EP4731425A1

EP4731425A1 - Method for sealing of packaging material and related apparatus

Info

Publication number: EP4731425A1
Application number: EP24739740.9A
Authority: EP
Inventors: Eugenio Navacchia
Original assignee: GD SpA
Current assignee: GD SpA
Priority date: 2023-06-26
Filing date: 2024-05-23
Publication date: 2026-04-29
Also published as: IT202300013191A1; WO2025004116A1

Abstract

Apparatus (2) and method for sealing of packaging material (1 ), the apparatus (2) comprising a sealing element (3) comprising at least one electrically conductive winding (5), an actuation system structured for moving said sealing element (3) and a contrast element (6) for compressing at least one electrically conductive portion (50) of said packaging material (1 ) between said sealing element (3) and said contrast element (6); an electric energy generator (7) and a control unit (8) programmed and configured for performing the following method steps: - compressing the portion (50) of the packaging material (1 ); - applying an electric voltage (V) to opposite ends of the winding (5) for creating, a passage of electric current having at least one component variable over time; - determining a time trend (i) of an intensity of electric current passing in the winding (5); - dynamically adjusting the electric voltage (V) as a function of said time trend (i) of the intensity of electric current.

Description

DESCRIPTION

Title: METHOD FOR SEALING OF PACKAGING MATERIAL AND RELATED APPARATUS

Technical field of the invention

The present invention relates to a method, and a related apparatus, for sealing of packaging material, for example of packaging material from which containers at least partially made of aluminum are produced.

State of art

For the transport, the storage, and the preservation of foods, typically of a liquid or semiliquid nature (milk, fruit juices, sauces, etc.) and personal care products (bath foam, shampoo, conditioner, mouthwash, etc.), it is known to use containers made from a packaging material.

Typically, to make the aforesaid containers, the packaging material, in form of a continuous planar tapes, is initially folded to assume a continuous tubular shape and subjected to an initial longitudinal seal. The packaging material in tubular shape is then filled with the desired product.

Subsequently, in correspondence of a sealing station, it is provided to clamp at regular intervals portions of the packaging material in tubular shape and to realize transverse seals in correspondence of these clamped portions to realize the individual containers. Summary of the invention

The term "dynamically" referring to an operation, such as a step of the method according to the present invention, is intended to mean that such operation is performed (e.g. iteratively over time) only in response to conditions occurring concurrently with the execution of a given operating cycle to which the operation belongs. In other words, the operation is not performed based on time trends, process parameters, preset and/or selected upstream of the operating cycle. However, this definition does not exclude that such operation may occur over two or more operating cycles to which the operation belongs, nor does it exclude that the same operation may be performed during factory adjustment.

The terms "perpendicular" and “orthogonal” respectively refer to a substantial perpendicularity and orthogonality between two elements, comprising both the ideal case in which such elements are arranged relative to each other to a right angle, and more frequent cases in which the two elements are arranged relative to each other to form an angle that deviates from the right angle (e.g., comprises in a range of ± 15°, more preferably ± 10°, relative to the right angle) but still negligibly or not relevantly affecting the operation of the present solution.

The terms “upstream”, “downstream”, refer to a direction of feeding the packaging material along an oriented feeding direction.

The term “pourable” refers to a fluid product lacking of a shape of its own (and thus assuming the shape of the object containing it), such as a liquid, powdery, granular, or semi-solid product.

In the context of the processes for producing containers from packaging material, particularly in the case of containers intended for food use, a fundamental phase is represented by the sealing of such containers, which must meet high-quality standards in order to preserve the integrity of the container and the relative content.

In this context, the sealing of the containers can be advantageously performed by exploiting the principle of electromagnetic induction to generate a localized thermal increase in correspondence of at least a portion of the container to be sealed. In summary, when a body, equipped with at least one electrically conductive portion, is subjected to a magnetic field, this magnetic field generates induced electric currents within the body, which, if the magnetic field is sufficiently intense, generate in turn a temperature increase in the body itself (typically due to Joule heating), which can be appropriately utilized to seal a portion of the body, for example of opposing flaps, typically in cooperation with a compressive force applied to the body.

