Patent Publication Number: US-2006018570-A1

Title: Plastic bag with integral tie strip

Description:
FIELD OF THE INVENTION  
      This invention relates generally to a plastic bag product having a perforated integral tie strip, and a method of manufacturing such a product.  
     BACKGROUND OF THE INVENTION  
      The most basic disposable plastic storage bags typically comprise a pair of overlapping plastic sheets closed along a bottom edge and a pair of side edges, or a tubular body welded along one end to form a closed bottom edge. The top edge is left open and can be temporarily closed by a separate closure mechanism such as wire twist ties. However, such closure mechanisms tend to be awkward to apply to the bag, often requiring both hands to handle the closure mechanism. Also, such separate closure mechanisms tend to get separated from the bag product and lost. Examples of such bag products with separate closure mechanisms are disclosed in U.S. Pat. Nos. 3,633,247; 3,662,434; 3664,575; 3,972,469; 3973,610; 3,997,943 and 4,077,562.  
      Attempts have been made to develop more complex bag products that integrate a closure mechanism into the bag&#39;s body. For example, tie elements have been integrated into the body of the bag. An example of such bag products are disclosed in U.S. Pat. No. 5,044,775. These bag products require manufacturing processes that are relatively complex and time consuming, and manufacturing machines that are relatively expensive.  
      It is also known to provide a channel in the bag body to receive a tie element. An example of such a design involves producing a hem on the bag that receives a tie element threaded therethrough. Such a hem requires additional bag material and manufacturing steps, thereby resulting in a more expensive and complex product that is relatively time consuming to manufacture. Examples of such bag products are disclosed in U.S. Pat. Nos. 3,029,853 and 3,506,048.  
      In yet another bag product design, the tie element is attached to the bag product by an adhesive, and can be separated from the bag for use in tying closed the bag opening. Such adhesives tend to add significant expense to the manufacture of the bag product. Also, when the tie element is separated from the bag, the user typically uses both hands to tie the tie element around the bag opening, which can be awkward as he must also hold the bag at the same time. U.S. Pat. Nos. 3,412,926 and 3,974,960 are examples of such bag products.  
      It is therefore an object to provide a closable bag product that is an improvement over known closable bag products and a method of manufacturing same.  
     SUMMARY OF THE INVENTION  
      According to one aspect of the invention, there is provided a bag product comprising a bottom-welded receptacle and a tie strip attached to the receptacle and used to close an opening of the receptacle. The receptacle is made of a flexible plastic material and comprises an opening at a top of the receptacle and a bottom line weld that closes a bottom of the receptacle. The tie strip is made of flexible plastic material and has a bottom end secured to the receptacle by the bottom line weld, a top end secured to the receptacle near the opening by a top line weld, and perforations extending transversely across the tie strip that enables enough of a top portion of the tie strip to be separated from a bottom portion of the tie strip to enable the top portion of the tie strip to wrap around and close the receptacle opening.  
      Such a bag product can be disposable and is particularly useful for storing items such as trash. Because the tie strip is attached to the receptacle, there is no risk of losing the tie strip. Also, because one end of the tie strip remains attached near to the opening of the receptacle and is exceptionally long when the tie strip is separated at the perforations, wrapping the tie strip around the receptacle opening is particularly convenient. Furthermore, because the bottom weld line closes the bottom of the receptacle as well as attaches the bottom end of the tie strip, manufacture of the bag product is made more efficient.  
      The top line weld can be a weld selected from the group of a continuous line weld, intermittent line weld and spaced spot welds. Also, the tie strip can be arranged substantially perpendicular to the receptacle opening.  
      The receptacle can be an extruded tubular film which is closed at the bottom by the bottom line weld. Alternatively, the receptacle can be a plastic coated material. In particular, the tubular film can comprise first and second overlapping sheets joined together by a pair of side edges and closed at the bottom by the bottom line weld. In such case, the top end of the tie strip can be welded to the first sheet by the top line weld. The top line weld can secure the tie strip to the first sheet without securing the first and second sheets together. Or, the top line weld can secure the tie strip to the first sheet, as well as securing the first and second sheets together.  
