PACKAGING METHOD AND APPARATUS

Abstract: A package is steam-shrunk while subjected to a substantially constant sub-atmospheric pressure generated by a suction fan 22 operating at the same time as steam is generated maintaining the prevailing pressure within a chamber interior 20 at sub-atmospheric pressure in the presence of steam. The low pressure of the steam ensures that its termperature is well below the boiling point of water and avoids thermal damage to the material of a container 12 being shrunk.

Description: The present invention relates to the packaging of articles in flexible containers, for example bags or pouches, made of heat-shrinkable material which can be caused to contract tidily around the product article being packed, leaving a sub-atmospheric pressure within the pack. GB-A-2078658 discloses a vacuum packaging cycle in which the extraction of air from within a vacuum chamber proceeds while the neck of a container of flexible heat-shrinkable material is constricted so as to allow only limited removal of air from within the package, causing the container material to balloon away from the product while residual air within the chamber is both heated and circulated to impart shrinking heat to the package. The heat transfer to the container walls proceeds due to conduction from the moving air flow but requires a relatively long cycle time. GB-A-2094745 discloses a modification in which the removal of gas from within the container in the chamber is impeded while shrinking heat is applied to the container by radiant heating, so that again the gas remaining within the container maintains the container wall clear of the product such that equilibrium between the shrinking forces in the container material and the pressure differential between the interior and the exterior of the closed container during evacuation of the chamber within which the container is placed results in the desired ballooning configuration during the application shrinking heat to the container, to allow subsequent release of the air or other gas from within the flexible container to permit the desired tidying shrink action. US-A-4567713 discloses a vacuum chamber packaging process in which when, during the cycle, the chamber evacuation stops the venting of the chamber occurs by means of introduction of steam from outside the chamber, initially while the pressure is low but also continuing during the build-up of pressure. The steam, superheated before entry into the chamber, is prevented from condensing on the chamber wall by means of wall heaters, but condenses onto the container, thereby heating the container with the latent heat of condensation and permitting the container to shrink into contact with the enclosed product. We now propose to provide a modified process and apparatus which enhances the appearance of the pack as compared with that of US-A-4567713, in that the likelihood of fogging of the container is reduced and the efficiency of heat transfer is maintained. Accordingly, one aspect of the present invention provides a method of heat-shrinking a package, including the steps of placing a product in a container; reducing the pressure prevailing on the surface of the container; causing the container to undergo heat shrinking onto the product and arriving at a closed shrunk package by contacting the container surface with steam while maintained at a sub-atmospheric pressure in order to impart shrinking heat to the container wall by virtue of the released latent heat of condensation of the sub-atmospheric pressure steam; and subsequently discontinuing the flow of steam and restoring the pressure; characterised in that the container is a bag; in that the bag is placed in the chamber spaced from the chamber walls; in that the sub-atmosphere pressure is maintained uniform throughout the vacuum chamber around the container; in that the uniform sub-atmospheric pressure is maintained substantially constant on said surface of the container during the steam shrinking step; and residual steam from around the bag is pumped away after said steam-shrinking step. A further aspect of the present invention provides apparatus for steam shrinking a package, comprising:- a vacuum enclosure within which the package is to be shrunk; means for generating steam and for introducing it into said enclosure; and means operable while the steam generator is in operation, for extracting air and/or steam from the enclosure; characterised in that the chamber includes a support for a bag spaced from the walls of the chamber so that the bag is completely enclosed within the chamber with a uniform pressure around the bag exterior; in that air extraction means comprise a fan operative to maintain a substantially constant subatmospheric pressure in the steam-filled enclosure around the package; and in that the fan is operative after termination of the steam generation so as to extract residual steam from the exterior of the bag before the enclosure is opened. In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings in which:- FIGURES 1A, 1B and 1C show the operating sequence of a first embodiment of apparatus for carrying out the process of the present invention, under manual control; FIGURE 2 is a schematic side