A method and equipment for securing the joined edges of a heat-sealable film material in wrapping machines

Abstract: In a packaging machine through which products (4) are advanced singly and in succession along a wrapping line (24) together with a heat-sealable wrapping film, the film (3) is folded around each product (4) to form a wrapper (5) of tubular appearance with two joined longitudinal edges (2), whereupon the temperature of the edges (2) is raised by electromagnetic induction to soften the material in readiness for heat-sealing; the pressure required to fuse the joined and heated edges together is applied evenly from either side.

Description: The present invention relates to a method of securing the joined edges of a heat-sealable film material in wrapping machines. The utilization of heat-sealable materials in the art field of packaging has long been known to have many advantages, given that it becomes possible to avoid the use of adhesives and similar substances for the purpose of securing wrappers around packages. In a typical wrapping process, each individual product is enveloped in a wrapper made initially to assume a tubular shape with two longitudinal edges joined and extending parallel to the feed direction. The joined edges are secured together by raising the temperature of the material locally to an optimum melting point for heat-seal purposes, and thereafter applying a compression or pinching force. The operation of heat-sealing the joined longitudinal edges of a wrapper in this way is performed currently by dynamic or static means consisting respectively in rollers or plates heated by electrical resistances. In the majority of cases, the operation is performed by a pair of rollers disposed on opposite sides of the joined edges, by which the two edges are warmed and compressed until mutual fusion occurs. Given the high operating speeds of such machines and the rate of feed at which the joined edges advance, the heat-seal rollers are raised to temperatures of a somewhat high order so that the edges will be sure to unite during the brief periods for which the rollers and the material are brought into contact. Heat-seal equipment constructed and operating in the manner outlined above tends to display a high thermal inertia, since the rollers remain at their operating temperature for a relatively long duration even after the electrical resistances have been deactivated. This signifies that most of the equipment currently in use is lacking in flexibility as regards control over the operating temperature of the rollers. In the US-A-4 731 980 is disclosed an apparatus for forming tube from polyfoil web and a seal machine. In one embodiment the web edges of the polyfoil are directly heated by a coil which generates eddy current in the metallic sheet of the polyfoil. In GB-A-1 161 902 the eddy current are generated in two metallic rollers which in turn heat the web of the material to be bonded. The rollers have a relative high thermal inertia and therefore it is required a constant rotation of the rollers and a constant advancing rate of the web. The impact of such a drawback is especially negative during those transient states, of whatever duration, in which the joined edges may be caused to advance at a rate of feed other than the optimum. In particular, it may happen that a reduction in the operating speed of the equipment occurs for one reason or another, and consequently, the time for which the heat-seal rollers and the joined edges remain in contact can be extended to the point that critical melt temperatures begin to register in the material. One way to shorten the duration of the contact between the rollers and the joined edges of the wrapper might be to distance the rollers one from the other and thus bring about their separation from the material; this is not possible in most instances however, since for reasons of structural and functional simplicity the rollers are also utilized as a means of advancing the wrappers along the feed direction. Another reason why the rollers cannot be separated from the material is that the two edges may fail to unite through lack of compression. Similar problems of failure to obtain a successful heat-seal can also occur during positive thermal transients, or when the machine of which the equipment forms a part is switched on, likewise when the operating speed is increased for any reason. The object of the present invention is to provide a method such as will allow of varying the temperature of heat-seal rollers by bringing them to an optimum heat-seal temperature according to the feed rate of an advancing material, and thus to overcome the drawbacks mentioned above. The stated object is realized according to the present invention in a method according to claim 1. The stated object is realized similarly in equipment according to claim 6. The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: fig 1 is a schematic plan view of equipment according to the present invention; fig 2 is the section through II-II in fig 1; fig 3 is a schematic plan view of the equipment as in fig 1, illustrating a different embodiment; fig 4 is a partly sectional schematic illustration showing details of fig 3, with certain parts omitted better to reveal others; fig 5 is a partly sectional schematic illustration showing details of fig 1, with certain parts omitted better to reveal others. With reference to the drawings, the present invention relates to a method of securing together the joined longitudinal edges, denoted 2, of a heat-sealable film material 3 as used typically in a machine designed to wrap products 4 fed along a relative wrapping line 24. The film 3 