Abstract:
A device ( 10 ) for welding thin-walled work pieces ( 52 ) such as e.g. food containers of foil, fibrous web or foil-fibrous web composites using ultrasound has at least two components, i.e. a sonotrode ( 12 ) and a counter tool ( 14 ) for the sonotrode ( 12 ). One component ( 12 ) has a working region ( 16 ) which faces a working region ( 18 ) of the other component ( 14 ). The working region ( 18 ) of one component ( 14 ) comprises at least one projection ( 41 ) and the working region ( 16 ) of the other component ( 12 ) comprises at least one recess ( 46 ) into which the projection ( 41 ) can engage during the welding process. In this manner the work piece is securely held without slippage.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns a device for welding thin-walled work pieces such as e.g. food containers made from foil, fibrous web, or foil-fibrous web composites using ultrasound, the device comprising at least two components, i.e. a sonotrode and a counter tool for the sonotrode, wherein one component comprises a working region which faces a working region of the other component, at least during the welding process. 
     A device of this type is known from DE 100 27 735 A1 (assigned to the assignee of the instant invention), which describes an ultrasound-welding device comprising a sonotrode and an opposing support (also called anvil). The sonotrode and the opposing support each have working edges between which the work piece to be welded is clamped during the welding process. 
     The conventional device is mainly used to produce containers for food such as e.g. milk, juices, etc. During welding, the maximum temperature is achieved in that region of the work piece to be welded where the amplitude and the mechanical pressure of the ultrasound waves introduced into the work piece reach their maximum. The sonotrode thereby couples the ultrasound energy into the work piece. 
     It is the object of the present invention to further develop a device of the above-mentioned type such that the welding seams produced thereby are of consistently high quality and density from work piece to work piece. 
     SUMMARY OF THE INVENTION 
     A device of the above-mentioned type achieves this object in that the working region of one component has at least one projection and the working region of the other component has at least one recess into which the projection can engage during the welding process. 
     In the inventive device, the sonotrode and the counter tool securely hold the work piece to be welded in a fixed position relative to the sonotrode and the counter tool during the welding process. This prevents the work piece from being displaced relative to the working regions of the sonotrode and/or the counter tools during the welding process, in a simple and inexpensive fashion. 
     Such a displacement of the work piece tends to occur; in particular, when the containers are filled with liquid, since during filling of the liquid into the containers in the filling device the container outer wall is always moistened with the liquid being filled which makes the surface of the work piece “slippery”. This reduces the friction between the work piece and the working regions of the sonotrode and the counter tool abutting the work piece, and has conventionally impeded secure holding of the work piece to be welded by the sonotrode and the opposing support. If the work piece is displaced during the welding process, the energy is coupled into a larger material region, which can produce a leaking welding seam. 
     During the welding process, the projection and the recess on the sonotrode or counter tool deform, clamp and thereby securely fix a region of the work piece directly adjacent to the region to be welded. Even for work pieces with smooth and/or slippery surfaces, reproducible welding seam quality is guaranteed. It is thereby clear that the dimensions of the projection and of the recess are matched such that a gap between the wall of the projection and the wall of the recess is produced in which the container wall can be received without being damaged. Additional tensioning and clamping means are not required for holding the work piece. This reduces the price of the inventive device and facilitates cleaning since fewer overall parts are required and no additional undercuts are produced. Since the work piece is safely held relative to the sonotrode and the counter tool, smooth welding lines can also be realized. Additional introduction of contours in the welding region is therefore not required. 
     Advantageous further developments of the invention are recited in the dependent claims. 
     In a first preferred embodiment of the inventive device, at least one projection is provided on the counter tool and at least one recess is provided on the sonotrode. This is advantageous for manufacture and produces optimum energy performance of the sonotrode in the region of the work piece to be welded. 
     A further development proposes that the working region of the counter tool has a cosmetic seam region and a sealing seam region and that the projection is disposed between the cosmetic seam region and sealing seam region. This arrangement provides the finished welded work piece with a particularly attractive visual appearance. At the same time, the region of the work piece, which is held by the sonotrode and the counter tool, is directly adjacent to the actual seam regions such that the welding quality of both seam regions is highly reproducible. 
