Abstract:
The invention relates to a device which is used for the heat-sealing of a thermoplastic synthetic film to a thermoplastic synthetic container. The inventive device has at least one thermal electrode ( 11 ) which is made from a material with high thermal conductivity. The electrode is equipped with a metal section ( 30 ) having electrical connection terminals ( 31 ) at its ends. A heat flux sensor ( 32 ) comprising two electrical connections ( 33 ) is also provided, and the lower face is fixed mechanically to the upper part of the above-mentioned section ( 30 ). In addition, the upper face of the heat flux sensor ( 32 ) is fixed to the lower face of a thermal capacitor ( 34 ) which is made from a material with high thermal diffusivity and conductivity. Furthermore, a thermocouple ( 35 ) is mounted in a cavity in the metal section ( 30 ).

Description:
[0001]     This application is a national stage completion of PCT/CH2004/000600 filed Sep. 24, 2004 which claims priority from French Application Serial No. 0311533 filed Sept. 30, 2003. 
     
    
     TECHNICAL DOMAIN  
       [0002]     The instant invention concerns a method for heat-sealing at least one film of synthetic thermoplastic material to a container made of at least one synthetic thermoplastic material, particularly a container for packaging products that are susceptible to microbiological contamination, more specifically, biological or perishable commodities such as agricultural produce, said method using at least a first and a second thermal electrode.  
         [0003]     It also concerns a device for heat-sealing at least one film of synthetic thermoplastic material onto a container made of at least one synthetic thermoplastic material, particularly a container for packaging products susceptible to microbiological contamination, more specifically, biological or perishable commodities such as agricultural produce, using at least a first and a second thermal electrode to implement this method.  
       PRIOR ART  
       [0004]     Numerous packages, particularly those designed for packaging food produce, are formed of a pouch consisting of two thermoplastic films sealed together or formed of a container made of one or more synthetic materials manufactured by heat-sealing and closed by sealing thermoplastic film onto the container using heating electrodes. Although steady improvements have been made with respect to barrier-type films, the weakest link in package sealing remains the joining of thermoplastic films to each other or joining a thermoplastic film or lid to a thermoplastic package. At high speed and using current techniques, neither the seal nor consumer safety standards relative to the microbiological aspect of food packaging are completely satisfactory.  
         [0005]     Thermoplastic film is normally composed of a sealing layer which, after heating and at a given pressure, forms tight contact with the other portion to which it is joined. During contact, heat sufficient to bring the sealing layer to its melting point is transmitted to the materials. The pressure maintained during sealing crushes the sealing layer, which spreads and thins out. When the thin layer of sealing material crystallizes upon application of some sort of mechanical constraint, it sometimes pulls away, causing the formation of cracks which destroy the microbiological integrity of the packaging.  
         [0006]     The principal problems contributing to this result have been identified.  
         [0007]     They relate primarily to the heat. Heat regulation is essentially arbitrary, with the result that there is little control over the energy transmitted by the thermal electrodes to the material, causing the sealing layer to possibly overheat, spread excessively, and leading to increased shrinkage by the material. Furthermore, the randomness of the heat control also results in excessively long production cycles, detracting from the efficiency of the production line.  
         [0008]     Various techniques exist for sealing film with heat, for example, the use of heating bars, hot wires, or heat impulsion. These different techniques are not suitable for all types of polymers used as synthetic heat-sealable material. It is necessary to take into account the surfaces to be sealed, their various thicknesses, the coating on the materials, etc. The high speeds requirements of current production techniques often limit sealing time to less than a second. The application of either excessive or insufficient amounts of heat detracts from the quality of sealing. Current technical improvements are principally based on more precise temperature control of the heating bars. Data on the behavior of sealed polymers is only available for laboratory settings using destructive protocols. There is currently no device for dynamic control of sealing on production lines.  
