Patent Publication Number: US-2004047939-A1

Title: Thermoforming tool

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates to an improved thermoforming device, and more particularly to a thermoforming device having a forming tool in which there is at least one mold opening and means with which a plastically deformable sheet material can be brought into contact with the innerwall of the mold opening. A first parting tool which is connected to a first support element has at least one first parting edge extending around the rim of the mold opening. A second parting tool is connected to a second support element and has at least a second parting edge which cooperates with the first parting edge on the first parting tool. At least one parting tool is made from a different material than its associated support element.  
       [0003] 2. Description of the Prior Art  
       [0004] A thermoforming tool of the type with which this invention is concerned is known for producing cup-shaped or lid-shaped end products from a thermoplastically deformable sheet. To that end, the sheet is first pressed into the mold opening in the forming tool, with a die-like prestretcher tool, and then the sheet is brought with compressed air into contact against the inner wall of the mold opening, and finally it is cooled down.  
       [0005] To enable separating the unmolded, cooled-down cup or lid from the remaining sheet, a cutting or stamping device is provided. It includes a cutting edge, which extends around the rim of the mold opening and is embodied on a cutting die plate. On the other side of the sheet, facing the cutting die plate, there is a cutting plate in which there are recesses that are complementary to the cutting edges of the cutting die plate and that are defined by cutting edges. When the cutting plate is moved toward the cutting die plate, the corresponding cutting edges cooperate to shear the finished cup from the remaining sheet.  
       [0006] In the known thermoforming tool, the support elements to which the cutting plate or cutting die plate are fastened are made of lightweight metal, such as aluminum. The cutting plate and the cutting die plate are each made of steel.  
       [0007] Because different materials are used for the parting tool and the corresponding support element, the optimal material for each function can be selected. For the parting tool, for reasons of stability and wear, a comparatively dense, wear-resistant material is used. On the other hand, by using a lightweight metal for the support element, the total weight of the entire structure comprising the parting tool and the support element can be reduced. This in turn makes it possible to achieve faster cycle times.  
       [0008] The reason for this is the fact that during and after the production process, the forming tool and the corresponding support element have to be moved in order to eject the finished workpieces. The lighter the corresponding structure is in weight, the faster this motion can be accomplished, and with less exertion of force. Furthermore, the bearings are also less heavily loaded because of the lower weight, so that either simpler, less-expensive bearings can be used, or the service life of the apparatus is lengthened. Moreover, the total structure, comprising the parting tool and the support element, comes to a stop more quickly after a movement because of the lower weight, which is also favorable for the cycle times.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0009] The object of the present invention is to refine a thermoforming tool of the type described above such that with it, the separation of the end product from the remaining sheet can be done even more reliably. This object is attained in that the first parting tool is not supported or fastened rigidly relative to the first support element, and/or the second parting tool is not supported or fastened rigidly relative to the second support element.  
       [0010] The novel thermoforming tool of the invention makes good, reliable separating action of the parting tool possible, since the exact relative positioning of the cooperating parting edges of the parting tools can be assured. This is attained by means of the aforementioned nonrigid support or fastening. The term “nonrigid” is understood to mean that one parting tool can expand (or shrink) thermally relative to its associated support element and thus without bulging, for instance, or warping. A nonrigid support or fastening of this kind can for instance comprise a floating support.  
       [0011] To enable the finished end product to be separated reliably from the remaining sheet, high dimensional stability of the relative positions of the cooperating parting edges of the parting tools is required. According to the invention, it has been found that despite careful calibration in the factory, this dimensional stability was not always assured. It was discovered that one cause of this was that a thermoforming tool is exposed to a different temperature at the usage site, for instance, from the temperature at its production site. This temperature difference, in structures made of different materials with different coefficients of thermal expansion, can lead to different thermal expansions.  
       [0012] It has furthermore been ascertained according to the invention that such different thermal expansions, in the previously conventional rigid fixation of the parting tool to the support element, can lead to an uneven deformation of the parting tool, such as bulging or warping, which causes shifting in the relative positions of the parting edges. Even permanent distortion of the parting tool relative to the support element can sometimes occur, so that the original relative positions of the parting edges are not regained even after a suitable temperature equalization.  
