Patent Publication Number: US-2009232923-A1

Title: Adjustable clamping table for thermoforming machines

Description:
FIELD OF THE INVENTION 
     The invention refers to thermoforming machines and, in particular, concerns an adjustable table for supporting and clamping sheets of thermoformable material, for examples plastic sheets, during a thermoforming process. 
     BACKGROUND OF THE INVENTION 
     Traditional thermoforming machines, both of single station and for production line plants, usually comprise a vacuum box for containing a shaping mold movably supported between a lowered position and a raised position where the mold protrudes slightly through a rectangular aperture of a window, provided in a clamping table which closes the top of the vacuum box. 
     During thermoforming, the sheet material must therefore be supported and sealingly locked against the edges of the window, by appropriate sealing means arranged between the supporting table, the thermoformable sheet material and the thermoforming mold, or its movable supporting platen. 
     Since the dimension of the sheet material to be thermoformed may vary in both length and width, the need exists to vary and adjust the longitudinal and cross dimensions of the window of the clamping table to each change in format, always ensuring a good seal, both in a vacuum and under pressurized air conditions, with the interior of the box. Various adjustable locking tables have been proposed, e.g. by EP 0,882,566, EP 0,623,449, EP 0,692,365 and EP 0,997,258. 
     In particular, EP 0,882,566 and EP 0,997,258, which constitutes the closest prior art, show an adjustable clamping table comprising a first and a second pair of movable plates at right-angled directions; the plates slide along overlapping surfaces, directly in contact with each other, above stationary plates defining lateral sides of the box. 
     In turn, EP 0,692,365 shows an adjustable clamping table comprising four coplanar support plates, sliding along dovetail guides positioned along longitudinal sides of the same support plates. 
     In contrast, EP 0,623,449 shows a clamping table comprising a stationary frame provided by elongated bars, and an adjustable frame comprising elongated bars slidably supported by the underlying stationary frame. 
     The clamping tables in the prior art, besides requiring use of appropriate supporting frames, with consequent costs and constructional complications, due to the high width and overlapping of the movable plates in respect to the stationary plates, makes achieving the necessary seal with the box for the thermoforming mold, extremely difficult; also, clamping tables with relatively high comparative dimensions are sometimes required, with the consequent need for modifying the molds, and greater waste of material. 
     OBJECTS OF THE INVENTION 
     One object of the invention is to provide an adjustable table for clamping sheet material in thermoforming machines which, besides having a simple construction, also guarantees a good seal with both the sheet of thermoformable material, and with the vacuum box for the mold, particularly in the most critical points of the support plates which define the edges of the window. 
     A further object of the invention is to provide an adjustable table for clamping sheet material in thermoforming machines, as previously mentioned, suitable for supporting and locking sheet material of different dimensions, within a wide adjustable range, always maintaining good seal conditions with both the sheet of thermoformable material and the vacuum box. 
     A further object of the invention is to provide an adjustable clamping table for thermoforming machines, which does not require any modification of the molds, allowing a reduction of waste of the thermoformable material to a minimum. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to a first embodiment of the invention, a clamping table for thermoforming machines has been provided, the clamping table comprising: 
     first and second pair of longitudinally and crosswise extending support plates which are movable in relation to each other on overlapping planes, said support plates having internal edges conformed to define a rectangular opening of an adjustable window through which a thermoforming mold extends, characterized by comprising: 
     a stationary base plate conformed with longitudinal and cross sides defining the rectangular opening of the window, said base plate having upper and bottom planar surfaces; 
     a pair of upper plates slidably supported by the base plate, in a first direction; 
     a pair of bottom plates slidably supported base plate, in a second direction at a right-angle to the sliding direction of the upper plates; 
     a clamping bar along an internal edge of each sliding plate, the clamping bars having co-planar upper edges defining an upper surface for supporting and locking a sheet of thermoformable material, and bottom edges defining a flat sealing surface for the thermoforming mold, coplanar to the bottom surface of the base plate; 
     metal sheets for supporting the clamping bars extending from the internal edge of each bottom sliding plate; and 
     elastically yielding sealing means between the mold, and said metal sheets for supporting the clamping bars. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and further characteristics of the adjustable clamping table for thermoforming machines according to the invention, will be more clearly evident from the following description and the examples of the accompanying figures, in which: 
         FIG. 1  is a longitudinal section of a vacuum box of a thermoforming machine, comprising an adjustable clamping table according to the invention; 
         FIG. 2  is a top view according to line  2 - 2  of  FIG. 1 , with the clamping table in a maximum opening condition; 
         FIG. 3  is an enlarged cross sectional view, according to line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a more enlarged detail of  FIG. 3 ; 
         FIG. 5  is an enlarged cross section according to line  5 - 5  of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the cross-section of  FIG. 1 , and top view of  FIG. 2 , a thermoforming machine for a thermoplastic sheet material, basically comprises a box shaped body  10  defining a chamber  11  connectable to a vacuum source and/or to pressurized air source. 
