Abstract:
A twin sheet thermoformer apparatus and method in which an extruder continuously produces a sheet of hot plastic which is cut into cut sheets while the sheet is exiting the extruder, which are loaded in pairs into a series of transfer cars which are moved successively to an oven and then to a forming station. Sets of mold assemblies include pairs of molds with cavities which are faced upwardly to receive sheets lowered thereon and molded in the cavities and are then pivoted to face each other and moved together to fuse the molded sheets together to form a hollow part.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional Serial No. 60/461,475, filed Apr. 8, 2003. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention concerns thermoforming, a well known process for molding articles from preheated plastic sheet material, using a vacuum and/or air pressure to assist in drawing the sheets into conformity with mold surfaces.  
           [0003]    In an extension of this process, twin sheet forming has heretofore been developed in which two sheets are thermoformed separately, and the two formed pieces are pressed together while still in their respective molds to fuse the same together and produce a complete part.  
           [0004]    This process is used in forming large hollow parts such as fuel tanks.  
           [0005]    Conventionally, the sheets are precut and stored prior to being thermoformed, and are at room temperature (or below if stored outside in cold weather). It thus is necessary to heat the sheets in the thermoforming apparatus to the temperature necessary for the molding to be carried out.  
           [0006]    Particularly for heavier multilayer sheets as are used to mold fuel tanks, preheating is required to slowly bring the sheets up to temperature for the reasons described in US 2002/0017745 A1.  
           [0007]    This complicates the apparatus and also slows the process considerably as the preheating takes substantial time.  
           [0008]    A transfer system, either linear or rotary, is typically used in twin sheet thermoformers to move a car holding two of the sheets from a loading station through a preheating, oven and to a forming station. See U.S. Pat. No. 6,454,557 B1, issued on Sep. 24, 2002 and U.S. Pat. No. 3,925,140, issued on Dec. 9, 1975 and U.S. Pat. No. 6,382,953 B1, issued on May 7, 2002, for examples.  
           [0009]    This extended preheating step has precluded a continuous thermoforming process for this type of thermoforming operation.  
           [0010]    The twin sheet forming process also must allow for insertion of components into the fuel tank during processing prior to fusing of the two molded pieces so as to seal the components in the tank and avoid any openings in the fuel tank wall through which fittings, etc., are extended.  
           [0011]    It is one object of the present invention to provide a thermoforming process in which preheating of the sheet material is not required.  
           [0012]    It is a further object to provide a continuous twin sheet thermoforming process which does not require storage and handling of precut stored sheets.  
         SUMMARY OF THE INVENTION  
         [0013]    The above recited objects and other objects which will become apparent upon a reading of the following specification and claims are achieved by combining an extruder with a thermoforming apparatus so that a hot extruded plastic sheet feeds directly into the thermoformer apparatus such that the continuously extruded plastic sheet is hot when received.  
           [0014]    The continuous hot sheet is sheared into discrete sheet lengths which are alternately loaded into two take away shuttles, conveyor sections which are alternately positioned ahead of the extruder die and a “flying” shear which cuts the extruded sheet into discrete lengths.  
           [0015]    The two section shuttles may be cooled to lower the temperatures of the hot sheets, depending on the operating requirements and conditions.  
           [0016]    Each of the conveyor section shuttles shifts between a position aligned with the extruder die and shear where it receives a discrete length hot sheet of plastic and a position aligned with a sheet support comprised of a fixed conveyor table aligned beneath a respective one of two clamping frames mounted on one of three sheet transfer cars, where it discharges its sheet onto the fixed conveyor table.  
           [0017]    When both fixed conveyor sections are loaded, the above located sheet transfer car is lowered by a lift/lower mechanism to the fixed conveyor table and grippers on each clamping frame clamp to a respective sheet on a respective fixed conveyor table, and the transfer car is then raised to be able to be advanced linearly along a track into an oven, where both sheets are heated to the proper final forming temperatures.  