      According to another aspect of the invention, there is provided a bag product comprising a side-welded receptacle and a tie strip attached to the receptacle and used to close an opening of the receptacle. The receptacle is made of a flexible plastic material and comprises an opening at a top of the receptacle, left and right sides respectively closed by left and right line welds, and a closed bottom. The tie strip is made of flexible plastic material and has a left end secured to the receptacle by the left line weld, a right end secured to the receptacle by the right line weld, and perforations extending transversely across the tie strip and between the secured ends such that the tie strip can be separated into two portions each having a free end and a secured end, and wherein at least one of the tie strip portions can wrap around and close the receptacle opening.  
      The receptacle can be a folded plastic film having a fold line that defines the closed bottom, and having side edges closed by the left and right line welds, and a pair of top edges that define the receptacle opening. The length of the tie strip can extend substantially parallel to and close to the bag opening A plurality of spot welds can be spaced along the length of the tie strip and serve to secure the tie strip to a surface of the receptacle. This reduces the tendency for the tie strip to sag relative to the receptacle surface.  
      The perforations can be located near one of the ends of the tie strip such that when the tie strip is separated, one of the tie strip portions is substantially longer than the other portion, making it easier to wrap the tie strip portion around the receptacle opening. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1 ( a ) is a schematic perspective view of a roll of flattened tubular plastic sheet and  FIG. 1 ( b ) a roll of flattened tubular plastic sheet with one longitudinal edge cut open.  
       FIG. 2  is a schematic side view of a prior art plastic bag manufacturing line.  
       FIG. 3  is a schematic side view of a plastic bag manufacturing line that joins a tie strip to a receptacle sheet according to one embodiment of the invention.  
       FIG. 4  is a schematic plan view of a welding and cutting assembly of the plastic bag manufacturing line.  
      FIGS.  5 ( a ) to ( c ) are schematic front views of different embodiments of a plastic bag with integral tie strip product as manufactured by the manufacturing line shown in  FIG. 3 , namely bottom-welded bags with a tie strip sealed to the top edge of a receptacle by a continuous line weld ( FIG. 5 ( a )), intermittent line welds ( FIG. 5 ( b )), and spot welds ( FIG. 5 ( c )).  
      FIGS.  6 ( a ) to ( c ) are schematic front views of different embodiments of a plastic bag with integral tie strip product as manufactured by the manufacturing line shown in  FIG. 3 , namely bottom-welded bags with a tie strip sealed to the top edge of a receptacle by a continuous line weld ( FIG. 6 ( a )), intermittent line welds ( FIG. 6 ( b )), and spot welds ( FIG. 6 ( c )), wherein the welds partially close the top opening of the bag.  
      FIGS.  7 ( a ) and ( b ) are schematic front views of another two embodiments of the invention, namely a side-welded bag with a tie strip sealed to the side edges of a receptacle and to the front of the receptacle ( FIG. 7 ( a )), and a side-welded bag with a tie strip sealed to the side edges of the receptacle only ( FIG. 7 ( b )).  
       FIG. 8  are schematic front views of steps of filling and securing closed the bag shown in  FIG. 5 ( a ) by its tie strip.  
       FIG. 9  are schematic front views of steps of filling and securing closed the bag shown in  FIG. 7 ( a ) by its tie strip. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
       FIG. 1 ( a ) shows a continuous tubular plastic sheet  53  flattened into a plastic strip comprising two overlapping sheets joined by a pair of longitudinally extending side edges with an opening in the direction of the front of the roll  26 A. This form of material is suited for the manufacture of bottom welded bags.  FIG. 1 ( b ) shows a continuous tubular plastic film  253  flattened into a plastic strip comprising two overlapping sheets joined by one longitudinal side edge. The other side edge is cut opened longitudinally. The plastic sheet  253  can also be a single layer plastic film folded longitudinally at its centre. The plastic sheet  253  has openings in the front and one side of the roll  26 B. This form of material is suited for the manufacture of side welded bags.  