is advanced along the wrapping line 24 and folded around the single products 4 in such a manner that each becomes enveloped in a wrapper 5 of tubular appearance with two joined longitudinal edges 2. The edges 2 are heated by electromagnetic induction and brought thus to an optimum softening temperature for heat-sealing purposes, then compressed together until completely united one with another. As discernible in figs 1 and 3, where F denotes the direction followed by the wrappers 5 along the wrapping line 24, the feed direction F coincides substantially with the length or longitudinal dimension of the edges 2. Heat is generated utilizing electromagnetic induction means denoted 7 in the drawings, and might be applied directly (see figs 1 and 2) by means 7 positioned on opposite sides of the advancing edges 2, with which the material 3 itself makes no contact. The step of compressing the edges follows the heating step and is performed by relative compression means 8. The joined edges 2 can be heated likewise indirectly, as illustrated in fig 3, locating the electromagnetic induction means 7 in close proximity to the revolving surfaces 10 afforded by respective rollers 9 of low thermal capacity positioned on opposite sides of the joined edges 2. The rollers 9 might be composed of an outer ring 11 fashioned from a material of low thermal capacity, and an inner ring or central core 12 of heat insulating material. The step of compressing together the joined and heated edges 2 is effected by relative compression or pinching means 8 that could be located following the rollers 9 of low thermal capacity along the feed direction F, or alternatively, coinciding with the selfsame rollers 9 as indicated in fig 3. In both of the solutions described, that is to say with heat applied to the joined edges 2 directly or indirectly, and in either instance utilizing the same electromagnetic induction means 7, thermal inertia is almost negligible and the temperature of the joined edges 2 can be regulated swiftly in response to any variation in the rate at which the material is caused to advance along the wrapping line 24. Equipment suitable for implementing the method thus described is denoted 1 in its entirety, and comprises a plurality of elements, arranged in sequence along the feed direction F followed by the wrappers 5 with their respective joined edges 2: enveloping means 13 by which the heat-sealable material 3 is bent around the products 4 to form a wrapper of tubular appearance with two joined longitudinal edges 2, electromagnetic induction means 7 serving to heat selected portions or areas 6 of the joined edges 2, compression or pinching means 8 engaging the selected portions or areas 6, and folding means 16 by which the joined and heat-sealed edges 2 are flattened against the wrapper 5 of which they form a part. The enveloping means 13 are illustrated as a pair of rollers, conventional in embodiment, positioned on opposite sides of the joined edges 2 and capable of fashioning the wrappers 5 around the products 4. In order to ensure that the structure of the equipment 1 can be readily appreciated, and since the disclosure is effectively pertinent only to the joined edges 2, the wrapping material 3 and the relative products 4 are indicated in figs 1 and 3 only along an incoming or initial stretch of the wrapping line 24 terminating substantially at the enveloping means 13, whilst only the joined edges 2 are shown along the remaining part of the line. The folding means 16 are similarly of conventional embodiment, consisting in a pair of rollers disposed on opposite sides of the joined edges 2. Also indicated in figs 1 and 3 are feed means 14 of which the function is to advance the wrappers 5 along the wrapping line 24. Such means 14 will be described further in due course. The electromagnetic induction means 7 are positioned following the enveloping means 13 with respect to the feed direction F, and in such a way as to heat the oncoming joined edges 2 either directly as illustrated in figs 1 and 2, or indirectly as in figs 3 and 4. Looking first at figs 1 and 2, the electromagnetic induction means 7 are disposed on opposite sides of the joined edges 2 and act directly on the material 3. In practice, the electromagnetic induction means 7 comprise a magnetic or magnetizable element 17 in the form of a ring or loop affording a small break, hence a gap 18, through which the joined edges 2 are able to pass freely and without contact, also a conductor 19 coiled around the magnetic or magnetizable element 17 and energized electrically by a monitoring and control unit 15 of which one input is connected to the output of sensing means 20 serving to indicate the speed at which the wrappers 5 and the relative edges 2 advance along the feed direction. The compression or pinching means 8, which in fig 1 are located following the electromagnetic induction means 7 along the feed direction F, comprise a pair of pressure rollers 21 disposed on opposite sides of the joined edges 2 and maintained permanently in contact one with another, so that the interposed edges 2 are squeezed or pressed firmly together. The revolving surfaces 10 of the pressure rollers 21, or at least the part of each roller offered to the joined edges 2 (see fig 2), will also present roughened and matched profiles generated for example by milling or knurling. The pressure rollers 21 serve additionally to advance the joined edges 2, and are one and the same as the aforementioned feed means 14. In the example of fig 1, accordingly, the speed sensing means 20 operate in conjunction with one of the pressure rollers 21. The equipment 1 disclosed is extremely simple