     Advantageously, at least one projection is formed as a bridge, which extends along the longitudinal extension of one component, with at least one recess being formed as a groove which extends along the longitudinal extension of the other component and which is complementary to the bridge. The bridge and the groove preferably extend along the entire length of the sonotrode and the counter tool. This guarantees optimum clamping of the work piece between the sonotrode and the counter tool during welding. Alternatively, at least one pin may be provided instead of the bridge and/or at least one hole may be provided instead of the groove. 
     In a particularly advantageous fashion, the projection is disposed on a part which is separate from the rest of the component and which can be exchanged. This takes into consideration the fact that the projection is worn with time by the numerous clamping processes which such a welding device must perform. The inventive further development permits easy and inexpensive replacement of the projection without having to replace the entire component. This reliably prevents damage during the clamping process. 
     It may be advantageous to production when the projection is integrated in the component as a single piece. The stability of the projection is also particularly good in this further development of the inventive device. 
     In one embodiment of the inventive device, the projecting end of the projection and/or the edge of the recess has an insertion bevel. This insertion bevel(s) prevent(s) damage to the work piece to be processed when a region adjacent to the region to be welded is clamped between the sonotrode and the counter tool. 
     The invention also concerns a method for welding thin-walled work pieces through ultrasound such as e.g. food containers of foil, fibrous web or of foil-fibrous web composites, wherein the work piece is disposed between the working regions of two components, namely a sonotrode and a counter tool, wherein the working regions face each other at least during the welding process. 
     To guarantee the quality of the welding seam between the two work pieces which is produced by this method, i.e. such that it is reproducible, the invention proposes loading and deforming a region of the work piece during the welding process using a projection which is provided on one component and which engages in a recess provided on the opposite component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Embodiments of the invention are explained below with reference to the enclosed drawing. 
     FIG. 1 shows a perspective view of a device for welding thin-walled work pieces using ultrasound and having a sonotrode and a counter tool; 
     FIG. 2 shows a perspective exploded view of the counter tool of FIG. 1; 
     FIG. 3 shows a perspective view of the sonotrode of FIG. 1; 
     FIG. 4 shows a cross-section through the sonotrode of FIG.  3  and the counter tool of FIG. 2 in the installed position with an interposed region of a work piece to be welded; 
     FIG. 5 shows a top view onto an alternative embodiment of a counter tool; 
     FIG. 6 shows a sectional representation along the line VI—VI of FIG. 5; 
     FIG. 7 shows a detailed illustration VII of FIG. 6; 
     FIG. 8 shows a side view of an alternative embodiment of a sonotrode; and 
     FIG. 9 shows a detail IX of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a device for welding thin-walled work pieces, designated in its entirety with reference numeral  10 . This device welds e.g. food containers that are filled with a liquid, via ultrasound and thereby tightly seals them. Towards this end, the device  10  has a sonotrode  12  and a counter tool  14  which is also referred to as an “anvil”. The sonotrode  12  comprises a working region  16 , and the anvil  14  has a working region  18 . Both working regions are flat and face each other. The exact configuration of the working regions  16  and  18  of sonotrode  12  and anvil  14  is explained in more detail with reference to FIGS. 2 through 4. 
     The anvil  14  comprises a basic body  20 , which has an overall approximately rectangular, longitudinal structure. When installed, its side facing the sonotrode  12  has a first opposing support surface  22 , which is in total flat and slightly raised. It is limited by a side surface  24  which extends at a right angle to the opposing support surface  22  and which is part of a step  26 . 
     A sheet metal plate  28  is inserted into the step  26  and positioned on the basic body via pins  30 . In the installed position, a region of the sheet metal plate  28  flatly abuts the side surface  24  of the basic body  20  of the anvil  14 . The pins  30  are introduced into corresponding bores  32  and  34  of the side surface  24  and of the sheet metal plate  28 . FIG. 4 shows that, when installed, the sheet metal plate  28  protrudes slightly past the first opposing support surface  22  to form a projection  41 . 
