Flaw detector for package closures

Abstract: A control unit (1) interacting with a strip packaging material to which are applied, to close respective openings (3) in the packaging material, a number of patch elements (5) and a number of tear-off tongues (6) heat sealed to respective patch elements (5). To determine the integrity of the patch elements (5), the unit has at least one contact surface (21) at an operating electric potential (Va) and cooperating with the side of the packaging material (4) having the patch elements (5) to define one plate of a capacitor (23), the other plate of which is defined by an aluminium film (4a) of the packaging material (4), and the dielectric of which is defined by the heat-seal plastic material interposed between the plates; and a detecting circuit (26) connected to the contact surface (21) and to the aluminium film (4a) to detect an electric quantity related to variations in the dielectric strength of the dielectric of the capacitor (23) and so determine the presence of any microholes in the patch elements (5).

Description: The present invention relates to a control unit for checking the integrity of tear-off closing devices applied to respective openings in strip packaging material for producing aseptic sealed packages of pourable food products. As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material. A typical example of such a package is the parallelepiped package for liquid or pourable food products known by the name of Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing strip-rolled packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of 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 defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material defining the inner face of the package eventually contacting the food product. As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube is formed from the packaging material supplied in strip form. The strip of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally to form a tube. The tube is filled with the sterilized or sterile-processed food product, and is sealed at equally spaced cross sections along which it is then cut into pillow packs, which are subsequently folded mechanically to form the finished, e.g. substantially parallelepiped, packages. The finished package comprises an opening through which to pour the product; and a tear-off closing device applied to the opening. The openings are formed and the respective closing devices applied to the strip packaging material before the material is folded into the vertical tube and filled with the food product. More specifically, each closing device comprises a "patch" element defined by a small sheet of heat-seal plastic material and applied, so as to close the respective opening, to the side of the packaging material eventually defining the inside of the package; and a tear-off tongue applied to the opposite side of the packaging material and heat sealed to the patch element. The tongue comprises a layer of aluminium; and a layer of heat-seal plastic material, normally polyethylene, which is sealed to the patch element. By virtue of the patch element and the tongue adhering to each other, the portion of the patch element sealed to the tongue is also removed, thus exposing the opening, when the tongue is torn off. In the course of the heat-seal operation, tears or microholes may be formed in the superheated patch element, particularly as a result of the patch element interacting with the normally fairly irregular edges of the opening formed in the packaging material. The presence of microholes in the patch element may impair the integrity and asepticity of the package by allowing outside air into the package. Document US-A-3 519 922 discloses the use of two spaced capacitors for the detection of abrupt changes in thickness of moving sheet material. It is an object of the present invention to provide a control unit for checking the integrity of tear-off closing devices applied to respective openings in strip packaging material, and which provides for quality controlling application of the closing devices to the packaging material. It is a further object of the present invention to enable said check to be made during operation of the packaging machine. According to the present invention, there is provided a control unit for checking the integrity of tear-off closing devices applied to respective openings in a strip packaging material for forming aseptic sealed packages of pourable food products; said packaging material having a multilayer structure comprising at least one electrically conductive sheet of barrier material covered on opposite sides with respective layers of heat-seal plastic material; each said closing device comprising a sheet patch element made of heat-seal plastic material and applied, so as to close a respective said opening, to a face of said packaging material eventually defining the inside of the package, and a tear-off tongue also made of heat-seal plastic material and which is applied to an opposite face of said packaging material and sealed to said patch element; said unit being characterized by comprising: at least one contact surface at an operating electric potential and cooperating with said packaging material at said patch elements to define one plate of a capacitor, the other plate of which is defined by said sheet of barrier material, and the dielectric of which is defined by the heat-seal plastic material of said patch elements and of the layer of packaging material interposed between said sheet of barrier material and the patch elements; and detecting means connected to said contact surface and to said sheet of barrier material of said packaging material to detect an electric quantity related to variations in the dielectric strength of the dielectric of said capacitor and so determine the presence of any microholes in said patch elements. 