and flexible, since the means by which the joined edges 2 are heat-sealed consist only in the electromagnetic induction means 7 and the pressure rollers 21, whilst the monitoring and control unit 15 is set up in such a way as to respond to a variation sensed in the rate at which the joined edges 2 are advanced by piloting a corresponding and proportional variation in the output of the electromagnetic induction means 7. Departing from the notion of utilizing electromagnetic induction means 7 to generate heat, the applicant has sought to simplify the equipment 1 further through the expedient of heating the joined edges 2 indirectly. In this instance the electromagnetic induction means 7 are located in close proximity to rollers 9 of low thermal capacity in such a way that electromagnetic energy emitted by the means 7 is transmitted first to the rollers 9 and thence to the joined edges 2. In the example of fig 3, electromagnetic induction means 7 comprise two magnetic or magnetizable elements 17 each located in close proximity to the revolving surface 10 of a relative roller 9, the rollers 9 of low thermal capacity being two in number and disposed on opposite sides of the advancing joined edges 2. More exactly, one magnetic or magnetizable element 17 of "U" shape is associated with each roller 9, occupying a position close to the part of the revolving surface 10 offered in contact to the relative edge 2. Also associated with each roller 9, preceding the relative magnetic or magnetizable element 17 in the direction of rotation, are respective sensing means 22 capable of detecting the temperature of the adjacent revolving surface 10 and transmitting a corresponding output signal to the monitoring and control means 15. In this solution, the monitoring and control means 15 are able to pilot a variation in the electromagnetic energy output of the electromagnetic induction means 7 not only according to the speed at which the joined edges 2 advance along the wrapping line 24, but also to the temperature of the revolving surface 10 coming within the compass of the electromagnetic induction means 7. More precisely, the thermal capacity of the rollers 9 is greater than that of the joined edges 2, and accordingly, the temperature of the surfaces 10 must always be known in order to avoid overheating. The functions of monitoring the temperature of the revolving surfaces 10 of the rollers 9 and thereupon regulating the level of electromagnetic induction hold a special significance in the equipment 1 disclosed, since the rollers 9 of low thermal capacity will cool measurably from the point of ceasing contact with the joined edges 2 to the point of returning within the operating compass of the electromagnetic induction means 7. These two operations had no significance in conventional types of equipment given that the heated rollers would effectively never cool down, by reason both of their high thermal capacity and of the high thermal inertia in the heating system overall. The rollers 9 of low thermal capacity can be embodied in one piece, or preferably, as illustrated in figs 3 and 4, composed of an outer ring 11 fashioned from a material of low thermal capacity and an inner ring or central core 12 of heat insulating material. In the particular case of fig 4, for reasons of engineering and construction, the inner ring or core 12 is shown as comprising a ring 12 of heat insulating material and a distinct central core 23. To maximize the simplicity of the equipment 1, the rollers 9 of low thermal capacity are structured in such a way as to perform the additional function of the compression or pinching means 8 so that the joined edges 2 can be heated and united in one and the same operation, hence instantaneously heat-sealed, without the drop in temperature that occurs when material is heated and compressed at separate stations. Again in this instance, the parts of the revolving surfaces 10 of the rollers 9 destined to come into contact with the joined edges 2 will be roughened with matching profiles, for example milled or knurled, as already intimated with reference to fig 2. As to the joined edges 2 themselves, moreover, these are heated across portions or areas 6 that typically will be continuous though of width less than the overall width of the edges 2. The monitoring and control unit 15 is programmed in such a way that the output from the electromagnetic induction means 7 will be just sufficient to heat the joined edges 2, or rather the portions or areas 6 of the edges 2 destined to come into contact with the revolving surfaces 10 of the rollers 9, to an optimum heat-seal temperature, i.e. the temperature at which the heat-sealable material of the portions or areas 6 will readily soften and then fuse under the pressure of the selfsame rollers 9. Naturally, the programming of the monitoring and control unit 15 will be based on a given temperature of the outer ring 11. On detecting a variation in temperature of the outer ring 11, the relative sensing means 22 relay a corresponding signal to the monitoring and control means 15, which in turn will vary the output of the electromagnetic induction means 7 by a corresponding amount. The same occurs whenever the monitoring and control means 15 receive a signal from the speed sensing means 20 indicating that the joined edges 2 have begun to advance at a faster or slower rate. It will be clear that the equipment 1 is able to pilot a rapid variation in the output of electromagnetic energy used to heat the joined edges 2 in response to a variation in the operating speed of the machine, and more exactly in the rate at which the joined edges 2 advance along the wrapping line.

