Patent Publication Number: US-2005142239-A1

Title: Thermoforming tool

Description:
FIELD OF THE INVENTION  
      The invention relates to a thermoforming tool for producing cup-shaped molded parts from a heated film of plastic, comprising an upper tool and a lower tool, which are movable in relation to each other for clamping the film of plastic between them, at least one mold cavity for the forming of a molded part from the film of plastic being formed in the lower tool, and at least one prestretching means which can be moved into a corresponding mold cavity being arranged in the upper tool inside a hold-down device.  
     DESCRIPTION OF THE BACKGROUND ART  
      Such a thermoforming tool is known from, inter alia, EP 1 103 365 A1. The known thermoforming tool comprises an upper tool and a lower tool, which are movable in relation to each other in order to clamp the heated film of plastic between them. Arranged on the upper tool, inside a hold-down device, is a prestretching device, which is fastened to the lower end of a rod which is displaceably mounted coaxially in the upper tool and is connected to a drive. For molding the molded part, the prestretching device is moved into the mold cavity, coming to bear against the heated film of plastic and pressing the latter downward into the mold cavity. Subsequently, forming air is introduced through an air duct provided in the prestretching device, for which reason the film of plastic comes to lie against the inner surface of the mold cavity. The contact with the cooled mold cavity walls has the effect that the thermoformed film of plastic cools down and becomes dimensionally stable.  
      The end face of the prestretching device that comes into contact with the film of plastic is relatively large, in order to achieve suitable pre-deformation of the film of plastic before the molding air is introduced. However, the film of plastic is cooled on the end face of the prestretching device, so that in the pre-deformation by the prestretching device the film of plastic is stretched to a lesser extent in the region of the end face of the prestretching device than in the region results in an uneven wall distribution, and in particular a considerable wall thickness in the bottom region of the container to be molded.  
     SUMMARY OF TH E INVENTION  
      The the object of the invention is to provide a thermoforming tool which makes it possible to produce molded parts with uniform wall distribution and small wall thickness.  
      This object is achieved according to the invention by a thermoforming toolfor producing cup-shaped molded parts from a heated film of plastic, comprising an upper tool and a lower tool, which are movable in relation to each other for clamping said film of plastic between them, at least one mold cavity for forming of a molded part from the film of plastic being formed in the lower tool, and at least one prestretching means which can be moved into a corresponding mold cavity being arranged in the upper tool inside a hold-down device, wherein said prestretching means comprises at least two prestretching levers pivotable about an associated pivot axis, which lies in a plane running perpendicularly to the direction of movement of said prestretching means, each of said prestretching levers has a stretching arm extending downward, and a stop arm extending upward, and on the inner surface of said hold-down device a guiding projection is formed opposite to a corresponding one of said prestretching levers, said stretching arm sliding on said guiding projection during movement of said prestretching means into said mold cavity in such a way that said stretching arm is pivoted outward.  
      Preferably a contact element with a convex contact area which comes into contact with the film of plastic when there is movement of the prestretching mechanism into the mold cavity is arranged at the free end of the stretching arm.  
      Advantageously the mold cavity has a polygonal cross section, and a prestretching lever is respectively assigned to a corner of the mold cavity.  
      In a preferred embodiment the mold cavity has a rectangular cross section.  
      In the case of the thermoforming tool according to the invention, the film of plastic is stretched outward from the bottom region in a specific manner by the prestretching lever and is available for side wall distribution. A movement into a respective corner of a molded part of polygonal cross section allows sufficient material to be brought into the corner for this region of the molded part also to have the necessary wall thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      An exemplary embodiment of the invention is explained in more detail below on the basis of drawings, in which:  
       FIG. 1  shows a longitudinal section through a mold device of a thermoforming tool according to the invention, in the region of two prestretching levers, a prestretching mechanism being located in a starting position;  
       FIG. 2  shows the mold device from  FIG. 1 , the prestretching mechanism being located in an intermediate position;  
       FIG. 3  shows the mold device from  FIG. 1 , the prestretching mechanism being located in an end position;  
       FIG. 4  shows the mold device from  FIG. 1 , the prestretching mechanism being located in an end position and molding air being introduced. 
    
