CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. Ser. No. 08/986,977, filed Dec. 8, 1997 now U.S. Pat. No. 5,980,231. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention concerns thermoforming machines of the type in which thermoplastic sheets are first heated in an oven and then transferred to a forming station where a heated sheet is drawn onto tooling forms by vacuum and/or air pressure to be formed into an article. 
     In some instances, a clamping frame is used to hold thinner sheets which would otherwise become too weak when heated to be transferred between stations in the machine. 
     When using a clamping frame, a continuous in-line process is not possible as the clamping frames must be returned to the first station and reloaded with a cut sheet. 
     Rotary transfer thermoforming machines are known such as described in U.S. Pat. No. 3,925,140 issued on Dec. 9, 1975, which use continuous rotation of a carrier member to carry the frames through the station and to return the frames to a load/unload station after successive rotation through the heating and forming stations. 
     For very large articles, rotary transfer thermoforming machines occupy excessive plant floor space. 
     An in-line shuttle transfer has also been used, but this either is too slow if a single clamping frame is used, or requires two molding stations on either side of an oven. 
     Another problem is encountered in thermoforming parts with substantially angled deep portions, as excessive stretching of the material would occur if the sheet is simply formed onto the tool shape. 
     Draping techniques are sometimes employed for closed shapes forming dished containers, with vacuum (or air pressure) causing a pronounced sagging of the sheet material preparatory to the forming step. Such technique is not useable with a nonclosed shape or with angled ends connected with a flat center section. 
     U.S. Pat. No. 4,744,848 issued on May 17, 1988, describes a frame with hinged side portions which floats to allow forming with minimal stretching. This arrangement relies on engagement with a male and female mold to cause hinging of the frame ends which in turn bends the sheets into closer conformity with the mold contours prior to forming. See also U.S. Pat. No. 3,574,807 issued on Apr. 13, 1971, which describes a frame hinged in the middle to form an angled article shape. 
     These patents do not disclose an adaptation of a hinged clamping frame to automated thermoforming apparatus. 
     It is an object of the present invention to provide a frame having a section pivot capability for sharply angled elongated parts which is adapted to an automated system and which does not require engagement with a male and female contour to be actuated. 
     SUMMARY OF THE INVENTION 
     These objects and others which will become apparent upon a reading of the following specification and claims are accomplished by a linear transfer system for a set of two clamping frames. 
     The clamping frames are each successively moved through a load station, heating or oven station, a forming station, and an elevator/return station by a reciprocated transfer bar structure. Each clamping frame is slightly elevated in the heating, forming, and elevator/return stations to allow retraction of the transfer bar structure to receive a trailing clamping frame for separate advance. 
     Each clamping frame is elevated to a height above the heating station oven, and transferred back over the top of the oven, to a position above the loading station. After partial lowering, a cut sheet is loaded into the frame, which is thereafter lowered onto the transfer bar structure. 
     Each clamping frame has pivoted end sections which are each engaged by pairs of extensible gear posts when each frame is in the forming station, the posts each advancing a cam roller into and out of engagement with one of a pair of side channels on either side of the frame by operation of a drive system. The pivoting of the end sections brings the clamped sheet into closer conformity to the mold surfaces. The gear posts are retracted to disengage the cam rollers with the side channels to allow advance of the clamping frame to the elevator/return station. 
     An array of pneumatic cylinders and a series of clamping bars are arranged around the inner perimeter of the clamping frame. The cylinders are operated to retract or advance the clamping bars by application of pneumatic pressure communicated through manifold bars engaging one of the lift plates protruding from each end of the clamping frame. 
     The manifold plates are mounted on elevating cylinders located at the load and form stations so as to establish a fluid connection to the cylinders when the clamping frame is raised off the transfer bars. 
     In the load station a cut sheet is raised within the clamping frame, and the cylinders operated to clamp the cut sheet in the clamping frame. 
     In the forming station, the cylinders are retracted as the sheet is formed to release the sheet and allow removal of the formed article. 
     This invention allows a very large sharply angled article to be efficiently formed in an automated system by pivoting of the clamping frame end sections in the forming station by a disengageable drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a thermoforming machine according to the invention. 
     FIGS. 2A through 2G are a diagrammatic view depicting the linear movement of two clamping frames through the machine of FIG.  1 . 
     FIG. 3 is a side elevational view of the lower clamping frame transfer system. 
     FIG. 4A is a view of the section  4 — 4  in FIG. 3 with a fragmentary portion of a clamping frame resting on the transfer bar structure. 
     FIG. 4B is the same section as FIG. 4A but showing the clamping frame raised off the transfer bars. 
     FIG. 5 is a view of section  5 — 5  in FIG.  3 . 
     FIG. 6 is a side elevational view of the upper clamping frame transfer system. 
