BACKGROUND OF THE INVENTION 
     The invention relates to methods and apparatus for producing bags having a recloseable fastener and in particular bags in which such a fastener extends in substantially the same direction as the top and bottom seals of the bag. 
     It is well known to make bags by a form, fill and seal (FFS) process. The FFS process is well known and will be described herein with reference to vertical, form, fill and seal (VFFS) machines. In such a machine, a web of flexible packaging material is fed over a forming shoulder to form a tube about a vertically oriented filling pipe. The edges of the web are sealed together in the longitudinal direction of the tube and then successive transverse seals are made at bag length intervals by means of opposed sealing jaws. One jaw of an opposed pair usually carries a knife which cuts through the tubular web at each transverse seal to provide a top seal for a completed bag and a bottom seal for the following bag. Product is fed down the forming tube in synchronicity with the sealing jaws so that each package contains a required amount of product. 
     It is also known to provide bags produced on FFS machines with a recloseable fastener. Such fasteners typically take the form of two elongate elements one of which comprises a rib extending along the length of the element and the other of which has a channel which receives the rib. The arrangement of the elements is such that the rib can be pushed into the channel and will be releasably retained therein to form a fastening. Such fasteners will be referred to herein as zips or zip fasteners. 
     One approach to applying zips to bags produced on VFFS machines has been to apply the zip to the web before it reaches the forming shoulder. The web approaches the forming shoulder in planes which may be horizontal but are at least transverse to the axis of the forming tube. The zip is fed from a supply reel across the web from one side thereof with the two elements of the zip engaged, ie with the rib received in the channel. At the location at which the zip is to be applied to the web, a length of the zip substantially equal to the width of the bag is cut and then the cut length is secured to the web by means of a heat sealing jaw. In more detail, one of the two elements of the zip is secured to the upwardly facing side of the web by heat welding. The element may be securely welded to the web or tacked onto the web. The web with attached zip then passes over the forming tube and when the opposed sealing jaws meet to form the transverse bag seals, the elements of the fastening strip are simultaneously welded to their respective sides of the bag just below the top seal of the bag. 
     SUMMARY OF THE INVENTION 
     The invention provides apparatus for securing a fastening strip to a web of packaging material, said apparatus comprising means defining a support surface for said web, said support surface having an elongate recess for receiving said fastening strip, movable sealing means disposed opposite and below said support surface, a retractable support element disposed between said support surface and said sealing means, means for feeding said fastening strip in a lengthwise direction of said recess and between said support element and said support surface and means for holding said fastening strip, the arrangement being such that said support element can be retracted to a position which permits said sealing means to move said web into engagement with said support surface and said fastening strip with said fastening strip received in said recess, said holding means being actuable to hold said fastening strip when said support element is retracted. 
     The invention also includes a method of securing a fastening strip to a web of packaging material, said method comprising the steps of feeding said fastening strip across said web such that a length of said strip is positioned at a predetermined position relative to said web, providing a support for said fastening strip during said feeding step, applying a holding force to said fastening strip and withdrawing said support, moving said web into engagement with said fastening strip and securing said web to said fastening strip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic side elevational of a vertical, form, fill and seal machine for making recloseable bags; 
     FIG. 2 is a schematic representation of a zip applying apparatus for the machine shown in FIG. 1; 
     FIG. 3 is a schematic side elevation of a zip which can be used with the zip applying apparatus of FIG. 2; 
     FIG. 4 is a section on line IV—IV in FIG. 2; 
     FIG. 5 shows details of a sealing device of the apparatus shown in FIG. 2; 
     FIG. 6 is a schematic section view of a zip strip guide of the zip applying apparatus. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic side elevation view of a VFFS machine  10  for making recloseable bags. The machine comprises a frame  12  to the rear of which is fixed a mounting device  14 . The mounting device  14  is arranged to receive a reel  16  of packaging film which is held so as to be rotatable whereby the film, or web  18 , can be gradually payed off from the reel by means of a nip roller drive  19  or, alternatively a motor drive (not shown). It will be understood that the position of the nip drive  19  indicated in FIG. 1 is purely exemplary and that it may be positioned at any suitable position on the machine. 