elevation of a semi-automated first embodiment of apparatus for carrying out the invention; FIGURE 3 is a cycle timing diagram illustrating variations of temperature and pressure within the vacuum chamber of Figure 2, as a function of time; FIGURE 3A is a cycle diagram similar to Figure 3 but showing a modified method; FIGURE 4 is a schematic side elevation of a second possible apparatus for carrying out the invention in an automatic manner; FIGURE 5 is a detail of the evacuation nozzle of Figure 5; FIGURE 6 is a schematic view of a third possible apparatus for carrying out the invention; FIGURES 7A TO 7D show the operation of the doors of the apparatus of Figure 6, and illustrate the automatic control of the doors; and FIGURE 8 is a schematic illustration of a variant of the apparatus of Figure 6. The manual apparatus of Figure 1 includes a vacuum chamber 2 comprising an upper portion 3 which can be lifted and lowered in order to open the chamber, and a fixed lower chamber part 4 having a peripheral flange 5 which seals against a corresponding peripheral flange 6 of the upper chamber part 3 in the closed configuration of the chamber. Within the lower chamber part 4 is a hollow space 7 bounded at its upper part by an upper panel 21 having perforations for release of steam into the chamber interior 20, and at its lower part by an electrical resistance heater 8 controlled by a thermostatic temperature controller 9. Across the open top of the lower chamber part 4 is a horizontal grid 10 to support a product 11 loaded in a flexible container, in this case a bag 12, of heat-shrinkable material. The support grid 10 is formed of polytetrafluorethylene plates, or plates coated with polytetrafluorethylene, which extend generally along parallel regularly spaced vertical planes to allow air and steam to pass up through the grid into contact with the bag thereon. Steam is released into the lower chamber part 4, and hence into the interior 20 of the entire chamber, by the introduction of water by way of a shut-off valve 17 into the space 7 heated by the electrical resistance heater 8 to a temperature which will generate steam within the rarefied atmosphere in the vacuum chamber. The reduced pressure in the vacuum chamber interior 20 induces the flow of steam through the apertures in the plate 21. This situation is illustrated in Figure 1B. The vacuum chamber includes a rotary suction/blower fan 22 which is able to reduce the internal pressure in the vacuum chamber to a residual pressure of the order of 600 millibars, and the line between the vacuum chamber 2 and the fan 22 includes an air shut-off valve 24. One complete cycle of operation of the apparatus of Figures 1A, 1B and 1C will now be described:- Initially, the chamber cover is lifted and the closed bag 12 enclosing the product 11 is placed on the grid 10. Then the upper chamber part 3 is replaced in position and the chamber interior 20 thus sealed from the atmosphere. At this point the start of the cycle illustrated in the left hand part of Figure 1A begins by operation of the fan 22, with the air shut-off valve open, to reduce the residual pressure in the chamber interior 20. This pressure reduction (from 1,000 millibars to 600 millibars) is depicted by the solid line graph while the constant temperature of the order of 20°C is illustrated by the dotted line graph. During this phase of operation, the electrical resistance heat 8 under the control of the temperature controller 9 maintains the air temperature within the enclosure 7 at a temperature high enough so that at the beginning of the phase illustrated in Figure 1B, when the water inlet valve 17 is opened, the arrival of the water in the space 7 (under the action of the pressure reduction within the chamber 20) causes that water to evaporate immediately and to spray as steam into the chamber interior 20. This starts at time T1 on the graph shown in Figure 1B and results in an increase of the residual pressure to a value of the order of 750 millibars at which it remains constant due to the continuing operation of the fan 22 extracting air and surplus steam from the chamber interior 20. During this steam introduction phase the temperature of the atmosphere in the chamber interior 20, and particularly in contact with the exterior surface of the bag 12, rises rapidly due to the conduction of heat from the enclosure 7 by the escaping steam. As shown in Figure 1B, by the time the end of that particular phase of operation is reached the temperature has mounted from about 20°C to about 88°C. Meanwhile the pressure holds at its value of approximately 750 millibars until the end of the water introduction phase at which the valve 17 is closed while the fan 22 runs on for a short while to cause a dip in the pressure. Although it is not evident from the dotted line temperature curve in Figure 1B, the degree of heat transferred to the atmosphere within the chamber interior 20, and in particular to the surface of the bag 12, is much more pronounced than would result simply from the temperature gradient shown, because instead of simply conducting the heat from the enclosure 7 to the bag surface by the thermal capacity of air, the process involves thermal conduction of the much higher