The present invention also relates to a packaging machine for continuously producing sealed packages of a pourable food product and featuring such a sealing device.

BACKGROUND ART Many pourable food products, such as fruit juice, UHT milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.

A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing laminated strip packaging material having a multilayer structure comprising a layer of fibrous material, e. g.

paper, covered on both sides with heat-seal plastic material, e. g. polyethylene.

In the case of aseptic packages for long-storage products such as UHT milk, the packaging material comprises a layer of barrier material, e. g. an aluminium or EVOH sheet, which is superimposed on a layer of heat- seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually forming the inner face of the package contacting the food product.

As is known, such packages are made on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine itself, e. g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e. g. vaporized by heating, from the surfaces of the packaging material; and the web of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form a vertical tube.

The tube is filled with the sterilized or sterile- processed food product, and is gripped at equally spaced cross sections by two pairs of jaws. More specifically, the pairs of jaws act cyclically and successively on the tube to heat seal the packaging material of the tube and form a continuous strip of pillow packs connected to one another by respective transverse sealing bands.

The pillow packs are separated by cutting the relative sealing bands, and are then conveyed to a final folding station where they are folded mechanically into the finished parallelepiped shape.

The tube portion gripped between each pair of jaws is heat sealed by heating means carried by one of the jaws and for locally melting the two layers of thermoplastic material gripped between the jaws.

More specifically, packaging material in which the barrier layer is defined by a sheet of electrically conducting material, e. g. aluminium, is normally sealed by a so-called induction heat seal process, in which, when the tube is gripped by the pair of jaws, leakage current is induced in the aluminium sheet to heat the aluminium sheet locally and so locally melt the heat-seal plastic material.

More specifically, in induction heat sealing, the heating means substantially comprise an inductor carried by one of the two jaws-known as the sealing jaw-and which is powered by a high-frequency current generator and substantially defined by a bar of electrically conducting material which interacts with and induces in the tube material a leakage current to bring the tube material up to the required sealing temperature; while the other jaw-known as the counterjaw-has pressure pads made of elastic material and which cooperate with the inductor to heat seal the tube along a relative sealing band.

Once the sealing operation is completed, a knife carried by one of the two jaws and interacting with the tube of packaging material is activated to cut the tube along the center line of the sealing band and so cut a pillow pack off the bottom end of the tube of packaging material. Since the bottom end is sealed transversely, the jaws, on reaching the bottom dead center position, can be opened to avoid interfering with the top portion of the tube. At the same time, the other pair of jaws, operated in exactly the same way, moves down from a top dead center position to repeat the gripping/forming, sealing and cutting operations described above.

It has recently been proposed to employ packaging material having a barrier layer defined by a sheet of electrically nonconducting material, e. g. EVOH, and to seal the packaging material using a so-called pulsation heat seal process wherein the heat required to locally melt the heat-seal plastic material is supplied by a ceramic heating element carried by the sealing jaw-in this application, also referred to as the heating jaw- and which, when the tube is gripped by the pair of jaws, is heated instantaneously by applying a current pulse, the amplitude and duration of which depend on the desired characteristics of the seal.

A sealing jaw with a ceramic heating element for pulsation heat sealing is described, for example, in US Patent 5,682,732 filed by the present Applicant.

More specifically, the sealing jaw comprises a steel

or copper bar; a layer of electrically insulating ceramic material-in particular, a mixture of zirconium oxide (ZrO2) and silicon carbide (SIC)-fixed to the bar by adhesive or other mechanical anchoring means; and a strip of electrically conducting ceramic material-in particular, a mixture of titanium boride (TiB2) and silicon carbide (SIC)-which is housed partly in a cavity formed in the layer of electrically insulating ceramic material, projects from the layer of electrically insulating ceramic material, and has metallized ends for connection to an electric current source for supplying a current pulse whenever the strip of electrically conducting ceramic material is to be heated to perform a heat-seal operation.

Though widely used, the sealing jaw described above has several minor drawbacks preventing its many advantages from being exploited to the full.

In particular, tests conducted by the Applicant have shown that packaging machines featuring such sealing jaws fail to provide for seals of the same high quality as those obtainable by induction heat sealing, which explains why pulsation heat sealing is adopted on so few, and in particular high-speed, packaging machines.

Heat dissipation of the above sealing jaw, in fact, is fairly slow, so that the sealing jaw cannot be cooled fast enough to ensure the same working temperature at the start of each sealing operation. As a result, packaging machines employing this type of sealing jaw fail to

reproduce the same working conditions at each sealing operation, and hence to provide for accurately controlling flow of the molten thermoplastic material within. the packaging material, which is indispensable for obtaining high-quality seals.

