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:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/400,290, filed Mar. 9, 2009 now abandoned, which is a continuation of U.S. patent application Ser. No. 12/030,049, filed Feb. 12, 2008 now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 11/073,559 filed Mar. 7, 2005 now U.S. Pat. No. 7,328,556, which is a continuation-in-part of U.S. patent application Ser. No. 10/345,763, filed Jan. 16, 2003, now U.S. Pat. No. 6,862,867, issue date Mar. 8, 2005, which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of the Related Art 
     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. 
     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. 
     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. 
     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. 
     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. 
     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”. 
     Heretofore there has not been available a bag sealing system and method with the advantages and features of the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     Another object is to provide a heat-sealing device that can withstand high pressure water wash-down. 
     A further object of the invention is to accommodate thermoplastics of various thickness, including relatively thick bags. 
     Yet another object of the invention is to provide bag sealing units adapted for stand-alone, endless-belt and circular conveyor types of operations. 
     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 
         FIG. 1   a  is a side elevational view of a bag sealing system embodying the present invention. 
         FIG. 1   b  is another side elevational view thereof, shown with the vacuum sealing units raised. 
         FIG. 2   a  is a longitudinal cross-section of a bag sealing unit in a closed-cover position. 
         FIG. 2   b  is a longitudinal cross-section section thereof with a sealing bar assembly engaged. 
         FIG. 2   c  is a longitudinal cross-section section thereof with the vacuum chamber raised. 
         FIG. 3  is a transverse cross-section thereof taken generally along line  3 - 3  in  FIG. 2   a.    
         FIG. 4   a  is a fragmentary, top plan view thereof, particularly showing the sealing bar assembly. 
         FIG. 4   b  is a fragmentary, side elevational view thereof, taken generally along line  4   b - 4   b  in  FIG. 4   a.    
         FIG. 5  is an orthographic view of a sealing bar thereof, shown with a cover plate removed. 
         FIG. 6  is a top plan view of the sealing bar, taken generally along line  6 - 6  in  FIG. 8 . 
         FIG. 7  is an orthographic view of the sealing bar. 
         FIG. 8  is an elevational view thereof. 
         FIG. 9  is an orthographic view of a modified, straight sealing bar, shown with a cover plate removed. 
         FIG. 10  is an orthographic view of the modified, straight sealing bar. 
         FIG. 11  is an orthographic view of a sealing support assembly. 
         FIG. 12  is orthographic view of a vacuum chamber cover. 
         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. 
         FIG. 14  is a top plan view of the sealed bag. 
         FIG. 15  is a top plan view of the sealed bag, shrunken to its final configuration. 
         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. 
         FIG. 17  is a top plan view thereof, showing the bag sealed. 
         FIG. 18   a  is a longitudinal cross-section of a modified embodiment bag sealing unit with a modified cutoff knife assembly. 
         FIG. 18   b  is a longitudinal cross-section thereof, showing the sealing bar and the cooling plates in their lowered, engaged positions. 
         FIG. 18   c  is a longitudinal cross-section section thereof, showing the vacuum cover raised and the bagged product being removed. 
         FIG. 19  is a top plan view of a circular, carousel-type bag sealing system. 
         FIG. 20  is a plan view of a bag sealing system comprising an alternative embodiment of the present invention. 
         FIG. 21  is another plan view thereof, showing various stations corresponding to the steps of the bag sealing method of the present invention. 
         FIG. 22  is a side elevational view thereof. 
         FIG. 23  is an enlarged, fragmentary, side elevational view taken generally within circle  23  in  FIG. 22 , and particularly showing a drive mechanism. 
         FIG. 24  is an enlarged, vertical, cross-sectional view of a bag sealing unit, with a dome thereof in its raised position. 
         FIG. 25  is an enlarged, vertical, cross-sectional view thereof, with the dome in its lowered position. 
         FIG. 26  is an enlarged, vertical, cross-sectional view thereof, with the dome and a sealing assembly thereof in their lowered positions for sealing a bag neck. 
         FIG. 27  is an enlarged, vertical, cross-sectional view thereof, showing the sealing bar slightly lifted above the bag neck. 