           [0018]    A retractable sheet squaring mechanism can be included in the loading area lift/lower mechanism to insure proper orientation of each sheet prior to being clamped in the clamping frames. Alternatively, sheet guides can be provided on the fixed conveyor tables.  
           [0019]    At the same time, a second transfer car previously in the oven is simultaneously linearly advanced into a forming station in a position located above a first set of two side-by-side forming mold assemblies to locate each of the two sheets in the respective clamping frames over a respective one of two molds in the first mold assembly set.  
           [0020]    A forming station lift/lower mechanism lowers the second transfer car in the forming station to bring the sheets carried in the associated clamping frames down onto the upturned molds. A mold plug set on the lift mechanism may also be carried down with the second transfer car, which mold plugs can be extended to assist the thermoforming of the sheets with an applied mold vacuum, to mold the sheets into conformity with the mold cavities.  
           [0021]    The clamping frame grippers are released from the sheets at this time so that the lift/lower mechanism can raise the second transfer car to an elevated position above the level of the top of the oven, so that a second linear transfer system can transfer the same back to the load station at a point above the fixed conveyor tables.  
           [0022]    A third transfer car has in the meantime previously been lowered over the conveyor tables and another two sheets have been clamped into its clamping frames.  
           [0023]    With the transfer car in the form station elevated out of the way, a robot can emplace inserts as necessary into the cavities in the formed sheets.  
           [0024]    Each mold in the first mold assembly set is pivotally mounted and able to be tilted as with hydraulic actuators to be rotated from an upward facing position of its cavities to a rotated down position to bring their respective mold cavities into an opposing or facing relative position.  
           [0025]    The two mold assemblies are also mounted for relative linear motion to bring exposed portions of the two formed sheets into abutment, as by moving one mold against the other which is held stationary. The molds are locked and hydraulically forced together to fuse the formed sheets together into a complete part.  
           [0026]    The first mold set assembly is transferred out of the forming station to an adjacent cooling unloaded area, while a second mold assembly set is simultaneously transferred into the forming station with its mold cavities in a tilted up position by a linearly movable platform which mounts both sets of mold assemblies.  
           [0027]    After sufficient cooling of the first set of mold assemblies, the molds are unlocked and separated. Upon pivoting back to a cavity up position the completed part retained in one of the molds is removed, as by a robot.  
           [0028]    The first mold assembly set is then ready to be shifted into position for another cycle when the second mold assembly is ready to be shifted into a cooling unload area adjacent thereto. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a simplified diagram of the major components of the apparatus according to the invention.  
         [0030]    [0030]FIG. 2 is a simplified diagram of the movement of three transfer cars through the stations included in the components shown in FIG. 1.  
         [0031]    [0031]FIG. 3 is a plan view of some of the sheet transfer components included in the apparatus according to the invention.  
         [0032]    [0032]FIG. 3A is an elevational view of one of the three sheet transfer cars used in an apparatus according to the invention.  
         [0033]    [0033]FIG. 4 is a side elevational view of a lift/lower system used int eh loading station and two sheet transfer cars, used in the apparatus.  
         [0034]    [0034]FIG. 5 is a plan view of the lift/lower system shown in FIG. 4.  
         [0035]    [0035]FIG. 6 is an elevational view of the components including in the forming station of an apparatus according to the invention.  
         [0036]    [0036]FIG. 7 is an enlarged elevational view of one of the mold assembly sets and the forming station lift/lower system and mold plug sets shown in FIG. 6.  
         [0037]    [0037]FIG. 8 is a further enlarged elevational view of the movable mold assembly in the mold assembly set shown in FIG. 7 and associated components.  
         [0038]    [0038]FIG. 9 is an enlarged view of the stationary mold assembly of the mold assembly set shown in FIG. 7 and associated components.  
         [0039]    [0039]FIG. 10 is a plan view of a clamping frame linear transfer system suitable to linearly transfer sheet transfer cars from the loading station to the oven and from the oven to the forming station.  
         [0040]    [0040]FIG. 11 is a side elevational view of the transfer system shown in FIG. 10.  
         [0041]    [0041]FIG. 12 is an enlarged fragmentary end view of portions of the transfer systems and portions of one of the transfer cars. 