Tests conducted by the Applicant have also shown that, using jaws of the above type, transverse sealing of the tube of packaging material is particularly sensitive to nonparallelism of the jaws in each pair and to the position of the longitudinal seal on the tube of packaging material, and that the jaws perform poorly on multiple-jaw packaging machines.

DISCLOSURE OF INVENTION It is an object of the present invention to provide a sealing device designed to eliminate the aforementioned drawbacks.

According to the present invention, there is provided a sealing device for pulsation heat sealing a thermoplastic packaging material for producing sealed packages of pourable food products, as claimed in Claim 1.

It is a further object of the present invention to provide a packaging machine for producing sealed packages of a pourable food product from a tube of sheet packaging material that can be pulsation heat sealed, and which provides for obtaining seals of the same quality as those obtainable by induction heat sealing.

According to the present invention, there is

provided a packaging machine for producing sealed packages of a pourable food product from a tube of sheet packaging material that can be pulsation heat sealed, as claimed in Claim 7.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a view in perspective, with parts removed for clarity, of a packaging machine for producing aseptic sealed packages of pourable food produces from a tube of packaging material; Figure 2 shows a cross section of a sealing jaw in accordance with the present invention and forming part of the Figure 1 packaging machine.

BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figure 1 indicates as a whole a packaging machine for continuously producing aseptic sealed packages 2 of a pourable food product, such as pasteurized or UHT milk, fruit juice, wine, etc., from a tube 4 of packaging material.

The packaging material has a multilayer structure (not shown) and comprises a layer of fibrous material, normally paper, covered on both sides with respective layers of heat-seal plastic material, e. g. polyethylene; and the side of the packaging material eventually contacting the food product in package 2 also has a layer of barrier material, in turn covered with one or more

layers of heat-seal plastic material.

Tube 4 is formed in known manner-not described in detail-by longitudinally folding and sealing a web of heat-seal sheet packaging material 6, is filled with the sterilized or sterile-processed food product by means of a fill pipe 8 extending inside tube 4 and having a flow- regulating solenoid valve 10, and is fed by known devices along a vertical path A to a forming station 12 where it is cut transversely and folded mechanically into packages 2.

More specifically, forming station 12 comprises a pair of forming assemblies 14 (only one shown in Figure 1) which move vertically along respective guides (not shown) and interact cyclically and successively with tube 4 to grip it at equally spaced cross sections and to perform on tube 4 heat-seal and cutting operations by which to form a continuous strip of pillow packs connected to one another by respective transverse sealing bands, and which are separated by cutting along the respective sealing bands, and are then conveyed to a finish folding station (not shown) where they are folded mechanically into the finished parallelepiped shape to form packages 2.

Each forming assembly 14 substantially comprises a slide (not shown) running along the respective guide; and a pair of jaws 16,18 hinged to the bottom of the slide about respective horizontal axes and movable between a closed position and a fully open position.

In the example shown, each jaw 16,18 is defined by a substantially L-shaped plate comprising a substantially quadrangular base portion 20 hinged by its bottom end to a bottom portion of the slide; and an arm 22 which interacts with tube 4, is fixed to the top end of portion 20, and projects from portion 20 in a direction perpendicular to a vertical longitudinal central plane through the axis defining the vertical path of tube 4.

More specifically, arms 22 of jaws 16,18 extend towards and beyond said central plane so as to be positioned on opposite sides of tube 4.

With reference to Figure 2, which shows a cross section of jaws 16,18 of one forming assembly 14 in the closed position gripping and heat sealing a cross section of tube 4, each forming assembly 14 also comprises a sealing device 24 and a cutting device 26, which, at each cross section of tube 4 gripped between jaws 16,18, heat seal the cross section and cut along a central parting line respectively.

More specifically, sealing device 24 comprises a cooling body 28, and a pair of ceramic heating elements 30 incorporated in cooling body 28.

Cooling body 28 is made of metal-preferably steel, copper, aluminium or an alloy of such materials-and has a pair of straight elongated cavities 32 in which ceramic heating elements 30 are embedded. More specifically, cavities 32 have roughly U-shaped cross sections narrowing towards the open ends, extend in a

direction crosswise to the axis defining the vertical path A of tube 4, and are located symmetrically on opposite sides of an intermediate plane B perpendicular to the axis defining vertical path A of tube 4 and coincident, in use, with the parting line of the cross section of tube 4 gripped between the two jaws 16,18.

Alternatively, cooling body 28 may be separate from jaws 16,18, may be fixed in known manner to one of them - in the example shown, jaw 16, hereinafter referred to as the sealing jaw-or may be defined by a portion of arm 22 of sealing jaw 16.