     FIG. 7 is a view of the section  7 — 7  in FIG. 6 with a portion of a captioned clamping frame shown in phantom lines. 
     FIG. 8 is a diagrammatic view of the belt drive for the upper transfer system. 
     FIG. 9 is a diagrammatic view of the belt drive for the lower transfer system. 
     FIG. 10 is a plan view of a clamping frame. 
     FIG. 11 is a view of section  11 — 11  in FIG.  10 . 
     FIG. 12 is an end elevational view of the clamping frame pivoting system. 
     FIG. 13 is a plane view of the clamping frame pivoting system. 
     FIG. 14 is a side elevational view of the clamping frame pivoting system. 
     FIG. 15 is an enlarged end view of one of the gear post mechanisms used in the pivoting system shown in FIGS. 12 through 14. 
     FIG. 16 is a layout view showing the operation of one side of the clamping frame pivoting mechanism. 
     FIG. 17 is a diagrammatic view of the manifold fluid connection to the clamping frame pneumatic cylinders. 
     FIGS. 18A and 18B are diagrammatic views depicting the cutting and loading of a sheet into a clamping frame. 
     FIGS. 19-24 are diagrammatic views of successive conditions of an alternate form of the invention. 
    
    
     DETAILED DESCRIPTION 
     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. 
     Referring to FIG. 1, a thermoforming machine  10  according to the present invention is shown. 
     The machine  10  includes a loading/return station  12 , a heating station  14 , a forming station  16  (shown without any tooling), and an elevating/return station  18 . The machine components are supported on a machine framework  20 . 
     Lower transfer system support rails  22  extend through the stations  12  through  18  and return or upper transfer support rails  24  extend back above and along the stations  12  through  18  (although extending below the upper platen forming station  16 ). 
     The loading/return station  12  includes a sheet cut off and loading apparatus  26  for loading a sheet to be formed into a clamping frame. The basic arrangement of this equipment will be described hereinafter. 
     A pair of clamping frames A, B, are separately and successively advanced from the loading station  12  through stations  14  through  18  on a transfer bar structure  40  (FIG. 2) the lower rails  22 , and hence returned via the upper rails  24 . 
     Upper and lower heating panels  28 ,  30  forming an oven are disposed in the heating station  14 , together with a lower shield, the lower shield and/or oven moveable to clear a sagging heated sheet. Such heating ovens and shields are well known and do not form a part of the present invention and hence are not themselves described in detail herein. 
     The forming station  16  includes upper and lower platens  32 ,  34  for receiving tooling for forming the sheet into an article and unloading the formed article. Since such tooling and article handling apparatus also well known, and do not themselves form a part of the invention, further details will not be set forth herein, except in connection with the pivoting of sections of the clamping frames to facilitate the forming process, as will be described hereinafter. 
     The elevating/return station  18  has an elevator mechanism which comprises a set of four elevator gear posts  36  driven simultaneously to raise a clamping frame A or B from the lower rails  22  to the upper rails  24  for return transfer to station  12 . A lowering mechanism also including a set of four lowering gear posts  38  are driven to lower the returned clamping frame A or B released from a transfer bar structure  41  on the upper rails  24  (FIG. 1) to a loading position and thereafter onto the lower rails  22  to be ready for another cycle. 
     FIGS. 2A through 2G diagrammatically depict the basic machine cycles and movement of the frames A and B through the machine. 
     In FIG. 2A, clamping frame A rests on a transfer bar structure  40 , and clamping frame B is raised out of engagement with the transfer bar structure  40  in station  16 . Lower transfer drive  42  is activated to advance clamping frame A from station  12  to station  14 , where it is lifted clear of the transfer bar structure  40 , as indicated in FIG.  2 B. 
     The transfer bar structure  40  is then retracted as shown in FIG.  2 C and the B frame is lowered there unto and then advanced to the return station (FIG. 2D after removal of the formed part). 
     The elevator gear posts  36  lift the B frame off the advanced transfer bar structure  40  upwardly to an overhead upper transfer bar  41  driven by upper transfer drive  44 . The B frame is captured by activation of gripper mechanisms on the upper transfer bar structure  41  preparatory to a return transfer. 
     In the meantime, the lower transfer bar structure  40  has returned to its retracted position to be positioned to receive the A frame lowered from the heating station  14  onto its middle section (FIG.  2 E). Upon the next forward motion of the transfer bar structure  40 , the A clamping frame is advanced to the forming station  16  and elevated (FIG.  2 F), while the upper transfer bar structure  41  is driven back to a point positioning the B frame over the load station  12 . 
     The lowering gear posts  38  lower the B frame when released from the upper transfer bar structure  41  to a loading position whereat a cut sheet is loaded therein to. 
