BACKGROUND ART Ultrasonic vibrations within the frequency range of 15-50 kHz are nowadays employed industrially for a plurality of purposes, for example the welding of different types of materials. The welding of plastic material is a common task within the packaging industry, and ultrasound welding is therefore finding increasingly wider fields of application in the manufacture of different types of packaging containers, not only of pure plastic film or plastic material, but also of various types of laminates which include outer layers of thermoplastic material. Originally, the ultrasound technique was employed exclusively for relatively simple, rectilinear welds of planar material, but progress in this art has entailed that ultrasound welding can now be utilised also for advanced welding, for example different combinations of materials and different types of non-linear welds in several dimensions.

A typical assembly or unit for ultrasound welding of the type which is utilised within the packaging industry comprises a converter or ultrasound source for generating ultrasound at the desired frequency. The ultrasound source may be of conventional type and include, for example, a piezoelectric crystal which is brought into oscillation by being connected to a suitable current source. Once the ultrasound source has thus catered for the conversion from electricity into mechanical reciprocating movement, this movement is normally transferred by direct contact between the ultrasound source and a supply section, a so-called booster, which, because of its geometric configuration, amplifies the amplitude of the mechanical movement so that it will be optimised for ultrasound welding of, for example, thermoplastic material. The nodal point of the booster is utilised so that the unit may be suspended in a frame with a minimum of transfer of vibrations to the frame. The one end of the supply section or booster is thus mechanically coupled to the ultrasound source, and the opposite end of the supply section or booster is similarly in mechanical abutment against a tool

or sonotrode which includes a working surface intended to be brought into contact with the material which is to be sealed. The working surface of the sonotrode may be linear, for example in the form of a straight line of limited length, or be curved, for example annular or circular in order, in the manufacture of, for instance, cylindrical packaging containers, to realise a closed seal line or seam which extends around the entire circumference of the packaging container. This is a common type of welding when a round packaging container is to be provided with end walls, or when the circular casing of a round packaging container is to be connected to a more or less conical upper portion. Naturally, a closed seal around the circumference of, for example, a cylindrical packaging container may be realised using known technology also employing a tool with a short, straight working surface, on condition that the sealing operation takes place in many stages during simultaneous rotation of the packaging container. However, this method is time-consuming and not always gives an adequate result, for which reason it is desirable to realise an annular tool with inner dimensions which substantially correspond to the outer dimensions of the packaging container at the welding site. With the aid of a counter mandrel or block located inside the packaging container which may, for example, be gently conical or expandable, the weldable parts of the packaging container are urged against the inner working surface of the annular tool, whereafter the ultrasound source is activated so that the tool vibrates with the desired welding frequency. This technique has been tested with limited results. For example USPS 3.438.824 discloses int. al. an assembly which utilises an annular tool which, via a supply section, is radially connected to a conventional ultrasound source which generates reciprocating oscillations which are axially transferable to the tool through the supply section. In an annular tool, the oscillations are propagated substantially uniformly around the circumference of the tool, in which event the axial ultrasonic waves reciprocating along the centre line of the supply section will, because of the annular form of the sonotrode, be converted into radial waves which reciprocate along the radii extending from the centre of the tool. The annular tool is, in such instance, dimensioned such that its mean circumference, i. e. the mean value of the outer and inner diameters of the tool, correspond to one wave length of the ultrasound. The movements of the ultrasound caused in the tool may be seen as compression and decompression waves which

alternatingly extend and shorten the annulus in its circumferential direction. In such instance, each individual point of the material of the tool will reciprocate radially, which results in the desired radial movement of the working surface. In such instance, it is of crucial importance that the circumference of the annular working surface is set to the pertinent wave length which is generated by the ultrasound source. More precisely, the working surface must display a mean diameter which has been selected such that exactly one wave length of the ultrasound is accommodated around the mean circumference of the tool. Since the oscillation pattern and frequency of the tool are, to some degree, also affected by the material from which the tool is manufactured (normally titanium), a certain adjustment of the mean diameter and mean circumference may be made by a suitable material selection. Naturally, it is also possible to manufacture ultrasound sources with varying ultrasonic frequencies, but normally the ultrasound sources are manufactured with certain standard frequencies which enjoy advantages as regards, for example, the degree of efficiency and the suppression of harmful audible sound. In this instance, 20 kHz is a common frequency. Because of the standard frequencies determined in practice and but limited possibilities of material selection in the tool, it will hence be necessary instead to adapt the diameter and the circumference of the objects which are to be welded or sealed to meet the possibilities which are available using current types of ultrasound units, which is clearly a disadvantage which limits freedom of choice as regards forming, for example, packaging containers. Thus, there is a general need in the art to realise an ultrasound unit which includes an annular tool with a working surface whose diameter and circumference may be simply adapted to the relevant working duty at hand, i. e. the circumference and diameter of the object which is to be welded.