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 lateral elevation of a control unit in accordance with the present invention for checking the integrity of tear-off closing devices applied to respective openings in strip packaging material; Figure 2 shows a front elevation of the Figure 1 control unit; Figure 3 shows a larger-scale section of a closing device applied to a respective opening in the packaging material. Number 1 in the accompanying drawings indicates as a whole a unit for checking the integrity of tear-off closing devices 2 applied to respective openings 3 in strip packaging material 4 fed along a path P. Unit 1 may be incorporated in a packaging machine (not shown) for continuously producing, from strip packaging material 4, aseptic sealed packages (not shown) of pourable food products, such as pasteurized or UHT milk, fruit juice, wine etc. In particular, packaging material 4 is folded and sealed longitudinally in known manner to form a vertical tube, which is filled with the sterilized or sterile-processed food product, is sealed along equally spaced cross section, and is then folded mechanically to form the finished packages. Unit 1 is located along path P of packaging material 4, downstream from a known station (not shown) for applying closing devices 2 to respective openings 3 in packaging material 4, and upstream from a station for forming the vertical tube of packaging material. Packaging material 4 has a multilayer structure and substantially comprises a layer of fibrous material, e.g. paper, covered on both sides with respective layers of heat-seal plastic material, e.g. polyethylene. The side of packaging material 4 eventually defining the inner face of the package, and therefore contacting the food product, is also provided with a layer of electrically conductive barrier material defined, for example, by an aluminium film 4a, which in turn is covered on both sides with respective layers of heat-seal plastic material, e.g. polyethylene. With particular reference to Figure 3, each closing device 2 comprises a patch element 5 defined by a small sheet of heat-seal plastic material and applied, so as to close a respective opening 3, to the face of packaging material 4 eventually defining the inside of the package; and a tear-off tongue 6 which is also made of heat-seal plastic material, is applied to the opposite face of packaging material 4, and is heat-sealed to patch element 5. In particular, patch element 5 and tongue 6 are joined along a sealing area extending close to and inwards of a lateral edge of respective opening 3 and defining a tear portion of patch 5 when tongue 6 is torn off. Tongue 6 also has a multilayer structure and comprises a layer of heat-seal plastic material, e.g. polyethylene, one face of which is joined to patch element 5; and a layer of barrier material, normally aluminium, which is fixed to the layer of heat-seal plastic material on the opposite side to patch element 5. With reference to Figures 1 and 2, unit 1 substantially comprises an outer, substantially parallelepiped casing 8 made of plastic material and having opposite lateral openings 9, 10 for the passage of strip packaging material 4; and two idle rollers 11, 12, which are housed inside casing 8 with respective axes A, B parallel to packaging material 4, are supported in rotary manner by respective pairs of brackets 13, 14 projecting from a lateral wall of casing 8, and cooperate on opposite sides with packaging material 4 at patch elements 5 and tongues 6 respectively. More specifically, roller 11 is fitted, in rotary manner about respective axis A, to the free ends of brackets 13, whereas roller 12 is fitted, in rotary manner about respective axis B, to one end of a rocker-arm lever 15 hinged at an intermediate point to brackets 14, and the opposite end of which is connected to the bottom wall of casing 8 by a cylindrical coil spring 16 which therefore exerts a turning moment on lever 15 to keep roller 12 in contact with packaging material 4. Each roller 11, 12 comprises a cylindrical intermediate portion 40 of diameter d; and rounded, outwardly-convex opposite end portions 41 with a radius of curvature r ranging between 0.25 and 0.5 times the diameter d of intermediate portion 40. Roller 11 is connected to an adjustable high-voltage generator 20, so that an outer surface 21 of roller 11 contacting packaging material 4 is at an operating potential Va, whereas aluminium film 4a of packaging material 4 is at ground potential. Surface 21 of roller 11 and aluminium film 4a of packaging material 4 define respective plates of a capacitor 23, the dielectric of which is defined by the heat-seal plastic material of patch elements 5 and of the layer of packaging material 4 interposed between aluminium film 4a and patch elements 5. To ensure safe grounding of aluminium film 4a of packaging material 4, there is provided a further idle roller 24 which is