HK1091439B - Induction sealing device and method which may be used for producing packages of pourable food products - Google Patents

Description

Induction sealing apparatus and method useful for producing packages of pourable food products

Technical Field

The present invention relates to an induction sealing apparatus and method which can be used to produce packages of pourable food products.

In particular, the invention may preferably, but not exclusively, be used in a forming and sealing unit for forming and sealing packages from a tube of sheet packaging material continuously filled with a pourable food product.

Background

It is well known that many pourable food products, such as fruit or vegetable juices, pasteurized or ultra-high-temperature treated (UHT) milk, wine, etc., are sold in packages made of sterilized packaging material.

A typical example of this type of packaging is the parallelepiped-shaped packaging for pourable food products, known as Tetra Brik Aseptic ®, which is made by folding and sealing a web of laminated packaging material.

Laminated packaging material comprises a plurality of layers of fibrous material, e.g. paper, covered on both sides with heat-seal plastic material, e.g. polyethylene, and, in the case of aseptic packages for long-storage food products, such as uht milk, an oxygen barrier material, e.g. a layer of aluminium or EVOH, on the side which will eventually contact the food product in the package, and one or more layers of heat-seal plastic material.

As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube can be formed from a web-fed (web-fed) packaging material; the web of packaging material can be sterilized on the packaging machine itself, for example, by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which is removed or evaporated from the surface of the packaging material by heating once sterilization is complete; the web of packaging material thus sterilized is kept in a closed sterile environment and is folded and sealed in the longitudinal direction to form a vertical tube.

The tube is then filled from the top with sterilized or sterile-processed pourable food product, and clamped in equally spaced transverse sections by two pairs of jaws (jaws). More specifically, the two pairs of jaws act peripherally and continuously on the tube, sealing the packaging material of said tube and forming a continuous pillow-shaped filling strip, which is mutually connected by respective transverse sealing bands.

The pillow packs are separated by cutting the relative sealing strips and then they are transferred to a final folding station where they are folded by a machine into the finished parallelepiped shape.

In the case of aseptic packaging using an aluminium layer as barrier material, the transverse portion of the tube is generally sealed using a sealing device which can induce a parasitic current in the aluminium layer in order to locally melt the heat-seal plastic material.

More specifically, one of the jaws in each pair of jaws includes a body made of a non-conductive material and an inductor received on a nose seal on the body; the other jaw has a pressure pad made of a material that generates elasticity, such as rubber.

This inductor is energized when the relevant pair of jaws is clamped onto the tube, so that the transverse portion of the tube can be sealed by the sealing plastic material cover.

More specifically, the seal apparatus includes, in addition to the inductor, an ac power signal generator and a matching circuit for optimizing power transfer between the generator and the inductor. In fact, the generator provides the maximum power when the current-voltage phase angle is close to 0.

Existing matching circuits are usually defined by inductive-capacitive circuits, in which a capacitive element (usually defined by a series of capacitors connected in parallel) is connected in parallel with an inductive element (usually defined by a transformer); the capacitance values of the capacitive elements and the inductance values of the inductive elements are selected so as to produce such phase states: the phase angle of the current-voltage can thereby be brought close to 0. However, such a phase state is only optimal for a predetermined electrical load associated with a given operating condition (e.g., package volume, filling machine output rate and operating speed, type of inductor, etc.).

Thus, since varying operating conditions will cause variations in electrical load, there is a significant gap from optimal phase conditions, thus reducing power transfer to the inductor.

Disclosure of Invention

It is an object of the present invention to provide a sealing device which can be used to eliminate the disadvantages of the known devices.

According to the present invention, there is provided an induction sealing apparatus for producing packages of pourable food products, for which a tube of sheet packaging material is to be transversely sealed, said sheet packaging material comprising at least one layer of an induction heatable material, said induction heatable material being covered by a plastics material, said sealing apparatus comprising: -a generating means for generating an alternating power signal S (ω); at least one inductor for receiving an alternating power signal S (ω) to induce parasitic currents in said layer and locally melt said plastic material to form a transverse seal; and a matching circuit for optimal power transfer between said generator and inductor; the method is characterized in that: said matching circuit comprising an inductor-capacitor circuit in which at least one inductor element is connected to at least one capacitor element having a variable capacitance; the capacitance of the capacitive element is adjustable so that the current-voltage phase angle can be made close to 0.

