Abstract:
A system for sealing thermoplastic film includes one or more bag sealing units, each comprising a lower vacuum platen and a vacuum chamber cover adapted for sealing engagement on the platen to form a vacuum chamber. A sealing bar assembly includes a sealing bar designed for constant heated operation and a pair of cooling plates which function as heat sinks. The sealing bar assembly is pneumatically reciprocated between a raised, disengaged position and a lowered position with the sealing bar engaging the neck of a bag for hermetically sealing same. The cooling plates clamp the bag neck against a sealing support assembly. A method of sealing a thermoplastic film bag includes the steps of placing a packaging object in a thermoplastic bag and placing the bag on a cradle with the bag neck extending over a bag support assembly. A. vacuum chamber cover is placed on the platen and evacuated to form a vacuum chamber. A sealing bar assembly melds the thermoplastic to form a sealed area across the bag neck. A cutoff knife blade severs the end of the bag beyond a sealed area, which extends across its neck.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to vacuum packaging, and more particularly to an apparatus and system for thermally sealing bags using a constant temperature heat source located adjacent to one or more heat sinks.  
           [0002]    It is known in the prior art to seal perishable items, such as food products, by placing the item in a plastic bag, evacuating a substantial portion of the air within the bag to form a partial vacuum, and heat-sealing the bag opening to hermetically seal the bag and preserve the vacuum. Typically, this process is performed within a vacuum chamber. The bag containing the item or items to be packaged is placed into the chamber, and the chamber is closed. Air is evacuated from the chamber and the open end of the bag is sealed using a heat-sealing bar. As the bar comes into contact with the plastic, the plastic of both walls of the bag is melted, thereby causing the walls to meld or adhere to one another.  
           [0003]    Ordinarily, the vacuum chamber comprises two major elements or assemblies, an upper lid or cover assembly that houses the heat sealing mechanism and a blade for trimming excess bag material, and a lower base or platen assembly that holds the bag and product to be packaged, valves, sealing support device, cutting support device, and vacuum pump.  
           [0004]    A significant problem in food packaging applications relates to “leakers”, which result from defective seals. For example, meats and other packaged foods commonly have natural juices, fat particles, preservatives and other substances trapped in their bags. These substances are sometimes trapped in the bag openings as they are sealing, and prevent the thermoplastic film from closing air-tight across the mouths of the bags. Bag closures can thus be compromised with leak channels that form where the bag portions do not completely seal, which create leakers allowing fluid to leak out and other substances to leak in and potentially contaminate the packaged food products. Leakers tend to be aesthetically unacceptable for retail merchandising because they create unattractive packages, which customers tend to avoid. They can also discharge substances onto surrounding packages, store displays, shipping containers, etc. Leakers can occur in approximately 7%-20% of the thermoplastic bags sealed with current technology. Therefore, achieving complete, fluid-tight seals with minimal “leakers” is an important criterion in the design and operation of bag sealing equipment. A design strategy for eliminating leak passages involves providing a relatively wide area of engagement with crisscrossing sealing lines whereby a leak passage would have to cross multiple sealing lines in order to compromise the bag. On the other hand, equipment designs which place total reliance on single seal lines for bag closures tend to be more susceptible to being compromised by leak passages. For example, much of the current bag sealing equipment provides sealed areas that are only about 3 mm wide, and are thus susceptible to leak channels.  
           [0005]    A heat sealing method commonly used in the prior art is known as impulse sealing. Impulse sealing includes the intermittent application of electric current “impulses” to a heating element in a sealing bar. The sealing bar was formed of metal or other materials that transmit heat to the plastic bag. As the sealing bar was brought into contact with the plastic to be melted, an impulse of electrical current was applied to the heating element, which heated the sealing bar long enough to fuse or melt-weld (“meld”) the plastic bag. The heating element was then deenergized, thus allowing the sealing bar to cool until the next heating/cooling cycle began.  