         [0009]     The principal flaws of these known systems are due to:  
         [0010]     Too much thermal inertia in the sealing systems;  
         [0011]     Very low thermal stability of the sealing bars;  
         [0012]     Too much pressure applied to the film to be heat-sealed;  
         [0013]     Lack of control over the heat-sealing process on the line;  
         [0014]     Lack of control over cooling the seal on the line; and  
         [0015]     No regulation on the basis of the state of the synthetic material used.  
       DESCRIPTION OF THE INVENTION  
       [0016]     The instant invention proposes overcoming the disadvantages of the prior art by offering a high quality heat-sealing method that respects the microbiological integrity of a package.  
         [0017]     At least the first electrode is stabilized by controlling the variation in thermal flux emitted by this electrode;  
         [0018]     Temperature variation between the two electrodes is regulated by controlling the thermal flux flowing between said first and second electrodes, said thermal flux resulting from the temperature disequilibrium between the two electrodes and the variation in thermal resistance corresponding to the physical state of the synthetic thermoplastic material.  
         [0019]     The pressure exerted by at least one of the electrodes on the synthetic thermoplastic material is regulated by controlling the instantaneous variation in thermal flux resulting from the thermal energy absorbed by the melting of the synthetic thermoplastic material.  
         [0020]     A device for cooling the synthetic thermoplastic material is regulated by controlling the instantaneous variation of thermal flux resulting from the thermal energy restored by the synthetic thermoplastic material when it crystallizes.  
         [0021]     Advantageously, said first thermal electrode is first stabilized and the temperature difference between the two electrodes is regulated by controlling the heat flux using at least one heat flux sensor associated with said thermal electrodes.  
         [0022]     Preferably the pressure exerted by at least one thermal electrode on the synthetic thermoplastic material is regulated using at least one cylinder associated with this electrode and cooling of the synthetic thermoplastic material is regulated by chilling at least one of the thermal electrodes.  
         [0023]     The device as defined in the preamble for implementing this method is characterized in that it comprises:  
         [0024]     A means for stabilizing at least the first thermal electrode by controlling the variation in heat flux emitted by said electrode;  
         [0025]     A means for regulating the temperature difference between the two electrodes by controlling the heat flux flowing between the first and the second electrode, said heat flux resulting from the temperature disequilibrium between the two electrodes and the variation in thermal resistance corresponding to the physical state of the synthetic thermoplastic material;  
         [0026]     A means for regulating the pressure exerted by at least one of the electrodes on the synthetic thermoplastic material by controlling the instantaneous variation in heat flux resulting from the thermal energy absorbed by the melting of the synthetic thermoplastic material; and  
         [0027]     A means for regulating a device for cooling the synthetic thermoplastic material by controlling the instantaneous variation in heat flux resulting from the thermal energy restored by the synthetic thermoplastic material when it crystallizes.  
         [0028]     In a preferred form of embodiment said means for stabilizing at least said first thermal electrode by controlling the variation in heat flux emitted by said electrode comprises a heat flux sensor and a thermal flux meter regulator associated with this thermal electrode.  
         [0029]     In this same embodiment, said means for regulating a temperature differential between the two electrodes by controlling the heat flux flowing between said first and said second electrode, said heat flux resulting from the temperature disequilibrium existing between the two electrodes and the variation in thermal resistance corresponding to the physical state of the synthetic thermoplastic material, comprises at least one heat flux sensor associated with each of the thermal electrodes and a thermal flux meter regulator connected to these sensors and to these electrodes.  
         [0030]     Advantageously, said means for regulating the pressure exerted by at least one of said electrodes on the thermoplastic material by controlling the instantaneous variation of heat flux resulting from the thermal energy absorbed by the melting of the synthetic thermoplastic material comprises a cylinder associated with said thermal electrode.  
         [0031]     Preferably said means for regulating a device for cooling the synthetic thermoplastic material by controlling the instantaneous heat flux variation resulting from the thermal energy restored by the synthetic thermoplastic material when it crystallizes comprises at least one cooling channel formed inside at least one of said thermal electrodes.  
         [0032]     In an advantageous embodiment, at least one of the thermal electrodes comprises a heating bar.  