       [0013] All this is prevented by the nonrigid support or fastening provided according to the invention in which the parting tool can expand unhindered in response to temperature relative to the support element. Uneven deformation is thus precluded. If both parting tools are nonrigidly supported or fastened relative to their support elements, then upon a temperature-caused expansion (on the condition that both parting tools have substantially the same temperature), a change in the absolute positions of the parting edges is possible without producing any change, or at least any significant change, in the relative positions of the parting edges.  
       [0014] If only one of the two parting tools is nonrigidly supported or fastened relative to the corresponding support element, then it can likewise expand unhindered, and after a suitable temperature equalization, it can also return unhindered to the position set exactly at the production site under production conditions. Permanent distortion and an attendant irreversible deformation of the parting tool is prevented reliably by the nonrigid support. In this case—for a suitable “standard temperature”—the parting edge on the parting tool is always in the optimal absolute position intended at the factory, even when different materials are used, because of the nonrigid support or fastening.  
       [0015] In a first refinement of this invention, the nonrigid support or mounting includes a plurality of bending elements which connect the first and/or second parting tool to the first and second support element, respectively, which bending elements are designed such that they are deformed upon a defined maximum temperature-dictated relative motion between the first parting tool and the first support element, and the second parting tool and the second support element, respectively. Such bending elements enable very exact positioning of the parting tool on the support element and nevertheless permit the distortion-free relative motion, provided according to the invention, of the parting tool relative to the support element. Additional self-centering exists when the bending elements are designed such that they deform only elastically.  
       [0016] In a further embodiment, at least one of the bending elements includes a screw and a threaded portion, which threaded portion has a bending portion into which the screw is screwed. A bending element of this kind can be produced inexpensively.  
       [0017] It is also advantageous if the nonrigid support or fastening includes a fixation device, by which the first parting tool is fixed to the first support element and/or the second parting tool is fixed to the second support element, in each case at least translationally and approximately rigidly at some point. A fixation device of this kind assures a place on the parting tool and on the support element whose relative position remains unchanged. Such a place makes possible very accurate, replicable calibration of the parting tool relative to the support element.  
       [0018] The thermoforming tool of the invention is especially inexpensive to construct if the fixation device includes a fixation bolt. Moreover, such a bolt can be fabricated very exactly and introduced into correspondingly exactly dimensioned bores in the support element or in the parting tool. As an alternative to this, it is also possible that the fixation device includes a spot weld (or some other one-piece connection) between the parting tool and the support element.  
       [0019] In an advantageous feature of the novel thermoforming tool, it is also proposed that the nonrigid support or fastening includes at least one guide device by which the first parting tool and/or the second parting tool is guided movably relative to the first support element and the second support element, respectively, translationally along a respective guide axis. By means of a guide device of this kind as well, exact, distortion-free positioning of the parting tool relative to the support element is made possible.  
       [0020] Such a guide device is especially simple to produce if it includes a groove, which is engaged by a corresponding guide element.  
       [0021] Exact positioning of the parting tool relative to the support element, optionally even without a fixation device, is possible whenever two guide devices are provided, whose guide axes are at a right angle to one another. Above all in this case, negative pressure and/or a magnetic force can for instance be used to cause the parting tool to “stick” to its support element.  
       [0022] If a fixation device is provided, it must be located on the guide axis of the guide device, or on the guide axes of the guide devices. The angular position of the parting tool relative to the support element is defined by the guide device or guide devices, while conversely the fixation device defines the translational position of the parting tool relative to the support element.  
       [0023] It is also proposed that at least one low-friction sliding layer is located between one parting tool and the associated support element. This facilitates the distortion-free motion of the parting tool.  
       [0024] In a refinement of this, it is proposed that at least one substrate material is present between one parting tool and the associated support element and has a low-friction sliding layer on both sides. This construction is stable and makes both manufacture and maintenance easier.  