     
    
     DETAILED DESCRIPTION  
       [0042]    In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims.  
         [0043]    Referring to the drawings, and particularly to FIGS. 1 and 2, the apparatus  10  according to the invention includes an extruder  12  which is capable of continuously creating multiple layers of plastic sheet, which may be of various compositions, and which are layered together into a single continuous sheet S exiting an extruder die  14 .  
         [0044]    The sheet S exits onto a shear conveyor  16  which may be comprised of powered roller type conveyor with the rollers cooled as necessary with a cooling system indicated to render the extruded sheet S capable of being handled.  
         [0045]    A commercially available “flying” shear  18  is driven back and forth over the sheet S to cut the sheet S into discrete lengths S′ while the sheet S is being extruded.  
         [0046]    The cut sheets S′ are conveyed alternately onto two conveyor shuttles  20 A,  20 B which are shifted rapidly in a lateral direction to bring each conveyor shuttles  20 A,  20 B alternately into alignment with the shear conveyor  16  and with respective sheet supports comprising stationary conveyor tables  22 A,  22 B.  
         [0047]    Each conveyor shuttle  20 A,  20 B alternately receives a cut sheet S′ and shifts into alignment with a respective fixed conveyor table  22 A or  22 B and discharges its sheet S′ thereon while the other conveyor shuttle  20 A,  20 B is being loaded.  
         [0048]    A shelf  24  can be provided to prevent sagging of the sheets S′ during transitions where moving the next shuttle conveyor into position, as the sheets S′ are continuously moving.  
         [0049]    The rate of feed of the extruder  12  of course must be set to the speed of operation of the other equipment.  
         [0050]    Positioned above and in alignment with the fixed table conveyors  22 A are a pair of sheet clamping frames  26 A,  26 B carried on one of three sheet transfer cars  28 A.  
         [0051]    The sheet transfer car  28 A is lowered by a lift/lower system  30  to bring the frames  22 A,  22 B down around the sheets S′ on each conveyor table  22 A,  22 B.  
         [0052]    A series of gripper clamps on the clamping frames engage the perimeter of the sheets S′.  
         [0053]    The clamping frames  26 A,  26 B are then transferred linearly on the sheet transfer  28 A by a linear transfer system  32  into an oven  34 .  
         [0054]    At the same time, a second sheet transfer car  28 B is transferred into a forming station  36  with previously heated sheets S′ clamped therein.  
         [0055]    The sheets S′ after transfer into the forming station  36  are lowered onto a set of molds  38  by a lift/lower system  40  after which a thermomolding process molds the sheets S′ in upper and lower part halves. The thermoforming is carried out by conventional methods involving a vacuum applied to the molds assisted by mold plugs described herein. Such techniques are well known in the art and do not themselves comprise the invention, and thus are not here described in further detail. A robot can emplace rings into the mold cavities prior to lowering the sheets S′ onto the molds.  
         [0056]    The gripper clamps in the clamping frames  20 A- 3 ,  20 B- 3  are released and the transfer car  28 -C is then raised by the lift/lower system  40  to a level above the oven  34 , and a second linear transfer system  42  returns the empty sheet transfer car  28 C to a position over the fixed conveyor tables  28 A,  28 B in the load station  25 .  
         [0057]    Inserts can also be emplaced into the molded cavities in the sheets S by a robot after the sheet transfer car has been raised out of the way.  
         [0058]    [0058]FIG. 3 shows additional details including a series of rollers  44  mounted on a frame  48 , the rollers rotated by the motor  46 .  
         [0059]    The conveyor shuttles  20 A,  20 B also have powered rollers  50  supported in frames  52 A,  52 B on a framework  52  driven by motors  54 A,  54 B.  
         [0060]    The fixed conveyor tables  22 A,  22 B similarly each have a series of rollers  56 A,  56 B powered by motors  58 A,  58 B.  