To generate the aforementioned magnetic fields of desired intensity, it is advantageous to use a coil subjected to an electrical voltage difference to the respective ends for creating a passage, within the coil windings, of a variable (e.g. oscillating) electric current over time. For example, a resonant circuit comprising at least one inductor and one capacitor can be utilized. Such resonant circuits are characterized by a respective resonance frequency, typically dependent on the capacitance C of the capacitor and on the inductance L of the inductor.

In such resonant circuits, the application of a step (i.e. , a sudden change from a first to a second constant value over time) of an input forcing function to the circuit (e.g., the electrical voltage applied to the circuit), typically corresponds to an oscillation of the electric current flowing through the inductor whose amplitude tends to damp over time. To ensure that the current intensity maintains a desired oscillation amplitude over time, it is necessary to impose on the circuit an input forcing function that is also variable over time. Furthermore, in order to maintain high circuit efficiency, the Applicant has found that the oscillation frequency of the aforementioned forcing function must remain as close as possible to, if not coincident with, the resonance frequency of the circuit over time. It is therefore of paramount importance to accurately know the resonance frequency of the circuit.

However, although the characteristic resonance frequency inherent to the given resonant circuit can be calculated with high precision during the design phase, the Applicant has found that, in practice, it may occur that this resonance frequency varies, typically due to variations in the total value of the capacitance C and/or the inductance L of the circuit.

Regarding the value of the capacitance C, it may be subject to static variations (e.g., due to the capacitor tolerances) and/or to dynamic variations caused by the increase in temperature of the capacitors during use.

Regarding the value of the inductance L, it depends not only on the specific inductor incorporated into the circuit but also on the contribution provided by the packaging material to be sealed and/or by its position relative to the inductor, which may vary from time to time.

Without wanting to limit itself to any theory, the Applicant indeed believes that the inductive coupling that occurs between the inductor of the circuit and the packaging material can be schematized as an air transformer (where the secondary side is represented by the packaging material), whose overall inductance, depending on both the elements, i.e. the inductor and the packaging material (and in particular on the physical/structural composition of the latter and/or its position relative to the former), must be taken into account for determining the characteristic resonant frequency of the circuit. Furthermore, the contribution to the overall inductance variation made by the packaging material can also vary over time as the same material passes through the sealing station, for example, due to changes in the thickness of the material (e.g., in the respective layers) and/or changes in the composition of the material itself, for example due to manufacturing tolerances, and/or spatial oscillations of the packaging material tape relative to the induction sealing components.

The aforementioned variations in inductance due to the packaging material are sudden (as the packaging material passes through the sealing station at a given speed) and are difficult to predict and/or measure in advance. This results in an effective difficulty in determining with the desired precision and in a timely manner the resonance frequency of the circuit to which to set the forcing function. The Applicant has therefore addressed the problem of sealing a food packaging material in a simple, rapid manner, particularly with times compatible with industrial series production regimes, and dynamically maintaining a desired efficiency over time (e.g., in terms of circuit efficiency) and sealing quality.

According to the Applicant, the above problem is solved by a method, and a related apparatus, for sealing containers according to the attached claims and/or having one or more of the following features.

According to one aspect, the invention relates to a method for sealing packaging material. Preferably said packaging material comprises at least one electrically conductive portion. Preferably said method comprises providing a sealing element, preferably comprising at least one electrically conductive winding.

Preferably said method comprises compressing said at least one portion of said packaging material between said sealing element and a contrast element.

Preferably said method comprises applying an electric voltage to opposite ends of said winding for creating, in said winding, a passage of electric current having at least one component variable over time, for sealing said packaging material by induction at said at least one portion.

Preferably said method comprises determining a time trend of an intensity of electric current passing in said winding.

