WO2000015412A1

WO2000015412A1 - An ultrasound unit

Info

Publication number: WO2000015412A1
Application number: PCT/SE1999/001392
Authority: WO
Inventors: Ulf Lindblad
Original assignee: Tetra Laval Holdings & Finance S.A.
Priority date: 1998-09-11
Filing date: 1999-08-18
Publication date: 2000-03-23
Also published as: SE511004C2; AU6375599A; JP2002524253A; SE9803091L; SE9803091D0

Abstract

The disclosure relates to an ultrasound unit with an annular tool (4) or sonotrode intended for welding packaging containers of irregular or hexagonal cross section. The tool has a flange-shaped projection (8) with a polygonal working surface (5) extending around the tool. The mass of the projection (8) is less than 20 per cent of the total mass of the tool (4) and, as a result, in practice does not affect the dissipation of the ultrasonic waves in the tool in any negative manner.

Description

AN ULTRASOUND UNIT

TECHNICAL FIELD

The present invention relates to an ultrasound unit of the type disclosed in the preamble to appended Claim 1.

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.

A closed seal around the circumference of, for example, a cylindrical packaging container is described in USPS 3.438.824 which illustrates a unit 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. 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.

Any possible deviations from the rotation-symmetrical configuration of the annular tool also entail negatively acting irregularities in the oscillation pattern and, in prior art applications, use has therefore been made of annular tools displaying wholly rotation-symmetrical configuration and with a substantially cylindrical or gently conical working surface facing towards the centre of the annulus. Specific measures have also been implemented in the mechanical connection between the ultrasound source/ supply section and the annular tool (PCT application PCT/ IB98/ 00897).

Since it is desirable within the packaging technology to manufacture packaging containers displaying other cross-sectional configurations than round or gently conical, there is a need in the art to realise an ultrasound unit with not only annular tools but also, for example, polygonal (e.g. hexagonal or octagonal) or oval cross-sectional configurations in order, for instance, to be able to weld bottom walls in hexagonally shaped packaging bodies. The term "polygonal" is here taken also to signify the more or less rounded transitional forms which are conceivable between a polygonal and a circular-cylindrical configuration. Nor are excessively "sharp" corners in practice desirable, since this occasions excessively high tension in the tool of the ultrasound horn.

OBJECTS OF THE INVENTION

One object of the present invention is therefore to realise an ultrasound unit with an annular tool which has an irregular, e.g. oval or polygonal working surface. A further object of the present invention is to realise an annular tool for an ultrasound unit, the tool, despite having an irregular, e.g. polygonal working surface, not placing demands on the specific adaptation either of the frequency of the ultrasound source or the material in the tool.

Yet a further object of the present invention is to realise an annular tool for an ultrasound unit which, despite freedom of choice as regards the configuration of the working surface, makes it possible, both efficiently and without undue losses, to transfer 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 finally to realise an annular tool with an irregular working surface, the tool being of uncomplicated configuration, being simple to manufacture and not being subjected to excessive stresses in connection with ultrasound welding.

SOLUTION The above an 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 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 polygonal working surface of the tool on a projection whose mass constitutes only a minor proportion of the total mass of the tool, it will be possible to realise, for example, an oval or polygonal working surface without appreciably changing the annular form of the tool. As a result, welding of, e.g. hexagonal objects, may be carried out without the previously experienced drawbacks occurring.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One preferred embodiment 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, a tool employed in the ultrasound unit of Fig. 1; and Fig.3 is a section through the tool according to Fig. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

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, hexagonal 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, the thermoplastic acting as an adhesive which permanently unites the included packaging container parts to one another in liquid-tight fashion. This technology is well-known from previously mentioned USPS 3.438.824 and 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 (horn or 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.

The preferred embodiment of the tool 4 according to the present invention which is shown in Figs. 2 and 3 is annular with a hexagonal working surface 5 which, however, may also be of other, more or less polygonal form (e.g. rounded triangle or square), oval, or be of other irregular configuration. Depending upon the frequency of the ultrasonic vibrations generated by the ultrasound source 2, the tool 4 is given a mean circumference 6 (Fig. 3) which corresponds to one ultrasonic wave length, i.e. one wave length of the ultrasound will "have room" along the mean circumference 6. The relevant length depends upon the material from which the tool 4 is manufactured, but, in a normally employed material, e.g. titanium, with an ultrasound source of standard type which generates ultrasonic vibrations of a frequency of 20 kHz, the mean diameter will be approx. 70 mm. Both the propagation and amplitude of the ultrasonic waves are thus dependant, on the one hand, on the mean circumference 6, and, on the other hand, the rotational-symmetry of the annular tool 4. The annular tool may naturally be made totally rotational-symmetric in those cases when the working surface is to be cylindrical, but in an irregular, e.g. polygonal working surface, the rotational-symmetry will unavoidably be disrupted. On condition that the symmetry is retained in other respects, e.g. between the upper and lower halves of the tool (Fig. 3, i.e. the sectors a and b located on either side of a mass plane 7 determined by the mass distribution and disposed at a right angle to the centre axis of the tool have equally large radial cross-sectional areas), it has proved that the presence of an asymmetric flange-shaped projection 8 (of relatively slight mass) located in the mass plane 7 for forming the polygonal working surface 5 in practice does not disrupt either the propagation or the amplitude of the ultrasonic waves. Naturally, the mass of the projection 8 is minimised as far as is possible, for example by ensuring, in a polygonal working surface, that the portions (the "corners") of the working surface 5 located most distally from the centre axis of the tool 4 coincide with the inner annular surface turned to face towards the centre.