housed inside casing 8 so as to rotate about a respective axis C perpendicular to packaging material 4, has an outer annular groove 25 cooperating with and laterally contacting a peripheral edge of packaging material 4, and is grounded. An important aspect of the present invention is that unit 1 also comprises a detecting circuit 26 for detecting variations in the dielectric strength of packaging material 4 fitted with closing devices 2, and so determining the presence of any microholes (Figure 3) in patch elements 5. More specifically, on account of the different insulating properties of air and the material defining patch elements 5, tests have shown that, when closing devices 2 are perfectly integral, the dielectric strength of packaging material 4 fitted with closing devices 2 equals a first value D1 - in the example shown, 20 kV/mm - and assumes a second value D2 - in the example shown, 3 kV - lower than value D1 in the presence of microholes in patch element 5. Detecting circuit 26 provides for detecting electric current flow through the dielectric of capacitor 23, which occurs as a function of the operating potential value Va supplied by generator 20 and when the dielectric strength of packaging material 4 fitted with closing devices 2 passes from value D1 to value D2, i.e. in the presence of microholes in patch element 5. In particular, the operating potential Va supplied by generator 20 is so selected as to produce no electric current flow through the dielectric of capacitor 23 when patch elements 5 are perfectly integral, but to be sufficient to produce such current flow in the presence of microholes in patch elements 5. With particular reference to Figure 1, detecting circuit 26 comprises a resistor 28 having a first terminal 29 connected to generator 20, and a second terminal 30 connected to roller 11; and a comparator 31 having a first input 32 connected to terminal 29 via the interposition of a voltage reducer 33 supplying a measuring potential Vm related to the potential of terminal 29 and therefore to operating potential Va, a second input 34 receiving a reference potential Vf, and an output 35 supplying a logic state signal Sp indicating the presence/absence of microholes in relative patch element 5, and which assumes a high logic level (Sp = 1) when measuring potential Vm is below reference potential Vf, and a low logic level (Sp = 0) in other conditions. In actual use, generator 20 supplies a square-wave operating potential Va, which assumes high values in the neighborhood of each closing device 2, and low values at the areas of packaging material 4 interposed between two successive openings 3, so as to produce, at such areas, no flow of electric current possibly caused by the presence of small cracks in the layers of heat-seal plastic material contacting the aluminium layer, and which may impair packaging material 4. In the event of a microhole (Figure 3) in the patch element 5 traveling between rollers 11 and 12, the dielectric strength of the material interposed between roller 11 and aluminium film 4a of packaging material 4 falls from value D1 to value D2, so that, in the presence of a high operating potential Va value from generator 20, current flows through packaging material 4 thus resulting in a fall in measuring potential Vm. As measuring potential Vm falls below reference potential Vf, state signal Sp switches from a low to a high logic level to indicate the presence of a microhole in patch element 5. When measuring potential Vm falls below a minimum threshold value related to the dielectric strength D2 of the material interposed between roller 11 and aluminium film 4a of packaging material 4, no electric current flows through packaging material 4, so that potential Vm increases up to a maximum threshold value above which electric current again flows through the dielectric of capacitor 23; which phenomenon is repeated until the microhole gets past rollers 11 and 12, i.e. for a time interval depending on the traveling speed of packaging material 4. When the microhole gets past rollers 11 and 12, measuring potential Vm returns to the nominal value and, hence, state signal Sp assumes a low logic level. Conversely, if the patch element 5 traveling between rollers 11 and 12 is perfectly integral, no electric current flows through packaging material 4 fitted with closing devices 2, so that measuring potential Vm remains at the nominal value and does not fall below reference potential Vf, and signal Sp remains at the low logic level (Sp = 0). The advantages of unit 1 according to the present invention will be clear from the foregoing description. In particular, unit 1 provides for quality controlling the operations performed on packaging material 4 to form openings 3 and apply respective closing devices 2. The information contained in state signal Sp indicating the presence of microholes in patch elements 5 may be used to reject, at a subsequent stage in the operating cycle of the packaging machine, at least the packages formed from the portion of packaging material 4 comprising defective patch elements 5. Clearly, changes may be made to unit 1 as described and illustrated herein without, however, departing from the scope of the accompanying Claims. In particular, roller 11 connected to generator 20 may be connected to casing 8 by rocker-arm lever 15 and spring 16, and roller 12 may be fitted in a fixed position to casing 8.