     A strip-like element  36  is disposed on the side of the sheet metal plate  28 . It is fastened by screws  38  in threaded bores in the side surface  24  of the basic body  20 . In this fashion, the sheet metal plate  28  is clamped between the strip-like element  36  and the basic body  20 . The strip-like element  36  has a second opposing support surface  40  which also faces the sonotrode  12  when installed but which, in contrast to the first opposing support surface  22 , is curved in a convex manner. The axis of curvature is thereby parallel to the longitudinal axis of the anvil  14 . The projection  41  of the sheet metal plate  28  protrudes past the “highest” point of the second opposing support surface  40 . The highest point of the second opposing support surface  40  is slightly offset with respect to the plane of the first opposing support surface  22  (approximately 0.1 mm). 
     The sonotrode  12  has a central region  42  which, when installed, tapers towards the working region  16  facing the anvil  14 . The working region  16  of the sonotrode  12  is delimited towards the anvil  14  by a completely flat working surface  44 . It contains a rectangular groove  46  which is slightly offset relative to its central axis, and which extends in the longitudinal direction. Each edge of the groove  46  adjacent to the working surface  44  of the sonotrode  12  has one insertion bevel  48 . Analog thereto, the projecting end of the projection  41  of the sheet metal plate  28  has corresponding bevels  50 . The width of the groove  46  is slightly larger than the thickness of the projection  41 . Moreover, the groove  46  is deeper than the projection of the sheet metal plate  28  past the opposing support surfaces  22  or  40  i.e. deeper than the height of the projection  41 . 
     The device  10  works as follows: The working region  16  of the sonotrode  12  is initially separated from the working region  18  of the anvil  14 . A region  52  of a container (not shown in detail), which is filled with food, is introduced into the gap between the sonotrode  12  and the anvil  14 . It consists of two closely adjacent layers  54  and  56  of a foil, a fibrous web or a foil-fibrous web composite. The cavity (not shown), which receives the food, is located in FIG. 4 below the region  52 . 
     The anvil  14  and the sonotrode are moved towards each other (arrows  58 ) until the region  52  is clamped between the sonotrode  12  and the anvil  14 . The projection  41  on the sheet metal plate  28  thereby presses a region  57  of the region  52  of the food container into the groove  46 , which is thereby deformed in a crimped manner. The gap between the projection  41  and the wall of the groove  46  (i.e. the “play” between the projection  41  and the groove  46 ) is sufficiently large that the material of the region  57  is not damaged. In this fashion, the region  52  is securely fixed relative to the anvil  14  and the sonotrode  12  even when its outer side is possibly contaminated by liquid food. 
     When the sonotrode  12  is oscillated with a frequency in the ultrasound region, the zones of the work piece loaded with pressure experience an extreme temperature increase. The two layers  54  and  56  thereby melt in the region of the opposing support surfaces  22  and  40 . A so-called “cosmetic seam” is produced in that region which is clamped between the working surface  44  of the sonotrode and the first opposing support surface  22  of the anvil  14 . The actual sealing seam is produced in that region that is clamped between the working surface  44  and the second opposing support surface  40 . After welding, the anvil  14  and the sonotrode  12  are withdrawn in opposition to the direction of the arrows  58 . The region  52  of the food container is released and the welded container can be removed from the device  10 . 
     FIGS. 5 through 7 show an alternative embodiment of an anvil  14 . Parts and regions that have equivalent functions to parts and regions of the above-described anvil  14  have the same reference numerals and are not explained again in further detail. 
     In contrast to the anvil  14  shown in FIGS. 3 through 4, the anvil  14  shown in FIGS. 5 through 7 has only one single opposing support surface  40 . It has zigzagged elevations  60  which are disposed in a row. 
     During operation, it simultaneously produces a sealing seam and a cosmetic seam. Instead of a separate sheet metal plate, the anvil  14  shown in FIGS. 5 through 7, has a bridge  41  integral with the basic body  20  of the anvil  14 . Moreover, a coolant channel  62  extends through the basic body  20 . 
     FIGS. 8 and 9 show an alternative embodiment of a sonotrode  12 . In this case as well, those parts and regions having equivalent functions to the parts and regions of the above-described sonotrode  12  have the same reference numerals. The sonotrode  12  shown in FIGS. 8 and 9 primarily differs with regard to the position of the groove  46 : It is disposed opposite to the longitudinal axis of the sonotrode  12 , in a central, non-eccentric manner.