         [0033]     According to a variation, at least one of the thermal electrodes may comprise a thermal capacitor.  
         [0034]     Preferably at least one of the thermal electrodes is attached to a flexible block and housed inside said flexible block which is attached to a support on the heat sealing device.  
         [0035]     Advantageously said thermal electrode may comprise an integrated resistor element.  
         [0036]     Said device is not intended uniquely for controlling and guiding the sealing of food packaging, but for any thermoplastic film sealing process where improved sealing quality is sought. Its applications are broad and may extend to medical devices (transfusion pouches), or to thick injected containers and lids, for example. It is also possible with this device to control the strength of seal delamination and peeling. 
     
    
     SUMMARY DESCRIPTION OF THE DRAWINGS  
       [0037]     The features of the present invention will be more apparent from the following description of different modes of implementing the method and different embodiments of the device of the invention, with reference to the attached drawings, in which:  
         [0038]      FIG. 1  is a schematic view of a heat-sealing device;  
         [0039]      FIGS. 1A and 1B  are perspectives of two embodiments of thermal electrodes that can be used with the heat-sealing device of  FIG. 1 ;  
         [0040]      FIG. 2  is a cross-section of one example of films made of synthetic thermoplastic material constituting multi-layer heat-sealable materials;  
         [0041]      FIG. 2A  is a cross-section of a package comprising a thermo-formed container and a heat-sealed lid;  
         [0042]      FIG. 3  is an elevation of a first embodiment of a thermal electrode that can be used with the device of  FIG. 1 ;  
         [0043]      FIG. 3A  is a cross-section of said first embodiment of a thermal electrode shown in  FIG. 3 ;  
         [0044]      FIG. 4  is an elevation of a second embodiment of a thermal electrode that can be used with the device of  FIG. 1 ;  
         [0045]      FIG. 4A  is a cross-section of said second embodiment of a thermal electrode shown in  FIG. 4 ;  
         [0046]      FIG. 5  is an elevation of a third embodiment of a thermal electrode that can be used with the device of  FIG. 1 ;  
         [0047]      FIG. 5A  is a cross-section of said third embodiment of a thermal electrode shown in  FIG. 5 ;  
         [0048]      FIG. 6  is an elevation of a fourth form of embodiment of a thermal electrode that can be used with the device of  FIG. 1 ;  
         [0049]      FIG. 7  is a view showing the zone where the two heat-sealable materials are joined;  
         [0050]      FIG. 8  is a view illustrating the heat-sealing principle for two heat-sealable materials at the same temperature;  
         [0051]      FIG. 8A  is a view showing the heat-sealing principle for two heat-sealable materials at different temperatures;  
         [0052]      FIG. 9  illustrates the heat-sealing device equipped with its heat flux control and regulation elements;  
         [0053]      FIG. 10  represents profile views of the thermal electrodes in the sealing zones;  
         [0054]      FIGS. 11 through 13  represent various forms of seals that can be obtained; and  
         [0055]      FIG. 14  represents a particular application of the heat-sealing device according to the invention. 
     
    
     HOW TO ACHIEVE THE INVENTION  
       [0056]     With reference to  FIG. 1  the heat-sealing device  10  shown may comprise two thermal electrodes  11  and  12 . A single thermal electrode may suffice for certain applications. These electrodes are generally made of a highly heat-conductive material such as, for example, aluminum or copper. Electrode  11  is held by a support  13  that is mounted on a pneumatic or electric pressure cylinder  14 . Electrode  12  is rigidly attached to a support  15  integral with the machine frame (not shown). Support  15  may also be attached to a cylinder for certain specific applications.  
         [0057]      FIG. 1A  shows a first embodiment of thermal electrodes  11  and  12 . They comprise a metal bar  11   a  and  12   a  each containing at least one integrated resistor element such as a heating wire  11   b,    12   b,  respectively, or a heating stick, or the like.  