Preferably said method comprises dynamically adjusting said electric voltage as a function of said time trend of said intensity of electric current.

According to another aspect, the invention relates to a sealing apparatus for sealing packaging material.

Preferably said apparatus comprises a sealing element, preferably comprising at least one electrically conductive winding.

Preferably said apparatus comprises an actuation system. Preferably said actuation system is structured for moving said sealing element and a contrast element for compressing at least one electrically conductive portion of said packaging material between said sealing element and said contrast element.

Preferably said apparatus comprises an electric energy generator connected to said sealing element.

Preferably said apparatus comprises a control unit.

Preferably said control unit is programmed and configured for commanding said actuation system for compressing said at least one portion of the packaging material between said sealing element and said contrast element.

Preferably said control unit is programmed and configured for commanding said electric energy generator for applying an electric voltage to opposite ends of said winding for creating, in said winding, a passage of electric current having at least one component variable over time, for sealing said packaging material by induction at said at least one portion.

Preferably said control unit is programmed and configured for determining a time trend of an intensity of electric current passing in said winding.

Preferably said control unit is programmed and configured for commanding said electric energy generator for dynamically adjusting said electric voltage as a function of said time trend of said intensity of electric current.

The Applicant has found that determining the time trend of the electric current (e.g., its respective time-varying component) flowing through the winding allows measuring a quantity directly related to the characteristic resonant frequency of the circuit, enabling to determine this characteristic resonant frequency entirely independently from calculating the current values of capacitance C and inductance L of the welding resonant circuit. In other words, the time trend (typically oscillatory) of the electric current in the inductor has, instant by instant, a period directly derived from the characteristic resonant frequency of the circuit, whatever it may be.

Therefore, by determining the time trend of the electric current flowing through the winding, it is possible to identify the characteristic resonant frequency of the circuit at the given moment or time interval, entirely independently from the values, and respective variations, of total capacitance and inductance of the resonant circuit.

The dynamic adjustment of the electric voltage at the ends of the winding as function of the time trend allows for dynamically adapting the input forcing function to the circuit keeping it at the characteristic resonant frequency of the circuit, allowing to autonomous track any variations.

The present solution therefore avoids the estimation and/or the calculation of the variations that may involve the values of capacitance and inductance of the resonant circuit.

Furthermore, the present solution avoids complex and/or costly alternative solutions to manage, such as the use of a fixed-frequency voltage generator and the adjustment of the characteristic resonant frequency of the circuit to keep it within a range of the fixed frequency of the generator by adding and/or subtracting capacitors from the circuit (connecting and/or disconnecting banks of capacitors in such a way as to balance the variation in total inductance L of the circuit with a corresponding voluntary variation of the total capacitance C).

The present invention in one or more of the aforesaid aspects can have one or more of the following preferred features.

Preferably applying said electric voltage comprises applying a time program of electric voltage.

Preferably said control unit is programmed and configured for commanding said electric energy generator for applying a time program of electric voltage.

In other words, the electric voltage is applied according to a function of time (any function, comprising for example a constant voltage over time in at least a time interval of application).

Preferably determining said time trend of the intensity of electric current comprises determining a sign (e.g. positive or negative) of said intensity of electric current.

Preferably said control unit is programmed and configured for determining a sign of said intensity of electric current.

In this way obtaining the characteristic resonance frequency of the circuit is further simple, direct and dynamic over time. In fact, the time interval in which the current has, continuously, positive or negative sign corresponds from time to time to an exact half of a period of oscillation of the intensity of current and therefore to a half of the current value of the resonance frequency of the circuit, whatever it is (and therefore independently of variations in the meantime which have occurred due to the effects described above).

Preferably dynamically adjusting said electric voltage is performed as a function of said sign of said intensity of electric current.

Preferably said control unit is programmed and configured for dynamically adjusting said electric voltage as a function of said sign of said intensity of electric current.

In this way the electric voltage is easily adjusted, as better described below.