         FIG. 28  is an enlarged, vertical, cross-sectional view thereof, showing the dome and the sealing assembly in their raised position, with the bag neck sealed. 
         FIG. 29  is an enlarged, fragmentary view of a modified sealing assembly of another alternative embodiment of the present invention, with the cutoff blade thereof mounted on the outside face of a cooling bar. 
         FIG. 30  is a longitudinal cross-section thereof showing a dome in a raised position. 
         FIG. 30   a  is an enlarged, fragmentary, side elevational view taken generally within circle  30   a  in  FIG. 30 , and particularly showing a sealing assembly and cut-off assembly. 
         FIG. 31  is a longitudinal cross-section thereof showing the dome in a lowered position, and a sealing assembly and a cut-off assembly in raised positions. 
         FIG. 32  is a longitudinal cross-section thereof showing the sealing bar and cooling bars in lowered positions for sealing a bag neck. 
         FIG. 33  is a longitudinal cross-section thereof showing the sealing bar, cooling bars, and cut-off blade in lowered positions. 
         FIG. 33   a  is an enlarged, fragmentary view of both the sealing bar and cooling bars in contact with the bag neck, and the cut-off blade in contact with the resilient contact surface. 
         FIG. 34  is a longitudinal cross-section thereof showing the cooling bars in lowered positions, the sealing bar in a slightly raised position, and the cut-off blade in a raised position. 
         FIG. 35  is a longitudinal cross-section thereof showing the dome in a raised position, and the sealing assembly and cut-off assembly in raised positions. 
         FIG. 36  is a transverse, cross-section thereof taken generally along line  36 - 36  in  FIG. 35  and particularly showing the sealing assembly and the cut-off assembly. 
     
    
    
     DETAILED DESCRIPTION 
     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 . 
     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. 
     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 . 
     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 . 
     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. 
     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. 
     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 . 
     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. 
     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.    
       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. 
     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. 
       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 . 
     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.    
       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 . 
     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.    
     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). 
     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. 
       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.    
     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.    
     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. 
       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. 
       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. 
       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 . 
       FIG. 12  is an upper, front, orthographic view of the vacuum chamber cover  300 . 
       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 . 
       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 . 
       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. 
       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. 
     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.    
     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. 
       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. 
     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. 
     Another alternative embodiment bag sealing system is shown in  FIGS. 20-27  and is generally designated by the reference numeral  1002 . The system  1002  generally includes a conveyor subsystem  1004 , which comprises a platen conveyor  1006  with a plan configuration defining a platen travel path generally in the shape of a racetrack or a rounded oblong, and a carousel-type dome conveyor  1008 . The dome conveyor  1008  has a generally circular plan configuration defining a circular dome travel path, which is generally located in a horizontal plane above the level of a horizontal plane generally containing the platen conveyor  1006 . The platen conveyor  1006  mounts a number of platens  1010 , which selectively receive respective domes  1012  suspended from the dome conveyor  1008  whereby bag sealing units  1014  are combined temporarily through vacuum and sealing stations  1016 ,  1018  ( FIG. 21 ). The platen conveyor  1006  also includes loading and discharge stations  1020 ,  1022 . The dome conveyor  1008  also includes a dome-up station  1024 . The bag sealing units  1014  are combined through the vacuum and sealing stations  1016 ,  1018 , and separate into disconnected platens  1010  and domes  1012  through the other conveyor stations. 
     Although particular configurations and types of conveyors are shown and described by way of examples, various configurations and types of conveyors can be utilized with the present invention. Thus, the platens  1010  are rollingy placed on tracks  1026  on the platen conveyor  1006 . At the loading station  1020 , an unsealed bag  1028  containing an object  1030  is placed on each platen  1010 . The loading station  1020  can be supplied with a suitable supply conveyor (not shown) for automatic loading of the platens  1010 , or the bagged objects  1030  can be placed thereon manually. 
