Abstract:
A twin sheet thermoforming method and apparatus of a type using a rotary wheel to carry each of a series of clamping frames for releasably holding sheets to be formed through successive processing stations. The clamping frames can be shuttled on the wheel to be alternatively located for alignment with one of two forming tooling sets at a forming station, and to be shiftable at an unload-load station so that unloading and loading do not occur at the same location.

Description:
BACKGROUND OF THE INVENTION 
     This invention concerns thermoforming methods and apparatus, and more particularly thermoforming parts using two sheets, each individually formed and the two pieces then fused together to complete the part. Thermoforming involves drawing a heated plastic sheet against a mold surface using vacuum and/or air pressure. In the twin sheet process, two pieces are thermoformed in a forming station, and upper and lower platens holding the two formed pieces are forced together to fuse the two pieces together and complete the part. 
     It is sometimes desirable to utilize a well known technique called “plugging” for deep formed parts, in which a complementary shaped tool assists in thermoforming the part against a mold surface. That is, there is a male and a female part to the tooling, one the mold, the other the plug assist tool. 
     This technique produces more uniform wall thickness in deep formed parts. 
     Plugging is difficult to practice in conventional twin sheet thermoforming, as the need to locate the plug tools opposite the mold would prevent the separately formed pieces aligned at the forming station from being pressed together. 
     Special arrangements have been devised to allowing plugging to be practiced in twin sheet thermoforming processes. 
     U.S. Pat. No. 5,658,523 issued on Aug. 19, 1997 for a “Method and Apparatus for Forming Twin Sheet Hollow Plastic Articles” describes a process in which two sheets are loaded into a pair of side by side clamping frames, which are both simultaneously transferred by indexing of a rotary wheel carrier through a heating oven, and then to a forming station, where side by side molds and plug assist tools form each part piece. One of the molds, with one of the part pieces held therein is shifted laterally (after withdrawal of the plug assist tool) to lie beneath the other mold holding the other part piece, and the two pieces are fused together. The joined pieces are transferred to a cold forming station where the part is completed by a final forming operation. 
     The side by side simultaneous transfer of two clamp frames and sheets requires a large oven for heating both sheets at once, and also makes it difficult to carry out different degrees of heating for different sheet thicknesses. 
     Another disadvantage is that removal of a finished workpiece is conducted at the same location where one of the new sheets must be loaded, complicating the sheet load mechanism. 
     It is the object of the present invention to provide a twin sheet rotary transfer thermoforming apparatus in which a side by side simultaneous transfer of two sheets is not required when using plug assist tooling to form each part piece. 
     SUMMARY OF THE INVENTION 
     The above object and others which will become apparent upon a reading of the following specification and claims are achieved by loading sheets one at a time into each of a series of clamping frames disposed about a rotary transfer wheel, which indexes to bring each clamping frame into each of the processing stations, including preliminary and final heating stations, a forming station, and an unload-load station. 
     Each clamping frame is mounted on the transfer wheel station to be able to be located in either of two shifted positions on the wheel structure by action of a shuttle transfer drive shifting clamping frame between either of these two positions. 
     In the unload-load station, unloading takes place in one position of the clamping frame, and the clamping frame is then shifted to the other position where loading of a new sheet takes place, simplifying the design of the sheet loading mechanism. 
     Each sheet is then successively carried by indexing of the transfer wheel into one or more heating stations. Preferably, a preheat and final heat stations are provided to reduce cycle time. 
     The clamping frame is shuttled to the one position for the first of each two succeeding sheets indexed into the form station, so that sheet is disposed in one of two forming tooling sets, including a mold and a plug assist, whereat forming of the first part piece is carried out. The clamping frame releases the piece to allow that piece to remain in the mold and be retracted with the mold platen. 
     The next successive sheet is moved into the other of the tooling sets by the next wheel index, since its clamping frame remains in the one position on the wheel, and the second piece is then formed. 
     After forming, the top platen remains lowered and only the bottom platen is retracted, the second formed piece remaining clamped in its clamping frame. The top platen of the other tooling set is then transferred over the bottom mold of the one tooling set, carrying the piece and clamping frame with it. The two pieces are then fused by operation of respective press platens to form a completed part. 
     The finished part is then rotated to the unload-load station with the next wheel index, and the clamping frame releasing the part to allow it to drop onto a conveyor. 
     The clamping frame is then shuttled to the other shifted position of the wheel for loading of a fresh sheet. 
     The one at a time transfer of the sheets makes individualized heating of the two sheets easier and reduces the oven size required, while using a preheat oven reduces cycle time. Since part unloading and sheet loading take place at two different locations, the loading mechanism is simplified. 
    
    
     DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a diagrammatic plan view of a rotary twin sheet thermoforming apparatus according to the present invention. 
     FIGS. 2A-2K are diagrammatic side elevational views of the main components of the apparatus shown in FIG. 1, in various stages of operation of the apparatus. 
    