Claims: A control unit (1) for checking the integrity of tear-off closing devices (2) applied to respective openings (3) in a strip packaging material (4) for forming aseptic sealed packages of pourable food products; said packaging material (4) having a multilayer structure comprising at least one electrically conductive sheet of barrier material (4a) covered on opposite sides with respective layers of heat-seal plastic material; each said closing device (2) comprising a sheet patch element (5) made of heat-seal plastic material and applied, so as to close a respective said opening (3), to a face of said packaging material (4) eventually defining the inside of the package, and a tear-off tongue (6) also made of heat-seal plastic material and which is applied to an opposite face of said packaging material (4) and sealed to said patch element (5); said unit (1) being characterized by comprising: at least one contact surface (21) at an operating electric potential (Va) and cooperating with said packaging material (4) at said patch elements (5) to define one plate of a capacitor (23), the other plate of which is defined by said sheet of barrier material (4a), and the dielectric of which is defined by the heat-seal plastic material of said patch elements (5) and of the layer of packaging material (4) interposed between said sheet of barrier material (4a) and the patch elements (5); and detecting means (26) connected to said contact surface (21) and to said sheet of barrier material (4a) of said packaging material (4) to detect an electric quantity related to variations in the dielectric strength of the dielectric of said capacitor (23) and so determine the presence of any microholes in said patch elements (5). A unit as claimed in Claim 1, characterized in that said detecting means (26) comprise indicating means for indicating electric current flow through the dielectric of said capacitor (23). A unit as claimed in Claim 1 or 2, characterized in that said detecting means (26) comprise comparing means (31) having a first input (34) receiving a reference signal indicating a reference potential (Vf), a second input (32) receiving a measuring signal indicating a measuring potential (Vm) related to said operating electric potential (Va), and an output (35) supplying a state signal (Sp) indicating the presence of microholes in a relative said patch element (5) in the event of a predetermined relationship between said measuring signal and said reference signal. A unit as claimed in Claim 3, characterized in that said state signal (Sp) is a logic signal assuming a first logic level indicating the presence of microholes in the relative said patch element (5) when said measuring potential (Vm) is less than said reference potential (Vf). A unit as claimed in any one of the foregoing Claims, characterized in that said contact surface (21) is connected to voltage-generating means (20) supplying said operating electric potential (Va); and in that said detecting means (26) comprise resisting means (28) interposed between said voltage-generating means (20) and said contact surface (21). A unit as claimed in any one of the foregoing Claims, characterized by comprising a first and a second idle roller (11, 12), which have respective axes (A, B) parallel to said packaging material (4), and cooperate on opposite sides with the packaging material (4) at said patch elements (5) and said tongues (6) respectively; said contact surface (21) being defined by the outer surface of said first idle roller (11). A unit as claimed in Claim 6, characterized in that said first and second idle rollers (11, 12) each comprise a cylindrical intermediate portion (40); and rounded, outwardly-convex opposite end portions (41) having a radius of curvature (r) ranging between 0.25 and 0.5 times the diameter (d) of said intermediate portion (40). A unit as claimed in any one of the foregoing Claims, characterized by comprising a (third) idle roller (24) which has an axis (C) perpendicular to said packaging material (4), cooperates laterally with a peripheral edge of the packaging material (4), and is at ground potential.

Inventors: SIGHINOLFI EUGENIO, SIGHINOLFI, EUGENIO
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 1999
Main classification: B31B1
IPC codes: G01M3, G01N27, B65B57, B31B1, B65H20