The invention also relates to an induction sealing method which can be used for producing packages of pourable food products by transverse sealing of a tube of sheet packaging material comprising at least one layer of an induction heatable material covered with a plastic material, said method comprising the steps of: generating an alternating power signal S (ω); providing said alternating power signal S (ω) to at least one inductor to induce parasitic currents in said layer and locally melt said plastic material to form a transverse seal; and optimizing power transfer between said generator and inductor by means of a matching circuit; the method is characterized in that: the optimizing step further comprises the steps of: adjusting a capacitance of at least one capacitive element connected to the at least one inductive element to bring the current-voltage phase angle close to 0.

Drawings

A preferred, non-limiting embodiment of the invention is described below, by way of example, with reference to the accompanying drawings, in which:

fig. 1 shows a simplified circuit diagram of an induction sealing device to be used for producing packages of pourable food products;

FIG. 2 shows a portion of the apparatus of FIG. 1 in more detail;

figure 3 shows a variant of the device of figure 1.

Detailed Description

The reference number 1 in fig. 1 indicates as a whole an induction sealing device for producing packages of pourable food products.

More specifically, the sealing apparatus 1 includes: a generator 3 for generating an alternating power signal S (ω); an inductor 4 for receiving an alternating power signal S (ω); and a matching circuit 7 for optimizing the power transfer between the generator 3 and the inductor 4.

More specifically, generator 3 may generate a variable voltage (e.g. sinusoidal) signal of intermediate frequency (e.g. 530 khz) in a conventional manner, with a peak voltage around a few hundred volts (e.g. 540 v), may generate a continuous or pulsed alternating power signal S (ω), and may provide a maximum power (e.g. 2500 watts) when the phase angle between the current and the voltage (both measured at the output of generator 3) is close to 0.

The inductor 4 is conventionally defined by a winding 10, which winding 10 receives an alternating power signal S (ω) and generates a pulsed magnetic field, which in turn generates a parasitic current on an aluminium sheet 12 forming part of a vertical tube 13 (shown in part and not to scale), which vertical tube 13 is made of a suitably shaped web of laminated packaging material.

The laminated packaging material comprises a central layer 15 of fibrous material (e.g. paper) covered on both sides with heat-seal plastic material 16, e.g. polyethylene; the aluminium sheet 12 is inserted between the central layer 15 of fibrous material and one of the layers 16 of plastic material; the parasitic current locally melts the plastic material 16 of the two contact portions of the vertical tube 13 to seal said vertical tube 13 laterally.

The matching circuit 7 comprises at least one first capacitor 20, the first capacitor 20 being interposed between the first and second wires 21, 22; a series of capacitors 24, 25, 26, 27 (4 in the example shown, but they can obviously be any other number) can be connected to the wires 21, 22 or disconnected from the wires 21, 22 depending on the control signals acting on the corresponding switches 24a, 25a, 26a, 27 a. Capacitor 20 may be defined by a series of capacitors (e.g., 3, not shown) connected in parallel in a conventional manner and may have a capacitance of about 14-40nF in a conventional manner.

More specifically, the first ends 21a, 22a of the wires 21, 22 define the input terminals of the matching circuit 7, the second ends 21b, 22b of the wires 21, 22 are connected to two terminals of a primary winding 23a of a transformer 23, the transformer 23 having a secondary winding 23b defining the output of the matching circuit 7. The transformer 23 preferably has a ferrite core and the windings 23a, 23b are made of litz wire in order to reduce the internal losses considerably.