    
     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, the thermoforming machine  10  according to the invention includes a rotary wheel structure  12  of the well known type which is able to be indexed to sequentially transfer each of a set of clamping frames  14 A- 14 D through a series of machine stations  16 A- 16 D whereat various operations are performed. 
     According to the invention, the clamping frames  14 A- 14 D are mounted on the wheel structure  12  so as to be capable of being shuttled thereon between two shifted positions,. i.e., between an advanced position and a retracted position for a purpose to be described herein. 
     Each clamping frame  14 A- 14 D is constructed with a series of releasable gripper clamps  20  arranged around the inner perimeter of a frame structure  22  thereof in the well known manner (depicted diagrammatically in station  16 A and FIGS. 2J,  2 K). Such clamping frame construction is well known, as described in U.S. Pat. No. 5,658,523 referenced above. See also U.S. Pat. Nos. 3,910,747; 4,097,035; and 4,938,678. The assignee of the present application also is the assignee of copending U.S. Ser. No. 09/332,702 filed on Jun. 14, 1999 describing a powered adjustable clamping frame. 
     Station  16 A is where unloading and loading of the clamping frames occurs. A completed part P is released from clamping frame  14 A by opening of gripper clamps  20  to be deposited on a conveyor  18  extending beneath the retracted position of the clamping frame  14 A. 
     After release of a part P, the clamping frame  14 A is shuttled to its advanced position by a shuttle drive  24  sliding the clamping frame  14 A on guides  26  to a loading position offset from the unload position. 
     A fresh sheet of plastic S is loaded into the clamping frame  14 A, and the gripper clamps  20  activated. 
     At station  16 B, the previously loaded sheets are transported by the wheel index movement into a preheat oven  28  which partially heats the sheet S preparatory to being moved into the next station  16 C, where final heating is carried out in another oven  30 . 
     The next wheel index carries the now fully heated sheet S into the forming station  16 D. 
     The forming station  16 D includes two mold-plug assist tooling sets  30 ,  32 . 
     A pair of sheets S 1 , S 2 , are successively indexed into the forming station  16 D, and individually formed in tooling sets  30 ,  32  to form the two respective part pieces fused together to produce the part P. 
     The first sheet S 1 , in each pair is shuttled to the retracted position during index to be received in tooling set  30 . 
     The next sheet S 2  remains in the advanced position and is thereby transported into tooling set  32  by the next indexing motion of the wheel structure  12 . 
     Reference is now made to FIGS. 2A-2L for further details concerning the forming steps. The tooling sets  30 ,  32  each include an upper tool  30 A,  32 A driven by upper press platen  34 A,.  36 A, and a lower tool  30 B,  32 B each driven by a lower platen  34 B,  36 B, separated as shown during wheel indexing. 
     The first sheet S 1  in each pair is disposed in tooling set  30  by shuttling the clamping frame  14  holding the sheet S 1  holding the sheet S 1  to the retracted position shown by the shuttle drive  24 . 
     The tooling sets can be comprised of male or female molds and male or female plug assists. In the arrangement depicted, a female mold  30 B is shown in tooling set  30  and a male mold  32 A in set  32 . 
     FIG. 2B shows the first sheet S 1  being formed into a first part piece P 1  by movement together of the platens  34 A,  34 B by respective platen drives  38 ,  40 . 
     FIG. 2C shows the clamps  20  on frame  14 D opened to release the part piece P 1  which remains in the female mold  30 B as the platens  34 A,  34 B separate. 
     FIG. 2D shows the second sheet S 2  carried in frame  14 C transferred into the tooling set  32  by the next indexing of the wheel structure. The clamping frame  14 D is carried out of the station  16 D by the same indexing cycle. 
     FIG. 2E shows the second part piece P 2  formed by driving the platens  36 A,  36 B together by platen drives  42 ,  44 . 
     FIG. 2F shows retraction of the lower platen  36 B only, the part piece P 2  still clamped in the clamping frame  14 C. 
     FIG. 2G shows an upper platen transfer drive  46  moving the upper platen  36 A, clamping frame  14 C and part piece P 2  over the other part piece P 1  and female mold  30 B and platen  34 B. At the same time, the upper tooling  30 A and platen  34 A are shifted to an idle position out of the way by the transfer drive  46 . 
     FIG. 2H shows the part P being formed by fusing of the pieces P 1 , P 2  together by raising platen  34 B. 
     FIG. 21 shows retraction of both platens  34 B,  36 A leaving the part P in the clamping frame  14 C. 
     In the next wheel indexing cycle, the part P and the clamping frame  14 C are carried to the unload-load station  16 A. The clamping frame  14 C is in the retracted position to be disposed over the conveyor  18  so that by release of the clamps  20 , the part P can drop onto the conveyor  18  (FIG.  2 J). 
     FIG. 2K shows shifting of the clamping frame  14 C to the advanced position by shuttle drive  24  where it is loaded with a fresh sheet S by a sheet loader  50 . The previously transferred frame  14 D has undergone the same shift-loading process. 
     The process is then repeated to produce additional parts P. 
     The sequential transfer of sheets S 1 , S 2  allows individual heating of these sheets. In addition, the shuttle system allows separate locations for unloading of the part and loading of the next sheet. 
     It should be appreciated that various part shapes are commonly made by the twin sheet process, including opposite formed cavities, such as are commonly used for making gas tanks, etc., in addition to the nested pieces as shown in the drawings, commonly used for making pallets.