     The box  10  is partially closed by a table  12  conformed with a large rectangular window aperture, through which a shaping mold  13 , movably supported inside chamber  11  can protrude. The shaping mold  13  is fixed in a removable manner, to a platen  14  which may be moved vertically between a lowered position, shown in  FIG. 1 , and a raised position in which the mold protrudes partially or totally from the window, to shape a sheet  15  of thermoplastic material locked between the table  12  and an upper vertically movable locking frame  16 . 
     A thermoforming machine also includes, in a known manner, a heating system for heating the sheets  15  of thermoplastic material, which has not been shown since it does not form part of the invention. 
     The table  12  comprises a steady central plate  17 , also called base plate, having planar upper and bottom surfaces and a large rectangular opening of a window  18  defined by longitudinally and crosswise extending peripheral sides; the base plate  17 , has a peripheral frame formed, for example, by upper section bars  19  and lower section bars  20  fastened to the external edges of the plate  17 . 
     The thermoforming table  12 , above base plate  17 , comprises a first pair of movable plates P 1 , P 2 , which extend in the direction of a first axis, for example in the direction of the longitudinal edges of window  18 ; the plates P 1  and P 2  are supported by the base plate  17  to slide crosswise in the direction of the second axis at a right-angle to the first one. 
     In an underlying position, the base plate  17  also supports a second pair of movable plates P 3 , P 4  which extend crosswise, in the transversal direction of table  12 , to slide in the first longitudinal direction of window  18 . Plates P 1  and P 2  are movably supported in relation to each other, between a spaced apart or maximum opening position, shown in  FIG. 2 , and a close or minimum opening position, to continuously vary the width of the window  18 ; similarly, plates P 3  and P 4  are movably supported in relation to each other, between a spaced apart and a close position to vary continuously the length of window  18 , entirely independently of plates P 1  and P 2 . 
     Plates P 1  and P 2  slide along lateral guides  21  and in their approaching and retreating movements are controlled by two screw shafts  22  having opposite threading, which extend transversally to table  12 ; each threading of the control shafts  22  engages with a nut screw of a respective block  23 , fixed above the plates P 1  and P 2 . 
     The cross plates P 3  and P 4  in turn are supported in a sliding manner and are controlled in their approaching and retreating movements by two screw shafts  24 , one only shown in  FIG. 3 , which extend longitudinally on the two sides of window  18 ; once again, each control shaft  24  has opposite threading which engages with nut screws of corresponding blocks  25  fixed to plates P 3  and P 4 . 
     The control shafts  22  and  24  may be actuated automatically and/or manually through any electrical or mechanical device, or the plates P 1 , P 2 , P 3  and P 4  may be moved by pneumatic cylinders or by any other drive means. 
     With reference to  FIGS. 3 to 5 , the characteristics of the movable plates P 1 , P 2  and P 3 , P 4  will be described in greater detail, and the relative sealing means in respect to the box  10  and the sheet  15  of plastic material to be thermoformed. 
     As previously indicated, the two plates P 1  and P 2  coplanar extend on the base plate  17 , in the longitudinal direction of the window  18 ; each longitudinal plate P 1 , P 2 , along its internal edge, has a clamping bar  26 , which longitudinally extends between the two cross edges of the rectangular opening of the window  18  of base plate  17 . 