       [0025] Especially when two guide devices but no fixation device are provided, it is preferred that at least one parting tool is urged against the associated support element by negative pressure or a magnetic force. This facilitates the mounting and removal of the parting tool. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0026] The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which:  
     [0027]FIG. 1 is a fragmentary section through a first exemplary embodiment of a thermoforming tool with an upper part and a lower part;  
     [0028]FIG. 2 shows a detail II of the thermoforming tool of FIG. 1;  
     [0029]FIG. 3 is a schematic perspective view of a support element and a parting tool of the lower part of the thermoforming tool of FIG. 1;  
     [0030]FIG. 4 is a schematic fragmentary section taken along the line IV-IV in FIG. 3;  
     [0031]FIG. 5 is an enlarged view of one region of FIG. 4;  
     [0032]FIG. 6 is a partly cutaway view of one region of a second exemplary embodiment of a thermoforming tool;  
     [0033]FIG. 7 shows a detail VII of the thermoforming tool of FIG. 6;  
     [0034]FIG. 8 is a view similar to FIG. 4 of a third exemplary embodiment of a thermoforming tool; and  
     [0035]FIG. 9 is a view similar to FIG. 8 of a fourth exemplary embodiment of a thermoforming tool. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0036] A thermoforming tool identified overall by reference numeral  10  in FIG. 1 includes an upper part  12  and a lower part  14 , which can be moved toward and away from one another by a device not shown in the drawing. The upper part  12  includes an upper support element  16 , which is made entirely of lightweight metal such as aluminum. An upper bracing structure  18  and an upper cooling block  20  are part of the upper support element  16 . An upper stamping plate  22 , which is made from steel, is fastened to the upper cooling block  20  of the upper support element  16 .  
     [0037] The upper cooling block  20  is penetrated by coolant conduits, not visible in FIG. 1. A plurality of recesses  24  on the order of blind bores are also present in it, and in each of them there is one cup-shaped insert  26 , which in operation of the apparatus  10  has a holding-down function. In the position of repose shown in FIG. 1, one cylindrical prestretcher  28 , which is rounded on its end toward the lower part  14 , is accommodated in each cup-shaped insert  26 . The prestretcher  28  is fastened to a push rod  30 , which can be moved longitudinally via a device not shown in FIG. 1.  
     [0038] The upper stamping plate  22  has circular openings  32 , which are aligned with the outer contour of the cup-shaped inserts and are each surrounded by a respective annular collar  34  (see also FIG. 2). The radially inner free edges of the annular collars  34  are each embodied as an upper parting edge  36 . The upper parting edge  36  protrudes somewhat axially past the free edge of the cup-shaped insert  26 .  
     [0039] The lower part  14  of the thermoforming tool  10  is constructed similarly to the upper part  12 . It includes a lower support element  38 , here also preferably of aluminum, which likewise includes a lower bracing structure  40  and a lower cooling block  42 . Once again, the lower cooling block  42  is penetrated by coolant conduits, not visible in the drawing. A lower stamping plate  44  of steel is fastened to the top of the lower cooling block  42 .  
     [0040] Recesses  46 , each approximately aligned with one another, are provided in both the lower cooling block  42  and the lower stamping plate  44 , and a forming insert  48  is inserted into each and centered in the lower stamping plate  44  by the corresponding recesses  46 . In the position of repose, shown in FIG. 1, of the thermoforming tool  10 , there is an ejection plate  50  in the region of the bottom of a forming insert  48 ; the ejection plate is fastened to a push rod  52 , which can be moved in its longitudinal direction by a suitable device, not shown in the drawing.  
     [0041] The forming insert  48  defines a mold opening  54 , which corresponds approximately to the outer contour of a workpiece to be produced. The lower stamping plate  44 , on its side toward the upper stamping plate  22 , has lower parting edges  56  extending around the rim of the mold openings  54  or forming inserts  48 . As will be described in further detail hereinafter, these lower parting edges  56  cooperate with the upper parting edges  36  on the upper stamping plate  22 .  
     [0042] The thermoforming tool  10  shown in FIGS. 1 and 2 is used to produce plastic cups from a thermoplastically deformable plastic sheet. It is operated as follows:  
     [0043] First, there is an interstice between the upper part  12  and the lower part  14 . A sheet of a thermoplastically deformable plastic material (not shown) is passed through this interstice; the sheet has been preheated to its forming temperature of, say approximately 100 (although sheets that have a processing temperature of from 10 to 150° C. can be used). With the sheet stationary, the prestretchers  28  are moved out of the upper part  12  and into the facing mold openings  54  in the lower part  14  as indicated in FIG. 1 by dot-dashed lines. As a result, the plastic sheet is pressed into the mold openings  54  and already comes partly into contact with the inner wall of the corresponding forming inserts  48 .  
     [0044] Via compressed-air conduits, not shown in FIGS. 1 and 2, the plastic sheet is additionally acted upon by compressed air and as a result is brought into full contact with the inner wall of a forming insert  48 . The plastic sheet resting on the inner wall of a forming insert  48  sets as a result of the cooling of the forming insert  48  by means of the cooling block  42 . The prestretcher  28  is now moved back into the position of repose shown in FIG. 1.  