    
     DESCRIPTION OF THE BACKGROUND ART  
      A thermoforming tool  10  has an upper tool  6  and a lower tool  8 , which are movable in relation to each other in order to clamp a film of plastic  14  between them. The thermoforming tool  10  comprises a number of mold devices  11 , only one of which is shown in FIGS.  1  to  4 . The mold devices  11  serve in each case for forming a molded part of a rectangular cross section.  
      The mold device  11  comprises a pot-shaped, downwardly open hold-down device  16 , which is arranged in the upper tool  6  and has a substantially rectangular cross section perpendicularly to the plane of the drawing. A prestretching mechanism  18  is movably arranged coaxially inside the hold-down device  16 . The prestretching mechanism  18  comprises a likewise cross-sectionally substantially rectangular carrier body  17 , which is connected on its upper side to a prestretching rod  20  which passes through the upper end plate  19  of the hold-down device  16  and is movable coaxially in relation to the hold-down device  16  by a drive (not shown).  
      Inserted in the lower tool  8  underneath the hold-down device  16  is a mold insert  12 , in which there is formed a cross-sectionally substantially rectangular mold cavity  13  for the forming of a molded part, which is bounded in the downward direction by a mold base  15 , which is fastened in a known way to an ejector rod (not shown).  
      Formed in the lower region of each of the four corners of the carrier body  17  are recesses  21 ,  22 , which are triangular when seen in longitudinal section. A prestretching lever  24 ,  26  is respectively mounted pivotably about a pivot axis  25  in these recesses  21 ,  22 . A total of four prestretching levers  24 ,  26  are provided, only two of which are shown in FIGS.  1  to  4 .  
      Each prestretching lever  24 ,  26  has a stop arm  28 , which extends upward with respect to the pivot axis  25 , and also a prestretching arm  30 , which extends downward beyond the carrier body  17  and to the lower free end of which a spherical contact element  32  is attached. The prestretching levers  24 ,  26  are respectively biased, for example by a torsion spring, in such a way that the stop arm  28  bears with its outer free end against the inside wall of the hold-down device  16 .  
      In the starting position shown in  FIG. 1 , in which the carrier body  17  is adjacent the end plate of the hold-down device  16 , the contact elements  32  are arranged symmetrically in relation to each other with very little distance between them and are located just above the film of plastic  14 . On the inner side of the side wall of the hold-down device  16 , in the lower region, guiding projections  34  are respectively arranged lying opposite the corresponding prestretching lever  24 ,  26 . The height of the guiding projections  34  increases steadily at first in the direction of the lower tool  8 . This is followed by an end portion of constant height.  
      For the molding of a molded part, the carrier body  17  is moved downward by means of the drive and the prestretching rod  20 , so that the contact elements  32  come into contact with the film of plastic  14 . As this happens, the stop arms  28  slide along the guiding projections  34 . On account of the slope of the guiding projections  34 , the stop arms  28  are moved inward, and consequently the stretching arms  30  with the contact elements are moved downward. The prestretching levers  24 ,  26  are thereby pivoted uniformly and, in the lowermost end position, shown in  FIG. 3 , the contact elements  32  are in the direct proximity of the corresponding corner of the mold cavity  13 .  
      On account of the the downwardly and outwardly directed movement of the contact elements  32 , the film of plastic  14  is stretched uniformly in the side regions and in the bottom region. The contact area of the contact elements  32  is relatively small, so that the film of plastic  14  is not prematurely cooled in the bottom region. The film bears well against the convex contact area. Sufficient material gets into the corners of the mold cavity  13  for this region of the molded part also to have the necessary wall thickness.  
      Subsequently, forming air is introduced into the mold cavity  13  in a known way, so that the film of plastic  14  comes to bear against the inside wall of the mold attachment  12  and against the mold base  15 . The film of plastic  14  cools down and becomes dimensionally stable. Then the upper tool  6  and the lower tool  8  are moved away from each other and the prestretching mechanism  18  is retracted into the hold-down device  16  again, into the starting position shown in  FIG. 1 . At the same time, the molded part that has been produced is ejected by moving out the mold base  15  fastened to the ejector rod.