Thermoforming machine with linear recirculation of sheet holding frames

Abstract: A thermoforming machine utilizes two clamping frames (A,B) to advance cut sheets through heating and forming stations (14,16), with the clamping frames (A,B) returned via an overhead path above the oven (28,30) in the heating station (14). Pneumatic cylinders arranged about the inner perimeter of the frames (A,B) are operated to grip the sheet and release the sheets, which are pressurized and released via fluid connections established when the frames (A,B) are lifted from a transfer bar structure at the loading and forming stations (12,16). The clamping frames (A,B) 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.

Description: 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. Patent 3,925,140 issued on December 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. Patent 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. Patent 3,574,807 issued on April 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. Accordingly, it is an object of the present invention to provide a linear transfer system for a plurality of clamping frames used in a thermoforming apparatus to hold sheets during heating and forming steps. The system is adapted to article sizes too large for practical application to rotary machines. 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 engagement with one of a pair of side channels on either side of the frame by operation of a drive system. The gear posts are retracted to be disengaged 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 frame 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 which requires relatively less floor space than a comparable rotary machine. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side elevational view of a thermoforming machine according to the invention. Figures 2A through 2G are a diagrammatic view depicting the linear movement of two clamping frames through the machine of Figure 1. Figure 3 is a side elevational view of the lower clamping frame transfer system. Figure 4A is a view of the section 4-4 in Figure 3 with a fragmentary portion of a clamping frame resting on the transfer bar structure. Figure 4B is the same section as Figure 4A but showing the clamping frame raised off the transfer bars. Figure 5 is a view of section 5-5 in Figure 3. Figure 6 is a side elevational view of the upper clamping frame transfer system. Figure 7 is a view of the section 7-7 in Figure 6 with a portion of a captioned clamping frame shown in phantom lines. Figure 8 is a diagrammatic view of the belt drive for the upper transfer system. Figure 9 is a diagrammatic view of the belt drive for the lower transfer system. Figure 10 is a plan view of a clamping frame. Figure 11 is a view of section 11-11 in Figure 10. Figure 12 is an end elevational view of the clamping frame pivoting system. Figure 13 is a plane view of the clamping frame pivoting system. Figure 14 is a side elevational view of the clamping frame pivoting system. Figure 15 is an enlarged end view of one of the gear post mechanisms used in the pivoting system shown in Figures 12 through 14. Figure 16 is a layout view showing the operation of one side of the clamping frame pivoting mechanism. Figure 17 is a diagrammatic view of the manifold fluid connection to the clamping frame pneumatic cylinders. Figures 18A and 18B are diagrammatic views depicting the cutting and loading of a sheet into a clamping frame. Referring to Figure 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 (Figures 2A to 2E) 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 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 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 (Figure 2) to a loading position and thereafter onto the lower rails 22 to be ready for another cycle. Figures 2A through 2G diagrammatically depict the basic machine cycles and movement of the frames A and B through the machine. In Figure 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 Figure 2B. The transfer bar structure 40 is then retracted as shown in Figure 2C and the B frame is lowered thereunto and then advanced to the return station (Figure 2D after removal of the formed part). The elevator mechanism 36 lifts 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 (Figure 2E). Upon the next forward motion of the transfer bar structure 40, the A clamping frame is advanced to the forming station 16 and elevated (Figure 2F), while the upper transfer bar structure 41 is driven back to a point positioning the B frame over the load station 12. The lowering mechanism 38 lowers the B frame when released from the upper transfer bar structure 41 to a loading position whereat a cut sheet is loaded thereinto. 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. Figures 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 Figures 4A, 4B 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 50A, 50B, 50C are disposed at intervals along the length of the transfer bar structure 40 corresponding to the station spacing. The set of pins 50A, 50B, 50C 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 50A, 50B, 50C 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 (Figure 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., 6,3 mm (one quarter inch), just sufficient to allow return of the transfer bar structure 40 after retraction of pins 50A, 50B, 50C. At station 18, the lifting is carried out by the elevator mechanism 36. Figure 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. A similar arrangement is formed in the lowering mechanism 38 at station 12. Figures 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 (Figures 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. Figure 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 90 connected together at their ends. U-shaped pieces 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 are a series of pneumatic sheet clamping cylinder mechanisms 100, arranged in parts to move pairs of clamping bars 102, 104 (Figure 11) together as a part to grip or release a rectangular sheet positioned within the rectangular space defined by the clamping cylinder mechanisms. The left end plates 64A 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. 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 sections 92 of the clamping frames A, B are adapted to be swung up in the forming section. 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 (Figures 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 righthand 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 24 enter one end of guide channels 144 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 Figure 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 64A have fluid ports to supply the cylinders 100. A manifold bar 146 is provided at the load station 12 and forming station 16. When the frame A, B is elevated the bar 146 has aligned parts sealed to parts on the plates 64A. 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. Figures 18A, 18B 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.

