jig frame  4  with the gripping unit  11  and raise it. To ensure the unclamping process to be carried out in a reliable manner, as the slide plate  64  moves leftward, a hooked portion  76   a  of the guide member  76  fixedly secured to the slide support member  63  engages the upper end of the parallel links  65  to retain them in a collapsed state at the end of the leftward stroke of the slide plate  64 .  
         [0046]    After the upper and lower jig frames  3  and  4  have bee thus separated from each other, the carriage  8  is returned to the load/unload station  2 , the tank  1  is removed from the lower jig frame  4 , and it is replaced by a next tank  1  to repeat the entire process described above. If desired, high pressure air may be manually impinged upon the tank before or after removing it from the lower jig frame  4  by using a hand-held nozzle or the like to more thoroughly remove moisture from the surface of the tank  1 .  
         [0047]    Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.