         [0058]      FIG. 1B  shows a second embodiment of thermal electrodes  11  and  12 . They are in the form of blades  11   c  and  12   c  with a longitudinal slot  11   d,    12   d,  respectively, covered with a heat-resistant film  11   e,    12   e,  respectively.  
         [0059]     The temperature of thermal electrodes  11  and  12  is regulated on the basis of data furnished by sensors measuring the thermal energy required to effect heat-sealing.  
         [0060]     As shown in  FIG. 2 , films  20  and  21  to be sealed are, for example, multi-layer films and may comprise a first exterior barrier layer  20   a,    21   a  respectively, a first impression layer  20   b,    21   b,  respectively, a second impression layer  20   c,    21   c,  respectively, a second interior barrier layer  20   d,    21   d,  respectively, and a sealing layer  20   e,    21   e,  respectively. The sealing layer has a lower melting temperature T F  lower than the other layers, particularly the barrier layers. The two contacting sealing layers  20   e  and  21   e  are sealed when they begin to melt, ensuring the cohesion of the unit.  
         [0061]      FIG. 2A  illustrates a package comprising a container  22  made from heat-formed or injected material and a barrier film  23  serving as a lid. This barrier film could also be replaced by an injected cover. Sealing can be effected with a single electrode applied to the lid, the sealing zone on container  22  having been previously preheated using hot air or an infrared beam.  
         [0062]      FIGS. 3 and 3 A respectively illustrate an elevation and a cross-section of an embodiment of a thermal electrode called the sealing electrode  11  of device  10 . It consists essentially of a metal section  30  that may be several millimeters wide and of variable length. It is made of electrically resistant material, for example, ferro-nickel that may or may not be coated with Teflon® film. Electrical connecting terminals  31  are located at the extremities of section  30 . A heat flux sensor  32  is mechanically attached by its lower surface to the upper portion of section  30 . Heat flux sensor  32  has two electrical connections  33 . The upper surface of heat flux sensor  32  is attached to the lower surface of a thermal capacitor  34  made of material with high thermal conductivity and diffusivity. A thermocouple  35  is mounted in a cavity formed in metal section  30 .  
         [0063]      FIG. 3A  shows more detail of the unit attached to a support connected to the heat-sealing device. Thermal capacitor  34  is housed in a flexible block  36  made of electrically insulating thermal material, for example, silicon rubber, said block being housed inside a recess in support  37  integral with the heat-sealing device. The unique feature of this flexible assemblage is its ability to overcome the tendency of thermal electrodes to be slippery.  
         [0064]      FIGS. 4 and 4 A represent another embodiment of a thermal electrode, called sealing electrode  11 , of device  10 . This sealing electrode consists of a metal section  40  made of thermally conductive and highly diffusive material joined to a heating bar  41  made of electrically resistant material. This heating bar  41  is equipped with electrical connection terminals  42 . The metal section  40  has a central groove  43  for housing a heat flux sensor  44 , the lower portion of which is attached to the upper surface of metal section  40 , and the upper surface of which is attached to thermal capacitor  45  made of the same material as metal section  40  which constitutes the thermal electrode called the sealing electrode. Thermal capacitor  45  is joined below electrical heating bar  41 . Heat flux sensor  44  has two electrical connections  46 . A thermocouple  47  is attached to the inside of the sealing electrode.  
         [0065]      FIG. 4A  represents a cross-section of this thermal electrode. As with the embodiment in  FIGS. 3 and 3 A, the unit consisting of metal section  40 , heating bar  41 , thermal capacitor  45 , and heat flux sensor  44  is housed in a flexible block  48 . Flexible block  48  itself is housed in a support element  49  for the heat-sealing device. The unique feature of this flexible assemblage is its ability to overcome the tendency towards slipperiness during heat-sealing.  