Claims: A thermoforming machine (10) including a heating station (14) for preheating a sheet of thermoplastic material, a forming station (16) whereat said preheated sheet is formed into an article by being shaped by tooling in said forming station (16), a plurality of clamping frames (A, B), each having clamping means (100, 102, 104) for gripping a sheet therein, and a loading station (12) ahead of said heating station (14) for placing a sheet into a clamping frame (A, B) moved into said loading station (12), characterized by a linear advance transfer system including means for successively advancing each clamping frame (A, B) loaded with a sheet out of said loading station (12) and through each of said heating and forming stations (14, 16), the means comprising a horizontally extending advance transfer structure (40) and drive means (42) for moving said advance transfer structure (40) back and forth between fully retracted and advanced positions to carry clamping frames (A, B) step by step through each of said stations (14, 16), a return station (18), a first elevator mechanism (36) raising each clamping frame (A, B) after being advanced out of said forming station (16) and into said return station (18), a linear return transfer system receiving each raised clamping frame (A, B) and transferring each raised clamping frame (A, B) back past said forming and heating stations (16, 14) to a position aligned with said loading station (12), a second elevator mechanism (38) lowering each clamping frame (A, B) to said advance transfer system, and means (56 to 64) for moving each clamping frame (A, B) out of engagement with said advance transfer structure in said heating station (14) and said forming station (16), whereby movement of said advance transfer structure (40) is caused so as to produce separate transfer movement of one clamping frame (A, B) remaining in engagement with said advance transfer structure (40) while another of said clamping frames (A, B) is out of engagement with said advance transfer structure (40), whereby said one clamping frame (A, B) is transferred ahead while the other clamping frame (A, B) remains in said heating station (14) or in said forming station (16). The thermoforming machine (10) according to claim 1 wherein said advance transfer structure (40) in said retracted position extending from said loading station (12) whereat said clamping frames (A,B) are loaded with a sheet to said forming station (16) and in said advanced position extending from said heating station (14) to said return station (18). The thermoforming machine (10) according to claim 1 or 2 further including lift means for raising said clamping frames (A, B) after being advanced by forward movement of said advance transfer structure (40) to allow return of said advance transfer structure (40). The thermoforming machine (10) according to claim 2 or 3 wherein said advance transfer structure (40) includes a series of retractable locator element sets (50A, 50B, 50C) allowing lengthwise securement of said clamp frames (A, B) in either of three lengthwise positions correspond to the locators beneath said stations (12 to 18) in said retracted and advanced positions of said advance transfer structure (40). The thermoforming machine (10) according to any of the foregoing claims where each of said clamping frames (A, B) has a pivotal portion (92) to which a portion of a loaded sheet is clamped, and further including mean (118) for pivoting said portion (92) when said clamping frame (A, B) is at said forming station (16), said means (118) including an extendible pivotal structure (120) normally not engaging said portion (92) but extendible to engage said pivotal portion (92) and pivot the same preparatory to a forming operation on a sheet loaded thereon. The thermoforming machine (10) according to claim 3, 4 or 5 wherein said clamping frames (A, B) have a series of pneumatically-operated gripper clamps (100), and further including a detachable fluid pressure connection(116) established at said load and forming stations (12, 16) when said clamping frames (A, B) are raised by said lift means to enable gripping and releasing operation of said grippers to enable engagement and release respectively of said sheet at said loading and forming stations (12, 16). The thermoforming machine according to any of claims 2 to 6 wherein said return transfer system includes an upper transfer structure (41) and means (74) for releasably grasping a clamping frame (A, B) elevated to the underside of said upper transfer structure (41). The thermoforming machine according to any of claims 2 to 7 wherein separate reciprocal strokes of advance transfer structure (40) separately advance each of said clamping frames (A, B) from station to station. A method of transferring sheets preparatory to each being clamped in one of a plurality of clamping frames (A, B) formed in a thermoforming machine (10), said transfer being from a heating station (14) to a forming station (16), comprising the steps of: loading one of said sheets into a first clamping frame (A) to support said sheet therein; advancing said first clamping frame (A) into a heating station (14); thereafter loading another sheet into a second clamping frame (B); advancing said first clamping frame (A) out of said heating station (14) and into a forming station (16) horizontally spaced from said heating station (14) and linearly aligned with said loading and heating stations (12, 14); advancing said second clamping frame (B) into said heating station (14); removing a formed article from said first clamping frame (A) and advancing said first clamping frame (A) out of said forming station (16) and into a return station (18); elevating said first clamping frame (A) above said heating station(14); returning said first clamping frame (A) to said load station (12) along a path extending above said heating station (14); advancing said second clamping frame (B) from said heating station (14) to said forming station (16); removing a formed article from said second clamping frame (B) and advancing said second clamping frame (B) out of said forming station (16) and into said return station (18); and repeating said steps to continually form articles from sheets loaded successively into said first and second clamping frames (A, B); wherein in said step of advancing said first and second clamping frames (A, B), a advance transfer structure (40) is reciprocated repeatedly, said first and second clamping frames (A, B) lifted therefrom in said stations during the return movement of said advance transfer structure (40). The method according to claim 9 further including the steps of extending locating elements into said first and second clamping frames (A, B) during advancing thereof.

Inventors: ARENDS ALBERT W, CHUN VICTOR L, KARKLIN ROLAND, ARENDS, ALBERT W., CHUN, VICTOR L., KARKLIN, ROLAND
Applicants: BROWN MACHINE LLC
Application year: 1998
Main classification: B29C51
IPC codes: B29C51