      A conventional plastic bag manufacturing line  81  is shown on  FIG. 2 , a roll of plastic tubular film  26 A is mounted on an unwind stand (not shown). As the roll unwinds, the plastic sheet  53  is released and threaded through rollers  25  and  50  and continue on to rollers  60  before it is fed into a pair of driven feed rollers  61 . The pair of feed rollers  61  controls the length of the bag to be made. During the bag making process the pair of feed rollers  61  pulls the plastic sheet forward by rotating in preset speed and preset number of revolutions. The pair of feed rollers  61  operate in intermittent cycles and is driven by a servo motor (not shown). The preset number of revolutions of the feed rollers  61  dictates the length of the bag. At each bag making cycle when the preset revolutions of the feed rollers are reached, the servo motor applies a brake on the feed rollers  61  so that plastics sheet  53  are always of the same length when fed Into a bag forming machine  62 .  
      Inside the bag forming machine  62  is a welding and cutting assembly  64 . When brake is applied to the feed rollers  61 , the welding and cutting assembly is activated and a welding iron  68  applies a transverse weld line and a cutting knife  66  cuts the plastic sheet  53  cleanly across the sheet  53 . The transverse weld and the clean cut edge are parallel to each other and closely spaced The transverse weld line defines the bottom weld of the bag and the adjacent cut edge defines the bottom edge of the bag. The plastic sheet  53  on the downstream side of the cutting knife  66  defines the top edge opening of a previously made bag  71 . A conveyor  69  conveys the bag  71  to a collection table  72 .  
      The above describes the conventional manufacture of a bottom weld plastic bag on a bag manufacturing line as shown on  FIG. 2 .  
      Alternatively, the roll  26  can be the continuous plastic sheet  253 , as shown in  FIG. 1 ( b ). With the plastic sheet  253  as material, the welding and cutting assembly can be modified to simultaneously apply a pair of transverse welds and the cutting knife  66  will cut the plastic sheet  253  cleanly in between the two transverse welds. This produces discrete bags having the openings perpendicular to the manufacturing line, eg in the direction of the cut longitudinal edge of the plastic sheet  253  with the side edges of each bag being defined by adjacent pairs of transverse welds and the bottom edge being define by the uncut longitudinal edge of the plastic sheet  253 . Such bags are known as side welded bags.  
      According to one embodiment of the invention and referring to  FIGS. 3, 5 ,  6 ,  8 , a plastic bag manufacturing line  82  is built and equipped to carry out a process of mass manufacturing plastic bags with integral tie strip  116 , each bag  100  comprises a receptacle  110  and a tie strip  116  securely attached to the receptacle  110  that can be use to tightly close the opening  114  of the bag  100 , e.g. after the bag has been filled up.  
      A roll  52  of a continuous plastic tie sheet (tie sheet  55 ) is installed on an unwind stand. The plastic tie sheet  55  may be single or multi ply or tubular. This tie sheet  55  is much narrower than receptacle sheet  53  and serves as material for the tie strips  116 . Manufacture of such tie sheet is well known in the art and is thus not discussed here.  
      The tie sheet  55  unwinds from roll  52  and passes tension rollers  25  and  50 . The tension rollers serves to position the tie sheet  55  generally parallel to and may or may not be centred over receptacle sheet  53 . The tie sheet  55  and receptacle sheet  53  are then pulled together into a perforation machine  54  by feed rollers  61 . The tie sheet  55  is pulled over a transversely extending plate  56  inside the perforation machine  54 , and the receptacle sheet  53  is pulled under the plate  56 . A perforator  58  positioned above the plate  56  is lowered onto the tie sheet timed when feed rollers  61  is at brake position, i.e. when both tie sheet and receptacle sheet  53  are at rest and not moving. As a result, a transversely extending perforation line  118  is applied across tie sheet  55  only.  