Preferably dynamically adjusting said electric voltage comprises applying a first value of said electric voltage when said intensity of electric current has positive sign.

Preferably dynamically adjusting said electric voltage comprises applying a second value of said electric voltage, different from said first value, when said intensity of current has negative sign.

Preferably said control unit is programmed and configured for setting (e.g. also indirectly, for example by acting on a control quantity, such as a supply voltage of the entire circuit by the electrical energy generator, from which the voltage across the winding electrically connected to it arises) said electric voltage to said first value when said intensity of electric current has positive sign and setting said electric voltage to said second value when said intensity of current has negative sign.

In other words, the voltage takes on respectively the first value throughout the entire time interval in which the electric current has positive sign and the second value throughout the entire time interval in which the electric current has negative sign (the first value and the second value can also be functions of time), oscillating between these two values. In this way, by aligning one half-cycle of the voltage oscillation (e.g., time interval during which the voltage takes on the first value or the second value) with one half-cycle of the electric current oscillation (e.g., time interval during which the electric current has positive or negative sign, as described above), a robust time-varying voltage variation is achieved with frequency that matches the resonance frequency of the circuit at each instance.

Preferably said first value and second value of said electrical voltage are constant over time. In this way the time program of electrical voltage is greatly simplified. Preferably, said time program of electric voltage comprises, more preferably consists of, a square wave oscillating between said first and second value.

Preferably said first value of electrical voltage is greater than zero.

Preferably said second value of electrical voltage is less than zero.

In this way, the forcing function is concordant with the intensity of electric current.

Preferably determining said sign comprises determining each zero crossing of said time trend of said intensity of electric current and each respective subsequent crossing of said time trend of said intensity of electric current respectively of a first threshold of intensity of current or of a second threshold of intensity of current, said first threshold of intensity of current being greater than zero and said second threshold of intensity of current being less than zero.

In this way, the determination of the sign is further precise and/or accurate. Indeed, by determining for example the positive sign of the intensity of the electrical current when such intensity has exceeded both zero and the first threshold, it is possible to establish with ample precision and/or accuracy that effectively the intensity of the current has indeed risen above zero, eliminating (e.g., appropriately calibrating the value of the first threshold) any errors due to disturbances caused by oscillations of the intensity of the current around zero (but below the first threshold). The same applies for determining the negative sign with reference to the second threshold. Preferably said packaging material has multilayer tape shape.

Preferably said packaging material comprises a cellulose-based layer. Preferably said packaging material comprises a thermoplastic layer.

In this way the material is lightweight and robust, particularly possessing the desired qualities of suitability for contact with food.

Preferably said at least one electrically conductive portion comprises (more preferably consists of) a layer of metallic material, such as aluminum.

Preferably said method comprises arranging said packaging material in a tubular shape having a main development direction disposed along a feeding direction (longitudinal). In this way the material is prepared to be filled.

Preferably compressing said at least one portion comprises clamping said at least one portion of said packaging material together with a respective (transversally) opposite portion for closing said tubular shape at a sealing section perpendicular to said feeding direction.

Preferably said actuation system is structured for clamping said at least one portion of said packaging material together with a respective (transversally) opposite portion for closing said tubular shape at a sealing section perpendicular to said feeding direction. In this way, the portion is prepared for sealing.

Preferably applying said electric voltage is performed after said compressing said at least one portion. Preferably said compressing said at least one portion is maintained over time during at least part of said applying said electrical voltage. In this way the sealing is carried out in a simply and quickly manner.

According to another aspect the invention relates to a machine for packaging.

Preferably said machine is a food product packaging machine. In an equally preferred manner, said machine is a personal care product packaging machine.

Preferably said packaging machine comprises said sealing apparatus according to the present invention.

Preferably said packaging machine comprises a feeding system of a tape (preferably continuous) of said packaging material along a feeding direction.

Preferably said machine comprises a conformation system.

Preferably said conformation system is arranged upstream of said sealing apparatus.