           [0006]    Such heating/cooling cycles tended to cause operating problems with prior art equipment. For example, delays occurred and energy was wasted as components, such as heating bars, were brought up to operating temperatures and then allowed to cool. Therefore, prior art components with substantial thermal mass tended to incur substantial operating delays and consumed considerable amounts of energy due to their cyclic operations. Moreover, heating/cooling cycles tended to expand and contract thermally conductive components, such as metals and ceramic-core heating elements. The resulting expansion/contraction cycles subjected the equipment to wear. Operators of prior art impulse-type bag sealing equipment thus incurred operating expenses for replacement parts, repairs and downtime.  
           [0007]    On the other hand, constant-temperature sealing bars can benefit from greater thermal mass because they tend to be less affected by heat loss to the workpieces. For example, equipment for sealing thermoset plastic bags tends to operate more efficiently and with less wear if operating temperatures are maintained relatively constant. However, thermal energy from constant-heat sealing bars can dissipate throughout the equipment and cause other problems. The present invention addresses these and other problems with the prior art by providing heat sinks on both sides of a heating bar, thus focusing and directing the radiant heat output along a relatively narrow strip or “heat zone”.  
           [0008]    Heretofore there has not been available a bag sealing system and method with the advantages and features of the present invention.  
         SUMMARY OF THE INVENTION  
         [0009]    In the practice of the present invention, a bag sealing system includes one or more bag sealing units, each comprising a lower vacuum platen and a vacuum chamber adapted for sealing engagement on the platen. A sealing bar assembly includes a sealing bar designed for constant heated operation and located between a pair of heat sink/cooling plates which function as heat sinks. The sealing bar assembly is pneumatically reciprocated between a raised, disengaged position and a lowered position with the sealing bar engaging the neck of a bag for hermetically sealing same. The cooling plates clamp the bag neck against a sealing support assembly. A cutoff knife blade severs the end of the bag beyond a sealed area, which extends across its neck. In the practice of the method of the present invention, a packaging object is placed in a thermoplastic bag, which is then placed on a cradle mounted on the platen with the bag neck extending over a sealing support assembly. A vacuum chamber is placed on the platen and a partial vacuum is drawn in the vacuum chamber, thus evacuating the bag. A sealing bar assembly melds the thermoplastic to form a sealed area across the bag neck. After the vacuum chamber is open, the closed bag is heat-shrunk to a final, reduced-volume configuration.  
         OBJECTS OF THE INVENTION  
         [0010]    It is, therefore, an object of the present invention to provide a constant temperature heat sealing device for vacuum packaging machines that avoids the problems of prior art impulse sealing devices such as oxidation of the element and mechanical stress due to rapid and frequent temperature fluctuations.  
           [0011]    It is a further object to provide a constant temperature heat-sealing device that hermetically closes a plastic bag after evacuation of the air inside the bag.  
           [0012]    Another object is to provide a constant temperature heat-sealing device wherein the sealing bar may be linear or curved, flat or crowned, as required by the material to be sealed.  
           [0013]    Another object of the present invention is to provide a continuous temperature heat-sealing device that works well using relatively large heating elements having an increased thermal mass.  
           [0014]    It is a further object of the invention to provide a continuous temperature heat-sealing device that yields a relatively low failure (“leaker”) rate in sealed bags.  
           [0015]    Another object is to provide a heat-sealing device that can withstand high pressure water wash-down.  
           [0016]    A further object of the invention is to accommodate thermoplastics of various thickness, including relatively thick bags.  
           [0017]    Yet another object of the invention is to provide bag sealing units adapted for stand-alone, endless-belt and circular conveyor types of operations.  
           [0018]    It is a further object to provide a heat-sealing device that is capable of creating a seal width in the range of about 2 mm to 10 mm. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 a  is a side elevational view of a bag sealing system embodying the present invention.  
         [0020]    [0020]FIG. 1 b  is another side elevational view thereof, shown with the vacuum sealing units raised.  
         [0021]    [0021]FIG. 2 a  is a longitudinal cross-section of a bag sealing unit in a closed-cover position.  
         [0022]    [0022]FIG. 2 b  is a longitudinal cross-section section thereof with a sealing bar assembly engaged.  
         [0023]    [0023]FIG. 2 c  is a longitudinal cross-section section thereof with the vacuum chamber raised.  
         [0024]    [0024]FIG. 3 is a transverse cross-section thereof taken generally along line  3 - 3  in FIG. 2 a.    
