Abstract:
An induction sealing head, an induction sealing system and a method for sealing a closure to an aperture of a container are disclosed. The induction sealing head is for sealing a closure to an aperture of a container, and includes an induction coil, wherein the induction coil is wound about an axis, and has a first portion lying substantially in a first plane perpendicular to the axis, and a second portion stepped out of the first plane, and lying substantially in a second plane parallel with the first plane.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to an induction sealing apparatus and a method for sealing a closure to a container. 
       BACKGROUND 
       [0002]    Induction sealing is a well-known process which is used to seal a closure to an aperture of a container. A known induction sealing system includes a sealing head which includes an induction coil wound in shape corresponding to the shape of the aperture to be sealed. A multi-layer closure is positioned over the aperture of the container, and the container is positioned under the sealing head. The closure typically includes a metal foil layer and a polymer layer. The sealing head is moved near to the closure and container, and the induction coil is supplied with an alternating current. When an electric current is applied to the coil, a magnetic field is generated, which induces eddy currents in the foil closure. As a consequence, the temperature of the foil closure increases, causing the polymer layer on an underside of the metal foil layer to melt and bond the closure to the aperture of the container, forming a hermetic seal between the closure and the container. 
         [0003]    Such a system is suitable for sealing closures having a circular shape and a uniform thickness to containers having circular apertures. In such cases, a circular coil is used. However, for a closure having a non-circular shape or a closure having a non-uniform thickness, such as a closure which includes a folded-back tab, a circular coil is not suitable, since some regions of the closure require different amounts of heat to create a hermetic seal with the closure. 
         [0004]    Another known induction sealing system achieves a varied heating profile in the closure by incorporating ferrites into the sealing head in order to concentrate the magnetic field generated by the induction coil at particular locations, thereby increasing the heating effect at corresponding locations within the closure. However, ferrites can behave in an unpredictable and inconsistent manner, and the exact position of a ferrite in an induction coil required to achieve a particular heating profile in a closure can be difficult to predict. 
         [0005]    Thus there is a problem that an induction coil may not induce heat in a closure in a desired manner. In some instances, for example, uneven heating could result in the closure not being hermetically sealed to the container and, therefore, contents of the container could be spoiled. 
       SUMMARY 
       [0006]    According to a first aspect, the present invention provides an induction sealing head for sealing a closure to an aperture of a container, the sealing head comprising an induction coil, wherein the induction coil is wound about an axis, and has a first portion lying substantially in a first plane perpendicular to the axis, and a second portion stepped out of the first plane, and lying substantially in a second plane parallel with the first plane. The second plane may be substantially parallel to the first plane. 
         [0007]    The induction coil may have a third portion stepped out of the first plane and/or the second plane, and lying substantially in a third plane. The third plane may be parallel with the first plane and with the second plane. 
         [0008]    The induction coil may include a third portion stepped out of the first plane or the second plane, and lying substantially in a third plane. The third plane may be substantially parallel to at least one of the first plane and the second plane. 
         [0009]    The induction coil may be a multi-turn coil. The induction coil may be at least partially enclosed within a magnetic material. The magnetic material may comprise a ferrite. 
         [0010]    The sealing head may further comprise a contact pad for engaging with a closure to be sealed. The contact pad may lie substantially in a plane parallel to the first plane and the second plane, and the distance between the contact pad and the first plane may be different from the distance between the contact pad and the second plane. 
         [0011]    The induction coil may have a shape that is substantially circular. The first and second portions of the induction coil may each constitute approximately half of the induction coil. 
         [0012]    The induction coil may have a shape that is substantially rectangular. The second portion of the induction coil may constitute at least one corner of the induction coil. 
         [0013]    According to a second aspect, the present invention provides an induction sealing system comprising an induction sealing head as described above; and means for moving the closure and the container into a position relative to the sealing head such that the sealing head can induce a current in the closure. The means for moving the closure and the container may comprise a linear conveyor, a rotary conveyor or a star wheel conveyor. The closure includes an electrically conducting layer, typically a metal foil, so as to be heated by electromagnetic induction by the induction sealing head. The closure also includes a polymer layer which when heated and subsequently cooled forms a bond with the container aperture. 
