MXPA96002488A - Method for regulating resonance frequencies of interferen oscilation modes - Google Patents

Abstract

The present invention relates to a method for regulating resonance frequencies of interferent oscillation modes in ultrasound sealing units, of the type having a centrally placed drive equipment, surrounded by reaction bodies, and a tip with a sealing surface , narrow, elongated, characterized in that symmetrically placed slots are provided in the reaction bodies, these slots are placed where the interferent oscillation modes have a maximum potential or alternatively a maximum displacement

Description

METHOD FOR REGULATING FREQUENCIES OF RESONANCE OF INTERFERING OSCILLATION MODES FIELD OF THE INVENTION The present invention relates to a method for regulating resonance frequencies of interferent oscillation modes in ultrasonic sealing units of the type having a centrally positioned drive equipment, surrounded by reaction bodies, and a tip with a sealing surface, narrow, elongated BACKGROUND OF THE INVENTION It is generally known to employ ultrasound units for sealing purposes. This method can also be used for sealing in filling machines of the type that pack liquid foods into disposable single-use type packagings. In the filling machine, a spool of packaging material is formed in a tube with a seam or longitudinal, overlapping seam. The packaging material may consist of a paper or cardboard core to which different thermoplastic layers are laminated, and possibly an aluminum foil. The spool of packaging material formed in a tube is filled with its proposed contents, and the material tube is sealed transversely in the regularly periodic space. The tube filled with the liquid is subsequently separated in the transverse seals to form individual packages. The ultrasonic sealing unit described in Swedish Patent Application No. SE 9300918-1 is particularly adapted for use in the cross-seam sealing of the tube of material in a filling machine of the type described above. The sealing unit has an elongated and narrow sealing surface whose width corresponds to the width of the junction seal, transverse. In order to make a cross seal sufficiently long, a number of sealing units must be constructed together to form a complete, composite sealing equipment unit. This ultrasound sealing equipment with a complex geometry must operate with a fundamental oscillation mode, defined, at a given working frequency. This oscillation mode must be able to be excited by the drive unit of the sealing unit, and in order to be able to obtain a uniform amplitude and high enough for the sealing, there must not be an interfering oscillation mode placed too close to the working frequency of the oscillation mode, fundamental. In the production of these ultrasound sealing units, there may be asymmetries in the material, or asymmetries may occur in the processing of the material. These asymmetries can occur in up to thirty percent of the units produced. These asymmetries cause the interferent oscillation modes at resonance frequencies close to the working frequency, which contribute to a non-uniform amplitude, which in turn result in a non-uniform sealing. additionally they cause fatigue and unfavorable stress on the units, which are a cooperative factor in reducing the service life of these units. The degree of efficiency of the sealing units is further reduced by the interferent oscillation modes.

OBJECTS OF THE INVENTION It is an object of the present invention to realize a method for regulating the resonance frequencies of the interferent oscillation modes that cause the aforementioned inconveniences. Using the method of the present invention, it is possible to move the resonance frequencies of the interfering oscillation modes, so that the oscillation modes no longer constitute an interference element. Still a further object of the present invention is to ensure that one hundred percent of the sealing units produced are operative and exhibit the desired production.

SOLUTION These and other objects according to the present invention have been achieved since the method of the type described by way of introduction has been given; the characteristic aspect is that symmetrically placed slots are provided in the reaction bodies, the slots that are placed where the interferent oscillation modes exhibit a maximum potential, or alternatively, a maximum displacement. Additionally, preferred embodiments of the invention have been given; the characteristic aspects that are as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention will now be described in greater detail below, with particular reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an ultrasonic sealing unit for which the method according to the present invention is proposed; Figure 2 shows a sealing unit according to the present invention; Figure 3 shows a symmetrical reflection of an oscillation; Y Figure 4 shows an antisymmetric reflection of an oscillation.

DESCRIPTION OF THE PREFERRED MODALITY Figure 1 is a schematic diagram of an ultrasonic sealing unit 1 for which the method according to the present invention is proposed.