Claims: A method of securing the joined edges (2) of a heat-sealable film material (3) in machines designed to wrap products (4) fed along a wrapping line, comprising the steps of: directing the film (3) along the wrapping line (24) at an advancing rate while enveloping each product (4) in a wrapper (5) of tubular appearance exhibiting two joined longitudinal edges (2); indirectly heating the joined edges (2)to obtain an optimum softening temperature in readiness for sealing, by electromagnetic induction means (7); compressing the joined edges (2) until completely secured one to the other; characterized in that the heating step is performed by induction means (7) having negligible thermal inertia, in such a way the temperature of the joined edges (2) is swiftly regulable in response to any variation of the advancing rate of the film (3). A method as in claim 1, wherein the electromagnetic induction means (7) operates in conjunction with rollers (9) of low thermal capacity, the rollers (9) being heated indirectly and interposed between the joined edges (2) and the electromagnetic induction means (7) for joining and sealing the edges (2) when subjected to the action of compression means (8). A method as in claim 2, wherein the rollers (9) of low thermal capacity are disposed in such a way as to operate on opposite sides of the joined and advancing edges (2), their revolving surfaces (10) engaging in contact with the joined edges (2) in close proximity to the electromagnetic induction means (7). A method as in claim 3, wherein the joined edges (2) are heated from each opposite side by a corresponding roller (9) of which the revolving surface (10) is the revolving surface afforded by an outer ring (11) of low thermal capacity encircling a heat-insulating inner ring or central core (12). A method as in claim 3, wherein the joined edges (2) are heated by the rollers (9) of low thermal capacity and sealed one to another through the agency of the selfsame rollers (9). Equipment for securing the joined edges of a heat-sealable film material in wrapping machines, comprising: means (13) by which to envelop products (4) individually in relative wrappers (5) of heat-sealable material presenting a tubular appearance and exhibiting two joined longitudinal edges (2); means (14) by which to feed the wrappers (5) together with the respective products (4) along a wrapping line (24) at an advancing rate, suitably proportioned electromagnetic induction means (7) by which the joined longitudinal edges (2) are indirectly heated to an optimum softening temperature in readiness for sealing one to another, and compression means (8) disposed and operating on opposite sides of the joined edges (2); characterized in that the electromagnetic induction means (7) have a negligible thermal inertia and are interlocked to a monitoring and control unit (15) able to sense the speed of the advancing joined edges (2) and to pilot the operation of the electromagnetic induction means (7) so that the temperature of the edges (2) is swiftly regulable in response to any variation of the advancing rate of the wrappers (5). Equipment as in claim 6, wherein the joined edges (2) are advanced by the feed means (14) and subject to the indirect action of electromagnetic induction means (7) disposed and operating in close proximity to the revolving surfaces (10) afforded by two rollers (9) of low thermal capacity maintained in direct contact with the joined edges (2). Equipment as in claim 7, wherein each roller (9) of low thermal capacity comprises an outer ring (11) in material of low thermal capacity and an inner ring or central core (12) of heat-insulating material. Equipment as in claim 7, wherein the rollers (9) of low thermal capacity are disposed in close contact with the joined edges (2) in such a way as to perform the function of compression means (8). Equipment as in claim 7, wherein each roller (9) of low thermal capacity operates in conjunction with means (22) by which to sense the temperature of the revolving surface (10) afforded by the selfsame roller (9), disposed immediately preceding the relative electromagnetic induction means (7) in the direction of rotation of the surface (10) and connected on the output side to a respective input of the monitoring and control unit (15).

Inventors: BERTI ANDREA, SPATAFORA MARIO, BERTI, ANDREA, SPATAFORA, MARIO
Applicants: ACMA SPA, AZIONARIA COSTRUZIONI MACCHINE AUTOMATICHE-A.C.M.A.-S.P.A.
Application year: 1996
Main classification: B65B51
IPC codes: B65B51