         [0014]    According to a third aspect, the present invention provides a method for sealing a closure to an aperture of a container, the method comprising providing a sealing head having an induction coil which is wound about an axis, the induction coil having a first portion lying substantially in a first plane perpendicular to the axis, and a second portion stepped out of the first plane, and lying substantially in a second plane parallel with the first plane; and supplying a current to the induction coil such that the induction coil is capable of inducing a current in the closure to thereby heat the closure and seal the closure to the aperture of the container. The sealing head may be a sealing head as described above. 
         [0015]    It will be appreciated that the features of any of the various aspects of the invention may be combined with features of any other aspects of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: 
           [0017]      FIG. 1  is a schematic view of an induction system constructed in accordance with the present invention; 
           [0018]      FIGS. 2, 3, 4, 5 and 6  are perspective views of closures suitable for being sealed to containers by the present invention; 
           [0019]      FIGS. 7, 8, 9 and 10  are perspective view of closures and corresponding induction coils constructed according to various embodiments of the invention; and 
           [0020]      FIG. 11  is a schematic view of an induction sealing head constructed in accordance with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to the drawings,  FIG. 1  shows, schematically, an induction sealing system  100  constructed in accordance with the present invention. The sealing system  100  includes a sealing head  102  which is connected to a generator  104  which contains control electronics and other components (not shown) for supplying an alternating current to an induction coil  106 . For example, in one embodiment, the induction coil  106  is connected to at least one capacitor in series or in parallel to form a resonant circuit. The resonant circuit is energised by a high frequency power source (not shown), and may include a matching transformer. The induction coil  106  may be a single or multi-turn wound conductor, and may be formed of any suitable material, for example litz wire. In some embodiments, the induction coil  106  is partially or fully enclosed or embedded within a potting compound (not shown). 
         [0022]    The sealing system  100  can be used to seal one of a plurality of closures  108  to a corresponding one of a plurality of containers  110  as the closure and the container pass under the sealing head  102 . The closure includes a metal foil layer and a polymer layer. When heated the polymer layer flows into the container aperture and when cooled the polymer layer forms a bond with the container aperture resulting in a hermitically sealed product. The sealing system  100  may include conveying means, such as a linear or rotary conveyor  112  or a star wheel conveyor (not shown) for moving each container  110  into a required position beneath the sealing head  102  for sealing. In this regard, the conveyor  112  of this embodiment is configured to move the containers  110  in a direction shown by arrow X, such that an unsealed container and its closure are moved along the conveyor  112  until it is beneath the sealing head  102 . The sealing head  102  is then lowered, and an alternating current is supplied to the coil  106  for a duration sufficient for the closure to be heated to a temperature sufficient to seal the closure to the container. The sealing head is then raised upwards, away from the container and the sealed container is moved away from the sealing head so that an unsealed container can be moved into position beneath the sealing head. 
         [0023]    In some embodiments, the induction coil  106  of the sealing head  102  is brought close to a closure  108  and container  110  to be sealed, but no contact is made between the induction coil and the closure. In other embodiments, the sealing head  102  includes a crimp rubber (not shown) positioned beneath the induction coil  106  such that, when the sealing head is lowered towards the closure  108  and the container  110  to be sealed, the crimp rubber engages the closure, applying pressure to the closure and the container to aid the sealing process. In some embodiments, the crimp rubber also serves to crimp and fold down an excess peripheral portion of the closure, known as a skirt. 
         [0024]    In known sealing systems used for sealing circular closures to containers having circular apertures, a ring-shaped induction coil is used. Such a coil is oriented substantially parallel to the closure to be sealed, and is wound in a single plane. In general, the shape of an induction coil used in a sealing head corresponds to the shape of the closure and/or the aperture of the container being sealed. 
         [0025]    In the sealing system  100  of the present invention, the induction coil  106  is wound such that the coil defines a central plane, but has at least one portion extending out of that plane, as will be described below. 
         [0026]      FIGS. 2, 3, 4, 5 and 6  show examples of closures of various shapes and configurations that can be sealed to containers of corresponding shapes using the present invention. 
         [0027]      FIG. 2  shows a substantially round closure  200 , which includes a main portion  202 , which serves to seal a round aperture in a container, and a tab portion  204  which is folded flat onto the main portion  202  during the sealing process. When it is necessary to open the container and gain access to the contents of the container, then the tab portion  204  can be lifted upwards into a position shown by the dashed line, thereby making it easier for a user to remove the closure  200  from the container to which it is sealed. 