thermal capacity steam medium and the donation of the latent heat of condensation when the steam condenses on the surface of the bag exterior. There is thus considerable heat transfer to the bag, but without requiring exposure of the bag to a temperature of more than about 88°C in doing so. The third stage of operation, shown in Figure 1C, involves closing the air shut-off valve 24 and venting the chamber interior 20 back to a residual pressure of 1,000 millibars while the temperature falls rapidly towards its starting value of 20°C (ambient) as shown in the graph of Figure 1C. As indicated above, the process cycle depicted in Figures 1A, 1B and 1C involves generous donation of shrinking heat to the bag 12 without excessive rises in temperature which might cause degradation of the bag film material. Furthermore, because the fan 22 runs on after the water shut-off valve 17 has been closed there is efficient extraction of surplus steam so that when ultimately, at the end of the process cycle, the upper chamber portion 3 is raised there is no noticeable escape of steam into the atmosphere of the packaging room. The automatic version of the apparatus illustrated in Figure 2 operates in a similar way, but includes additional elements. The chamber upper and lower portions 103 and 104 includes the same flanges 106 and 105 to effect sealing, and when sealed encloses an inner space 120. However, in this particular case the bag 112 with its enclosed product 111 does not become sealed until a particular point during the process when the upper sealing bars 127 close against the lower sealing bars 126 and sealing heat is applied to close the bag. In this case air extraction is achieved by way of a centrifugal fan 137 communicating with the chamber interior 120 by way of an air shut-off valve 136, and also by way of a vacuum pump 138 (in this case a rotary vane pump) communicating with the chamber interior 120 by its respective air shut-off valve 139. Under the grid 110 of the chamber is an annular steam generator comprising an annular water plenum 108 fed with water by way of a water shut-off valve 117 and arranged to liberate the water into the interior of an annular heater chamber surrounding an externally finned annular electrical resistance heater 131. Within the annular heater 131 is a fan 129 driven by a motor 130, for the purposes of inducing circulation of atmosphere throughout the chamber interior 120. As in the case of Figure 1, the annular electrical resistance heater is controlled by a thermostatic temperature control unit 109. The process cycle of the apparatus shown in Figure 2 is illustrated in Figure 3, again using solid lines to indicate the variation of pressure and a dotted line to indicate the variation of temperature in the chamber interior 120. Before instant T₀ the upper chamber part 103 is raised to remove the upper sealing bar 127 clear of the lower sealing bar 126 and to allow a loaded but unclosed bag 112 around the product 111 to be placed on the grid 110 as shown in Figure 2. Then the chamber cover 103 is replaced in position with the neck of the bag 110 loosely confined between the spaced upper and lower heat sealing bars 127 and 126, respectively. At point T₀, the chamber has been closed, and the fan 129 is rotated by its motor 130 in order to circulate air over the fins of the heater 131 to raise the temperature in the chamber interior 120 without changing its pressure to any substantial extent. At instant T₁, the pressure is still at about 1,000 millibar whereas the temperature has risen to about 70°C. At this point the air shut-off valve 139 is opened and the vacuum pump 138 is operated in order to reduce the pressure within the chamber interior 120. This continues until instant T₂ at which point the residual pressure is under 100 millibars and the temperature has correspondingly fallen to approximately 68°C. Meanwhile the air shut-off valve 136 is in a closed state to isolate the fan 137 from the chamber interior 120 so that the reduction in pressure is solely due to the operation of the suction pump 138 (with the air shut-off valve 139 open). Between instant T₂ and instant T₃, the upper heat sealing bar 127 is pressed downwardly to compress the bag neck material between the two heat sealing bars, and sealing heat is applied to weld the bag material. It will, of course, be appreciated that during the pressure reduction phase from instant T₁ to T₂ the pressure both inside and outside the bag 112 will be reducing, and that by the time the bag neck has been sealed at instant T₃ the residual pressure inside the bag 112 will be less than 100 millibars. At instant T₃ the application of shrinking heat is initiated by opening the water shut-off valve 117, allowing the arrival of water into the annular water plenum 108 and corresponding discharge of steam into the circulating current of low pressure air induced by virtue of the continuing rotation of the fan 129 (driven by the motor 130). This accounts for the rapid rise in pressure following instant T₃, and the corresponding rise in the temperature to about 88°C which is obtained by the time the pressure has risen to, and stabilised at, 750 millibars. In order to maintain the release of steam into the