As is known, many pourable food products, such as fruit or vegetable juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for pourable food products known as Tetra Brik Aseptic®, which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer sheet structure substantially comprising one or more stiffening and strengthening base layers typically made of a fibrous material, e.g. paper, or mineral-filled polypropylene material, covered on both sides with a number of heat-seal plastic material layers, e.g. polyethylene film. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a gas- and light-barrier material layer, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a heat-seal plastic material layer, and is in turn covered with another heat-seal plastic material layer forming the inner face of the package eventually contacting the food product.
Packages of this sort are produced on fully automatic Packaging Machines 1, also known as Filling Machines, of the type shown in FIG. 1, wherein a continuous vertical tube 2 is formed from the web-fed packaging material 3, which is sterilized by applying a chemical sterilizing agent such as a hydrogen peroxide solution, which, once sterilization is completed, is removed, e.g. evaporated by heating, from the surfaces of the packaging material. The web-fed packaging material 3 is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form the vertical tube 2.
The vertical tube 2 is then filled downwards with the sterilized or sterile-processed pourable food product by means of a filling pipe 4 extending inside the tube 2 and equipped with a flow-regulating solenoid valve 5, and is fed by known devices along a vertical path to a forming station 6, where it is gripped along equally spaced cross sections by a jaw system including two or more pairs of jaws, which act cyclically and successively on the tube 2, and seal the packaging material of the tube 2 to form a continuous strip of pillow packs 7 connected to one another by transverse sealing strips. Pillow packs 7 are then separated from one another by cutting the relative sealing strips, and are conveyed to a final folding station (not shown) where they are folded mechanically into the finished, e.g. substantially parallelepiped-shaped, packages 8.
In the case of aseptic packages with an aluminium layer as the barrier material, the tube 2 is normally sealed longitudinally and transversely by an induction sealing device, which induces parasitic electric current in the aluminium layer to locally melt the heat-seal plastic material. More specifically, for transverse sealing, one of the jaws in each pair comprises a main body made of non-conducting material, and an inductor housed in a front seat in the main body; and the other jaw is fitted with pressure pads made of elastically yielding material, such as rubber.
When the relative pair of jaws grips the tube 2, the inductor is powered to seal a cross section of the tube 2 by heat sealing the plastic cover material. When powered, the inductor generates a pulsating magnetic field, which in turn produces parasitic electric current in the aluminium sheet in the packaging material from which the vertical tube is made, thus locally melting the heat-seal plastic cover material.
In the case of packages without an aluminium layer or other electrically conductive materials, the tube 2 is normally transversely sealed by a hot plate which locally heats the packaging material from the outside to the inside. More specifically, one of the jaws in each pair is equipped with the hot plate, and the other jaw is fitted with one or more pressure pads made of elastically yielding material. In this type of sealing, known as hot plate sealing, a relatively long time is needed for the hot plate to locally melt the heat-seal plastic cover material, which results in a low package production rate.
In order to improve the performance of the Filling Machines, ultrasonic sealing devices of the type disclosed for example in EP-B-615907 in the name of the present Applicant have been introduced, which essentially comprise an anvil and an ultrasonic transducer, also known as sonotrode, operable to convert electrical energy into ultrasonic mechanical vibratory energy, which are mounted on respective jaws in each pair and cooperate in heating the packaging material by means of ultrasonic vibrations.