         [0061]    As seen in FIG. 3A, the sheet transfer car  28 A comprises an outer frame  60  having a pair of rectangular sheet support frames  62 A,  62 B supporting members making up gripper clamping frames  26 A- 1 ,  26 A- 2  so as to allow for an adjustment in the size thereof by removal of pins received in perforated members of the sheet support frames  62 A,  62 B to allow repositioning the members of the clamping frames  26 A- 1 ,  26 A- 2  in an adjusted position of those members. Such adjustable gripper clamping frames are very well known in the art and one thus not here described in further detail. See for an example, U.S. Pat. No. 4,938,678.  
         [0062]    Arrays of fluid pressure operated clamps or grippers  64 A,  64 B are arranged around the interior of the clamping frames  26 A- 1 ,  26 A- 2 . The gripper cylinders  66  (FIG. 3A) in the array  64 A,  64 B are described in copending application U.S. Ser. No. 10/654,278, filed Sep. 2, 2003, incorporated by reference herein, those cylinders being of a commercially available type in which the clamping jaws are opened by fluid pressure and closed by a spring force acting through on over center linkage when the fluid pressure is relieved. The linkage insures that the grippers remain closed even if air pressure is lost. Suitable manifolding and pressure connections are carried on the sheet transfer cars  28 A,  28 B,  28 C for opening the gripper cylinders, by a known power actuator operated connection, such as described in U.S. Pat. No. 6,454,557 B1, incorporated herein by reference.  
         [0063]    [0063]FIGS. 4 and 5 show further details of a lift/lower system  30  and lift/lower system  40 . Lift/lower system  30  comprises a framework  60  having four gear rack posts  70  supporting a platform  72  onto which is rolled each sheet transfer car  28 A,  28 B,  28 C from an adjacent track  74  (FIG. 2) extending over the oven  34 .  
         [0064]    A supporting connection between the gear rack posts  70  and platform  72  comprises a series of pinion gears  76  which allow raising and lowering of the platform  72  to raise or lower the sheet transfer car  28 A, B, C. Such a vertical drive is shown in U.S. Pat. No. 5,814,185 and copending application Ser. No. 10/218,982 and also hereinafter in connection with the lift/lower system  40 .  
         [0065]    Also preferably included in the lift/lower system  30  is a sheet squaring mechanism  84  having movable members  78  forming a rectangular array having angle tabs  80  attached engageable with the edges a sheet S′ on the fixed table conveyor  22 A,  22 B to square the same in a similar manner to the mechanism described in detail in copending application U.S. Ser. No. 10/654,278, filed Sep. 2, 2003 incorporated by reference herein. An array of gear racks  82  are driven to lower and raise the sheet squaring mechanism  84  and a supporting sub-framework  86 . In and out synchronized movement of the members is produced by motor driven gear rack shafts  88 ,  90 .  
         [0066]    Alternatively, fixed guides on the sheet transfer cars  28 A, B, C could be used to square the sheets S′.  
         [0067]    The platform  72  is lowered in the load station to allow the held open elongated bar jaws of the grippers  66  to be aligned with the edges of the sheet S′, and the air pressure is relieved to cause the jaws to close to grip the sheet S′ securely in its squared up orientation. An actuator (not shown) makes an air connection at this station to the clamping frame  28 A, B, C to allow opening of the gripper jaws. When the air pressure is disconnected the jaws close under the influence of springs, with an over center linkage insuring that the sheet S′ remains clamped even if air pressure is lost, driven by a motor drive gear units  77  driven by motor  92  connected by cross shafts  79  to be in synchronism with each other.  
         [0068]    After the sheet S′ are clamped into the clamping frames  26 A- 1 ,  26 A- 2 , the platform  72  is raised slightly to clear the fixed conveyor tables  22 A, B and be aligned with the track  74 A and to be ready for linear transfer into the oven  34 .  
         [0069]    [0069]FIG. 6 depicts the major components of the forming station  36 . Two sets of side-by-side mold assemblies  38 A- 1 ,  38 A- 2  and  38 B- 1  and  38 B- 2  are used alternately, each set alternately driven into position beneath the mold plug-platen set  106 - 1 ,  106 - 2 , while the other mold assembly set is in a cooling/part removal position off to one side by an actuator arrangement.  