     [0045] To separate the completed cup from the remaining sheet, the upper part  12  is moved against the lower part  14 . In the process, the upper parting edges  36  on the upper stamping plate  22  cooperate with the lower parting edges  56  on the lower stamping plate  44  and separate the finished plastic cups, located in the mold openings  54 , from the remaining sheet. Depending on the design of the parting edges  36  and  56 , this can involve a stamping or a cutting operation.  
     [0046] The upper part  12  is now moved away from the lower part  14  again, so that the parting edges  36  and  56  move free of one another again. Then the lower part  14  is pivoted about a substantially horizontal axis (not shown), and the finished plastic cups are expelled from the mold openings  54  by the corresponding ejection plates  50  into a collecting or stacking container.  
     [0047] To allow the separation of the finished plastic cups from the remaining sheet to proceed quickly and without damage to a finished plastic cup, the parting edges  36  and  56  must be positioned very precisely relative to one another. The gap between the two parting edges  36  and  56  must be very uniform and should typically be in the range of 10 μm. Particularly for a plastic sheet of polypropylene, which has only comparatively little brittleness, clean separation of the finished plastic cup from the remaining sheet cannot be assured otherwise.  
     [0048] For this reason, a very precise positioning of the upper stamping plate  22  on the upper support element  16  and of the lower stamping plate  44  on the lower support element  38 , as well as a very precise positioning of the support elements  16  and  38  relative to one another, are done in the factory. This high-precision relative positioning must not be impaired either by shipping or in operation of the thermoforming tool  10 .  
     [0049] To assure that, it must be taken into account that the support elements  16  and  38 , which in the present exemplary embodiment are also made of aluminum, have a different thermal expansion behavior than the stamping plates  22  and  44  that are made of steel. So that the stamping plates  22  and  44  will not become distorted (bulging or warping, for instance) relative to the respective support elements  16  and  38  if the thermoforming tool  10  is exposed to a different temperature than during the calibration in the factory, the stamping plates  22  and  44  are nonrigidly supported relative to the respective support elements  16  and  38 . This is understood to mean that a stamping plate  22  or  44  can expand or shrink in response to heat unhindered relative to its associated support element  16  and  38 , respectively, and that upon a return to the initial temperature, it can return to its original outset position unhindered as well. Various possibilities for this kind of nonrigid support are shown as examples in FIGS.  3 - 8 .  
     [0050] In FIG. 3, the lower part  14  of the forming tool  10  is shown only schematically. The lower support element  38  can be seen, which includes the lower bracing structure  40  and the lower cooling block  42 , and the lower stamping plate  44 , nonrigidly supported on the lower cooling block  42 , can be seen as well. In the exemplary embodiment shown in FIG. 3, the nonrigid support includes a fixation device, which is realized in the form of a central fixation bolt  58  (see FIG. 4). By means of this central fixation bolt  58 , the lower stamping plate  44  is fixed translationally and approximately rigidly at a point relative to the lower cooling block  42 . However, beginning at the central fixation bolt  58 , the lower stamping plate  44  can expand unhindered relative to the lower cooling block  42  along two axes  60  and  62 , which are at a right angle to one another and pass through the central fixation bolt  58 . The corresponding possibilities of motion are represented by the arrows  64  and  66  in FIG. 3.  
     [0051] As can be seen from FIGS. 4 and 5, the nonrigid support, shown in FIG. 3, includes a plurality of pinlike bending elements  68   a - 68   d.  These connect the lower stamping plate  44  to the lower cooling block  42 . They are so stiff that high-precision positioning of the lower stamping plate  44  relative to the lower cooling block  42  is assured.  
     [0052] At the same time, however, they are also designed such that at a defined maximum temperature-dictated relative motion between the lower stamping plate  44  and the lower cooling block  42  of the lower support element  38 , they deform only elastically and not plastically. A correspondingly deformed state is shown in dashed lines in FIG. 5.  
     [0053] By means of the nonrigid support, shown in FIGS.  3 - 5 , of the lower stamping plate  44  on the lower cooling block  42  of the lower support element  38 , the lower stamping plate  44  cannot become distorted upon a relative motion, caused by a temperature change, relative to the cooling block  42  of the lower support element  38 . Moreover, the relative motion between the lower stamping plate  44  and the lower cooling block  42  returns to approximately zero again, and the geometric conditions established at the factory are restored, as soon as the lower part  14  of the thermoforming tool  10  has returned to the “standard temperature” specified at the factory.  