         [0025]    [0025]FIG. 4 a  is a fragmentary, top plan view thereof, particularly showing the sealing bar assembly.  
         [0026]    [0026]FIG. 4 b  is a fragmentary, side elevational view thereof, taken generally along line  4   b - 4   b  in FIG. 4 a.    
         [0027]    [0027]FIG. 5 is an orthographic view of a sealing bar thereof, shown with a cover plate removed.  
         [0028]    [0028]FIG. 6 is a top plan view of the sealing bar, taken generally along line  6 - 6  in FIG. 8.  
         [0029]    [0029]FIG. 7 is an orthographic view of the sealing bar.  
         [0030]    [0030]FIG. 8 is an elevational view thereof.  
         [0031]    [0031]FIG. 9 is an orthographic view of a modified, straight sealing bar, shown with a cover plate removed.  
         [0032]    [0032]FIG. 10 is an orthographic view of the modified, straight sealing bar.  
         [0033]    [0033]FIG. 11 is an orthographic view of a sealing support assembly.  
         [0034]    [0034]FIG. 12 is orthographic view of a vacuum chamber cover.  
         [0035]    [0035]FIG. 13 is a top plan view of a bag containing a poultry carcass, with the sealing bar and cutoff blade shown in position for sealing and cutting off the bag.  
         [0036]    [0036]FIG. 14 is a top plan view of the sealed bag.  
         [0037]    [0037]FIG. 15 is a top plan view of the sealed bag, shrunken to its final configuration.  
         [0038]    [0038]FIG. 16 is a top plan view of a rectangular product, such as a block of cheese, shown in a bag with a seal bar and cutoff blade shown in position for sealing and cutting off the bag.  
         [0039]    [0039]FIG. 17 is a top plan view thereof, showing the bag sealed.  
         [0040]    [0040]FIG. 18 a  is a longitudinal cross-section of a modified embodiment bag sealing unit with a modified cutoff knife assembly.  
         [0041]    [0041]FIG. 18 b  is a longitudinal cross-section thereof, showing the sealing bar and the cooling plates in their lowered, engaged positions.  
         [0042]    [0042]FIG. 18 c  is a longitudinal cross-section section thereof, showing the vacuum cover raised and the bagged product being removed.  
         [0043]    [0043]FIG. 19 is a top plan view of a circular, carousel-type bag sealing system.  
     
    
     DETAILED DESCRIPTION  
       [0044]    Turning to the figures, FIGS. 1 a  and  1   b  illustrate an automated multiple-chamber vacuum packaging machine  100 . The machine includes a continuous, driven chain or belt  101  supported on and driven by an idler roller  102   a  and a drive roller  102   b . As illustrated, a circuitous train of lower vacuum platens  200  are fastened at their leading edges to the belt  101 . Preferably, the platens  200  are made of stainless steel. As illustrated in FIG. 1 b , the platens are moving counterclockwise in a direction from right to left across the top. The belt  101  is driven by sprocket and bearing assemblies that are fixed to a drive shaft and a free wheeling shaft (not shown). The drive shaft is driven by a servo drive gear reduction motor  110 . Three vacuum chambers  300  (individually denoted by numbers  300   a ,  300   b  and  300   c ) are mounted above the belt  101 . The platens  200  and respective vacuum chambers  300  collectively form respective bag sealing units  106 , which are capable of automated or semi-automated operation (FIGS. 1 a, b ), or stand-alone operation as individual bag sealing units  106 .  
         [0045]    The vacuum packaging machine  100  operates as follows. The belt  101  moves counterclockwise (i.e., from right-to-left across the top). Movements can be continuous or intermittent, the latter being adapted for “batch”-type operations, thereby moving the lower vacuum platens  200  underneath the vacuum chambers  300 . The packaging machine  100  rate of output is generally governed by the number of vacuum chambers  300  usable simultaneously in operation, together with the duration of the process steps in each unit. Preferably, each vacuum chamber  300  operates independently and simultaneously. The packaging machine  100  uses all available empty vacuum chambers  300  by means of sensors  224  that monitor various operating parameters, such as timing, temperature and pressure with respect to the vacuum chambers  300  and the bag sealing units  106 , the rate of chain  101  movement and availability of vacuum chambers  300 . A programmable microprocessor controller  222  can be connected to the sensors  224  and other components of the system  100  for controlling its operation, particularly in automated and semi-automated operating modes.  