atmosphere within the chamber interior 120, the air shut-off valve 136 is opened at instant T₃ and the fan 137 resumes operation, thereby ensuring the pumping of any excess steam that may be generated inside the chamber and hence the further release of fresh steam into the chamber interior from the annular water plenum 108. As in the case of the Figure 1 embodiment of the apparatus, the water shut-off valve 117 is closed before instant T₄ and hence before the air shut-off valve 136 is closed, causing a dip in the pressure to about 700 millibars at instant T₄. At instant T₄ the chamber is vented to atmosphere so that the pressure rises rapidly towards a residual value of 1,000 millibars (ambient) while the temperature falls progressively towards a value of about 25°C which is due to the restoration of ambient conditions but subject to continuing circulation of air through the chamber interior 120 by virtue of the fan 129. It will be appreciated that one fundamental difference between the Figure 1 embodiment and the Figure 2 embodiment of the apparatus is that Figure 1 provides an apparatus which allows post-shrinking to a bag which has already been sealed, whereas Figure 2 provides a pre-shrinking action by virtue of air circulation between instant T₀ and instant T₂, plus a subsequent more pronounced shrinking operation between instants T₃ and T₅ after sealing of the bag, the sealing itself having been achieved inside the vacuum chamber 120. Although the apparatus shown in Figure 1 uses a steam generator inside the lower chamber portion 4, it could equally be attached to the exterior of the lower chamber portion 4, or even be positioned in a steam line to the lower chamber portion 4. As a further variation of the Figure 1 embodiment, the water control valve 17 may be controlled by an adjustable timer which enables the quantity of water entering the steam generation enclosure 7 to be adjustable at will for varying the quantity of steam used for the post-shrink operation. In both Figure 1 and Figure 2, the temperature to which the electrical resistance heater is heated can be adjusted by suitable adjustment of the control temperature of the thermostatic controller 9 or 109 respectively. The suction fan 22 in Figure 1 or 138 in Figure 2 may be a side channel blower. The valves 24 and 17 of Figure 1 may be activated in the proper sequence and with suitable timing by suitable means such as a "programmable logic control" (usually known 10 as a PLC) not shown in the drawing. In order to avoid the possibility of steam condensing on the side walls of the upper and lower chamber parts 103 and 104, it may be desirable to incorporate thermal insulation so as to allow the inside surface of the chamber to remain hot from one cycle to the next despite the existence of a much lower ambient temperature around the exterior of the chamber during the process cycle. In any case, the tendency for condensation on the warm chamber walls is reduced by virtue of the fact that the steam is introduced in to the chamber interior 120 at a low residual pressure which will delay the onset of condensation, except as regards the much cooler surface temperature of the exterior bag 112 where condensation is in any case desirable in order to promote the donation of latent heat of condensation to the bag surface. Although in Figure 2 the upper and lower sealing bars 127 and 126 themselves effect loose confinement of the bag to allow extraction of air from around the product 111, it is possible for additional resiliently biased clamping elements to be arranged downstream of the sealing bars 126 and 127 to exert a yieldable clamping effort by virtue of biasing springs so that as the pressure in the chamber interior 120 drops there will be a lag in the pressure reduction in the interior of the bag 112 due to the yieldable holding of the bag neck by these additional clamping bars, promoting ballooning of the material of the bag 112 clear of the product 111 and hence guarding against the formation of a pocket of residual gas behind the product 111 (i.e. to the right of it as viewed in Figure 2). Venting in the case of either the Figure 1 or Figure 2 embodiment may be effected by a separate vent line with a shut-off valve (which are not shown in the drawings). An alternative embodiment of the automated method using the apparatus of Figure 2 is possible as shown in the modified cycle diagram of Figure 3A. Here the pressure reduction is divided into two separate phases which coincide with two separate shrink heating applications. In Figure 3, the pressure reduction only starts at instant T₁ after an initial rise in temperature in the interval between T₀ and T₁ resulting from the motor-driven fan 129 circulating air having been heated by the heater 131 through the interior of the chamber and over the product to effect a heat shrinking step. By contrast, in Figure 3A the pressure reduction starts at instant T₀, while the fan is still operating to circulate the residual air over the heater and the product, but the extraction of this air by means of the fan 137, following opening of the valve 136 at instant T₀, is accompanied by a generation of steam in view of the water control valve 117 opening at instant T₀, and