Ultrasonic welding device

An ultrasonic welding device includes a sonotrode emitting vibratory energy and an anvil having a work face cooperating with the sonotrode for supporting a workpiece positioned for welding between the sonotrode and the anvil. The anvil has a housing and a plurality of laminae individually shiftably supported in the housing in a face-to-face relationship for forming a lamina stack. Each lamina has an edge face at a side of the lamina stack; the edge faces together constitute the work face of the anvil. There is also provided a force exerting arrangement for imparting individually to each lamina a force oriented towards the sonotrode for individually urging each lamina outwardly of the housing in the direction toward the sonotrode.

BACKGROUND OF THE INVENTION 
This invention relates to an ultrasonic welding device, particularly for 
welding together superposed films and comprises an elongated energy 
concentrating horn (hereafter "sonotrode"). 
Swiss Pat. No. 613,170 discloses vibration welding for welding a cardboard 
coated with a thermoplastic material. In this arrangement mechanical 
energy is transformed to local heat energy in order to reduce the required 
heat and energy quantities. Further, U.S. Pat. No. 3,526,792 discloses an 
ultrasonic transducer which comprises a converter for changing electrical 
energy into mechanical energy and a sonotrode, by means of which the 
mechanical energy is concentrated as an oscillating energy with a 
predetermined frequency of oscillation. In operation, the sonotrode 
cooperates with an anvil whereby the coated parts are pressed together 
between the work face of the sonotrode and the work face of the anvil. The 
oscillation of the sonotrode generates in the compressed workpiece 
frictional heat which is sufficient to activate the thermoplastic layers 
in such a manner that the thermoplastic material of the layers flow 
together. Upon stopping the oscillation, the thermoplastic material cools 
and solidifies and, as a result, the superposed layers are bonded to one 
another. 
U.S. Pat. No. 4,279,675 describes an application of the above-outlined 
welding method in the packaging industry. The anvil is, at the edge of its 
work face and in a central portion thereof, provided with a cutout to 
accommodate the increased thicknesses of superimposed cardboard portions. 
Between the cutouts there are provided longitudinally arranged grooves to 
allow the liquefied thermoplastic material to be forced aside at the 
crests between adjoining grooves and to allow it to gather in the grooves 
to form a weld. 
It has been found that the above-outlined arrangement is insufficient in 
case there are multilayer portions at some locations as it is the case, 
for example, in longitudinal seams of tubular foil wrappers. Thus, there 
may be locations which remain unwelded which would cause air leaks in an 
evacuated package. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved welding device of 
the above-outlined type from which the described disadvantages are 
eliminated and wherein an airtight weld is ensured independently from the 
number of layers to be welded to one another. 
This object and others to become apparent as the specification progresses, 
are accomplished by the invention, according to which, briefly stated, the 
anvil of the welding device is formed of a plurality of individual, 
relatively displaceably supported laminae whose end faces constitute a 
counter support for the sonotrode and which are each exposed to a force 
directed toward the sonotrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to FIGS. 1 and 2, a sonotrode 10 of a conventional ultrasonic 
welding device has a work face 11 which engages a work face 23 of an anvil 
generally designated at 20. The latter is formed of a plurality of 
face-to-face arranged individual laminae 21 which are accommodated in a 
housing 24 formed of two side walls 24a, 24b, a bottom wall 24c, a front 
wall 24d and a rear wall 24e. The two side walls 24a and 24b have, at 
their top, longitudinally extending respective support flanges 25a, 25b 
which serve as retaining supports for the laminae 21. Within the housing 
24 there are arranged a support base 26 and a pressure wall 27 which, by 
means of a screw 28a may be urged against the stack of laminae 21 for 
pressing the lamina stack together. Each lamina 21 engages, with an edge 
portion 29 which is remote from the work face 23, a cushion 22 of elastic 
material positioned on the support base 26. The latter and thus the bias 
exerted by the cushion 22 may be adjusted by two further setscrews 28b and 
28c. 
The mass of the individual laminae 21 is so designed that dynamic reaction 
forces of the oscillations can be taken up thereby. Thus, the laminae may 
be made of 1 mm thick brass sheets and for a given width of the work face 
23 of 10 mm, the overall height and width of one lamina may be 45 mm and 
40 mm, respectively. The cushion 22 may be a rubber insert having a 
rebound hardness of 40-45 Shore A. The length of the work face 23 of the 
lamina stack is in the order of magnitude of 65 mm. 
It is thinkable that the laminae 21 clamped by the pressing wall 27 by 
means of setting a pressing force with the aid of setscrews 28a may loosen 
because of temperature fluctuations. In order to avoid such an undesirable 
occurrence, between the pressing wall 27 and the lamina stack there is 
provided a spring element 32 which may be an elastic cushion and which 
compensates for such a slack. 
Tests with an anvil 20 formed of laminae as described above have shown that 
in case of a herringbone seam or a transverse fold comprising four layers 
of material each, a gastight weld could be obtained independently whether 
the outer layer of the material was paper or a thermoplastic material. 
FIGS. 3, 4 and 5 show three further preferred embodiments of the invention 
as an alternative to the resilient cushion 22 of the embodiment described 
in connection with FIGS. 1 and 2. In these three embodiments, a housing 
identical to the housing 24 may find application and also, a slack 
compensator spring element 32 may used. 
Turning now specifically to FIG. 3, a resilient displacement of the 
individual laminae 30 is effected by separate arcuate springs 31, 
associated with each laminae 30 and engaging the bottom edge of the 
laminae 30 and the bottom surface of the housing 24. 
The embodiment illustrated in FIG. 4 is in principle similar to that 
described in connection with FIG. 3 except that instead of a separate 
spring, each lamina 40 is provided with resilient legs 41 and 42 which 
form an integral, one-piece part with the lamina 40. 
Turning now to FIG. 5, each lamina 50 is provided with two elongated 
cutouts (slots) 51 and 52 which are spaced from one another and which 
extend from opposite side edges of the lamina parallel to the bottom edge 
54 which is opposite from the work face 53. The slots 51 and 52 are in an 
overlapping relationship approximately along two-thirds of their length. 
Turning now to the embodiment illustrated in FIG. 6, there is shown an 
angle profile 60 and a counter support rod 61 which are held together by 
means of two lateral walls (not shown) and which serve for supporting a 
plurality of laminae 62 each having a work face 63 and a cutout 64 for 
forming a support portion for a resilient positioning of the work face 63. 
Each lamina 62 has an edge face 65 which is opposite the work face 63 and 
which bounds the resilient support portion of the lamina. A slack 
compensator indentical to component 32 of the FIG. 2 embodiment may be 
included in the construction of the FIG. 6 arrangement. 
It is a common characteristic of all embodiments that each individual 
lamina is capable of setting itself--by virtue of its individual resilient 
support--to the thickness of the material to be welded. In this manner, a 
uniform counter pressure is exerted in the direction of the sonotrode 
along the entire seam length so that the material of the foil wrapper can 
be uniformly welded together independently from the localized thicknesses 
of longitudinal seams, pinch folds, etc. 
In order to ensure that the laminae may shift with respect to one another 
with ease in order to render effective solely the pressure exerted by the 
various resilient force generating arrangements, advantageously at least 
one of the two surfaces of adjoining laminae are provided with an 
anti-friction coating. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims. Such a coating may be, for example 
polytetrafluorethylene.