     [0054] It will be understood that the nonrigid support, shown in FIGS.  3 - 5  for only one stamping plate, can be provided analogously for the other stamping plate as well. This is correspondingly true for the exemplary embodiments described below. It will also be understood that a nonrigid support would also be possible if even only one of the bending elements  68 , without the fixation bolt  58 , were used.  
     [0055] In FIGS. 6 and 7, an alternative and especially preferred embodiment of a bending element  68  is shown, with which the upper stamping plate  22  is fastened to the upper cooling block  20  of the upper support element  16 . Those elements and regions of FIGS. 6 and 7 that have functions equivalent to elements and regions of the exemplary embodiment shown in FIGS.  1 - 5  are identified by the same reference numerals. They will not be explained again in detail.  
     [0056] The fastening of the upper stamping plate  22  to the cooling block  20  is effected, in the thermoforming tool  10  shown in FIGS. 6 and 7, by means of a plurality of screws  70 , whose head  72  is braced on a shoulder  74  of a stepped through bore  76  in the upper stamping plate  22  (see FIG. 7). A threaded insert  80  is screwed into a blind threaded bore  78  in the cooling block  20 . The end region, toward the stamping plate  22 , of this insert is spaced apart from the wall of the blind threaded bore  78  and forms a collarlike bending portion  82 .  
     [0057] In the threaded insert  80 , there is a comparatively short threaded bore  84 , into which the screw  70  is screwed. The threaded bore  84  and the connection between the screw  70  and the threaded insert  80  are essentially located in the region of the bending portion  82 . If upon a temperature change, because of the different coefficients of thermal expansion, a relative motion occurs between the stamping plate  22  and the cooling block  20 , then the bending portion  82  of the threaded insert  80 , and with it the screw  70 , can bend elastically, thereby preventing a deformation of the stamping plate  22 .  
     [0058] In FIG. 8, yet another version of a nonrigid support between the upper stamping plate  22  and the upper cooling block  20  is shown. Once again, those elements and regions that have equivalent functions to elements and regions of the exemplary embodiments shown in FIGS.  1 - 7  are identified by the same reference numerals and will not be explained again in detail.  
     [0059] In the exemplary embodiment of a thermoforming tool  10  shown in FIG. 8, the upper stamping plate  22  is fixed relative to the upper cooling block  20  at a point by a spot weld  58 . However, a guide device  86  is additionally present, by which the upper stamping plate  22  is guided translationally and movably relative to the upper cooling block  20  along a guide axis  60  (see FIG. 3) located in the plane of the drawing. The guide device  86  includes a sliding block  88 , which is fastened to the upper stamping plate  22  and which is engaged by a suitably complementary guide element  90  that is fastened to the cooling block  20 . In an axis perpendicular to the plane of the drawing in FIG. 8, there is a further guide device, which enables a motion of the stamping plate  22  along a guide axis  62  (see FIG. 3). The guide axes  60  and  62  are thus at a right angle to one another and pass through the welding point  58 .  
     [0060] If a guide device as in the exemplary embodiment shown in FIG. 8, which has two guide axes orthogonal to one another, is used, then it is optionally possible to dispense with a rigid fixation device. The adhesion between the two elements can then be accomplished for instance by means of negative pressure and/or a magnetic force instead.  
     [0061] In FIG. 9, yet another version of a nonrigid support between the upper stamping plate  22  and the upper cooling block  20  is shown. Once again, those elements and regions that have equivalent functions to elements and regions of the exemplary embodiments shown in FIGS.  1 - 8  are identified by the same reference numerals and will not be explained again in detail.  
     [0062] In FIG. 9, a thin substrate material  92  (for instance a thin metal sheet) is disposed between the stamping plate  22  and the upper cooling block  20 ; it is provided on both sides with a low-friction slide coating  94  and  96 , respectively, for instance of Teflon. As a result, sliding between the stamping plate  22  and the upper cooling block  20  with very low friction is made possible, with at the same time only a slight spacing between the two elements. As in the above exemplary embodiments already, it is naturally possible for the teaching of FIG. 9 to be applied to the conditions in the lower part  14  of the thermoforming tool  10 .  
     [0063] The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.