remaining open until instant T₁. Thus, during the interval between T₀ and T₁ the fan 129 is circulating a mixture of hot air and steam at sub-atmospheric pressure over the product to initiate the shrinking operation. This has the advantage of increasing the amount of bag shrinkage even over and above the shrinkage evident from the cycle of Figure 3 because of the higher thermal capacity of the steam used in the interval T₀ to T₁. Subsequently, in the interval T₁ to T₂ the steam generation has ceased and the suction pump 138 operates to reduce the residual pressure from 0.75 bar to a value just under 100 millibar and during this time the temperature falls from the peak value attained at instant T₁ to a dip occurring just after instant T₃ when the temperature has dropped to a value between 60 and 70°C. The remainder of the cycle of Figure 3A after recovery of the temperature to its maximum value of 88°C is identical to the later parts of the cycle of Figure 3. An alternative embodiment of automated apparatus is shown in Figures 4 and 5 in which the "suction nozzle" principle is used. Those components of Figure 4 which are also shown in Figure 1 are denoted with the same reference numeral increased by 200. In the alternative embodiment shown in Figure 4 the chamber 202 has the upper chamber portion 203 once again lifted mechanically in timed relation to the operating cycle of the machine. As with the embodiment of Figure 2, there is a fixed sealing bar set 226 in the fixed lower chamber part 204 and a movable upper sealing bar set 227 which is carried by the movable upper chamber part 203 for movement towards and away from the bag neck. However, in this particular embodiment there is additionally means for moving the upper sealing bar 233 vertically relative to the upper chamber part 203 because initially an air extraction nozzle 232 is positioned inside the mouth of the bag 212 in the early part of the chamber evacuation phase. The bag 212 rests on a support plate 234 coated with polytetrafluoroethylene in order to avoid the bag sticking to the plate. As with the earlier embodiments, the steam may be generated either in situ by water injection into the chamber lower part or by an optional steam generator 213 which uses a water heater 214 operating on a water supply line 215 having a solenoid-operated water control valve 217. With the Figure 4 embodiment, the steam valve controls the admission of steam to not only the chamber interior 220 but also the nozzle 232 (by way of a nozzle steam line 235). A pressure responsive control unit 236 is linked to a pressure transducer 237 on the floor of the lower chamber part 204 and controls the air extraction valve 224 between the chamber interior 220 and the suction fan 222. The nozzle 232 is shown in more detail in Figure 5 and comprises a generally flat tubular structure divided into three longitudinally extending side-by-side passages of which one (in this case the central passage) is a steam injection passage 238 while the other two lateral passages 239 are open at both ends so as to communicate the interior of the bag 212 with the exterior for removal of air from within the bag 212. The operation of the apparatus of Figures 4 and 5 is as follows:- Initially, starting with the upper chamber portion 203 raised, and the product support plate 234 vacant, an open-mouthed bag 212 enclosing a product 211 is introduced into the chamber and placed on the product support plate 234. The neck of the bag is arranged around the generally flat end portion of the nozzle 232, just above the lower heat sealing bar set 226. This may, for example, require the nozzle 232 to be movable to an out of the way position to allow the bag neck to be arranged carefully over the lower heat sealing bar set and then swung back into position to enter the bag neck to arrive at the configuration shown in Figure 4. The vacuum chamber is then closed by lowering of the upper chamber part 203. Once the chamber has been closed the suction fan 222 is energized and the air extraction valve 224 is opened by means of a controller 236. The suction fan 222 thus reduces the pressure in the chamber interior 220. The water feed valve 217 or any alternative water injection control valve, is then opened to allow water to flow to the steam generator point such as the heater 214. Where fitted, the steam inlet valve 228 is opened to allow simultaneous ingress of the generated steam into the bag interior by way of the steam injection passage 238 of the nozzle 232 and into the chamber interior 220 around the bag exterior. The contact of the low pressure steam with both the interior and the exterior surfaces of the bag walls efficiently transfers heat to the bag material to promote shrinking, but at a temperature which is significantly less than the boiling point of water (indeed less than 90°C) because of the sub-atmospheric pressure prevailing in the chamber at the time of steam injection, and at a high heat transfer rate by virtue of the latent heat of condensation liberated by the steam on contact with the cool bag wall. This sub-atmospheric pressure is maintained by continued operation of the suction fan 222 throughout the period of generation of steam. At a desired instant the