     The web  18  passes upwardly from the mounting device to a first roller unit  20 , also mounted at the rear of the frame. The roller unit  20  comprises one or more rollers (illustrated schematically as roller  22 ) around which the web is guided and is arranged such that the web is directed generally horizontally towards the front of the frame  12 . 
     A cylindrical filling tube  24  is supported on the front of the frame and carries a forming shoulder  26 . A weigher or other control device  28  is mounted above the filling tube  24  and serves to drop predetermined amounts of product into the filling tube. 
     The web  18  is directed from the roller  22  towards the filling tube  24  and the forming  26  shoulder is arranged to cause the web to adopt a tubular configuration about the filling tube  24 . Downstream of the forming shoulder, there is a heat sealing unit  36  which is positioned at the front of the machine and connected to the frame  12  by means of an L-shaped connecting arm  38 . The heating sealing unit may be of any known form including a hot air unit such as that disclosed in WO98/40202, the content of which is incorporated herein by reference. 
     Two drive bands  40 , or alternatively draw-off rollers, are arranged on opposite sides of the forming tube  24  to draw the web  18  downwardly towards a transverse sealing and cutting device  41 . The drive bands  40  and nip roller drive are controlled by a control system  42  and provide a web feed arrangement for the VFFS machine  10 . A photoelectric cell  43 , or other suitable sensing device, is positioned adjacent the forming shoulder to detect registration marks on the web  18 . The control system uses the signals provided by the sensor  43  to control the web feed. 
     The sealing and cutting device  41  comprises opposed reciprocating sealing jaws  44 , which serve to form transverse seals in the film to define a succession of packages. The sealing jaws are preferably actuated by respective pneumatic cylinders (not shown). One jaw of the pair includes a blade (not shown) by means of which the individual packages  45  are separated from the film. The arrangement and form of the jaws may be of any suitable conventional type and since suitable sealing jaw arrangements will be familiar to those skilled in the art, the sealing jaws and knife arrangement will not be described in detail herein. 
     The control system  42  is arranged to provide suitable control signals for synchronising the operations carried out by various parts of the machine. The control system can be of any known type and may, for example, comprise a PLC. Control systems for bag making machinery are well know to those skilled in the art and will not therefore be described in any further detail herein. 
     A zip applying apparatus  46  is provided for applying a length of zip to the web  18  upstream of the forming shoulder  26 . 
     FIG. 3 is a schematic representation of a typical configuration of zip fastener  47 . The zip fastener  47  comprises two elements  48 , 49 . The lowermost element  48  defines a recess  50  that is arranged to snap-fittingly engage with a projection  51 , which is a part of the upper element  49 . In order to seal the zip fastener to the bag, three seals will be made. The seals will be made in the regions  48 ( 1 ),  48 ( 2 ) and  49 ( 1 ) of the zip, which will be applied to the web such that the region  48 ( 1 ) forms the leading edge of the zip as the web is fed downstream of the zip applying apparatus. 
     It is to be understood that the zip fastener  47  shown in FIG. 3 has been shown simply to illustrate the functioning of the zip applying apparatus  46  and that the apparatus and invention are not limited to use with that particular configuration of zip. 
     Referring to FIGS. 2,  4  and  6 , the zip applying apparatus  46  includes a reel stand  52  for mounting a reel  54  of zip  47  in strip form and a nip drive  56  for drawing the zip strip  58  from the reel. The nip drive draws the zip strip over a roller mounted on a banjo arm  59  and feeds it into a reservoir  60  which houses a roller  62  supported by the zip strip. The roller  62  is free to move in the directions indicated by the arrow  64 . An upper limit sensor  66  and a lower limit sensor  68  are arranged to detect the presence of the roller  62  when it reaches respective limit positions. 