         [0046]    In operation, each independent vacuum chamber  300  performs the following functions. The vacuum chamber cover  302  descends upon a vacuum platen  200  positioned directly below (see  300   a , FIG. 1). The vacuum chamber cover  302  forms a seal with the upper surface  202  of the vacuum platen by means of a seal gasket  304  (see FIGS. 2 a  and  2   b ). Air within the sealed chamber  300  is then evacuated by means of an exhaust valve  306  located in the top surface of the cover  302  and connected to a suitable vacuum source, such as a compressor. A vacuum sensor (not shown) monitors the air pressure in the chamber  300  and reports the pressure value to the microprocessor controller  222 . An air pressure set point has been previously programmed into the microprocessor controller  222 . When the set point is reached, the microprocessor controller  222  triggers an air compressor (not shown) to inflate a bladder  308  located on the inner, upper surface of the cover  302 . The bladder  308  fills with compressed air, provided through bladder air supply line  318 , and expands downward, forcing the sealing bar assembly  310  downward (FIG. 2 b ) and overcoming the return springs  384 . The sealing bar  350  is mounted on the lower extremity of the sealing bar assembly  310 .  
         [0047]    As illustrated in FIG. 2 a , prior to closure of the cover  302 , an item  104  to be vacuum sealed, in this case a poultry carcass, has been placed inside a plastic vacuum seal bag  120  upon a cradle  204  located on the upper surface  202  of the vacuum platen  200 . The bag  120  is made of a thermoplastic film known in the industry for heat sealing and heat shrinking applications. The bag  120  is oriented so that the open neck  122  lies on top of a sealing support assembly  205  with spring-loaded engagement gaskets  206   a ,  206   b  and  206   c . In addition to lying over the tops of the gaskets  206   a,b,c , the neck  122  is fitted over a set of neck retention pins  209  that hold the neck  122  of the bag open so that air may be drawn out of the bag  120  by the vacuum created in the chamber  300 .  
         [0048]    After closing the cover  302  against the platen  200  and evacuating the air inside the chamber  300  to the pre-programmed set point, the sealing bar  350  is forced downward by the expanding inflatable bladder  308 , thereby coming into contact with the plastic of the neck  122 . The sealing bar  350  continues to move downward, overcoming the upward spring  216  bias of the engagement gaskets  206   a,b,c . As the sealing bar  350  moves downward the neck  122  is pushed against a fixed cutoff blade  124 . The neck  122  of the bag  120  is thereby sheared or cutoff by the cutoff blade  124 , which separates a neck cutoff portion  122   c . The device is calibrated so that downward motion of the sealing bar  350  ceases shortly after the neck  122  of the bag is driven against the cutoff blade  124  and severed.  
         [0049]    The sealing bar  350  includes a contact surface  354 , which contacts the plastic of the neck  122 , thus transferring thermal energy to the plastic film, melting the plastic and causing the upper wall  122   a  and the lower wall  122   b  to meld or fuse together, creating a thermocompressive bond at  122   d . Shortly before the sealing bar  350  comes into contact with the neck  122 , two heat sink/cooling plates  360   a,b  also come into contact with the surface of the neck  122 , one on either side of the sealing bar  350 , along their respective cooling plate lower edges  362   a,b . The cooling plates  360   a,b  are attached to the seal bar assembly  310 , and are driven downward along with the sealing bar  350  by the force of the inflated bladder  308 . The heat sink/cooling plates  360  provide means for cooling the portion of the neck  122  proximate the area of contact between the sealing bar  350  and plastic film, thereby minimizing shrinkage of the neck  122  during heat sealing. The cooling plates  360  also serve to hold the neck  122  in position by clamping same against the engagement gaskets  206   a,c  during the sealing operation.  