nozzle 232 is automatically withdrawn, by means not shown, until its tip has just passed the sealing bars 226 and 227 and the heating element 233 of the upper sealing bar set 227 is energized as the bar 227 is driven downwardly into contact with the corresponding lower sealing bar 226 to close the bag. At this point steam introduction to both the chamber interior 220 and the bag interior 212 will have terminated. The condensing of the steam on the interior of the bag 212, which assists transfer of the latent heat of condensation to the bag to promote shrinkage, has the important effect that the condensation of the steam reduces to about 1/1700 the volume of the contents surrounding the product and within the closed bag so as to suck the bag material back more effectively onto the surface of the product 211, and to be free to shrink back, particularly as the chamber is vented. Once venting of the chamber has been completed, the upper chamber portion 203 can be lifted to open the chamber to allow removal of the tidy-shrunk package and the valve 224 opened in order to clear the chamber of residual steam which might otherwise escape into the packaging room. An alternative embodiment shown in Figure 6 has provision for facilitated introduction and removal of the bagged product and has those components which are in common with the Figure 1 embodiment increased by 300. The chamber interior 320 is defined within an enclosure including inner chamber doors 340 and 341 at the inlet and outlet ends, respectively of the chamber, and through which loaded bags 312 are carried on a foraminous support element 342, in this case an endless conveyor element which may be formed of either a stranded belt or rods. An optional steam generator 313 communicates with the top of the chamber interior 320 and has the steam therefrom distributed from within the chamber 320 by means of an upper baffle 343. A similar lower baffle 344 ensures that the air extraction current is distributed over the entire floor area of the chamber interior 320 as the extracted air is withdrawn by the suction fan 322. Alternatively, the steam may be generated in the upper chamber part by injection of water into the space between the upper baffle 343 and upper heating means (not shown). Around the exterior of the inner chamber space 320 is an air circulation conduit 345 which allows air to be heated and circulated by means of fan heaters 346 so as to pass both over the entering loaded bag 312 before it arrives at the chamber inlet door 340, and around the exterior of the discharging bags 312 after they have left the discharge chamber door 341. The air circulation conduit 345 thus provides a high velocity air curtain to preserve the low pressure in the chamber interior 320. In order to conserve air in the outer circulation conduit 345, there is an outer sliding door 347 at the inlet end and a further outer sliding door 348 at the discharge end. There is thus a type of air lock formed at the inlet, between the doors 347 and 340, and at the outlet, between the doors 341 and 348. The conveyor surface 342 may operate continuously if there is some means present for allowing the conveyor element to pass under the doors 340 and 341 in their substantially closed position. Alternatively the conveyor surface 342 may be advanced intermittently so that while the surface 342 is stationary one of the two doors 347 and 340 of the inlet and one of the two doors 341 and 348 of the outlet end may be closed while the other is opened because of the presence of a bagged product thereunder, as shown in Figure 6. One possibility for controlling the doors 340, 341, 347 and 348 is illustrated in Figures 7A, 7B, 7C, and 7D which only illustrate the inlet doors 340 and 347 but where the operating principle can be the same for the outlet doors 341 and 348. At the foot of each of the doors is a horizontal photoelectric beam generated by a transmitter 349 at one side of the product feed path and a receiver at the other side of that path, and control circuitry is provided which will cycle the door in question to rise at any stage when the beam is interrupted, and to continue that rising movement until the beam is restored. The beam is positioned somewhat in advance of (i.e. to the left of) the foot of the door so as to ensure that the beam becomes interrupted before any product article moving towards the door becomes impeded by the presence of the door itself. Because of the fact that the photoelectric detector and emitter are carried by the respective door 340, 341, 347 and 348, the door has a tendency to follow the profile of the article in that, as soon as the door has lifted sufficiently to raise the beam above the upper surface of the product article, that door will stop rising and will be driven to descend until the beam is once again interrupted. Figure 7A shows the outer door 347 beginning to open while the inner door 340 remains in its substantially closed position (i.e. just clear of the surface of the product support surface 342). In Figure 7B the door 347 has risen just far enough to allow the product article 312 b to pass therebelow, but the inner door 340 remains substantially closed. In Figure 7C the product article 312 b has passed the