         [0028]      FIG. 3  shows a shows a round closure  300  having a main portion  302  which serves to seal a round aperture in a container, and a semi-circular tab portion  304  which is folded flat onto the main portion  302  during the sealing. Again, when it is necessary to open the container and gain access to the contents of the container, then the semi-circular tab portion  304  can be lifted upwards into a position shown by the dashed line, thereby making it easier for a user to remove the closure  300  from the container to which it is sealed. 
         [0029]      FIG. 4  shows a round skirted closure  400  having a main portion  402  which serves to seal a round aperture in a container, and a peripheral skirt portion  404 , which is not intended to be sealed to a container, but which may be crimped and folded down onto a container using a crimp rubber as discussed above. 
         [0030]      FIG. 5  shows a closure  500  which is substantially rounded rectangular in shape—that is to say substantially rectangular with straight sides  502  and rounded corners  504 —but does not include a tab portion. 
         [0031]      FIG. 6  shows a closure  600  which similar in shape to the closure  500 , but which has a main portion  602  and a tab portion  604 . The rounded rectangular closure  600  includes two relatively longer substantially straight sides  606  and two relatively shorter substantially straight sides  608  which are joined to the longer straight sides by curved portions  610 . 
         [0032]    Each of the closures shown in  FIGS. 2 to 6  includes at least one feature which affects the heating profile in the closure resulting from a current induced by an induction coil, as will now be discussed. 
         [0033]    The closures  200 ,  300  and  600  shown in  FIGS. 2, 3 and 6  respectively include fold-back tab portions which increase the overall thickness of those portions of the closures. Consequently, when each of closures  200 ,  300  and  600  undergoes a sealing process, a magnetic field generated by an induction coil would be attenuated or shielded by the additional tab portion, resulting in a reduced heating effect in the portion of the closure beneath the tab portion. 
         [0034]    When the closure  400  shown in  FIG. 4  undergoes a sealing process, the skirt portion  404  affects a magnetic field generated by an induction coil by drawing some of the magnetic field away from the main portion  402  of the closure, and towards the skirt portion  404 , thereby reducing the heating effect in the main portion of the closure. The skirt portion  404  of the closure  400  is substantially uniform and continuous around the main portion  402  of the closure. However, in other embodiments, the non-uniform skirt portions may be used, for example having regions of different lengths. Such non-uniform skirt portions would further affect the magnetic field generated by an induction coil and, hence, the heating profile within the closure. 
         [0035]    In the closures  500  and  600  shown in  FIGS. 5 and 6  respectively, the corner portions of each of the closures affect the heating profile resulting from an induced current from an induction coil. When such closures undergo a sealing process, the currents induced in portions of the closure along the straight sides  606 ,  608  differ from the currents induced at the curved portions  610  of the closure. At the corner portions  610  of the closure  600 , the magnetic field generated by the induction coil is weaker than at the straight portions  606  of the closure. Thus, the heating effect resulting from the induced current at the curved portions  610  of the closure is not as great as the heating effect at the straight side portions  606  of the closure. 
         [0036]    The present invention enables control of the heating effect of the closure at particular locations in the closure. A sealing head constructed in accordance with the present invention includes an induction coil which is wound substantially in a plane parallel to the closure and to the aperture of the container to be sealed, but which has at least one region extending out of the plane. Put another way, the induction coil has a first portion lying substantially in a first plane, and a second portion stepped out of the first plane, and lying substantially in a second plane. The first and second planes are necessarily substantially parallel to the aperture of the container to be sealed. The magnetic field generated from whichever portion of the induction coil that lies in a plane relatively closer to the closure will have a greater effect on the closure that the portion of the induction that lies in a plane relatively further away from the closure. Thus, by stepping portions of the induction coil into planes lying closer to, or further away from, the closure, it is possible to accurately control the amount of heat induced in particular portions of the closure. 
         [0037]      FIGS. 7, 8, 9 and 10  show examples of arrangements of induction coils suitable for sealing the closures of  FIGS. 2, 3, 5 and 6 , respectively. It will be appreciated that, in use, each induction coil will form part of a sealing head, the rest of which is not shown in  FIGS. 7 to 10  for clarity. 