Preferably said conformation system is structured for conforming said tape into a tubular shape having a main development direction along said feeding direction. In this way the material is prepared. Preferably said machine comprises a dispensing system.

Preferably said dispensing system is positioned upstream of said sealing apparatus.

Preferably, said dispensing system is positioned substantially corresponding to said conformation system. Dispensing the product is indeed advantageous to occur concurrently, or immediately after, the conformation system has configured the packaging material into the tubular shape capable of containing the dispensed product.

Preferably said dispensing system is structured to dispense a product (e.g., food), for example, in fluid form (e.g., liquid, semi-liquid, granular, etc.) inside said tubular shape. In this way the product to be stored is provided before the sealing.

It is specified that some steps of the method described above may be independent of the reported execution order, unless expressly indicated as requiring sequentially or simultaneity between two or more steps. Furthermore, some steps may be optional. Additionally, some steps may be performed repetitively, or they may be carried out in series or in parallel with other steps of the method.

Brief description of the figures:

Figure 1 schematically shows a packaging machine according to one embodiment of the present invention; figure 2 schematically shows a detail of the machine of figure 1 . figure 3 schematically shows a time trend of current and electric tension in accordance with a method according to the present invention.

Detailed description of some embodiments of the invention

The characteristics and advantages of the present invention will be further elucidated by the following detailed description of some embodiments, provided for illustrative purposes and not limiting the scope of the invention, with reference to the attached figures.

In Figure 1 , with the number 99 a packaging machine is exemplarily shown, purely schematically through the representation of its functional components. Specifically, the packaging machine 99 is a machine structured to form containers 200 from a continuous tape 90 of packaging material 1 , which is appropriately folded and sealed with specific welds, and to fill each container 200 with a pourable product, in a fluid nature, such as a food product (e.g., wine, milk, fruit juices, syrups, beverages, sauces, creams, yogurt, purees, preserves, vegetables, legumes, peeled tomatoes, etc.).

Exemplarily (not shown) the packaging material 1 comprises at least one electrically conductive portion useful for generating induced currents within the packaging material to exploit the aforementioned principle of induction welding. Exemplarily, the packaging material is multi layered, comprising at least an outer layer of transparent polyethylene (e.g., heat-sealable, for example because it melts at relatively modest temperatures), a cellulose-based supporting layer, such as cardboard, an aluminum layer (which exemplarily realizes the electrically conductive portion), and an inner layer of transparent polyethylene (e.g., heat-sealable). Optionally, the packaging material 1 may be provided (typically through mechanical deformation performed with appropriately shaped rollers) with pre-weakened folding lines that guide the subsequent shaping through folding.

Exemplarily the packaging machine 99 comprises a feeding system 91 that unwinds from a coil 11 the continuous tape 90 of packaging material 1 advancing it along a processing path 101 that extends in space. Exemplarily, in a respective end section 102, the processing path 101 lies along a linear and vertical feeding direction 100.

Exemplarily the feeding system 91 comprises a plurality of rollers 911 arranged successively along the processing path 101 and configured to move the continuous tape 90 while keeping it under a desired mechanical tension.

Exemplarily the packaging machine 99 further comprises a conformation system 92 arranged at the end section 102 of the processing path and structured for conforming the tape 90 into a tubular shape 60 having a main development direction along the feeding direction 100. In particular, at the conformation system 92, two opposite longitudinal flaps of the continuous tape 90 of packaging material 1 are overlapped and then thermally sealed together (e.g., through induction sealing).

Exemplarily the packaging machine 99 also comprises a dispensing system 93, arranged substantially in correspondence with the conformation system 92 and structured to dispense the food product (or other products, such as personal care products) inside the tubular shape of the packaging material. For this purpose, the dispensing system 93 exemplarily comprises a dispensing nozzle 933 with a dispensing mouth (not visible) positioned along the feeding direction 100 so as to be at least partially housed within the tubular shape of the packaging material formed by the conformation system 92.