         [0070]    The mold assembly sets  38  are all supported on a platform  142 , resting on linear bearings  140  and driven by a motor-pinion gear drive  144  (FIG. 7) comprising a part of the actuator arrangement to shift the mold assembly sets  38  to the right or left to bring one of the sets  38  beneath the lift/lower system  40 .  
         [0071]    Each mold assembly set  38 A- 1 ,  38 A- 2 ,  38 B- 1 ,  38 B- 2  includes a stationary mold  38 A- 2 ,  38 B- 2  affixed relative to the platform  142  and a mold  38 A- 1 ,  38 B- 1  movable relative to the platform  142 . The movable mold assembly sets  38 A- 1 ,  38 B- 1  are supported on a respective pedestal  162 ,  164  affixed to a respective movable platform  146 ,  148 , supported on linear bearings  150 ,  152  and driven by an actuator arrangement which may be comprised of a respective motor pinion gear drive  154 ,  156  towards and away from one of the adjacent stationary mold assembly sets  38 A- 2  or  38 B- 2  which are mounted on pedestals  158 ,  160  affixed to main platform  142 .  
         [0072]    The lift/lower mechanism  40  shown in FIG. 7 is similar to mechanism  30  in that a set of four gear rack posts  94  is supported in a framework  96 , with a horizontal framework platform  98  supported and driven up and down thereon by a drive motor gear unit  100  and pinion  102  connected by cross shafts  104 .  
         [0073]    A clamp frame  28 A, B, C is rolled on and off the support platform  98  from aligned tracks.  
         [0074]    Also carried on the framework platform  98  is a mold plug platen assembly  106  mounted on a sub-framework  108  affixed to the framework  98 . A mold plug platen assembly  106  includes a pair of mold plug platens  110  carrying plugs  111 , each platen  110  supported for up and down movement on an array of gear rack posts  112  driven by a motor, gear unit, cross shaft system  114  to allow the platens  110  to be lowered.  
         [0075]    [0075]FIGS. 10-12 show a suitable linear transfer system, comprised of slidable horizontal gear racks  118  attached to shuttle bars  120  supporting gripper mechanism  122 .  
         [0076]    The gear racks  118  and shuttle bars  120  are supported on bearings  124 . A pinion gear  126  is engaged with the gear racks  118  driven by a motor drive unit  128  supported on a frame  130  to be reciprocated when the motor drive unit  128  is activated by the machine controls.  
         [0077]    The grippers  122  are engageable with fingers  136  on the sheet transfer cars  28 A, B, C to cause the cars to be linearly advanced by the motion of the gear racks  118  and shuttle bars  122 .  
         [0078]    The sheet transfer cars  28 A, B, C have roller sets  132  mounted on its sides to be engaged with fixed tracks  134  to support the weight thereof. The rollers and tracks may be shaped in the well known manner to guide linear movement of the sheet transfer cars  28 A, B, C.  
         [0079]    Such a linear transfer system has been used in prior designs for linear transfer of sheet transfer cars and may be used for both systems  32 ,  40 .  
         [0080]    Alternative arrangements are described in U.S. Pat. No. 5,980,231 and U.S. Pat. No. 3,669,594 incorporated by reference herein, which also shows suitable mating track and roller shapes to guide the motion along a straight path.  
         [0081]    Mold assembly sets  38 A- 1  and  38 B- 1  are identical to each other as are mold assembly sets  38 A- 2  and  38 -B- 2 .  
         [0082]    Referring to FIG. 8, movable mold assembly  38 B- 1  includes a platen  166  mounted on a pivot connection  168  attached to the pedestal structure  164  to be rotatable between a horizontal position with a mold cavity  172 - 1  facing up to a vertical position where cavity  172 - 1 ′ is facing a mold cavity  172 - 2  in the mold  170 B- 2  of the relatively fixed mold assembly  38 B- 1 .  