         [0050]    The three engagement or support gaskets  206   a,b,c  are spring biased, so that they maintain upward pressure against the neck  122  while yielding to the downward force of the sealing bar  350  and the cooling plates  360   a,b . In addition, the cooling plates  360   a,b  are also spring biased so that towards the end of the downward stroke of the sealing bar assembly  310  the sealing bar  350  may move past the cooling plates  360   a,b , driving further downward and causing the neck  122  to be cut against the bag cutoff blade  124 .  
         [0051]    After the sealing bar  350  has achieved its full downward stroke (FIG. 2 b ), compressing engagement gasket  206   b , an inlet valve  312  is activated and the chamber  300  returns to atmospheric pressure. The cover  302  is then raised and the chain  101  advances the platen  200  with the sealed bag  120  further down the line.  
         [0052]    As referenced above, the neck  122  of the bag  120  is held open during the sealing process by a pair of neck retention pins  209   a  and  209   b . A side view of pin  209   b  may be seen in FIGS. 2 a  through  2   c.    
         [0053]    [0053]FIG. 2 b  illustrates the downward travel of the sealing bar assembly  310  with arrows  313   a ,  313   b  and  313   c  indicating the downward direction of travel. Arrow  314  indicates the direction of the final evacuation of air from the bag  120 , which is achieved just prior to incision of the neck  122  by the cutting blade  124 . Dashed line  120   a  indicates the relative size of the bag  120  prior to the final expulsion of air which reduces it to the size indicated by the solid line  120   b . FIG. 2 b  also illustrates the bladder  308  in its inflated state.  
         [0054]    As shown in FIGS. 2 a - c , the cradle  204  may be formed with a concave upper surface to receive an item  104  having a curved or rounded shape.  
         [0055]    [0055]FIG. 2 c  illustrates the apparatus at the conclusion of a cycle, in which the cover  302  has been lifted off of the platen  200 . The sealed bag  120  is shown being removed from the cradle  204 . Arrow  315  indicates the upward direction of travel of the bag  120  as it is being removed. It should be appreciated that removal of the sealed bag  120  typically occurs after full retraction (lifting) of the cover  302 . Arrow  313   d  indicates the upward direction of travel of the seal bar assembly  310  as it is retracted upwards by expulsion of air from the bladder  308 . Arrow  316  indicates the upward direction of travel of the cover  302  as it is raised above the platen  200 .  
         [0056]    In FIG. 2 c  the neck  122  is shown after being separated by the cutting blade  124 . The portion of the neck  122  remaining attached to the body of the bag  120  contains the sealed portion of the neck  122   d  (see FIG. 14 for a top view of the sealed portion  122   d  of the neck  122 ). The cut-off remnant  122   c  of the neck  122  is ejected from the neck retention pins  209 , as shown by arrow  317  indicating the upward direction of travel, and phantom lines indicating the ejected neck remnant  122   c.    
         [0057]    [0057]FIG. 3 is a partial cross-sectional view along line  3 - 3  in FIG. 2 a . The cover  302  and the platen  200  are shown in cross section and the plastic bag  120 , the neck and in the in a and to the  122  and the pins  209   a,b  are shown in phantom lines. As illustrated, the bladder  308  is located on the upper inside surface of the cover  302  and is in communication with an air supply hose  318  which is in further communication with an air pump or compressor (not shown). A seal bar assembly suspension  380  comprises spring biased bolts  382  that support the seal bar assembly  310  by attachment to the upper inside surface of the cover  302 . The springs  384  force the assembly  310  upward, squeezing against the bladder  308  when the assembly  310  is in the retracted position. When air pressure to the bladder  308  is increased through the air supply hose  318 , the force exerted by the expanding bladder walls overcomes the tension of the springs  384 , causing the assembly  310  to slide downward along the shafts of the bolts  382 .  
         [0058]    A cooling plate suspension system  390  is also illustrated in FIG. 3. The cooling plates  360   a,b  are attached to the sealing bar assembly  310  via bolts  392  mounting return springs  394 . When the cooling plates  360   a,b  contact respective engagement gaskets  206   a,c , the tension in the springs  394  may be overcome by a greater force associated with the downward travel of the cooling plates  360   a,b.    