     Downstream of the reservoir  60 , the zip strip passes over a guide roller  70  which guides the strip into a first sealing device  74 , which is arranged to form spot welds at spaced intervals along the strip. The sealing device  74  is described in more detail hereinbelow. 
     Downstream of the sealing device  74 , there is a further guide roller  76  which guides the strip to a nip drive  78 . Intermediate the sealing device  74  and guide roller  76 , there is a registration adjustor, which includes a roller  80 . The roller  80  is releasably mounted in a pair of opposed guide slots  82 , whereby its position relative to the sealing device  74  can be adjusted. 
     The nip drive  78  is arranged to feed the zip strip  58  over the web  18  in a direction, indicated by arrow  84 , which is transverse to the direction in which the web is feeding. 
     As best seen in FIG. 6, the nip drive  78  preferably comprises two opposed rollers  85  of matched diameter and each having a knurled circumferential surface for engaging the zip strip  58 . The rollers  85  have a width (W) which is significantly less than that of the zip strip and are arranged to grip on the center portion of the zip strip. For example in one embodiment, the rollers  79  have a width of 5 mm for use with a zip strip having a width of 25 mm. Preferably, both rollers  79  are driven synchronously. It will be appreciated that this arrangement ensures that both elements of the zip strip are positively fed so that there is no tendency for slippage between them. A zip strip guide  86  is associated with the rollers  79  and serves to prevent the zip strip wrapping around the rollers  79 . The zip strip guide comprises two opposed plates secured together and shaped to define a channel  87  for the zip strip  58 . Each plate is provided with an aperture into which a portion of the respective roller  85  protrudes so that it can engage the zip strip. 
     A second sealing device  88  is positioned in the path of the web  18  for sealing lengths of the zip strip to the web. A zip knife  90  is provided adjacent the sealing device  88  for severing the lengths of zip from the strip  58 . The zip knife includes a blade  92  and a pneumatic cylinder  94  arranged to reciprocate the blade as indicated in FIG.  2 . 
     As best seen in FIG. 4, the second sealing device  88  comprises an anvil  100  which defines a vacuum chamber  102 . The anvil  100  additionally defines a profiled slot  104  which extends across the entire width of the anvil. The slot  104  is connected with the chamber  102  by means of a series of bores  106  spaced along the length of the slot. The vacuum chamber is connected with a vacuum source  108  by means of a pipe  110  and with a source of pressurised air  112  by means of a pipe  114 . 
     The second sealing device also includes a sealer  116  which comprises a clamping block  118  (FIG. 4) carrying two impulse sealers  120 . 
     The clamping block  118  is arranged to be reciprocated in the direction indicated by the arrow  122  (FIG. 2) by means of a pneumatic cylinder  124 . 
     The clamping block is provided with rubber cushions  125 . The clamping block  118  includes two bores  126  extending across the width of the sealer  116  and connected to the pressurised air source  112  by means of a pipe  127 . Each bore  126  communicates with a respective series of angled bores  128 , which are directed inwardly and upwardly of the clamping block. 
     A support plate  130  is arranged between the housing  102  and clamping block  118  and is connected with an arm  132  which is in turn connected with a pneumatic cylinder  134 . The cylinder  134  is actuable to retract the support plate from its position between the housing and clamping block to permit the clamping block to be moved by the pneumatic cylinder  124  to a position in which it presses the web  18  against the underside of the anvil  100 . The pneumatic cylinder may be arranged to move the guide plate against a return spring  136  or may be a double-acting cylinder, in which case, a return spring is not required. 
     As shown in FIG. 5, the first sealing device  74  comprises an anvil  140  which defines two projections  142 . The anvil is arranged to be reciprocated by means of a pneumatic cylinder  144 . In transverse cross-section perpendicular to the plane of the drawing, the projections are preferably substantially rectangular. 
     The first sealing device  74  also includes a horn  146  disposed opposite the anvil  140  and a source of ultrasound  148 . 