         [0083]    The platen  166  in turn mounts the mold  170 B- 1  having the mold cavity  172 - 1 . Suitable tool locks  174  and a cylinder operated locating pin  176  insure secure, precise location of the mold  170 B-  1  on the upper surface of a top plate  178  of platen  166 .  
         [0084]    Prior to lowering of the sheets S′ onto the cavities  172 , a robot can emplace rings in the cavity  170  for creating an access opening in the completed fuel tank.  
         [0085]    The platen  166  is caused to pivot 90° on the pivot connection  168  by a an actuator arrangement which may includes pair of double acting power cylinders  176 ,  178 , a power cylinder  176  pivoted at a lower end to an anchoring structure  180  fixed to platform  148  and a power cylinder  178  anchored at its lower end to platform  148 .  
         [0086]    The actuator rod  182 ,  184  of the power cylinders  176 ,  178  are pinned to the platen  166 , such that when the power cylinders  176 ,  178  are stroked so as to retract the rods  182 ,  184 , the platen  166  and mold  170  are tilted down 90° so that the mold cavity  172 - 1  faces the stationary mold assembly  38 B- 2  and mold cavity  172 - 2 .  
         [0087]    The power cylinders and other components to be described may be hydraulically operated, and a suitable hydraulic accumulator  186  may be mounted to the platen  166 .  
         [0088]    The drive unit motor  156  advances the mold assembly  38 B- 1  towards the stationary mold assembly  38 B- 2  to bring the two mold assemblies  38 B- 1 ,  38 B- 2  together, bringing flanges on the formed sheets in the molds  170 B- 1 ,  170 B- 2  into abutment.  
         [0089]    Receivers  188  for mating with guide pins  190  on the platen  192  for the other mold assembly  38 B- 2  (FIG. 9) are mounted to the platen  166  to insure that the molds  170 B- 1 ,  170 B- 2  are in precise alignment.  
         [0090]    The mold assemblies  38 B- 1 ,  38 B- 2  incorporate the platen locking and clamping system described and claimed in U.S. Pat. No. 5,814,185 and in U.S. Ser. No. 10/218,982, referenced above.  
         [0091]    In this arrangement, bayonet couplings are established when bayonet receivers  194  receive bayonet fittings  192  (FIG. 9) on the ends of rods  198  fixedly mounted to the platen  192 .  
         [0092]    Rotation of the receivers  194  creates a positive locking together of the mold assemblies, a pneumatic rotary actuator  200  acting on the lower end of each rod  198  to carry out the locking and unlocking rotation.  
         [0093]    Large diameter hydraulic cylinders  202  are coupled to the mold assembly  38 B- 1  and receivers  194  to generate large squeezing forces drawing the molds  170 B-  1  together to fuse the abutting flanges together.  
         [0094]    A position sensor  206  tracks the travel of the molds  170 B-  1 ,  170 B- 2  as the cylinders  202  are operated, and the system controller (not shown) actuates the drive motor  156  to advance the mold assembly  38 B- 1  on the linear bearing  152  as the drawing action of the cylinders  202  proceeds. This action prevents the motor  156  from being overloaded by attempting to itself carry out the squeezing action.  
         [0095]    The set of mold assemblies  38 B- 1 ,  38 B- 2  is shifted to a cooling/unload area shown on the left in FIG. 6 until sufficiently cooled, with the other set of mold assemblies  38 A- 1 ,  38 A- 2  is simultaneously shifted below the mold plugs  106 - 1 ,  106 - 2  to begin the next cycle by activation of the motor  144  and linear movement of the platform  142 .  
         [0096]    When sufficiently cooled, the molds  170 - 1 ,  170 - 2  are separated by retraction of the movable mold assembly  38 B- 1  by operation of motor drive  156 , the completed part staying in mold  170 B- 1 .  
         [0097]    The mold assemblies  38 B- 1 ,  38 B- 2  are again pivoted up, and a robot or other device may be employed to remove the part at that time. The mold assemblies  38 B- 1 ,  38 B- 2  are then in oriented for another forming cycle when again shifted into position beneath the lift/lower system  40 .