         [0038]      FIG. 7  shows an arrangement  700  which includes an induction coil  702  and a closure  704 . The closure  704  is similar to the closure  200  shown in  FIG. 2  in that it is a round closure which has base portion  706  and a folded-back tab portion  708  joined to the base portion. The tab portion  708  can be lifted upwards to provide means for a more easily remove the closure  704  from a container to which it is sealed. When the tab portion  708  is laid flat against the base portion  706 , the thickness of the closure  704  in the region including the tab portion  708  is greater than the thickness in the un-tabbed region. In other words, the total thickness of closure  704  in the region including the tab portion  708  includes the thickness of the base portion  706  and the thickness of the tab portion  708 , whereas the thickness of the un-tabbed region of the closure includes only the thickness of the base portion. 
         [0039]    To compensate for the extra material in the part of the closure  704  having the tab portion  708 , the induction coil  702  has a first portion  710  wound in a first plane substantially parallel to the closure  704 , and a second portion  712  which is stepped out of the first plane, and lies in a second plane such that separation between the second portion  712  and the closure is less than the separation between the first portion  710  and the closure. As a consequence, when the closure  704  is moved under a sealing head which includes the induction coil  702  in order to be sealed, the magnitude of the currents induced in the base portion  706  of the closure will be less than the magnitude of the currents induced in the portion of the closure which includes the tab portion  708 . Although the magnitude of the magnetic field generated from the second portion  712  of the induction coil  702  is equal to the magnitude of the magnetic field generated from the first portion  710  of the induction coil  702 , the reduced separation between the second portion of the induction coil and the closure compensates for the increased thickness of the portion of the closure  704  which includes the tab portion  708 , resulting in a relatively uniform heating effect around the edge of the closure. 
         [0040]      FIG. 8  shows an arrangement  800  including an induction coil  802  and a closure  804 . The closure  804  is similar to the closure  300  shown in  FIG. 3  in that it is a round closure which has base portion  806  and a semi-circular tab portion  808  joined to the base portion  806 . The tab portion  808  can be lifted upwards to provide means for more easily removing the closure  804  from a container to which it is sealed. When the tab portion  808  is laid flat against the base portion  806 , the thickness of the closure  804  on the tabbed half is greater than the thickness on the other, un-tabbed half. In other words, the total thickness of one half of the closure  804  includes the thickness of the base portion  806  and the thickness of the tab portion  808 , whereas the thickness of the other half of the closure includes only the thickness of the base portion. 
         [0041]    To compensate for the extra material on the side of the closure  804  having the tab portion  808 , the induction coil  802  has first portion  810  wound in a plane substantially parallel to the closure  804 , and a second portion  812  which is stepped out of the first plane, and lies in a second plane such that separation between the second portion  812  and the closure is less than the separation between the first portion  810  and the closure. As a consequence, when the closure  804  is moved under a sealing head which includes the induction coil  802  in order to be sealed, the magnitude of the currents induced in the base portion  806  of the closure will be less than then magnitude of the currents induced in the portion of the closure which includes the tab portion  808 . Although the magnitude of the magnetic field generated from the second portion  812  of the induction coil  802  is equal to the magnitude of the magnetic field generated from the first portion  810  of the induction coil, the reduced separation between the second portion of the coil and the closure compensates for the increased thickness of the portion of the closure  802  which includes the tab portion  808 , resulting in a relatively uniform heating effect around the edge of the closure. 
         [0042]      FIG. 9  shows an arrangement  900  including an induction coil  902  suitable for sealing a closure  904 . The closure  904  is tab-less, and has a rounded rectangle shape, similar to the closure  500  of  FIG. 5 . As can be seen in  FIG. 9 , the induction coil  902  has straight portions  906  which lie in a first plane substantially parallel to the closure  904 . The induction coil  902  has corner portions  908  which are stepped out of the first plane, and lie in a second plane such that the separation between each of the corner portions and the closure  904  is smaller than the separation between each of the straight portions  906  and the closure. As a consequence, when the closure  904  is moved under a sealing head which includes the induction coil  902  in order to be sealed, the magnitude of the currents induced in the corner regions of the closure  904  will be greater than the magnitude of the currents induced in the straight portions of the closure. The magnitude of the magnetic field generated from the straight portions  906  of the induction coil  902  is greater than the magnitude of the magnetic field generated from the corner portion  908  of the induction coil. Thus, the reduced separation between the corner portions  908  of the coil and the closure compensates for the weaker magnetic field in those regions, resulting in a relatively uniform heating effect around the edge of the closure  904 . 