     The loaded platens  1010  next proceed towards the dome carousel  1008  and into alignment underneath respective domes  1012 . The platen conveyor  1006  passes beneath the dome conveyor  1008  at approximately the location where the vacuum station  1016  commences. The bags  1028  are evacuated through the vacuum station  1016 , as described above. At the sealing station  1017  the bags  1028  are sealed and the bag sealing units  1014  proceed to the ventilation station  1018  whereat the domes  1012  are ventilated and lifted from the platens  1010 . The platens  1010  next proceed to the discharge station  1022  whereat the sealed items can be discharged by any suitable mechanism, including without limitation manual, semi automatic and automatic. The rails  1026  of the platen conveyor  1006  deflect upwardly at  1032  and thereby tilt the platens  1010  for sloping downwardly and outwardly and discharging the bagged items therefrom at the discharge station  1022 . 
     A drive mechanism  1034  is shown in  FIG. 23  and generally includes a drive motor and gearbox  1036  driving a main drive shaft or live axle  1038  through a belt drive assembly  1040 . The carousel-configuration in dome conveyor  1008  is supported by multiple casters  1042 , whereby the dome conveyor  1008  rotates about a drive axis extending through the main drive shaft  1038 . 
     Each platen  1010  includes a base  1040  with an upper surface  1042  mounting an item support  1044  adapted to receive a bagged items  1030  and a neck support  1046  adapted to receive necks  1048  of respective bags  1028 . The neck support  1046  includes a resilient contact service  1050 , which can be mounted on springs are comprises a compressible material in order to provide resiliency. A ridge  1052  is located on top of the contact surface  1050 . 
     Each dome  1012  includes a top  1060 , a perimeter sidewall  1062  with a lower edge  1064  mounting a perimeter sealing gasket  1066  adapted for forming a sealing engagement with the base upper surface  1042 . A sealing assembly  1068  is mounted on the underside of the top  1060  generally within a vacuum chamber  1070  formed by the dome  1012 . The sealing assembly  1068  includes a pneumatic raise/lower actuating mechanism  1072  with a bladder  1074  connected to an air inlet port  1076 , which in turn can be connected to a pressurized air source  1078 , such as a compressor or a packaging plant air source. 
     Return springs  1080  are provided for returning the raise/lower actuating mechanism  1072  to its raised position, e.g. when the bladder  1074  is deflated. The actuating mechanism  1072  includes a seal bar  1082  adapted for constant high-temperature operation, e.g. with nichrome electrical resistance heating elements, as described above. Without limitation on the generality of useful heat sources, other types such as circulating liquid, combustion, resistance heating, infrared, etc. could be provided. The seal bar  1082  mounts a cut-off blade  1084 , which protrudes slightly below a lower contact surface  1086  of the seal bar  1082 . The seal bar  1082  is located between a pair of heat-sink cooling bars  1088 , each of which includes coolant passages for receiving and circulating coolant, which enters and exits through cooling bar coolant ports  1090  connected to sealing assembly coolant inlet ports  1092 . The cooling bars  1088  are biased downwardly by compression springs  1094 , which compress the bag neck  1048  against the gasket  1050  for sealing. As shown in  FIG. 24 , the cooling bars  1088  normally extend below the level of the seal bar lower contact surface  1086 , whereby they function as heat sinks absorbing the heat generated by the seal bar  1082 . The entire sealing assembly  1068  is thus maintained at a relatively low temperature. The seal bar  1082  can be maintained at a constant temperature because exposure to the plastic bag neck  1048  only occurs in the sealing position, with the seal bar  1082  at its lowermost position relative to the cooling bars  1088  ( FIG. 26 ). In this position, which corresponds to the activation of the sealing process at station  1017 , the seal bar lower contact surface  1086  is in close proximity to the compressed double plastic layers forming the bag neck  1048 , which are thermally welded and sealed. The cut-off blade  1084  separates a neck cut-off portion  1049 , which can be discarded. The seal bar  1082  is then slightly raised to the position shown in  FIG. 27 , with its lower contact surface  1086  position slightly above the bag neck  1048  whereby the welded seam  1087  is allowed to cool or set, generally for about 0.1 to about 0.5 seconds. 