This ultrasound unit 1 is only half a wavelength long, since the space available in the filling machines, in which it is to be used, is very limited. This is so that the sealing unit 1 can be used in the existing filling machines without the need for it to be redesigned and rebuilt to any considerable degree. The filling machine is proposed to pack liquid foods in single-use disposable packages. A spool of packaging material consisting of a rolled product of paper or carbon, different thermoplastic layers that possibly an aluminum sheet is formed in the filling machines in a tube that is filled with the compound contents. The coil of material is sealed transversely through the liquid tube and the coil of material is separated in the transverse seals to form individual packages. It is for the cross sealing operations that this sealing unit 1 has been developed. The unit 1 of ultrasonic sealing of the Figure 1 consists of a centrally positioned actuating device 2 which is connected to a DC source. (not shown). The drive equipment 2 is centrally positioned at the point of intersection of the two planes of symmetry (see Figure 2) into which the sealing unit 1 can be divided. The drive equipment 2 is surrounded by symmetrically placed reaction bodies 3. The sealing unit 1 further consists of a tip 4 which is terminated by a narrow, elongated sealing surface 5. The width of the sealing surface 5 corresponds to the width of the cross seals. To achieve the length of the cross seals, a number of sealing units 1, described above, must be combined together to form a complete unit of ultrasonic sealing equipment. The nodal plane 6 of the sealed unit, ie a plane where the amplitude of unit 1 is zero, is to be found between the tip 4 and the reaction bodies 3. The nodal plane 6 is provided with an edge 7 of firm support and this is where unit 1 is fixed in the sealing machine. The purpose of the reaction bodies 3 is to cause counter-oscillations that compensate for the forces that occur when tip 4 oscillates. An ultrasonic sealing unit 1 having a complex geometry as described above, can oscillate in a large number of different oscillation modes, and each oscillation mode has a specific resonance frequency. In order for unit 1 to be able to operate satisfactorily, a fundamental, specific oscillation mode, or work mode with a defined working frequency must be isolated from the remaining oscillation modes. This working mode is to give a uniform amplitude all along the sealing surface 5, complete, a sufficiently high amplitude to obtain a uniform and reliable seal, and a well defined nodal plane 6, so that the unit 1 can fix on the edge 7 of firm support. It should not cause unduly high stresses on the material or construction, and it should be possible for the drive equipment 2 to excite this working mode. By using the FEM calculations (Finite Element Method) it is possible to determine the appearance of all the different oscillation modes that occur in a sealing unit 1, as well as locate the resonance frequencies of the oscillation modes. The sealing unit 1 has two planes of symmetry 8 as shown in Figure 2, and in both of these planes, symmetric reflections S may occur, as illustrated in Figure 3, and antisymmetric reflections A, as shown in FIG. Figure 4. As a result of the two planes of symmetry 8, four different cases, SS, AS, SA and AA, can occur in this way. Both Figure 3 and Figure 4 show greatly simplified examples of the oscillatory movement of the reaction bodies. A sealing unit 1 can have up to 35 different oscillation modes between 0 and 40 KHz. The fundamental oscillation mode or work mode is of the SS type. This will favor and isolate as far as possible from the other modes of oscillation. In particular, other oscillation modes of the SS type must be maintained at a distance of at least + 5 KHz. To ensure that the sealing is achieved using an ultrasonic sealing unit 1, verification must be maintained in all oscillation modes occurring at + _ 10 KHz of the working mode frequency. Because asymmetries may occur in the material of the sealing unit 1 or may appear when it is processed, oscillation modes of the AS, SA and AA types can be generated. These oscillation modes will cause interference since they occur at + 1 KHz of the working mode. The closer they are, the more interference they cause. These interferent oscillation modes give a non-uniform amplitude, which results in a non-uniform seal. In the worst case, the interfering oscillation mode may become completely dominant over the working mode and sealing will not occur at all. The interfering oscillation modes additionally cause unfavorable stresses on the material and construction, which shortens the service life of the sealing unit 1. As a result of these interfering oscillation modes, the sealing unit 1 will be less efficient due to the energy losses that dissipate as heat. An oscillation mode has several potential maximums and several displacement maximums. In order to be able to move the resonance frequency of an interfering oscillation mode, a maximum must be selected that does not coincide with any potential maximum or displacement of the oscillation, fundamental or