         [0043]      FIG. 10  shows an arrangement  1000  including an induction coil  1002  suitable for sealing a closure  1004 . The closure  1004  is similar to the closure  600  shown in  FIG. 6  in that it is a substantially rounded rectangular closure which has two relatively longer substantially straight sides  1006  and two relatively shorter substantially straight sides  1008  which are joined to the longer straight sides by curved corner portions  1010 . The closure  1004  has a base portion  1012  and includes a folded-back tab portion  1014  joined to the base portion. The tab portion  1014  can be lifted upwards to provide means for a more easily remove the closure  1004  from a container to which it is sealed. When the tab portion  1014  is laid flat against the base portion  1012 , the thickness of the closure  1004  in the region including the tab portion  1014  is greater than the thickness in the un-tabbed region. In other words, the total thickness of closure  1004  in the region including the tab portion  1014  includes the thickness of the base portion  1012  and the thickness of the tab portion, whereas the thickness of the un-tabbed region of the closure includes only the thickness of the base portion. 
         [0044]    To compensate for the extra material in the part of the closure  1004  having the tab portion  1014 , and for the relatively weaker magnetic field generated by the corner portions  1010  of the induction coil, the induction coil  1002  has a first portion  1016  corresponding to each of the straight sides of the closure, wound in a first plane substantially parallel to the closure  1004 , and a second portion  1018  corresponding to each of the three corners, the second portion being stepped out of the first plane, and lying in a second plane such that separation between the second portion  1018  and the closure is less than the separation between the first portion  1016  and the closure. The induction coil  1002  also has a third portion  1020  corresponding to the tab portion of the closure, the third portion being stepped out of the first plane and/or the second plane, and lying in a third plane such that the separation between the third portion and the closure is less than the separation between the second portion and the closure. As a consequence, when the closure  1004  is moved under a sealing head which includes the induction coil  1002  in order to be sealed, the magnitude of the currents induced in the straight sides  1006  of the closure will be less than the magnitude of the currents induced in the corner portions  1010  of the closure which, in turn, will be less than the magnitude of the currents induced in the corner portion of the closure which includes the tab portion  1014 . Although the magnitude of the magnetic field generated from the first portion  1016  of the induction coil  1002  is equal to the magnitude of the magnetic field generated from both the second portion  1018  and the third portion  1020  of the induction coil, the reduced separation between the second and third portions of the induction coil and the closure relative to the first portion compensates both for the increased thickness of the portion of the closure  1004  which includes the tab portion  1014  and for the weaker magnetic field in the corner regions, resulting in a relatively uniform heating effect around the edge of the closure. 
         [0045]    In some embodiments, the induction coil includes stepped regions configured such that a non-uniform heating profile is created in a closure, for example to create regions of relatively greater heating within a closure. In other embodiments, a multi-turn induction coil formed of a plurality of turns of a wire wound substantially in a first plane might include a subset of the plurality of wires which are stepped out from the first plane, and lie in a second plane. 
         [0046]      FIG. 11  shows, schematically, an induction sealing head  1100  constructed in accordance with an alternative embodiment of the invention. The sealing head  1100  includes an induction coil  1102  having a first portion  1104  wound in a first plane and a second portion  1106  stepped out of the first plane and lying in a second plane substantially parallel to the first plane. In this embodiment, the sealing head  1100  includes magnetic field concentrator elements  1108  in the form of ferrites, which serve to concentrate the magnetic field generated by the induction coil at particular regions. The sealing head  1100  further includes a contact pad  1110  which is configured to engage the closure to be sealed. The contact pad  1110  serves to apply even pressure onto the closure around the aperture of the container to aid the sealing process. 
         [0047]    Thus, the present invention is suitable for creating a desired heating profile within closures having a non-circular shape, closures having skirted portions, and closures having tab portions, due to the ability to control the separation between the portions of the induction coil and corresponding portions of the closure to be sealed, and hence the resulting change in the heating effect at particular regions of the closure. 
         [0048]    So far, the invention has been described in terms of individual embodiments. However, those skilled in the art will appreciate that various embodiments of the invention, or features from one or more embodiments, may be combined as required. It will be appreciated that various modifications may be made to these embodiments without departing from the scope of the invention, which is defined by the appended claims.