outer door 347 which has now once again closed because the beam is no longer interrupted, and has begun to pass under the inner door 340 which has risen automatically in the manner described above for door 347. Finally, the configuration shown in Figure 7D is the one in which the two doors 340 and 347 are substantially closed after the product 312 b has just entered the inner chamber portion and before the next product 312 c passes the outer door 347 to enter the "air lock" space 345. Although not shown in Figure 6, there may be means linking the interior of the air circulation conduit 345 with the air extractor fan 322 for the chamber interior 320, in order to ensure that the pressure of the air circulating within the air circulation conduit 345 is lower than atmospheric, thereby limiting the amount of leakage of air into the low pressure steam treatment chamber 320 at the centre of the apparatus, and supplementing the barrier function of the doors 340, 341, 347 and 348 which never quite close. The apparatus of Figures 6 and 7 operates in the following manner:- Initially all four doors 340, 341, 347 and 348 are closed. The door 347 opens to an extent sufficient to allow the first product to pass, and the doors 340 and 341 are meanwhile almost closed, i.e. they are open sufficiently to allow the continuously advancing support surface 342 to pass thereunder. This movement of the support surface 342 introduces a bagged product into the Figure 6 position under the open inlet door 347. The circulating air in the conduit 345, being pre-heated, carries out an initial pre-shrink phase on the entering sealed bag 312. Very soon thereafter, the door 340 opens slightly to allow the product support surface 342 to carry the first product thereunder. As soon as the bagged product has cleared the underside of the outer inlet door 347 this door substantially closes to allow the pressure of the air within the air circulating conduit 345 to begin to drop towards the low pressure in the chamber interior 320. Shortly after, the closed door 347 will be just clear of the support surface 342 while the door 340 is open to an extent sufficient to allow the product to enter the chamber interior 320. Once the product article has cleared the inner door 340, and is totally within the chamber interior 320, the door 340 may close fully for a time, and the steam may be generated and allowed to circulate within the chamber interior 320 to achieve shrinking but simultaneously with maintenance of the low pressure within the chamber interior 320 by virtue of operation of the suction fan 322. Shortly afterwards, the next product article 312 will begin to pass under the door 347 which must for this reason begin to open automatically, and gradually the previously described sequence will repeat until there are product articles at the various positions shown in Figure 6 of which:- (a) the lefthandmost is being pre-shrunk as it emerges into the hot air circulation conduit 345, (b) the righthandmost is being post-shrunk by the hot air in the air circulation conduit 345 as that product emerges from the circulation conduit, and meanwhile (c) the two central products within the chamber interior 320 are being more intensively shrunk by virtue of the low pressure steam circulating within the chamber interior 320. It will of course be appreciated that the apparatus of Figure 6 is, as with the embodiment of Figure 1, simply a mechanism for shrinking an already sealed bag but with minimum energy consumption. The difference between the process employed in Figures 1 and 6 and that of the prior art, for example in US-A-3567713, is that the generation of steam coincides with the extraction of gas from within the chamber, with the result that the generated steam is withdrawn by the suction fan whereas in the prior art the steam was used as a venting medium. It has been found, however, that by maintaining the pressure of the steam at well below atmospheric it is possible to maintain its temperature well below the boiling temperature of water and thus to avoid any deleterious effects on the heat-shrinkable plastic material of the packaging bag. The apparatus shown in Figure 8 is very schematically illustrated and includes an optional steam generator 413 and steam control valve 417 operating in conjunction with a two-part chamber comprising the upper chamber part 403 and the lower chamber part 404 both of which, in this embodiment, are movable laterally as well as able to be opened vertically. Alternatively water may be injected directly to a hot spot behind a diffuser, for example above the upper baffle 443, and allowed to circulate within the chamber 420. The upper and lower chamber parts 403, 404 each include steam-distributing and flow-controlling baffles 443 and 444, respectively, in order to homogenize, as far as possible, the flow of low pressure steam induced through the chamber interior 420 by virtue of the operation of the suction fan 422. As with the embodiment of Figures 6 and 7, there is a foraminous product-support surface 442 which in this case comprises an endless conveyor belt whose upper run cooperates with the chamber 402. The path of movement of the upper chamber part 403 is illustrated schematically by a