OBJECTS OF THE INVENTION One object of the present invention is thus to realise an ultrasound unit including an annular tool which has a working surface whose circumferential length and diameter may, within certain limits, be freely selected in response to the working frequency of a connected ultrasound source.

A further object of the present invention is to realise an annular tool for an ultrasound unit, the tool-despite the possibility of varying the

circumferential length of the working surface-not placing any demands on specific adaptation either of the frequency of the ultrasound source or the material of the tool. Given an ultrasound source and working material (normal design and construction prerequisites), the present invention hereby affords greater freedom of choice than hitherto as regards the diameter of the package.

Yet a further object of the present invention is to realise an annular tool for an ultrasound unit, the tool making it possible-despite freedom of choice as regards the circumferential length and diameter of the working surface-to transfer, efficiently and without unnecessary losses, the oscillations from the ultrasound source to desired parts of a processed object, e. g. a packaging container.

Still a further object of the present invention is to realise an annular tool for an ultrasound unit, the tool having a working surface possessing a mean diameter which differs from the mean diameter of the tool.

Yet a further object of the present invention is finally to realise an annular tool for an ultrasound unit, the tool having uncomplicated configuration, being simple to manufacture and not being subjected to extensive stresses in connection with ultrasound welding.

SOLUTION The above and other objects have been attained according to the present invention in that an ultrasound unit of the type described by way of introduction has been given the characterizing features as set forth in the characterizing clause of appended Claim 1.

Preferred embodiments of the ultrasound unit according to the present invention have further been given the characterizing features as set forth in the appended subclaims.

ADVANTAGES By placing the annular working surface of the ultrasound unit on a projection whose mass constitutes but a minor proportion of the total mass of the tool, it will be possible to realise a working surface possessing a mean diameter which is greater than the mean diameter of the mean circumference (in a working surface facing towards the centre of the tool), and a working surface whose diameter is less than the mean diameter (in a working surface

which is turned to face away from the centre of the tool). As a result, the annular tool may be adapted to the circumference and diameter of the object to be welded, without incurring the drawbacks inherent in prior art technology.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Preferred embodiments of the ultrasound unit according to the present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings which show only those parts and details essential to an understanding of the present invention. In the accompanying Drawings: Fig. 1 is a schematic side elevation of an ultrasound unit according to the present invention; Fig. 2 shows, on a larger scale and in section, a section through a part of the tool in the ultrasound unit of Fig. 1; and Fig. 3 shows, on a larger scale and in section, a section through a second embodiment of the tool in the ultrasound unit of Fig. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS An ultrasound unit 1 according to the present invention is intended, in its preferred embodiment, for fusing or welding together packaging container parts displaying annular or circular cross section, which is a typical example of the practical application of ultrasound technology within the packaging industry. One precondition for the ultrasound technology to be usable for fusing or welding together packaging material is that at least one of the material layers included comprises a material which may be plastified by ultrasonic waves. In practice, packaging containers (in any event those which are intended for wholly or partly liqueform contents), often include layers of thermoplastic material, which is extremely suitable for ultrasound welding. The material layers which are to be united to one another must, in this instance, include a contact surface which contains thermoplastic material, e. g. polyethylene, which makes it possible, preferably after compression of the two material layers against the working surface of the welding unit with the aid of some form of abutment mandrel or block, to ultrasound-vibrate the material so that the thermoplastic layers are plastified and fuse together in order, on completed ultrasound heating, once again to

cool and harden in order permanently to unite the included packaging container parts to one another in liquid-tight fashion. This technique is well- known, e. g. from previously mentioned USPS 3.438.824, as well as PCT application PCT/IB98/00897, to which reference is made for further information and technical details.