      Except when the tie sheet  55  is moving over and the receptacle sheet  53  is moving under the plate  56 , the tie sheet  55  is at all times on top of receptacle sheet  53  and the two sheets are always moving forward or stopping together. In this way the length of the receptacle  110  and the length of the tie strip  116  attached to the receptacle  110  are always the same length.  
      The perforation machine  54  is not fixed. It is constructed to be positionable along the length and across the width of the manufacturing line  82 . Any movement of the perforation machine  54  along the length of the bag manufacturing line will alter the centre distance between the perforation blade  58  and the welding iron  68 . The centre distance between perforation blade  58  and welding iron  68  defines the location of perforations  118  on the tie strip  116 .  
      After the perforation on the tie strip  116  is made, the feed rollers  61  are activated The tie sheet  55  and receptacle sheet  53  rejoin and exit the perforation machine  54 . The tie and receptacle sheet  53 ,  55  then go through a series of tension rollers  60  prior to being fed by feed rollers  61  into a bag forming machine  62 .  
      Inside the bag forming machine  62  is the welding and cutting assembly shown in detail in  FIG. 4 . The welding and cutting assembly  64  comprises three closely spaced, transversely-extending components, namely a cutting knife  66  sandwiched between a pair of welding irons  68 ,  70 . The cutting knife  66  spans the width of the receptacle sheets  53  and is lowered at timed intervals to cut the receptacle sheets  53  into discrete receptacles  110 , and the tie sheet  55  into discrete tie strips  116 . The upstream welding iron  68  (“bottom-welder”) extends the width of the receptacle sheets  53  and applies a line weld (“bottom line weld”) transversely across the receptacle  110  and tie strip  116 , thereby securing a bottom end of the tie strip  116  to one sheet of the receptacle  110 , as well as sealing together the two sheets  53  of the receptacle  110 , thereby closing the bottom of the receptacle  110  that is immediately upstream of the cutting knife  66 . The downstream welding iron  70  (“tie strip welder”) has a width corresponding to the width of the tie sheet  55  and applies sufficient heat to weld a top end of the tie strip  116  to the receptacle sheets  53  along a continuous line weld  120  (“top tie line weld”)  
      Certain properties of the tie strip welder  70  can be controlled such that sufficient heat is applied to secure the tie strip  116  to the top sheet of the receptacle  110 , but not secure the top sheet to the bottom sheet, thereby leaving the receptacle opening  114  open. The properties that can be controlled to effect such a weld include the temperature of the tie strip welder  70 , and the amount of force that the tie strip welder  70  contacts the tie strip  116 , and the duration of contact between the tie strip welder  70  and the tie strip  116 .  
      Alternatively, certain properties of the strip welder  70  can be controlled such that sufficient heat is applied to secure the tie strip  116  to the top sheet of the receptacle  110 , and secure the two sheets  53  of the receptacle  110  together at the top tie line weld  120 . This may be desirable, for example, to ensure that the tie strip  116  is securely fastened to the receptacle  110 . As the length of the top tie line weld  120  is small relative to the overall width of the receptacle opening  114 , it has been found that the weld between the two sheets  53  of the receptacle  110  can be broken without causing significant damage to the receptacle  110  and the functionality of the bag remaining intact.  
      While the embodiment shown in  FIG. 4  shows the strip welder  70  configured to apply a continuous line weld across the tie sheet  55 , the strip welder  70  can be configured to apply different types of top tie welds, such as an intermittent line weld, or a line of spaced spot welds.  