The matching circuit 7 thus defines an inductive-capacitive circuit comprising an inductive element (defined by the winding 23a of the transformer 23) connected in parallel with a capacitive element having a variable capacitance, said capacitance being variable by connecting one or more capacitors 24, 25, 26, 27 connected in parallel with the capacitor 20.

According to the present invention, the capacitance value of the capacitive element is adjusted so that the phase angle of current-voltage approaches 0.

The capacitance is adjusted in a conventional manner by a control circuit 30, which control circuit 30 is used to measure a parameter (for example, the instantaneous value of the current-voltage phase angle * at the output of the generator 3, and/or the impedance at the output of the generator 3, i.e. the input impedance of the matching circuit 7) during the packaging production process and to determine the electricityTarget capacitance C necessary for capacitor element_targTo obtain a current-voltage phase angle close to 0. The current and voltage are measured at the output of the generator 3 by a known instrument (not shown) which measures the instantaneous values of the voltage V, the current I, and the phase angle *.

In this way, a control signal may be transmitted to one or more switches 24a, 25a, 26a, 27a, connecting one or more capacitors 24, 25, 26, 27 connected in parallel with capacitor 20, and achieving the determined target capacitance C_targ. Thus, a change in operating conditions changes the parameters provided to the control circuit 30, which will open/close a predetermined combination of switches 24a, 25a, 26a, 27a, so that the overall capacitance meets the above-mentioned conditions.

Fig. 2 illustrates an exemplary embodiment of one of the switches 24a-27 a. More specifically, each switch 24a-27a includes first and second IGBT transistors 40a, 40b, the emitters (E) of which are connected to each other, and the collectors (C) of which are connected to the wire 21 and one end of the corresponding capacitor 24-27, respectively. The gates (G) of the IGBT transistors 40a, 40b are connected to each other by a wire 42 and receive a command in a voltage Vdc (e.g., 24 volts) through a resistor 44, which command is used to determine a control signal to turn on/off the IGBT transistors 40a, 40 b. A resistor 46 is inserted between the gate (G) and emitter (E) of the IGBT transistors 40a, 40b to discharge the current stored in the internal capacitors of the IGBT transistors when these transistors are turned off. A zener diode 48 is also inserted between the gate (G) and emitter (E) of the IGBT transistors 40a, 40b to limit the IGBT transistor voltage Vge to a predetermined maximum value (e.g., 16 volts).

A recirculation diode is inserted between the collector (C) and emitter (E) of each IGBT transistor to allow current flow during the half-wave flow opposite to the direct half-wave flow through the IGBT transistor, which is a unidirectional device.

Alternatively, first and second MOSFET transistors (metal oxide semiconductor field effect transistors) (not shown) may be used, their sources (S) being connected to each other, and their drains (D) being connected to the electric wire 21 and one terminal end of the corresponding capacitors 24 to 27, respectively.

In the variant of fig. 3, the device 1 also defines an inductive element with variable inductance connected in parallel with the capacitive element with variable capacitance.

In the non-limiting embodiment of fig. 3, the variable inductance is determined by a transformer 23, the transformer 23 having a primary winding 23a, the primary winding 23a having a series of inputs 50, the inputs 50 being connected to corresponding numbers of turns, and when selected, may produce different transformation ratios of the transformer 23. Input 50 is selectively connected to lines 21 and 22 according to a control signal from control circuit 30. More specifically, selecting input 50 may vary the inductance of the matching circuit such that the input impedance of matching circuit 7 (i.e., the impedance "seen" by signal generator 3) assumes a value close to the optimal impedance value Z_ottE.g., 50 ohms, to maximize power transfer from the generator 3 to the inductor 4.