Fig. 1 shows an ultrasound unit 1 according to the present invention.

The unit 1 includes an ultrasound source or converter 2 of known type which, by means of, for example, a piezoelectric crystal, converts electric current variations into mechanical movement in the form of reciprocating ultrasonic waves or vibrations which, in the described practical application, i. e. the welding of paper/plastic packages, typically have a frequency range of approx. 15-50 kHz, normally 20 kHz. The ultrasound source 2 which is connected in a per se known manner (not shown) to a current source, is also mechanically connected to a supply section or booster 3 for amplifying or converting the ultrasonic waves generated by the ultrasound source, the supply section 3 also defines the nodal point of the waves and is utilised in a conventional manner also for suspending the ultrasound unit 1 in a frame (not shown). The supply section 3 is thus, at its one end, mechanically connected to the ultrasound source 2 and the opposite end of the supply section 3 is mechanically connected to a closed or annular tool 4 (sonotrode) which, through its indirect mechanical connection with the ultrasound source 2, is drivable via the supply section 3 or booster so that the annular tool is subjected to radially directed ultrasonic vibrations throughout its entire circumference.

Two preferred embodiments of the tool 4 according to the present invention are shown in Figs. 2 and 3, where Fig. 2 illustrates a tool for external welding and Fig. 3 illustrates a tool adapted for internal welding.

The two tools 4'and 4"in Fig. 2 and Fig. 3, respectively, are annular and, as shown in Fig. 1, are connected via a radial supply section 3, to the conventional ultrasound source 2. Depending on the frequency of the ultrasonic vibrations generated by the ultrasound source 2, a mean diameter 5 for the tools 4 is selected which is determined by the mass distribution, the mean diameter being adapted such that the length of the mean circumference 6 corresponds to one ultrasonic wave length, i. e. one wave length of the ultrasound will"have room"along the mean circumference 6.

The pertinent length is dependent upon the material from which the tool 4 is

manufactured, but in a normally employed material, such as titanium, and with an ultrasound source of standard type which generates ultrasonic vibrations at a frequency of 20 kHz, the mean diameter will be approx. 80 mm. This entails that packaging containers which are welded using one of the prior art apparatuses must, under these conditions, have a maximum diameter of approx. 70 mm if a working surface at the inside of the tool 4 is utilised, and a diameter of at least approx. 90 mm if a working surface at the outside of the tool is employed. In both prior art constructions to which reference has been made herein (USPS 3.438.824 and PCT/IB98/00897), which both utilise a working surface at the inside of the tool 4, the packaging containers which are to be welded must thus have a maximum diameter of approx. 70 mm. If it is to be possible using this type of apparatus to weld packaging containers displaying greater or smaller diameter than 90 and 70 mm, respectively, either another material must be selected for the tool 4 or an ultrasound source of a different frequency must be employed. Both of these alternatives entail disadvantages. The commonly employed material for tools for ultrasound welding (sonotrodes) is titanium, which has superior properties as regards the transmission of oscillations and its mechanical strength, and it is in practice difficult to find any other material which is correspondingly suitable for this type of practical application. The ultrasound sources which are used for ultrasound welding are normally of standard type and then operate most generally at a frequency of 20 kHz, which has proved to give good efficiency at the same time as frequencies lying within the audible range are avoided, which places lower requirements on screening-off in order to avoid harmful auditory effects. A reduction of the frequency below the accepted standard range places the ultrasonic oscillations within the audible range, and an increase of the frequency above the previously mentioned accepted standard range gives reduced efficiency, for which reason nor is the frequency of the employed ultrasound source readily adaptable so that ultrasound welding of packaging containers displaying other diameters than those previously employed can be readily put into effect. In both of the prior art methods to which reference has been made, a conventional annular tool is thus employed with an inner cylindrical or conical working surface and a mean diameter 5 which substantially is located centrally between the inner and outer surfaces, respectively of the

tool. This design of annular tools for ultrasound welding has hitherto universally prevailed.

In order to make it possible, without changing the standard frequency of the ultrasound source, and without replacement of the material in the annular tool, to vary the diameter of the objects, e. g. packaging containers, which are to be welded, the present invention calls for the employment of annular tools in which the working surfaces have been moved or displaced from the customary positions that have hitherto been employed. This makes it possible to depart from the previously universally prevailing principle of placing the working surface centrally about the symmetry or mass plane determined by the mass distribution of the supply section, and as a result the mean diameter located in the mass plane will no longer constitute a limitation in the selection of the diameter of the annular working surface.