         [0059]    The elongated, convex side of the cooling plate  360   a  is illustrated in FIG. 3, including a notch  366  in the upper surface of the cooling plate  360   a  which provides egress for electrical supply wiring  400 . The wiring  400  conducts a controlled current to the heating element  352  (FIG. 5). The heating element  352  supplies thermal energy to the sealing bar  350 , which is thus maintained at a selected, relatively constant temperature. Typically, the thermal energy supplied to the sealing bar  350  is regulated by controlling the current applied to the heating element  352  through setting a desired temperature value in a microprocessor-controlled thermostat (not shown).  
         [0060]    Water inlet and outlet lines  370 ,  372  lead to and from the cooling plates  360   a,b . During operation of the vacuum packaging machine  100 , cool water (or other suitable coolant) is provided to the interior of the cooling plates  360   a,b  for circulation through internal coolant passages  370   a,b . The temperatures of the surfaces of the cooling plates  360   a,b  are thereby reduced, concurrently lowering the temperature of the portion of the plastic bag  120  contacted by the cooling plates  360   a,b  during sealing.  
         [0061]    [0061]FIG. 4 a  is a top plan view of the preferred embodiment of the neck retention structure  208 . It comprises a pair of pins  209   a  and  209   b  that extend outward from a neck retention bracket  210  that holds a guide tube  212  in which the pins  209   a,b  are urged outwardly by respective springs  214   a,b . The pins  209   a,b  travel along the guide tube  212  during operation of the device. When the bag neck  122  is placed over the engagement gaskets  206 , the pins  209   a,b  are compressed inwardly towards the center of the guide tube  212 . Releasing the pins  209   a,b  stretches the bag opening to its full open, extended position for maximum effective sealing at  122   d.    
         [0062]    The neck  122  is held open during the sealing process and, as illustrated in FIG. 4 a , has just been severed by the cutting blade  124 . FIG. 4 b  is an end view of the neck retention structure  208 , including a side view of neck retention pin  209   b.    
         [0063]    As an alternative to the spring-biased neck retention structure  208 , a motorized configuration with a screw-threaded rod driven by a suitable servo motor controlled by the microprocessor controller  222  can be provided and can reciprocate the neck retention pins  209   a,b  inwardly and outwardly.  
         [0064]    [0064]FIG. 5 is an orthographic view of a curved sealing bar  350  with the cover plate removed to show the tubular heating element  352  that provides constant sealing temperature. FIG. 5 also shows the contact surface  354  of the sealing bar  350  designed to provide a cross-hatch pattern when melting the sealed plastic of a vacuum bag  120 . FIG. 6 is a bottom view of the sealing bar  350  showing the cross-hatch pattern in greater detail. This cross-hatch pattern permits the device to form a seal through contaminated plastic as well as through gathered layers of plastic created by irregularly shaped products. In particular, multiple, crisscrossed meld lines are formed and tend to cut across contaminated substances and gathered plastic layers, forming multiple barriers to leakage. FIG. 7 is an orthographic view of the sealing bar  350  of FIG. 5 with the cover plate  356  in place. FIG. 8 is an isometric view of the front of the sealing bar  350  with the top portion of the sealing bar tilted slightly toward the viewer.  
         [0065]    [0065]FIG. 9 is an orthographic view of a straight or linear sealing bar  350  with the cover plate  356  removed to show the straight tubular heating element  352  used to create a constant temperature heat source. The contact surface  354  of the sealing bar  350  shown in FIG. 9 has a cross-hatch pattern. FIG. 10 is an orthographic view of the sealing bar  350  of FIG. 9 with the cover plate  356  in place.  
         [0066]    [0066]FIG. 11 is an orthographic view of the sealing support assembly  205  including the engagement gaskets  206   a ,  206   b  and  206   c , and the bag cutoff blade  124 . A sealing support base  220  includes secondary channels  222  for receiving springs  224 , a primary major channel  226   a  within which is mounted the cutoff blade  124 , and a secondary major channel  226   b  which defines and separates engagement gaskets  206   b  and  206   c . The gaskets  206   a ,  206   b  and  206   c  fit over channels  222  and rest upon springs  224 . The gaskets  206   a ,  206   b  and  206   c  may include a contact surface having a cross-hatched pattern. The arrangement shown in FIG. 11 would be appropriate for use with a curved sealing bar as shown in FIG. 7.  