Exemplarily the packaging machine 99 comprises a sealing apparatus 2 for the packaging material 1 , wherein the sealing apparatus 2 substantially constitutes a sealing station of the packaging machine 99 in correspondence with the end section 102. Exemplarily the sealing apparatus 2 is positioned downstream of the conformation system 92 and the dispensing system 93.

Exemplarily the sealing apparatus 2 comprises a sealing element 3, comprising a containment body 4, for example made of resin, and (at least) one electrically conductive winding 5 (only partially shown in section in Figure 2), encapsulated within the containment body 4.

Exemplarily the winding 5 is part of an electrical circuit (not shown) suitably powered (e.g., with voltage), which electrical circuit can be schematized as an RLC resonant circuit. The resistive and capacitive load of the circuit are not shown nor will be described in further detail, as they are of a known type, for example.

Exemplarily the sealing apparatus 2 also comprises a contrast element 6, opposed to the sealing element.

Optionally (not shown), the contrast element may comprise a respective electrically conductive winding encapsulated therein. Exemplarily the sealing element and the contrast element have respective main developments along a transverse direction substantially orthogonal to the feeding direction 100.

Exemplarily the apparatus comprises an actuation system (not shown) structured to reciprocally move the sealing element 3 and the contrast element 6 to compress one electrically conductive portion 50 of the packaging material 1 , together with an opposite portion, between the sealing element 3 and the contrast element 6 (as shown in figure 2). In other words, the sealing element 3 and the contrast element 6 exemplarily realize a pair of opposing sealing heads that are cyclically pressed, by the actuation system, against the continuous tape 90 in tubular shape to clamp opposite portions of the tape 90 together, forming a transverse seal of the tubular tape, sealing the tubular tape hermetically at a sealing section 300. Exemplarily the sealing is performed utilizing the aforementioned principle of magnetic induction which generates a heating of the packaging material at the respective pinched portions which, without limiting to any theory, at least partially melt, for example at the aforementioned low melting polyethylene layers (thermoplastics). The localized melting, along with the compression force exerted by the sealing and contrast elements, allows for mutual intimate joining of the opposite portions, which stabilizes upon subsequent cooling (e.g., cessation of the induction action by the magnetic field).

The sealing heads may also comprise respective cutting elements (not shown) to perform, concurrently with the sealing, also the transverse cutting of the tape 90 of packaging material tubular shaped.

The sealing apparatus 2 may also comprise a pair of movable shaped bodies (not shown) together with the sealing heads to embrace the tape 90 from opposite sides in order to shape it and give it a final shape, typically a parallelepiped. Exemplarily the sealing apparatus 2 also comprises an electric energy generator 7 electrically connected to the sealing element 3 (in figures such connection is only schematically shown).

Exemplarily the sealing apparatus 2 comprises a control unit 8. The control unit 8 is exemplarily operatively connected to the electric energy generator 7 and the actuation system.

For example, the control unit 8 can be data communication connected to the control unit of the machine (not shown), or it can be comprised within such a control unit of the machine, or it can even coincide with the control unit of the machine, in a way that harmonizes the control of these machine systems with the operation of the sealing apparatus, ensuring that the entire process of container production is synchronized, starting from the feeding of the packaging material 1 to the final unloading of each filled container 200.

In use, the sealing apparatus 2 allows for the execution of a sealing method for the packaging material 1 , particularly to create transverse seals on the tubular packaging material 1 as described above.

The sealing method exemplarily fits within the context of a broader method for the production and filling of containers 200 by the packaging machine 99.

Exemplarily, once the packaging material 1 is fed, the tubular shape is formed, and the desired food product is dispensed, the sealing method can be performed.

Exemplarily the control unit 8 is thus configured to command the actuation system to compress the portion 50 of packaging material 1 , along with the respective opposite portion of the tubular shape, between the sealing element 3 and the contrasting element 6 (figure 2).