It will be apparent from both Fig. 2 and Fig. 3 how a radial section through the annular tool in both embodiments has been given a cross sectional configuration which no longer coincides with the customary, substantially rectangular or square cross sectional configuration. As a result, it will be possible to dispose the working surfaces 7 in an unconventional manner in relation to the mean circumference 6, as will be described in greater detail hereinbelow with particular reference to each respective embodiment.

While both of the embodiments of the annular supply section or booster 3 according to the invention illustrated in Figs. 2 and 3 are intended for annular welding, the embodiment according to Fig. 2 is intended for external welding, while the embodiment according to Fig. 3 is intended for internal welding of, for example, packaging containers. Corresponding parts in both of the embodiments have been given identical reference numerals, apart from that, when necessary, a supplementary'symbol has been employed to indicate reference numerals relating to the firsts embodiment (Fig. 2), and a supplementary"symbol has been employed to indicate reference numerals relating to the second embodiment (Fig. 3).

In each one of the illustrated embodiments of the tool 4 according to the present invention, there is thus an annular projection 15 with a preferably cylindrical working surface 7 which, in the first embodiment intended for external welding, is turned to face towards the centre axis 8 of the tool, and, in the second embodiment intended for internal welding, is

turned to face away from the centre axis 8 of the tool. The working surface 7 is substantially centred about a working plane 9 which extends at a right angle to the centre axis 8 of each respective tool. Both of the embodiments of the supply section 3 also include a symmetry or mass plane 10 which is located a distance from the working plane 9 and which is similarly oriented at a 90° angle to the centre axis 8 and is placed in the axial direction such that the radial section through the annular tool shown in the Figures is divided into two sectors, namely an upper sector A and a lower sector B, which are in equilibrium in relation to one another as regards surface (mass) and geometric configuration (decisive for the rigidity of the annular supply section). In the two illustrated, preferred embodiments of the supply section according to the invention, the cross sectional sectors A and B, respectively, on either side of the mass plane 10 both have identical configurations and identical surfaces (mass), which is to be preferred since it results in a uniform, symmetric wave propagation in the supply section. In asymmetric design, for example, if the sector A has a smaller surface than the sector B, the sector B must, for the sake of equilibrium, instead have a geometric configuration which gives this part of the annular supply section a greater rigidity and thus compensates for the imbalance occasioned by the smaller surface (mass) of the sector A. In other words, both of the sectors A and B axially discrete and separate from one another by the mass plane 10 must mutually be balanced in respect of a factor which is determined by both the mass of the sector and the rigidity of the sector determined by the geometric configuration.

Thanks to the above-described, substantially symmetrical construction of the annular tool 4, the propagation and amplitude of the ultrasonic waves are not negatively affected by the fact that the working surface is not centred around the mass plane, i. e. by the axial distance between the mass plane and the working plane. The projection 15 which, in both embodiments, supports the working surface 7, has a mass which is negligible in relation to the total mass of the tool 4, and preferably amounts to only approx. 10 per cent of the total mass of the tool 4. It has practice proved that the wave propagation in the mass plane 10 is not negatively affected by the distance between the mass plane 10 and the working plane 9 if the mass of the projection 15 amounts to less than approx. 20 per cent of the total mass of the tool 4. Preferably, the tool 4 is designed to be symmetric

about the mass plane 10, but it is also possible to permit a certain asymmetry on condition that both of the sectors A and B are mutually balanced in respect of mass and rigidity. Greater mass in the one sector thus requires that the opposite sector has less rigidity, which balances out the otherwise uneven oscillation relationships between the two sectors located on either side of the mass plane 10. It is also crucial that the supply section 3 of the ultrasound source connects in the mass plane 10, since otherwise the oscillation balance between the two sectors is displaced.