Thanks to the described, substantially symmetrical construction of the annular tool 4, the propagation and amplitude of the ultrasonic waves will thus not be negatively affected by the fact that the working surface is polygonal. The projection 8 which supports the working surface 5 is of a mass which is negligible in relation to the total mass of the tool 4, and preferably amounts to only approx. 2-6 per cent of the total mass of the tool 4. It has, in practice, proved that the wave propagation in the tool 4 is not negatively affected if the mass of the projection 8 amounts to less than approx. 10 per cent of the total mass of the tool 4. In projections which comprise a maximum of approx. 10 per cent of the total mass of the tool, the disruptions in the wave propagation which are nevertheless occasioned by the asymmetry are negligible. As a result, in practice they do not affect the oscillation node of the annular tool. However, when the mass of the projection exceeds approx. 10 per cent, the disruptions become so great that the oscillation node is affected, leading to uneven wave propagation and consequential uneven welding or a total failure to weld at all.

Preferably, the formation of the tool 4 is symmetric about the mass plane 7, but it is also possible to permit a certain asymmetry, on condition that both 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 is of less rigidity, which balances out the otherwise uneven oscillation relationships between the two sectors located on either side of the mass plane 7. It is also essential that the supply section or booster 3 of the ultrasound source connects radially and in the mass plane 7, since otherwise the oscillation balance between the two sectors will be offset.

To the left in Fig. 3, ghosted lines indicate how the unit according to the present invention is intended to be employed for the sealing together of parts of packaging containers, preferably a hexagonal casing portion in the form of a sleeve 9 and an end wall 10 located at its one end. Both the sleeve and the end wall are preferably manufactured from laminated material comprising, e.g. a central or core layer of fibre material, for example paper, which is coated on either side with thermosealable material, e.g. a thermoplastic such as polyethylene. Fig. 3 also indicates how a counter mandrel or block 11 is utilised for compressing the one end of the sleeve 9 and an upwardly folded edge of the end wall 10 between the block and the working surface 5 of each respective tool 4 during the sealing operation.

On operation of the unit according to the present invention, the one end of the sleeve 9 is placed in the upper end of the annular tool 4 and with the outside of the end of the sleeve in contact with the working surface 5 facing towards the centre axis of the tool. The end wall 10 placed at the lower end of the sleeve 9 is urged with its upwardly folded edge against the inner surface of the sleeve with the aid of the block 11 which, as shown in Fig. 3, for example may be gently "conical" and thereby realise the desired compressive force on axial upward displacement as indicated by the arrow 12. The block 11 may also be of other known type, e.g. expandable and include a number of segments disposed closely adjacent one another which are urged radially outwards by means of a suitable power source, e.g. pneumatic. When the sleeve 9 and the end wall 10 have thus been placed in the correct position and, in adjacent parts, urged against one another with the aid of the block, the ultrasound source 2 is activated so that axial ultrasonic oscillations are propagated and amplified via the supply section or booster 3 and are transferred to and distributed around the annular tool 4. Since the mean circumference 6 located in the mass plane 7 is of a length which corresponds to one complete wave length, the annular tool 4 will, in its entirety, vibrate or pulse radially at a sufficient amplitude to heat the parts of the packaging container located between the working surface 5 and the block 11 to such a temperature that the surface layers of the packaging material consisting of thermoplastic fuse in the abutment surface between the sleeve 9 and the upwardly folded end of the end wall 10. 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 and the end wall. Hereafter, the block is removed axially and the packaging container parts may be taken out of the tool 4 together.

By ensuring, according to the present invention, that only a "negligible" proportion of the mass of the annular tool 4 is asymmetrically distributed around the otherwise rotationally symmetric, annular tool, the symmetry and balance around the mass plane important for the propagation of the ultrasonic waves is maintained, together with the retained, optimum mean circumference, it will thus be possible to seal polygonal objects, which has hitherto not been possible. As a result, the possibilities are expanded of employing ultrasound welding without necessitating expensive or impractical retro-constructions of either the ultrasound source or the tool proper.

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.

Claims

WHAT IS CLAIMED IS:

1. An ultrasound unit including an annular tool (4) and a supply section (3) which is connectable to an ultrasound source (2) for generating ultrasound at a predetermined wavelength, the tool (4) having a mass plane (7) located at a right angle to the centre axis of the tool and with a mean circumference (6) whose length is adapted to said wavelength, characterized in that it includes a flange-shaped projection (8) extending around the tool (4), with a working surface (5) and of a mass which is less than 20 per cent of the total mass of the tool (4).

2. The ultrasound unit as claimed in Claim 1, characterized in that the working surface (5) is turned to face towards the centre axis of the tool (4).

3. The ultrasound unit as claimed in any of Claims 1 to 2, characterized in that the projection (8) is located in the mass plane (7).

4. The ultrasound unit as claimed in any of Claims 1 to 3, characterized in that the supply section (3) connects radially to the tool (4) and in its mass plane (7).

5. The ultrasound unit as claimed in any of Claims 1 to 4, characterized in that the working surface is polygonal.

6. The ultrasound unit as claimed in Claim 5, characterized in that the portion of the working surface (5) located most distally from the centre axis substantially coincides with an annular surface on the tool (4) turned to face towards the centre axis.