      Referring now to FIGS.  5 ( a )-( c ), the bags  100  produced by the aforementioned approach are “bottom-welded” bags  100  having the receptacles  110  formed from a tubular plastic film and sealed along the bottom edge by the bottom line weld  112 . The bag  100  also has the tie strip  116  attached longitudinally to the receptacle  110 . That is, the tie strip  116  is attached at its bottom end to the bottom edge of the receptacle  110  by the bottom line weld  112 , and at a top end to the top edge of the receptacle  110  by the top tie line weld  120 . The perforation line  118  is located near the bottom of the tie strip  116  and receptacle  110 .  
      As shown in  FIG. 5 ( a ), the top line weld  120  is a continuous line weld that was applied with sufficient heat to weld the tie strip  116  to one sheet of the receptacle  110  but not weld the two sheets together. Alternatively, the top line weld can be an intermittent line weld  122  as shown in  FIG. 5 ( b ), or a line of spaced spot welds  122  as shown in  FIG. 5 ( c ).  
      According to another embodiment of the invention and referring to  FIG. 6 ( a ), the bag  100  is partially closed at its opening  114  by the top line weld  120 . This line weld  120  is a continuous line weld, but is sufficiently short that the two sheets  53  can be separated to reveal the receptacle opening  114  without causing significant damage to the receptacle  110 . Alternatively, the top line weld can be an intermittent line weld  122  as shown in  FIG. 6 ( b ), or a line of spaced spot welds  124  as shown in  FIG. 6 ( c ).  
      According to yet another embodiment of the invention and referring to  FIGS. 3, 7  and  9 , a plastic bag  200  is manufactured having a side-welded receptacle  210  and a tie strip  216  attached transversely across the top edge of the receptacle  210 . The tie strip roll  52  is positioned so that the tie sheet  55  is laid over the receptacle sheets  253  near the cut edge, and the perforator  58  is repositioned accordingly. The welding and cutting assembly  64  is modified so that the tie strip welder  70  is replaced by a duplicate of welding iron  68  that extends transversely across the receptacle sheets  253 . The welding and cutting assembly  64  is operated to cut the receptacle sheets  53  into discrete receptacles  210  and the tie sheet  55  into discrete tie strips  216  by the cutting knife  66  and to form the side edges of each receptacle by a continuous line weld applied by each of the welding irons  68 ,  70 , as well as to secure the ends of the tie strip  216  (“left” and “right” ends) to one sheet of the receptacle  210 . Optionally, a spot welder (not shown) can be installed inside the bag forming machine to apply spaced spot welds along the length of the tie strip  116 .  
      Each resultant bag  200  as shown in  FIG. 7 ( a ) comprises the side-welded receptacle  210  having a bottom edge  212  defined by the uncut edge of the receptacle sheets  253 , a pair of side edges defined by left and right transverse welds  226 ,  228 , and an opening  214  defined by the cut edge of the receptacle sheets  253 . The tie strip  216  is attached transversely across the top of the receptacle  210 . That is, a left end of the tie strip  216  is welded to the receptacle  210  by the left transverse weld  226  and the right end of the tie strip is welded to the receptacle  210  by the right transverse weld  228 . A perforation line  218  extends transversely across the tie strip  216  near the right end of the tie strip  216 . Optionally, a series of spaced spot welds  230  attach the middle of tie strip  216  to receptacle  210  to maintain the tie strip  216  in place.  FIG. 7 ( b ) shows the bag  200  without the spot welds.  
      Referring to FIGS.  8 ( a ) to ( d ), the bottom-welded bag  100  is filled with items such as garbage ( FIG. 8 ( b )); once filled, the bottom end of tie strip  116  is separated from the receptacle  110  by tearing along the perforated line  118  ( FIG. 8 ( c )). Then, the opening  114  is closed and the tie strip  116  is wrapped or encircled around the top of the receptacle  110 , and secured with a knot. Similar steps are shown for filling the side-welded bag  200  in FIGS.  9 ( a ) to ( d ).  
      The invention is not to be limited by the embodiment shown in the drawings and described in the description, which is given by way of example and not limitation, but only in accordance with the scope of the appended claims.