working mode. The fundamental oscillation mode should preferably have a minimum of potential or displacement where the modification takes place. In order to avoid these interfering oscillation modes, it is possible, according to the present invention, to regulate the resonance frequencies of the interfering oscillation modes, so as not to cause interference for a longer time. This is done by providing the grooves 9 in the reaction bodies 3 in the sealing unit 1. These slots 9 must be placed and formed symmetrically. Slots 9 are placed where there is a maximum potential, ie a maximum of bending of the unwanted oscillation mode, at the same time as the working mode should not change the character or be shifted away from the working frequency. Alternatively, the grooves or bevels placed symmetrically are made where the interferent oscillation modes exhibit a maximum displacement. Also in this case, the fundamental oscillation mode should not change the character or be shifted away from the working frequency. The fundamental oscillation mode retains its character and frequency of resonance, if the change is made where the fundamental oscillation mode exhibits a slight potential and smaller displacements. The height, width and length of the slots 9 control the travel distance. If there are several interferent oscillation modes, it may be necessary to provide the slots 9 on each side, for each oscillation mode. Alternatively, a maximum of potential or a maximum of displacement for several different modes of interferent oscillation may coincide, and then this makes it possible to provide only one common slot 9 on each side for the oscillation modes. By providing the slots 9 in the reaction bodies 3 where a specific oscillation mode has a maximum potential, the rigidity of the mode is reduced. The distribution of mass and rigidity in a body is decisive for a body to assume the modes of oscillation. It can be expressed such that a specific oscillation mode has a given rigidity and mass inertia. The areas with a maximum potential constitute the rigidity of the oscillation mode and the areas with a maximum displacement constitute the mass inertia of the oscillation mode. If the rigidity of an oscillation mode is reduced, its resonance frequency is reduced and if the rigidity is increased, its resonance frequency is increased. Correspondingly, the reduced mass inertia for an oscillation mode causes its resonance frequency, and an increased mass inertia to cause a reduced resonance frequency. It is an advantage of the present case to move the resonance frequencies of the interfering oscillation modes to a lower level, since the working frequency of the fundamental oscillation mode can be increased up to 1 KHz in the appropriate working phase. Since it is a relatively simple measure to reduce the stiffness in relation to the increase in mass, this alternative will be preferred. At the same time, as the interfering oscillation modes are regulated, a check must be made to ensure that no other oscillation mode is affected so that it may interfere, by providing the slots 9 in the actuating bodies 3. which are put into effect according to the invention. It is vital that these measures are carried out, that is to say the production of the grooves 9, as slight as possible while maintaining the regulation effect. As will have been apparent from the above description, the present invention performs a method for regulating the interfering resonance frequencies in the ultrasonic sealing units 1, so that the sealing unit 1 can give a junctional seal, transverse, uniform and reliable. The method also helps in reducing unfavorable stresses and fatigue in the material and construction of the sealing unit 1. The present invention should not be construed as restricted to that described above and shown in the drawings, many modifications which are conceivable without departing from the spirit and scope of the appended claims.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, the content of the following is claimed as property:

Claims (5)

1. A method for regulating resonance frequencies of intermittent oscillation modes in ultrasonic sealing units, of the type having centrally positioned drive equipment, surrounded by reaction bodies, and a tip with a narrow, elongated sealing surface, characterized in that grooves symmetrically placed in the reaction bodies are provided, these grooves being placed where the interferent oscillation modes have a maximum potential or alternatively a maximum displacement.

2. The method according to claim 1, characterized in that the distance of movement of the frequencies depends on the height, depth and length of the slot.

3. The method according to claim 1, characterized in that the regulation of a resonance frequency involves a movement of an interfering oscillation mode whose rigidity is reduced to perform the movement.

4. The method according to any one of the preceding claims, characterized in that the interfering resonance frequencies move downwards in the frequency range.

5. The method according to any of the preceding claims, characterized in that the interfering resonance frequencies move at least 1 KHz.