         [0066]      FIGS. 5 and 5 A represent another embodiment of this thermal electrode, called a sealing electrode, that consists of a metal section  50  made of thermally conductive, highly diffusive material. Said section  50  is joined to heating bar  51  made of electrically resistant material. At its extremities heating bar  51  is equipped with electrical connection terminals  52 . Metal section  50  has a groove  53  for receiving a heat flux sensor  54 . A threaded groove  55  traverses heating bar  51  coaxially in relation to groove  53  to receive head  56  of heat flux sensor  54 . A thermocouple  57  is attached in a suitable housing in the sealing electrode consisting of metal section  50 .  
         [0067]      FIG. 5A  shows how this thermal electrode is attached. Note that heating bar  51  and the metal section are housed in a flexible block  58 , with the block itself housed in a support element  59  for the heat-sealing device. The unique feature of this flexible assemblage is its ability to overcome the slipperiness of the elements intervening directly in the heat-sealing process, i.e. the sealing electrode or electrodes and/or the opposing contact element, as the case may be.  
         [0068]      FIG. 6  shows another embodiment of the thermal electrode called the sealing electrode. It consists of a metal section  70  comprising an interior channel  71  through which cooling fluid circulates on command. The purpose of this channel for the flow of cooling liquid is to control temperature and more specifically, thermal energy transmitted to the material for heat-sealing, thereby regulating the crystallization rate of this material in the sealing zone as it cools.  
         [0069]     This regulation is particularly important with large seals. Metal section  70  is associated with a thermal capacitor  72 . A heat flux sensor  73  is attached between the metal section  70  and thermal capacitor  72 .  
         [0070]     The operation of the heat-sealing electrodes is based on the following principle: when two thermoplastic materials are joined with heat, gradient pressure ΔP is applied so as to create a tight contact between these materials. The tight contact created in this way is necessary for the passage of quantities of heat ΔQ transmitted by the sealing electrodes, which may be from the hot zones at a temperature T 1  towards the compressed thermoplastic material constituting the cold zone at a temperature T 2  lower than T 1 . The quantities of heat are stored in the thermoplastic material and cause its temperature to rise. The temperature rises until it attains the temperature T F  at which heat sealing materials melt.  
         [0071]     From this point on, several phenomena occur. The first one is desirable, that is, auto-adhesion, which is very rapid, of the order of several milliseconds, ensuring molecular bonding between the two materials in the sealing zone.  
         [0072]     The second one undesirable, that is, flowing, which, due to the sudden change in viscoelasticity in the pressurized sealing zone, tends to reduce the thickness of the material in this same zone, making it mechanically fragile.  
         [0073]     The third one is the formation of the seal that begins with the cooling of the materials in the sealing zone. At this stage it is known that if cooling can be controlled, the crystallization rate (X C %) can also be controlled as a function of the slope of the cooling curve. The crystallization rate of the materials affects recrystallization and the shrinking phenomenon that may lead to formation of cracks and serious microbiological flaws in the heat-sealed package when it may subsequently be exposed to mechanical constraints.  
         [0074]     The challenge in heat-sealing consists of regulating these various phenomena. To accomplish this, the invention proposes to effect real time control over the exchange of quantities of heat flowing at a variable rate. According to the prior art, the temperatures were controlled, that is, the final condition, making real time regulation difficult or even impossible.  
         [0075]     As shown in  FIG. 7 , in a variable pattern, heat accumulates over a period of time dt in sealing zone dx at temperatures that vary over time. When sealing zone dx reaches the melting temperature T F  of the material, sealing zone dx is the location of energy absorption −PI.  
         [0076]     When sealing zone dx cools down and reaches the crystallization temperature T c , it becomes the location of energy restoration +PI. This variable pattern can be detected with a heat flux sensor correctly positioned on the thermal electrode.  
         [0077]      FIG. 8  presents a symbolic schematic of a heat-sealing device. During time t+a the equivalent thermal capacity Cp of the heat-sealable materials is charged by sealing electrodes  11  and  12  with quantities of heat ΔQ flowing from the hottest point of electrodes  11  and  12  toward the coldest point, sealing zone dx. Heat fluxes φ 1  and φ 2  migrate from thermal electrodes  11  and  12  towards sealing zone dx through thermal resistors Rth. A heat flux sensor  32  measures the variation in thermal flux. The heat fluxes are equal when the temperature of electrodes  11  and  12  is identical, such that T 1 =T 2  and are then nullified when the materials are charged.  