     Each bar  26  extends upwards from a respective longitudinal plate P 1 , P 2 , ending with an upper edge  27  which, with the upper edges of the other clamping bars defines a flat resting surface for supporting and locking a sheet  15  of thermoformable material. 
       FIG. 5  also shows how each locking bar  26  extends downwards, ending with a bottom edge  28  coplanar to bottom surface  29  of base plate  17 , in contact with the cross plates P 3  and P 4 . 
     In the case of longitudinal plates P 1  and P 2 , the locking bars  26  are of non-extendable type and have a length equal to the maximum opening of window  18 . 
     Otherwise, as shown in  FIGS. 2 ,  3  and  4 , the cross plates P 3  and P 4 , on their internal edge, have a telescopically or axially extendable clamping bars, maintained in contact with the longitudinal bars  26  to vary the length as the reciprocal position of plates P 1  and P 2  varies. 
     The telescopic clamping bars for plates P 2  and P 3  may be shaped in any way; in the example shown, each telescopic bar comprises a central section  30 , fixed to the internal edge of the plate P 2  and P 3 , and two lateral sections  31  freely and longitudinally sliding in respect to the central section  30 . In this regard, each lateral section  31 , on the side facing the central section  30 , has a T-shaped longitudinal ribbing  32 , or dovetail shaped, which slide along a correspondingly shaped guide slot  33  open at both sides of central section  30 . A linear actuator, for example a pneumatic cylinder  34 , or, alternatively, helicoidal thrust springs act on the two lateral sections  31  of each cross bar to maintain them in contact with the bars  26  of the longitudinal plates P 1  and P 2 . 
     Also in the case of the cross plates P 3  and P 4 , the central section  30  and the two lateral sections  31  of each telescopic clamping bar have upper edges  34  and  35 , coplanar to the upper edges  27  of the two longitudinal bars  26 , defining with the latter an upper flat surface for supporting and locking a sheet of thermoformable material  15 . 
     In turn, the central section  30  and the lateral sections  31  of each telescopic bar have bottom edges  36  and  37  coplanar to the bottom edges  28  of the bars  26  to define a bottom flat surface for sealing against a shoulder surface of the mold  13  or its supporting platen  14  in the raised position of the same mold. 
       FIG. 4  shows a further feature of table  12  for thermoforming machines according to the invention, which allows a high seal degree on the side of box  10 , with the mold or its supporting platen, whatever the opening of the window  18  of the thermoforming table. 
     In particular,  FIG. 4  shows that each telescopic clamping bar  30 ,  31 , is supported by the respective transversal plate P 3 , P 4 , through a laminar or thin metal sheet  39 , for example a one-millimeter thick steel sheet, just higher or just lower, fastened into a seat, flush to the bottom surfaces  28 ,  29 , and along the internal edge of plates P 3  and P 4 ; the bar support sheet  39  extends for a short length towards the inside of the window,  18  beyond the inner edge of plates P 3  and P 4 . 
     In this manner, when an edge of the platen  14  supporting the thermoforming mold  13 , or an edge of the same mold comes into contact with the laminar or thin sheet  39 , the small step formed by sheet  39  is sealed by a gasket  40  along the peripheral edges, which elastically yield to allow good seal both against the edge of the support sheets  39 , and against the bottom edge  28  of longitudinal bars  26 . 
     The seal under vacuum and/or pressured air conditions of the box  10 , along the edges of the window  18  may also be allowed by gaskets, for example of the double-lipped type, not shown, housed in seat of the mobile plates, on the side facing the base plate  17 . 
     It is understood from what has been described and shown in the accompanying drawings that a clamping table for sheets of thermoformable material has been provided suitable for sealing the box of thermoforming machines, which practically is of universal use, since it may be applied to any type of box without the need for any additional support frame, guaranteeing in all cases a high seal degree. 
     It is understood that what has been described and shown in the accompanying drawings has been given purely in order to illustrate the clamping table according to the invention. Therefore, other variations or modifications may be made to the entire table or its parts, without thereby departing from the claims.