     In use, the web  18  feeds from the reel  16  onto the forming shoulder  26  which causes it to wrap around the filling tube  24  and adopt a tubular configuration corresponding substantially to the shape of the forming tube. At bag length intervals, the web feed is halted and the sealing unit  36  applies heat to the longitudinal edges of the tubular web to form a continuous seal extending parallel to the axis of the forming tube. At the same time, the sealing jaws  44  move towards one another to engage the web and form transverse seals each of which defines the top seal of a completed bag  45  and the bottom seal of a bag being formed immediately upstream thereof. The blade carried by one of the sealing jaws severs the web through the transverse seal to separate the bag  45  from the web. The zip is applied to the web, whilst the web feed is halted to allow the formation of the transverse seals. 
     The zip applying apparatus  46  operates by drawing zip strip  58  from the reel by means of the nip drive  56  which feeds the strip into the reservoir  60 . The length of strip contained in the reservoir supports the roller  62  against the influence of gravity and the nip drive  56  is actuated in response to signals from the sensors  60 ,  62 . 
     The nip drive  78  is actuated to draw bag width lengths of zip strip from the reservoir. When the nip drive  78  is not feeding, the sealing device  74  is actuated to form spot welds  150  between the two elements of the zip strip. The arrangement of the sealing device  74  is such that the zip strip is positioned close to the horn  146  and to form a seal the anvil  140  is raised to bring the projections into engagement with the zip whilst the source  148  is activated to generate ultrasound vibrations which are fed into the horn. Each sealing operation provides a pair of spot welds  150  and the sealing operations are timed so that the pairs of spot welds are formed at spaced locations along the length of the strip, the spacing between adjacent pairs corresponding approximately to the width of the completed bag  46 . 
     Each time the nip drive  78  is actuated, it feeds a bag width length of zip strip across the web in a direction  84  which is perpendicular to the direction in which the web is fed. Whilst the zip is feeding across the web  18 , it is supported by the support plate  130  so that it does not contact the web  18  which can be feeding at the same time without the problem of the web movement affecting the tracking of the zip strip, which can therefore be accurately aligned across the web. As it moves across the support plate, the zip is guided by the slot  104 . 
     When the web  18  is halted to form the transverse seal, the vacuum source  108  is connected with the vacuum chamber so that the zip strip is held in the recess  104 . The pneumatic cylinder  134  is actuated to retract the support plate and the clamping block is raised by means of the cylinder  124  to push the web  18  into contact with the zip contained in the recess and the underside of the anvil which acts as a support surface for the web whilst the impulse sealers are activated to seal the web to the underside of the lowermost zip element  48 ; seals being formed between the web and the regions  48 ( 1 ) and  48 ( 2 ) of the zip. Substantially simultaneously, the blade  92  is moved by the cylinder  94  to cut through the zip strip between adjacent pairs of spot welds  150 . 
     It will be understood that by providing pairs of spot welds  150  at bag length intervals and cutting between welds of a pair so that the leading weld is at the trailing end of the length of strip attached to the web and the trailing weld is positioned ready to be fed across the web the next time the nip drive  78  is actuated, each length of zip strip attached to the web has a spot weld at its end regions, which will be at opposite sides of the bag opening. This arrangement of the welds in the bag opening serves to maintain registration of the zip elements when the zip is opened making it easier to close the zip and reseal the bag. 
     Where necessary, for example when applying zip to polyethylene and similar materials, cooling air can be directed at the web via the bores  128 . 
     Once the cutting of the zip is complete, the vacuum source is disconnected and pressurised air is fed into the chamber  102  to drive the zip from the recess  104 . The web  18  can then feed forward towards the forming shoulder  26 . 
     The second sealing device  88  preferably forms a complete seal between the zip element  48  and the web. However, the seal may only be partial such that it is sufficient to hold the zip in place during the forming processes which occur downstream of the zip applying apparatus. The sealing of the other zip element  49  to the web  18  takes place between the sealing jaws  44  as is already known in the art. If the zip-applying device has only partially sealed the zip element to the web, the complete seal is formed between the sealing jaws. Sealing jaws for forming seals between one or both elements of the zip are known in the art of bag making and will not be described in further detail herein. 