Claims (11)

1. An induction sealing device for producing packages of pourable food products by transverse sealing of a tube (13) of sheet packaging material, said packaging material comprising at least one layer of an induction heatable material (12), said induction heatable material being covered by a plastic material (16), said sealing device comprising:

-generating means (3) for generating an alternating power signal S (ω);

at least one inductor (4) for receiving an alternating power signal S (ω) to induce a parasitic current in said layer (12) of inductively heatable material and locally melt said plastic material (16) to form a transverse seal; and

-a matching circuit (7) for optimal power transfer between said generating means (3) and said inductor (4);

the method is characterized in that: said matching circuit (7) comprises an inductive-capacitive circuit in which at least one inductive element (23a, 23) is connected to at least one variable-capacitance capacitive element (20, 24, 25, 26, 27); the capacitance of the capacitive element is adjustable so that the current-voltage phase angle can be made close to 0.

2. The sealing apparatus of claim 1, wherein: the inductance element (23) and the capacitance element (20, 24, 25, 26, 27) are connected in parallel with each other.

3. The sealing apparatus of claim 1 or 2, wherein: said capacitive element (20, 24, 25, 26, 27) comprising at least one main capacitor (20); a series of auxiliary capacitors (24, 25, 26, 27), the series of auxiliary capacitors (24, 25, 26, 27) selectively connecting/disconnecting in parallel said main capacitor (20).

4. The sealing apparatus of claim 3, wherein: the switching devices (24a, 25a, 26a, 27a) are connected to the corresponding auxiliary capacitors (24, 25, 26, 27) so as to turn on/off the corresponding auxiliary capacitors (24, 25, 26, 27).

5. The sealing apparatus of claim 4, wherein: each switching device (24a, 25a, 26a, 27a) comprises a first and a second IGBT transistor (40a, 40b), the emitters (E) of the first and second IGBT transistors (40a, 40b) being connected to each other, their collectors (C) being in communication with the electric line (21) connected to the main capacitor (20) and with one terminal end of the corresponding auxiliary capacitor (24, 25, 26, 27), respectively; the gates (G) of said IGBT transistors (40a, 40b) are connected to each other and receive a voltage command V_daSo that the IGBT crystalThe pipes (40a, 40b) are turned on/off.

6. The sealing apparatus of claim 5, wherein: at least one resistor (46) is interposed between the gate (G) and the emitter (E) of the IGBT transistor (40a, 40 b); the resistor (46) ensures the discharge of the current stored in the internal capacitor of the IGBT transistor when the latter is turned off.

7. The sealing apparatus of claim 5, wherein: -inserting at least one zener diode (48) between the gate (G) and the emitter (E) of each IGBT transistor (40a, 40 b); the Zener diode (48) is used for limiting the voltage V of the IGBT transistor_geSo that it is a predetermined maximum value.

8. The sealing apparatus of claim 1, wherein: said inductive element (23, 23a) having a variable inductance; adjusting said inductance value so that the impedance of said matching circuit can be brought close to the optimum impedance value Z_ottTo a value such that the power transfer from said generating means (3) to said inductor (4) is maximized.

9. The sealing apparatus of claim 8, wherein: the inductive elements (23a, 23) comprise a transformer (23) having a primary winding (23a), the primary winding (23a) having a series of inputs (50) associated with respective numbers of turns, so that when selected, different transformer (23) transformation ratios can be produced.

10. An induction sealing method for producing packages of pourable food products by transverse sealing of a tube (13) of sheet packaging material comprising at least one layer (12) of induction heatable material covered with a plastic material (16), said method comprising the steps of:

generating an alternating power signal S (ω) by means of a generating device (3);

-providing said alternating power signal S (ω) to at least one inductor (4) to induce parasitic currents in said layer (12) of inductively heatable material and locally melt said plastic material (16) to form a transverse seal; and

optimizing power transfer between said generating means (3) and said inductor (4) by means of a matching circuit (7);

the method is characterized in that: the optimizing step further comprises the steps of: adjusting the capacitance of at least one capacitive element (20, 24, 25, 26, 27) connected to the at least one inductive element (23a, 23) to bring the current-voltage phase angle close to 0.

11. The sealing method of claim 10, further comprising the steps of: the inductance value of the inductive element is adjusted so that the impedance seen by the generating means can assume a value close to the optimum impedance value Zott, thereby maximizing the power transfer from the generating means (3) to the inductor (4).