         [0067]    [0067]FIG. 12 is an upper, front, orthographic view of the vacuum chamber cover  300 .  
         [0068]    [0068]FIG. 13 is a fragmentary plan view of the bag  120  containing the item to be packaged  104 , the sealing bar  350  positioned above the neck  122  of the bag  120 , the cutting blade  124 , and a severed portion (remnant)  122   c  of the neck  122 .  
         [0069]    [0069]FIG. 14 is a plan view of the bag  120  of FIG. 13 showing the neck remnant  122   c  severed and removed from the main portion of the bag  120  and the seal  122   d  formed across the neck  122 . After vacuum sealing according to the method of the present invention, a subsequent process occurs in the packaging process. The sealed bag  120  is deposited in a hot water bath or steam tunnel causing the thermoplastic material of the bag  120  to shrink as illustrated in FIG. 15.  
         [0070]    [0070]FIG. 16 is a fragmentary plan view of an alternative configuration sealing bar  350 . In this embodiment the sealing bar  350  is straight rather than curved as is the cutoff blade  124 . The embodiment shown in FIG. 16 is advantageous for use with rectangular shaped items, such as the cheese block shown. FIG. 17 is a top view of the bag  120  of FIG. 16 with a portion of the neck  122  removed after vacuum sealing and with the bag  120  shrunk after hot water immersion.  
         [0071]    [0071]FIGS. 18 a - c  illustrate an alternative embodiment of the vacuum packaging machine  500 . By way of example, the illustrated embodiment differs from that illustrated in FIGS. 2 a  through  2   c  primarily in that the engagement gaskets  506   a,b  are fixed rather than spring-biased. Also, the cutoff blade  524  is movable rather than fixed and is mounted on a cutoff blade platform  526  mounted on bolts  528  with springs  529  biasing the cutoff blade platform  526  downwardly.  
         [0072]    The platform  526  and the associated cutting blade  524  are moved upward during the cutting operation by means of a secondary bladder  528 . Air supply to the secondary bladder  528  is regulated by a three-way valve  530 . The valve  530  is activated by a pin  534 . During operation of the vacuum packaging machine  500 , the pin  534  is depressed by the descending cooling plate  560   b . The pin  534  moves downward through the platform  526  and activates the valve  530  causing the bladder  528  to be opened to ambient air pressure outside the vacuum chamber  500  through a vent opening  531  formed in the platen  600 . Due to the pressure differential between the outside (ambient) pressure and the partial vacuum within the chamber  500 , the secondary bladder  528  fills with outside air, pushing the platform  526  and the cutoff blade  524  upward, and severing the neck  122  of the bag  120  as shown in FIG. 18 b.    
         [0073]    Upon activation of the vent valve  312 , the chamber  500  returns to ambient atmospheric pressure, and the secondary bladder  528  is deflated by downward pressure from the platform  526  as exerted by springs  529 . FIG. 18 c  illustrates the vacuum packaging machine  500  at the conclusion of the cycle. The cover  502  has been lifted off the platen  600  and the sealed bag  120  is shown being removed from the cradle  604 . Arrow  615  indicates the upward direction of travel of the bag  120  as it is being removed.  
         [0074]    [0074]FIG. 19 shows an alternative configuration rotary chamber system  700  comprising a circular conveyor  702  with multiple bag sealing units  106  mounted thereon in radially-spaced relation. The conveyor  702  is rotated by a motor whereby the bag sealing units  106  perform sealing operations at appropriate workstations for different steps of the process.  
         [0075]    The components of the system  100  are preferably constructed of suitable materials, such as stainless-steel or aluminum, which can accommodate power washing for cleaning purposes and tend to resist rust and corrosion in working environments with relatively high humidity and temperature levels.  
         [0076]    It is to be understood that while certain embodiments of the invention have been shown and described, the invention is not to be limited thereto and can assume a wide variety of alternative configurations, including different materials, sizes, components and methods of operation. Moreover, the system and method of the present invention can be adapted to various applications, including the manufacture of bags and other products from thermoplastic film, forming multiple seals on bags and sealing the sides and ends of bags.