     The lower transfer bar structure  40  in the meantime is retracted to allow the loaded B frame to be lowered onto the transfer bar structure  40  to be ready for another cycle. 
     The transfer bar structure  40  spans three stations and is reciprocated the distance between the stations to enable step by step incremental successive advance of each clamping frame A and B through the stations  12  through  18 . FIGS. 3 through 5 show further details of the lower transfer system including lower rails  22  and a transfer bar structure  40  mounted for guided movement on the rails  22  by way bearings  46  (right side only shown in FIGS. 4A,  4 B and  5 ). The rails  22  are supported on the machine fixed framework including support stanchions  48  spaced along the length of the rails  22 . 
     The length of the transfer bar structure  40  corresponds to three of the stations  12  through  18 , and sets of spaced locator pins  50 A,  50 B,  50 C are disposed at intervals along the length of the transfer bar structure  40  corresponding to the station spacing. The set of pins  50 A,  50 B,  50 C may be raised and lowered by corresponding power cylinders  52  so as to be received or withdrawn from holes  54  in the bottom of the clamping frames A or B. 
     When raised, the pins  50 A,  50 B,  50 C secure the clamping frame A or B in position as the transfer bar structure is advanced. 
     The clamping frames A or B are raised out of contact with the transfer bar structure  40  when advanced into each station  14 ,  16 ,  18  (FIG. 4B) to allow retraction. In stations  14  and  16 , this raising is accomplished by power cylinders  56  each having an operating rod  58  to which is attached a bar  60  carrying tapered plugs  62  matched to openings in lift plates  64  welded to either end of the clamping frames A or B. 
     The clamping frames A or B are raised only a short distance in stations  14 ,  16 , i.e., one quarter inch, just sufficient to allow return of the transfer bar structure  40  after retraction of pins  50 A,  50 B,  50 C. 
     At station  18 , the lifting is carried out by the elevator mechanism  36 . 
     FIG. 5 shows one of the four gear posts  66 , connected in pairs, with bars  68  welded to the ends to prevent turning and to ensure equal stroking when gear  70  is driven to raise the posts  66  by a drive system  72 . 
     A similar arrangement is formed in the lowering mechanism  38  at station  12 . 
     FIGS. 6 and 7 show the upper or return transfer system including rails  24  and upper transfer bar structure  41 , which carries grippers  74  adapted to engage lift plates  64  of the clamping frames A, B to capture the clamping frames A, B. Downwardly driven locating pins  76  engage holes in the top of the clamping frame A, B. 
     Both upper and lower transfer bar structures  40 ,  41  are driven in either direction by belt pairs  78 ,  80  passed around sheaves  82 ,  84  (FIGS. 8,  9 ) and clamped to cross bars  86 ,  88 . Electric motors (not shown) are used to power the sheaves  82 ,  84  via drive shafts (not shown) in a conventional manner. 
     FIG. 10 shows one of the two identical clamping frames A, B which consists of a rectangular open framework  94  formed by lengths of aluminum extrusions forming parallel side and end members  90  connected together at their ends. 
     U-shaped end sections  92  have legs  96  pivoted at  98  to the inside of each end of the rectangular framework  94 . 
     Arrayed around the inside perimeter of the clamping frames A, B including the end piece  92  are a series of pneumatic sheet clamping cylinder mechanisms  100 , arranged in pairs to move pairs of clamping bars  102 ,  104  (FIG. 11) together as a part to grip or release a rectangular sheet positioned within the generally planar rectangular space defined by the array of clamping cylinder mechanisms  100 . 
     The left end plates  64 A have ports connected to fittings for establishing a fluid connection to tubing  116  and manifold plates  106  to actuate or release the pneumatic cylinders  100  (FIG.  10 ). 
     The pairs of cylinders  100  are mounted on opposite sides of posts  108 , and drive over center linkages  112  associated with respective bars  102 ,  104 . 
     The bars  102 ,  104  have complementary interfitting edges which engage the plastic sheet edge  110  when brought together. 
     The pivoted end sections  92  of the clamping frames A, B are adapted to be swung up in the forming station. 
     This is accomplished by pivot actuation mechanisms  118  each comprised of pairs of inclined gear posts  120  arranged on either side of the clamping frame A or B (FIGS.  12 - 16 ). The gear posts  120  slide within guide tubes  122  and have a cam roller  124  at their upper end. 
     One side of each gear post  120  has a gear rack formed along its length engaged by a drive gear  126  to be extendible upwardly and retracted thereafter. 
     A single motor-gear box  128  drives all of the drive gears  126 . A drive belt  130  recirculates around a drive pulley  132  driven by the motor gear box  128  which in turn circulates around an adjustable tensioner idler  134  and a pulley  136  on a cross shaft  138  fixed to opposite drive gears  126 . 
