TECHNICAL FIELD  
         [0001]    This invention relates to heat seal dies used to form a heat seal between two heat sealable surfaces such as thermoplastic film. More particularly, this invention relates to heat seal dies for use in portion control sized packaging machines to form top and bottom seals of packages made from thermoplastic film.  
         BACKGROUND OF THE INVENTION  
         [0002]    Flexible packaging such as thermoplastic film is commonly used in form/fill/seal equipment for packaging fluid material such as food products. In form/fill/seal systems, material is placed inside tubular thermoplastic film and the top and bottom of the tubular film is heat sealed with a heat seal die to encase the material. Condiments such as ketchup and mustard are often packaged this way.  
           [0003]    Temperature variation across the length of the heat seal die can produce packages that leak fluid material or serum over a period of time. This phenomenon occurs when the sealing temperature of the end seal dies exceeds or falls below the sealing range of the material. These suspect packages may not be evident to the operator of the form/fill/seal machines since the temperature controller only senses the temperature on one small area of the die. This condition often results in contamination of entire boxes of packaged goods containing large numbers of individual packets of product. Therefore, there is a need for more reliable form/fill/seal equipment.  
         SUMMARY OF THE INVENTION  
         [0004]    This invention encompasses a heat seal die incorporating heat tube technology for controlling the variation of temperature across a heat seal die. The invention also encompasses a system and method for portion control sized packaging using such a heat seal die.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0005]    [0005]FIG. 1 is a perspective view of a heat seal die made according to an embodiment of this invention.  
         [0006]    [0006]FIG. 2 is an end view of one die member illustrated in FIG. 1.  
         [0007]    [0007]FIG. 3 is a schematic diagram of a form/fill/seal system in accordance with an embodiment of this invention.  
         [0008]    [0008]FIG. 4 is a plan view of a heat sealed package formed in accordance with an embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0009]    This invention encompasses a novel heat seal die and a novel system and method for making portion control sized packaged goods.  
         [0010]    Turning to FIG. 1, a heat seal die  10  for forming a heat seal between two heat sealable surfaces such as a thermoplastic film or laminate comprising a heat sealable thermoplastic component is illustrated and generally comprises a first die member  12 , a mating second die member  14 , respective first and second heating elements  16  and  18  disposed in the first and second die members, and respective first and second heat tubes  20  and  22  disposed in the first and second die members.  
         [0011]    Each of the first and second die members  12  and  14  comprise a body  24  made of rigid heat conducting material such as metal. A wide variety of metals can be used to form the die member body  24 , but 440 stainless steel is particularly preferred because of its hardness and wear resistance.  
         [0012]    The die member body  24  extends from a first end  26  to a second end  28  along a longitudinal axis. First and second opposing side members  30  and  32  extend from a first end  26  to the second end and a raised portion  34  extends from between the first and second longitudinal sides. The raised portion  34  has sloping walls  36  extending outwardly and inwardly from the respective first and second sides  30  and  32  to a die face  40 .  
         [0013]    The die faces  40  of the first and second die members  12  and  14  include alternating lands  42  and grooves  44  which mate when the first and second die members engage to heat seal material such as thermoplastic polymer. A desirable land and groove configuration is illustrated in U.S. Pat. No. 4,582,555, the disclosure of which is incorporated herein by reference in its entirety. Although a land and groove configuration is desirable, it should be understood that other die face configurations such as flat surface may also be used.  
         [0014]    Each of the first and second die members  12  and  14  have a first longitudinal bore  46  extending from one end  26  of the die member body  24  to the other end  28  proximate the die face  40 . The first and second longitudinal heat tubes  20  and  22  are tightly disposed in the respective first bores  46  of the first and second die members  12  and  14 . A second longitudinal bore  48  extends from the first end  26  to the second end  28  of each of the first and second die members  12  and  14  and receive the respective first and second longitudinal heating elements  16  and  18 .  
         [0015]    The first and second heat tubes  20  and  22  desirably extend the length of the first and second die members  12  and  14 . Generally described, heat tubes comprised a closed metal tube such as a copper or aluminum tube. The tube contains a wick, with the remainder of the internal volume filled with a liquid under its own pressure. As the temperature along the heat tube changes, the fluid in the hotter area of the tube boils, picking up latent heat of vaporization. This high pressure gas travels to the lower pressure (cooler) area of the tube and condenses, thus transferring heat to the cooler area. This cycle in the heat tube equilibrates the temperature across the heat seal die members  12  and  14  and keeps such temperature substantially uniform. The heat tube suitable for any particular embodiment of this invention depends on the desired temperature range of the heat seal die and the length of the heat seal die. The heat seal tubes  20  and  22  can be chosen to limit the temperature variation across the length of the die members  12  and  14  as desired. It is often desirable to keep the temperature variation across the length of the die members  12  and  14  less than 10° F. and even more desirably less than 5° F. Although the temperature of the die members  12  and  14  will vary depending on the particular heat sealable material being sealed, the temperature normally ranges from 200° to 400° F.  
         [0016]    Suitable heating elements  16  and  18  include a variety of conventional heating elements. A particularly desirable heating element is a heating cartridge which fits tightly within the second bores  48  of the first and second die members  12  and  14 .  
         [0017]    As shown in FIG. 2, the heat seal die  10  also comprises a temperature sensor  50  disposed in the first side  30  of each die member  12  and  14 . A thermocouple is one suitable temperature sensor. Desirably, the temperature sensor  50  is disposed in the first side of each die member  12  and  14  because this side faces downwardly during operation of the heat seal die  10 . Accordingly, when the heat seal die  10  is washed with water, as is sometimes necessary, the water flows over the heat seal die without cooling the first side  30  of the die members and therefore does not affect the temperature sensor. Washing of the heat seal die  10  only cools the outer surface of the heat seal die and if the water contacts the area of the heat seal die near the temperature sensor, it can cause the temperature sensor to give a false reading.  
         [0018]    The heat seal die  10  is particularly suitable for heat sealing a laminate of film, foil, and thermoplastic sealant in a conventional form/fill/seal packaging unit  52  as illustrated in the schematic diagram in FIG. 3. A film feeder  54  feeds heat sealable laminate to the form/fill/seal unit  52  between the opposing die faces  40  of the heat seal die  10 . A flowable material feeder  56  delivers flowable material for the packaging to the form/fill/seal unit  52 . The unit  52  is useful for packaging of variety of flowable materials including liquids, sauces, purees, jells, pastes, and in particular, condiments, such as mustard, ketchup, and relish. This system is particularly suitable for packaging portions within the range of one to five ounces each.  
         [0019]    The packaging system of this invention produces sealed flexible packages  58  such as illustrated in FIG. 4. Each package  58  comprises a heat sealable packaging material  60  with a top seal  62 , a bottom seal  64 , and side seals  66  and  68  formed by the heat seal die  10  and the form/fill/seal unit  52 . As the heat sealable packaging material passes between the opposing die faces  40 , the heat seal die  10  forms the top heat seal edge of one package and the bottom heat seal edge of another package. A knife housed in the form/fill/seal unit  52  separates the two packages by severing the film just after the heat seals are formed.  
         [0020]    The described embodiment reduces the occurrences of serum leakers in portion controlled sized packages. Serum leakers are packages which leak liquid through the package heat seal.  
         [0021]    It should be understood that the foregoing relates to particular embodiments of the present invention, and that numerous changes may be made without departing from the scope of the invention as defined in the following claims.
This invention encompasses a heat seal die incorporating heat to technology for controlling the variation of temperature across a heat seal die. The invention also encompasses a system and method for portion control size packaging using such a heat seal die.