Figs. 2 and 3 indicate how both of the embodiments of the apparatus according to the present invention are intended to be employed for sealing together parts of the packaging container, preferably a cylindrical or conical jacket or casing part in the form of a sleeve 11 and an end wall 12 located at one end of the sleeve. Both the sleeve 11 and the end wall 12 are preferably manufactured from laminated material comprising, for example, a central core layer of fibre material, e. g. paper, which is coated on either side with thermosealable material, e. g. a thermoplastic such as polyethylene. In both of the Figures 2 and 3, it is also indicated how an abutment mandrel or block 13 is utilised in order, during the sealing operation, to compress one end of the sleeve 11 and an upwardly folded edge of the end wall 12 between the block and the working surface 7 of each respective tool.

On operation of the first embodiment of the apparatus according to the present invention which is illustrated in Fig. 2, the one end of the cylindrical sleeve 11 is placed in the upper end of the annular tool 4 and with the outside of the sleeve end in contact with the similarly cylindrical working surface 7. The end wall 12 placed at the lower end of the sleeve 11 is urged with its upwardly folded edge against the inner surface of the sleeve with the aid of the block 13 which, as is shown in Fig. 2, for example may be gently conical and thereby realise the desired compressive force on axial downward displacement, which is indicated by means of the arrow 14. The block 13'may also be of any other known type, for example expandable, and include a number of closely adjacent segments which are urged radially outwards with the aid of a suitable power source, e. g. a pneumatic source.

When the sleeve 11 and the end wall 12 have thus been placed in the correct position and in adjacent parts urged against one another with the aid of the block 13', the ultrasound source 2 is activated so that axial ultrasonic oscillations are propagated and amplified via the supply section 3 and

transferred to and distributed in the annular tool 4. Since the mean circumference 6 located in the mass plane 10 and determined by the mean diameter 5 is of a length which corresponds to one whole wave length, the annular tool 4 will, in its entirety, vibrate or pulse radially with a sufficient amplitude to heat the parts of the packaging container located between the working surface 7 and the block 13 to such a temperature that the surface layers of thermoplastic of the packaging material fuse together in the abutment surface between the sleeve 11 and the upwardly folded edge of the end wall 12. After the desired heating time, the supply of current to the ultrasound source 2 is discontinued, whereupon the vibrations cease and the temperature of the molten thermoplastic layers falls until such time as the fused layers harden and form a liquid-tight seal between the lower end of the sleeve 11 and the end wall 12. Hereafter, the block 13 is removed axially upwards and the packaging container parts may be jointly removed out of the tool 4.

The work cycle in the second embodiment of the apparatus according to the present invention illustrated in Fig. 3 is similar, with the difference that the contact between the tool 4"and the packaging container whose sleeve 11 and end wall 12 are to be fused or welded together takes place at the inside of the end wall, i. e. by contact between the folded edge of the end wall 12 and the working surface 7"located on the outside of the tool 4. More precisely, the end wall 12 is placed over the upper end of the tool 4 so that the end wall rests against it and the downwardly folded edge of the end wall abuts against the external working surface 7". Thereafter, the cylindrical sleeve 11 of the packaging container is displaced downwards until such time as its lower edge arrives flush with the lower end of the downwardly folded edge region of the end wall 12, whereafter an external block 13"compresses the overlapping parts of the sleeve 11 and the end wall 12 during simultaneous urging against the working surface. The block 13 may, for example, consist of a ring divided into sectors which is mechanically urged radially in a direction towards the centre axis 8 of the tool 4. In this position, the ultrasound source 2 is activated so that ultrasonic waves are propagated via the supply section 3 and distributed in the tool 4 whose radial vibrations heat up the mutually abutting thermoplastic layers so that the sleeve 11 and the end wall 12 are fused or welded together and, after cooling of the heated, united thermoplastic layers, form a liquid-tight seal at the bottom end of the

packaging container. Hereafter, the block 13"is removed and the packaging container is lifted axially upwards and released from the tool 4.

By displacing the working plane in accordance with the present invention axially in relation to the mass plane, while maintaining the balance around the mass plane which is important for the propagation of the ultrasonic waves, and with retained, optimum mean circumference, it will thus be possible to seal or weld objects of different diameters, which has previously not been possible, being prevented by the more or less given mean circumference resulting from the substantially coinciding mass and working planes. As a result, the possibilities of employing ultrasound welding are broadened without necessitating expensive or impractical retro- constructions and re-designing either of the ultrasound source or of the tool.

The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being conceivable without departing from the scope of the appended Claims.