     It will be appreciated that another advantage of the arrangement of the zip applying apparatus  46  is that when the zip feed process is complete and the zip is correctly positioned over the web  18 , withdrawal of the support plate  130  whilst the zip is held from above, permits access to both the regions  48 ( 1 ),  48 ( 2 ) so that the zip element  48  can be finish sealed to the web. Thus when the zip reaches the sealing jaws  44 , the only operation additional to the transverse sealing of the web is the forming of a seal between the web and the upper zip element  49 , thereby permitting the sealing process to be carried out more quickly should this be desired. This possibility has been found to be particularly advantageous in use of the machine  10  to form so-called block bottom bags. It is, however, to be understood that the apparatus is not limited to this feature and that it is possible for the second sealing device to be configured to produce only a seal at the region  48 ( 1 ) should this be desired. 
     It will be appreciated that another advantage of the provision of the support plate is that it allows the web  18  and zip strip to be fed simultaneously and the web feed only need be halted for long enough for the various seals to be formed. 
     The zip applying apparatus may be used with a VFFS machine in which the jaws are continuously rotated and the web  18  is fed continuously from the reel  16  and between the rotary sealing jaws. The rotary sealing jaws may be guided so as to follow a generally D-shaped or rectangular (box-like) path. If the apparatus is to be used with such a continuous machine it would be necessary to provide an accumulator to permit the web feed to be interrupted upstream of the forming shoulder to allow the zip to be applied to a stationary web portion whilst continuous motion of the web between the sealing jaws is maintained. Such accumulators and other lost motion devices which would permit the apparatus to be used in a continuous motion bag making machine will be well known to those skilled in the art and will not therefore be described in any further detail herein. 
     The registration device  82  allows the length of zip strip between the sealing device  74  and the cutting blade  92  to be varied manually according to the desired bag width so that the cutting blade will always sever the strip between a pair of adjacent spot welds  150 . Although, the roller  80  could be servo driven in order to provide a dynamic adjustment, it is envisaged that the roller will be moved to a desired position and clamped in place as part of a setting up procedure. 
     In a modification to the zip applying apparatus  46 , a photoelectric cell  200  may be provided for detecting registration marks on the web adjacent the zip applying apparatus  46 . The signals from the photoelectric cell  200  are used to control the nip roller drive  78  independently of the signals from the sensor  43 . Providing separate sensors located at optimum positions relative to the zip applying apparatus and the transverse sealing device  41 , permits more accurate positioning of the zip and the transverse seal relative to any printed matter on the web. 
     A further modification would be to provide a further nip drive for the web immediately adjacent the zip applying apparatus which could be driven independently or in synchronisation with the web feed arrangement. If this further nip drive is to be driven independently, an intermediate reservoir, for example an arrangement such as the roller mounted on a banjo arm  59 , would be employed to accommodate any differences in the web feed provided by the web feed arrangement and the further nip drive. 
     It will be understood that the machine  10  could be readily modified to feed a continuous stream of bags to a coiling device to form a reel of performed bags which can be fitted to another machine for filling and scaling.
An apparatus ( 50) for securing a fastening strip ( 58) to a web ( 18) of packaging material comprises an anvil ( 100) which defines a support surface for the web. The support surface has an elongate recess ( 104) for receiving the fastening strip. There is a moveable sealing device ( 116) disposed opposite and below the support surface and a retractable support element ( 130) disposed between the support surface and sealing device. A feeding arrangement feeds the fastening strip along the recess and between the support element and the support surface arrangement. Suction is applied via a vacuum chamber ( 102) to hold the fastening strip in the recess and permit the support element to be retracted to a position which permits the sealing device to move the web into engagement with the support surface and the fastening strip with the fastening strip received in the recess to permit sealing of the fastening strip to the web.