Cooling body 28 also has a pair of cooling conduits 31, each extending parallel to and over a respective cavity 32; and a further cavity 33 located between cavities 32 and housing a stop 35 for cutting device 26.

Each ceramic heating element 30 substantially comprises a straight elongated body 34 made of electrically insulating ceramic material, preferably a mixture of zirconium oxide (Z, O,) and silicon carbide (SIC), and having an integral straight longitudinal ridge 36 of electrically conducting ceramic material, preferably a mixture of titanium boride (TiB2) and silicon carbide (SIC).

More specifically, body 34 of electrically insulating ceramic material of each ceramic heating element 30 has a cross section of substantially the same shape and size as respective cavity 32, and is clamped inside cavity 32 by deforming cooling body 28.

More specifically, body 34 of electrically insulating ceramic material of each ceramic heating element 30 is housed inside respective cavity 32 with its base and lateral walls closely contacting the bottom and lateral walls of cavity 32, and has a head portion 38 projecting slightly from cavity 32 and on which ridge 36 extends integrally.

More specifically, ridges 36 of ceramic heating elements 30 extend symmetrically on opposite sides of and parallel to said intermediate plane B along substantially the whole length of head portions 38 of respective bodies 34 of electrically insulating ceramic material, and project towards the other jaw 18-hereinafter referred to as the counterjaw-to interact with tube 4 and increase the gripping pressure on tube 4 during heat sealing.

In the example shown in Figure 2, each of ridges 36 of ceramic heating elements 30 preferably comprises a wide base portion 40 connected to head portion 38 of respective body 34 of electrically insulating ceramic material, and having a substantially flat rectangular cross section; and a narrow head portion 42, which is connected to and located centrally with respect to base portion 40, has a substantially rectangular cross section with rounded corners, and is roughly twice the height and half the width of base portion 40.

Each of ridges 36 of ceramic heating elements 30 is connected in known manner-not described in detail-to

a current generator (not shown) for supplying a current pulse whenever ridge 36 is to be heated to perform a heat-seal operation.

Sealing device 24 also comprises a pair of pressure pads 44, which are made of heat-resistant elastomeric material, preferably nitrile rubber, and are housed in respective similarly shaped front cavities 46 formed in a supporting body 48 carried by counterjaw 18 and preferably made of steel. More specifically, cavities 46 are located symmetrically on opposite sides of central plane B, and are aligned with cavities 32 to enable pressure pads 44 to cooperate with ridges 36 of ceramic heating elements 30 to grip and heat seal tube 4 on opposite sides of central plane B.

Cutting device 26 substantially comprises a substantially flat, knife-shaped cutting member 50, which is housed to slide inside a front cavity 52 in counterjaw 18, is movable in a direction lying in central plane B and perpendicular to the longitudinal vertical central plane through the axis defining vertical path A of tube 4, and is activated by a hydraulic cylinder (not shown) incorporated in the counterjaw.

More specifically, cutting member 50 is normally maintained by a spring in a withdrawn rest position completely inside cavity 52, and, when activated by the hydraulic cylinder, moves into a forward cutting position projecting frontwards from counterjaw 18 and cooperating with stop 35 carried by cooling body 28.

The advantages of the present invention are as follows.

In particular, incorporating ceramic heating elements 30 in metal cooling body 28 greatly improves the heat dissipation of sealing device 24 as compared with that of the sealing jaw described in the aforementioned American patent, thus enabling sealing device 24 to be cooled more or less instantaneously and brought to the same optimum working temperature at the start of each sealing operation.

On packaging machines featuring sealing devices 24 in accordance with the present invention, the same working conditions can therefore be reproduced at each sealing operation, thus enabling precise control of the flow of molten thermoplastic material within the packaging material and, hence, the formation of seals of the same high quality as those obtainable by induction heat sealing.

Moreover, heating and cooling sealing device 24 more or less instantaneously reduces sealing time considerably, thus enabling pulsation heat sealing to be employed on high-speed packaging machines, and in particular on last-generation machines capable of producing 20,000 packages and hour.

Tests conducted by the Applicant have also shown that, using sealing devices 24 in accordance with the present invention, transverse sealing of tube 4 of packaging material is less sensitive to nonparallelism of

jaws 16,18 and to the position of the longitudinal seal on tube 4 of packaging material, and that sealing device 24 according to the present invention lends itself perfectly to use on multiple-jaw packaging machines.

Clearly, changes may be made to what is described and illustrated herein without, however, departing from the scope of the present invention as defined in the accompanying Claims.

In particular, ridges 36 of ceramic heating elements 30 may be of any shape other than that described and illustrated herein, so as to adapt to specific applications and desired sealing characteristics.