     A drive shaft  140  transfers the drive to the opposite right-hand activation mechanism  118 . 
     A connecting bar  142  joins each pair of gear posts  120  together to prevent rotation and ensure joint travel together. 
     The cam rollers  124  enter one end of guide channels  144  (FIG. 16) on each side of each end of the clamping frames A, B when the gear posts  120  are extended. 
     Continued travel causes the pivoted section  92  to swing up, bending the sheet portions clamped therein. 
     The sheet is thus bent into much closer conformity with the male tool T preparatory to the vacuum stretching onto the tool T as seen in FIG. 16, greatly reducing the stretching of the sheet material. 
     The gear posts  120  can then be retracted, releasing the clamping frame A or B to be lowered onto the transfer bar structure  20  and advanced to the return station. 
     The clamping frames A, B lifting plates  64 A have fluid ports to supply the cylinders  100 . 
     A manifold bar  146  (FIG. 17) is provided at the load station  12  and forming station  16 . 
     When the frame A, B is elevated the bar  146  has aligned ports sealed to ports on the plates  64 A. 
     A separate clamping mechanism  150  ensures a sealing engagement and also, in the forming station  16 , prevents movement of the clamping frame A or B. This establishes a fluid connection to a pneumatic supply circuit  152 . 
     FIGS. 18A,  18 B show the principle of sheet loading in station  12 . 
     Material is pulled from a roll of sheet material and over a lift table  154 , and then cut off. The material overlaps all four edges. The table  154  is driven up to position the sheet S inside the frame A or B with gripper clamping bars open. 
     The grippers are activated to grip the sheet edge and complete loading of the sheet into the frame A or B, which is then lowered onto the transfer bar structure  40  as described above. 
     While two clamping frames have been described, additional clamping frames could be employed in other applications as to reduce the time required to transfer an empty frame back and to reload the same preparatory to a new cycle of heating and forming. 
     In addition, a series of ovens are sometimes employed, each partially heating the sheets which are transferred successively through the series of ovens to reduce the cycle time. In this design, additional clamping frames will be employed for each oven. 
     Also, the arrangement is applicable to other thermoforming processes such as “twin sheet” forming, where two sheets are formed into article components and then pressed together to form an integral article. 
     Finally, this invention allows unlimited additional stations to be easily added by lengthening the line, while still using a reasonable amount of floor space. The stations can be readily added, changed, or deleted to afford great flexibility of the concept to a wide variety of applications. 
     FIGS. 19-24 diagrammatically depict an alternate form of thermoforming machine  160  in which a pair of frames A, B are successively transferred through a load station, heat station, form station, and standby station as indicated. 
     In this machine, a pair of rail systems  162 ,  164  are provided one for each frame A, B and located one above the other. The frames A, B are movable along the associated rail system  162 ,  164 , and the lower rail  164  is movable up and down as will be described, both passing through the heating station between upper and lower ovens  166 ,  168 , both of which being able to be raised and lowered in this embodiment. 
     In the condition shown in FIG. 19, frame A is being loaded with a sheet by table  170  preparatory to movement into the heating station. Frame B is in the forming station ready for transfer to the standby station. 
     FIG. 20 shows the frame A in the heating station with lower oven  168  raised. The lower rail system  164  is lowered from position A to B, and lower oven  168  is lowered to clear sag of the sheet in frame A. 
     In FIG. 21, frame A has been transferred to the forming station and oven  168  lowered. Forming is being carried out on the sheet in frame A. Frame B has been transferred to the load station and rail  164  raised from position A to B to be loaded by table  170 . 
     In FIG. 22, frame B is in the heating station and upper oven  166  lowered and lower oven  168  raised. 
     Frame A is being readied for transfer to the standby station, after forming and removal of the part and scrap in the forming station. 
     In FIG. 22, the frame B is heated and lower and upper ovens  166 ,  168  returned preparatory to transfer to the forming station. The frame A has been transferred to the standby station. 
     In FIG. 24, the frame A has been returned to the loading station and frame B to the forming station. 
     This version eliminates the need for making and breaking connections to the frame clamp actuators, but involves movement of the frames back through the oven which could affect the heat cycle, and also requires both upper and lower ovens to move vertically, as well as the lower rail system  164 .
A thermoforming machine utilizes two clamping frames to advance cut sheets through heating and forming stations, with the clamping frames returned via an overhead path above the oven in the heating station. Pneumatic cylinders arranged about the inner perimeter of the frames are operated to grip the sheet and release the sheets, which are pressurized and released via fluid connections established when the frames are lifted from a transfer bar structure at the loading and forming stations. The clamping frames have pivoted end sections which are engaged by extendible gear posts at the forming station to bend the clamped sheet into greater conformity with the tooling shape prior to forming.