         [0078]     In the example in  FIG. 8A  thermal electrodes  11  and  12  are no longer at the same temperature. For example T 1 &gt;T 2  The charging fluxes are different: φ 1 &gt;φ 2 . When the materials are charged, the thermal flux rate is no longer nil. A quantity of heat flow φ 3  is established from the hottest electrode  11  at temperature T 1  toward the coldest electrode at temperature T 2  through sealing zone dx. The flux level φ 3  is a function of the difference in temperature between electrodes ΔT=T 1 −T 2.    
         [0079]     A heat sensor  32  correctly positioned on electrode  12  will detect a flow φ 2  as the material begins charging, and when it has been charged, an inverse flux φ 3 .  
         [0080]     By fixing the temperature of one of the thermal electrodes at a higher value than the melting temperature T F  in the sealing zone dx and the temperature of the other thermal electrode at a lower value, the resulting heat flux detected by the heat flux sensor varies constantly as a function of small temperature differences, with the result that for the purpose sought, either the delaminating force or the peeling force is modified, which risks breaking the fragile mechanical seal. This can be overcome and the delaminating and peeling forces stabilized depending upon the various properties of the materials and the environment on the one hand, by regulating the temperature of one electrode using a heat flux regulator operating on the basis of data furnished by the heat flux sensor associated with it and delivering through this electrode only the necessary and sufficient quantities of heat; and on the other hand, by regulating the temperature of the other thermal electrode using a heat flux regulator operating on the basis of data furnished by the heat flux sensor associated with it and delivering through this electrode only the necessary and sufficient quantities of heat.  
         [0081]     It is therefore possible to make a controlled lid for a package and to regulate the strength of the seal by controlling either the force of delaminating or of peeling through the use of a heat flux regulator to control the thermal electrodes.  
         [0082]      FIG. 9  is a schematic illustration of the means for regulating a thermal electrode  80  associated with a heating bar  81  as a function of the data communicated by heat flux sensor  82 . The connecting terminals  84  on heating bar  81  are connected at outputs  85  of a thermofluximetric regulator  86 , heat flux sensor  82  is connected to inputs  87  of thermofluximetric regulator  86  by means of its connectors  89 , and thermocouple  90  is connected to input  91  of thermofluximetric regulator  86 .  
         [0083]     Flow is prevented in the sealing zone by using heat flux sensor  82  to detect melting in the zone, with the sensor delivering data processed by thermofluximetric regulator  86  which generates on opto-coupled circuit  92  a signal that passes from 0 to 1. This signal reduces the gradient pressure ΔP of cylinder  14  (see  FIG. 1 ) on the sealing zone. An opto-coupled output  93  on thermofluximetric regulator  86  passes from 0 to 1 at the same time. This signal controls injection into channel  71  (see  FIG. 6 ) on the thermal electrode of a cooling fluid during seal formation.  
         [0084]      FIG. 10  illustrates a series of thermal electrodes  100  with distinct profiles, the sealing surfaces  101  of which may have various possible configurations depending upon the desired application.  
         [0085]      FIGS. 11 through 13  illustrate different types of sealing zones obtained using different electrodes.  FIG. 11  represents a sealing zone with spaced apart points,  FIG. 12  represents a honeycomb sealing zone, and  FIG. 13  represents a multilinear sealing zone.  
         [0086]     In certain instances it is impossible to use juxtaposed thermal electrodes, especially when joining thick pieces, for example, a container  110  and a lid  111  as shown in cross-section in  FIG. 14 . In this case the sealing zone is heated in advance, either by infrared beam or by hot air heat convection.  
         [0087]     The problems are identical to those described previously. The temperature of the surface of the sealing zone is regulated using a radiant type heat flux sensor  112  and a thermofluximetric regulator as described above.