     The process is completed by retracting the actuating mechanism  1072  to its fully-raised position ( FIG. 28 ). The dome  1008  is lifted from the platen  1006  and the latter, with the vacuum-packaged product  1030  thereon, proceeds to the discharge station  1022 . Various additional steps and processes can be included in the method of the present invention and performed with appropriate modifications to the system of the present invention. For example, in food-handling operations certain procedures may be needed in order to maintain sanitary conditions. These can include washing and otherwise cleaning and sterilizing the equipment, which can comprise stainless steel or other suitable construction with desired characteristics such as resistance to rust and corrosion. Moreover, additional operations can be performed on the packaged products, such as weighing, labeling, additional packaging, freezing, drying, cooking, etc. 
     A suitable control system  1096 , which can include a microprocessor  1098 , can be provided and located in a control system and enclosure  1100  with a touch screen display  1102  mounted on a door  1104  of the enclosure  1100 . The control system can include various sensors, which can be connected to its inputs, for monitoring and interactively controlling the operation of the system  1002 . Operation of the various system components can be controlled by outputs from the control system  1096 , such as speed and load control for the drive motor  1036 , temperature control for the sealing bar  1082 , and coolant circulation pump control. The bag sealing system  1002  is suited for operation in an automated plant, whereby the product supply, loading, discharge and packaging procedures can be automated. The control system  1096  is also connected to an electrical power source  1095  powering the seal bars  1082 , a vacuum source  1097  for controlling the selective application of vacuum to the vacuum chambers  1070 , the pressurized air source  1078  for controlling the selective application of pressurized air through the air inlet ports  1076  and a coolant source  1099  for controlling the selective application of coolant through the coolant inlet ports  1090 . Vacuum, pressurized air and coolant are distributed from their respective sources  1097 ,  1078  and  1099  through a rotary manifold distribution mechanism  1093  to the domes  1012 . 
     A modified sealing assembly  1106  of another alternative embodiment of the present invention is shown in  FIG. 29  and includes the cut-off blade  1084  mounted on the outside of the cooling bar  1088 . In this configuration any cutoff portion  1049  of the neck  1048  would occur further away from the seam  1087 , whereby less or possibly no waste or scrap material is produced from each bag  1028 . It will be appreciated that the cut-off blade  1084  can be eliminated altogether, or deactivated by raising it to an inoperative position on the sealing assembly, whereby no waste or scrap bag material is produced in the sealing operation and the original neck is simply left in its original, full-length configuration. 
     Another alternative embodiment bag sealing system  2002  is shown in  FIGS. 30-36 . Referring to  FIG. 30 , the system  2002  includes an alternative embodiment bag sealing unit  2014  generally including a platen  2010  and a dome  2060 . Each platen  2010  includes a base  2040  with an upper surface  2042  mounting an item support  2044  adapted to receive a bagged item  2030  and a neck support  2046  adapted to receive the neck  2048  of a respective bag  2028 . The neck support  2046  includes a resilient contact surface  2050 , which can be mounted on springs and comprises a compressible material including, but not limited to silicone, in order to provide resiliency. 
     Each dome  2060  includes a lower sealing edge with a gasket  2066  adapted for forming a sealing engagement with the base  2040  upper surface  2042 . A sealing assembly  2068  is mounted on the underside of the dome  2060  generally within a vacuum chamber  2070  formed by the dome  2060  when it is placed in sealing engagement with the base  2040  and upper surface  2042 . Referring to  FIG. 30   a , the sealing assembly  2068  generally includes a pneumatic raise/lower actuating mechanism  2072  with a bladder  2074  connected to an air inlet port  2076 , which in turn can be connected to a pressurized air source, such as a compressor or a packaging plant air source, as described above. Return springs  2080  are provided for returning the raise/lower actuating mechanism  2072  to its raised position, e.g. when the bladder  2074  is deflated. The actuating mechanism  2072  includes a seal bar  2082  adapted for constant high-temperature operation, e.g. with nichrome electrical resistance heating elements, as described above. Without limitation on the generality of useful heat sources, other heat sources such as circulating liquid, combustion, resistance heating, infrared, etc. could be provided. The seal bar  2082  is located between a first and second heat-sink cooling bars  2088 ,  2089  each of which includes first and second coolant ports  2090 ,  2091  respectively for receiving and circulating coolant, which enters and exits through cooling bar coolant ports  2090 ,  2091  each of which includes coolant passages (such as passages  370   a  in  FIG. 2   a ) for receiving and circulating coolant. 