Substantially contemporaneously or subsequently, while maintaining the compressed portion, it is exemplarily envisaged to apply an electric voltage V across opposite ends of the winding 5 to generate, within the winding 5, a passage of electric current having at least one time-varying component.

It is therefore envisaged to determine a time trend i of an intensity of electric current passing through the winding 5. Exemplarily the electric current passing through the winding 5 has a sinusoidal time trend, with a frequency equal to the resonance frequency proper to the RLC circuit of which the winding 5 is part, whatever it is, also as a result of changes in time due to the phenomena described above. An example of the time trend of the intensity of electric current is shown in Figure 3. Purely as an example, in figure 3, the sinusoidal time trend of the intensity of current is shown to undergo, at a certain instant t1 , a change in period, specifically a reduction in period, resulting in an increase in oscillations per unit of time. This means that at instant t1 , the resonance frequency of the circuit to which winding 5 belongs has changed (increased) due to an external effect, such as the variation in the overall inductance L of the circuit caused by the variation in inductance due to the portion of continuous tape 90 subjected to sealing (as described above).

The method according to the present invention, which involves dynamically adjusting the electric voltage based on the time trend of the current intensity, therefore allows to autonomously adjust the forcing function of the circuit to the variations of resonance frequency of the circuit, ensuring that the forcing function oscillates over time with a frequency that continuously matches the current value of resonance frequency of the circuit, thus achieving the advantages described above.

Exemplarily determining the time trend of the electric current intensity comprises determining the sign (e.g., positive or negative) of the intensity of electric current. In particular, determining the sign of the intensity of electric current exemplarily comprises identifying each zero-crossing point of the time trend of the intensity of electric current and each respective subsequent crossing of the time trend of the intensity of electric current with either a first intensity of current threshold or a second intensity of current threshold. The first intensity of current threshold is greater than zero, and the second intensity of current threshold is less than zero.

Exemplarily dynamically adjusting the electric voltage is performed as function of the determined sign of the intensity of electric current. More specifically, dynamically adjusting the electric voltage exemplarily comprises applying, to the opposite ends of the winding 5, a first value V1 , exemplarily constant over time, of electric voltage when the intensity of electric current has positive sign, and applying a second value V2 of electric voltage, constant over time and different from the first value, when the intensity of electric current has negative sign.

Exemplarily the first value V1 is greater than zero and the second value V2 is less than zero, as shown in figure 3. Therefore, the time trend of the electric voltage across the winding 5 exemplarily takes on a substantially square wave form.

Operatively, the voltage across the winding 5 can be regulated by appropriately controlling the supply voltage of the circuit, for example using the electric energy generator 7 in the form of a voltage generator. In this case, the supply voltage of the circuit can be regulated to a square wave oscillating between 0 Volts and a maximum voltage Vcc.

The aforementioned steps of the method are preferably carried out by the control unit 8, which is appropriately programmed and configured for this purpose. Advantageously the control electronics are properly calibrated to ensure that the electric voltage switches with maximum time precision in synchronization with the changes in the sign of the intensity of electric current. To this end, it may also be advantageous to measure (e.g., in advance) the response times of the electronics so that they can be appropriately taken into account during the switching.

Claims

1. Method for sealing of packaging material (1 ) comprising at least one electrically conductive portion (50), said method comprising the steps of:

- providing a sealing element (3) comprising at least one electrically conductive winding (5);

- compressing said at least one portion (50) of said packaging material (1 ) between said sealing element (3) and a contrast element (6);

- applying an electric voltage (V) to opposite ends of said winding (5) for creating, in said winding (5), a passage of electric current having at least one component variable over time, for sealing said packaging material (1 ) by induction at said at least one portion (50);

- determining a time trend (i) of an intensity of electric current passing in said winding;

- dynamically adjusting said electric voltage (V) as a function of said time trend (i) of said intensity of electric current.

2. Method according to any one of the previous claims, wherein applying said electric voltage (V) comprises applying a time program of electric voltage, wherein determining said time trend (i) of the intensity of electric current comprises determining a sign of said intensity of electric current, and wherein dynamically adjusting said electric voltage (V) is performed as a function of said sign of said intensity of electric current.

3. Method according to any one of the previous claims, wherein dynamically adjusting said electric voltage (V) comprises applying a first value (V1 ) of said electric voltage when said intensity of electric current has positive sign and applying a second value (V2) of said electric voltage, different from said first value (V1 ), when said intensity of current has negative sign.

4. Method according to claim 3, wherein said first value (V1 ) and second value (V2) of said electrical voltage (V) are constant over time and wherein said first value (V1 ) of said electrical voltage is greater than zero.

5. Method according to any one of claims from 2 to 4, wherein determining said sign comprises determining each zero crossing of said time trend (i) of said intensity of electric current and each respective subsequent crossing of said time trend (i) of said intensity of electric current respectively of a first threshold of intensity of current or of a second threshold of intensity of current, said first threshold of intensity of current being greater than zero and said second threshold of intensity of current being less than zero.

6. Method according to any one of the previous claims, wherein said packaging material (1 ) has multilayer tape (90) shape and it comprises a cellulose-based layer and a thermoplastic layer.

7. Method according to any one of the previous claims, comprising arranging said packaging material (1 ) in a tubular shape (60) having a main development direction disposed along a feeding direction (100), wherein compressing said at least one portion (50) comprises clamping said at least one portion (50) of said packaging material (1 ) together with a respective opposite portion for closing said tubular shape (60) at a sealing section (300) perpendicular to said feeding direction (100).

8. Sealing apparatus (2) for sealing packaging material (1 ), said apparatus (2) comprising:

- a sealing element (3) comprising at least one electrically conductive winding (5);

- an actuation system structured for moving said sealing element (3) and a contrast element (6) for compressing at least one electrically conductive portion (50) of said packaging material (1 ) between said sealing element (3) and said contrast element (6);

- an electric energy generator (7) connected to said sealing element (3);

- a control unit (8) programmed and configured for:

- commanding said actuation system for compressing said at least one portion (50) of the packaging material (1 ) between said sealing element (3) and said contrast element (6);

- commanding said electric energy generator (7) for applying an electric voltage (V) to opposite ends of said winding (5) for creating, in said winding (5), a passage of electric current having at least one component variable over time, for sealing said packaging material (1 ) by induction at said at least one portion (50);

- determining a time trend (i) of an intensity of electric current passing in said winding (5);

- commanding said electric energy generator (7) for dynamically adjusting said electric voltage (V) as a function of said time trend of said intensity of electric current.

9. Sealing apparatus (2) according to claim 8, wherein said control unit (8) is programmed and configured for commanding said electric energy generator (7) for applying a time program of electric voltage, wherein said control unit (8) is programmed and configured for determining a sign of said intensity of electric current and for dynamically adjusting said electric voltage (V) as a function of said sign of said intensity of electric current, and wherein said control unit (8) is programmed and configured for setting said electric voltage to a first value (V1 ) of said electric voltage when said intensity of electric current has positive sign and setting said electric voltage to a second value (V2) of said electric voltage, different from said first value (V1 ), when said intensity of current has negative sign.

10. Machine for packaging (99) comprising said sealing apparatus (2) according to claim 8 or 9, wherein said machine for packaging (99) comprises a feeding system (91 ) of a tape (90) of said packaging material (1 ) along a feeding direction (100).

11 . Machine (99) according to claim 10, comprising a conformation system (92) arranged upstream of said sealing apparatus (2) and structured for conforming said tape (90) into a tubular shape (60) having a main development direction along said feeding direction (100), and wherein said actuation system is structured for clamping said at least one portion (50) of said packaging material (1 ) together with a respective opposite portion for closing said tubular shape (60) at a sealing section (300) perpendicular to said feeding direction (100).