     A cut-off assembly  2202  is mounted on top of the dome  2060 . The cut-off assembly  2202  includes a pneumatic raise/lower actuating mechanism  2204  with bladders  2206  connected to air inlet ports  2208  which in turn can be connected to a pressurized air source, such as a compressor or a packaging plant air source, as described above. Return springs  2210  are provided for returning the raise/lower actuating mechanism  2204  to its raised position, e.g. when the bladders  2206  are deflated. The actuating mechanism  2204  includes guide rods  2214  mounting a cut-off blade  2212  located between the first cooling bar  2088  and the seal bar  2082 . The guide rods  2214  extend from the cut-off blade  2212 , extending through the return springs  2210  terminating at an upper end in contact with the cut-off assembly bladders  2206 . 
     In operation of the bag sealing system  2202 , a bag  2028  is sealed by first positioning a dome  2060  directly above a platen  2010  with the sealing assembly  2068  and the cut-off assembly  2202  in their raised positions ( FIG. 30 ). An unsealed bag  2028  containing an object  2030  is placed on each platen  2010 , and the platen  2010  and the dome  2060  are sealingly engaged temporarily through application of a vacuum removing air from the bag  2028  and chamber  2070 , as described above ( FIG. 31 ). When the appropriate vacuum is achieved, the bladder  2074  is inflated to force the sealing assembly  2068  downward ( FIG. 32 ), compressing the return springs  2080 . First, the cooling bars  2088 ,  2089  are biased downwardly by the compression springs  2094  bringing the lower contact surface  2092  in contact with the bag neck  2048 , which compress the bag neck  2048  against the resilient contact surface  2050 . As shown in  FIG. 31 , the cooling bars  2088 ,  2089  are disposed on either side of the sealing bar  2082 , whereby they function as heat sinks absorbing the heat generated by the seal bar  2082  before, during and after the sealing operation. The entire sealing assembly  2068  is thus maintained at a relatively low temperature during operation. The seal bar  2082  can be maintained at a constant temperature because exposure to the plastic bag neck  2048  only occurs in the sealing position with the seal bar  2082  at its lowermost position relative to the cooling bars  2088 ,  2089  ( FIG. 32 ). Next, the bladder  2074  is inflated to bring the seal bar lower contact surface  2086  in contact with the compressed double plastic layers forming the bag neck  2048 , which are then thermally welded and sealed by the seal bar  2082 . Shortly thereafter, the cut-off assembly  2202  is then engaged by inflating the bladders  2206  to force the cut-off blade  2212  downward through the bag neck  2048  and into contact with the resilient contact surface  2050  thereby creating a neck cut-off portion  2049 , which can be discarded ( FIGS. 33-33   a ). Next, the bladders  2206  are deflated returning the cut-off blade  2212  to the raised position by the return springs  2210 . The seal bar  2082  is then slightly raised to an intermediate position shown in  FIG. 34  by the return springs  2080  when the bladder  2074  is partially deflated. In the intermediate position, the lower contact surface  2086  of the seal bar  2082  is positioned slightly above the bag neck  2048  while the cooling bars  2088  remain in contact with the bag neck  2048  enabling the welded seam  2087  to cool and set, generally for about 0.1 to about 0.5 seconds. The bag sealing process is completed by retracting the sealing assembly  2068  to its raised position by the return spring  2080  when the bladder  2074  is fully deflated. The vacuum chamber  2070  is then returned to atmospheric pressure and the dome  2060  is lifted from the platen  2010  permitting removal of the sealed bag  2028  and placement of an unsealed bag  2028  upon the platen  2010  and neck support  2046 . 
     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, number of bag sealing units  2014 , components and methods of operation. For example, the complete stand-alone system  2002  consisting of a plurality of bag sealing units  2014  can be dimensioned to fit within a standard shipping container. 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.