rectangular set of vector arrows 449 from which it can be seen that when the chamber 402 is closed the upper chamber part 403 is moving rightwardly parallel to the direction of the upper run of the conveyor surface 442, after which the chamber part 403 rises to open the chamber and to free the heat-shrunk bagged products for further advance along the path of the conveyor surface 442, followed by which the upper chamber part 403 moves leftwardly back to its start position ready to descend over the next two product articles for steam-shrinking them. Conversely, the lower chamber part 404 moves rightwardly, then descends, then moves leftwardly, and then rises again to close around the next two product articles during these four operating movements of the upper chamber part 403 illustrated by the vector arrows 449. The cycling of the activation and de-activation of the steam generator 413, the steam control valve 417, and the air extractor fan 422 are much as described in connection with the Figure 1 embodiment. There are various alternative possibilities for the method of operation of the automatic or semi-automatic apparatuses for carrying out the process in accordance with the present invention, but each of them will involve the basic operating principle described above with reference to Figure 1 and will enjoy the benefits of a maintained substantially uniform sub-atmospheric pressure during the steam shrinking step, thereby ensuring that the temperature of the steam is well below the boiling point of water and hence well below any temperature which it is likely to cause thermal damage to the heat-shrinkable film being processed.

Claims: A method of heat-shrinking a package, including the steps of placing a product in a container; reducing the pressure prevailing on the surface of the container; causing the container to undergo heat shrinking onto the product and arriving at a closed shrunk package by contacting the container surface with steam while maintained at a sub-atmospheric pressure in order to impart shrinking heat to the container wall by virtue of the released latent heat of condensation of the sub-atmospheric pressure steam; and subsequently discontinuing the flow of steam and restoring the pressure; characterised in that the container is a bag; in that the bag is placed in the chamber spaced from the chamber walls; in that the sub-atmosphere pressure is maintained uniform throughout the vacuum chamber around the container; in that the uniform sub-atmospheric pressure is maintained substantially constant on said surface of the container during the steam shrinking step; and residual steam from around the is bag is pumped away after said steam-shrinking step. A method according to claim 1 characterised in that the bag is closed before the heat-shrinking operation by use of steam at sub-atmospheric pressure is effected. A method according to claim 1, characterised in that the bag is closed only after the pressure in the interior of the bag has been lowered by extraction of air from around the exterior of the bag while the bag is still open. A method according to claim 1 or 3, characterised in that the interior surface of the bag is contacted with the steam at sub-atmospheric pressure by leaving the bag open until the steam shrinking operation is well underway, following which the bag is closed and the residual steam in the bag allowed to condense to permit a tidy shrinking of the bag material into contact with the enclosed product. A method according to claim 4, characterised in that steam is also applied to the outer surface of the bag to impart shrinking heat by virtue of the released latent heat of condensation of the steam around the bag. A method according to claim 4 or claim 5 characterised in that the introduction of steam into the bag is by way of a nozzle equipped with a passage for introducing sub-atmospheric pressure steam and a passage for passing discharged gas and subsequently the steam out of the bag into a low pressure chamber surrounding the bag. A method according to any one of claims 1 to 6, characterised in that the bags are subjected to a pre-shrinking operation (T₀-T₁) using hot air before they are subjected to the steam shrinking operation (T₃-T₄). A method according to any one of claims 1 to 6, characterised in that the bags are subjected to a two-stage shrinking operation using steam at sub-atmospheric pressure, the first stage (T₀-T₁) being at a higher absolute pressure than the subsequent stage (T₃ T₄). A method according to any one of claims 1 to 8, characterised in that the steam is generated by injecting water to a heated location in a low pressure enclosure in which the package is disposed. A method according to claim 9, characterised in that the steam contacts the bag after passing through a diffuser screen. Apparatus for steam shrinking a package, comprising:- a vacuum enclosure (3, 4; 103, 104; 203, 204; 344; 403, 404) within which the package is to be shrunk; means (17, 8; 117, 131; 213; 313; 413) for generating steam and for introducing it into said enclosure; and means (22; 222; 322; 422) operable while the steam generator is in operation, for extracting air and/or steam from the enclosure; characterised in that the chamber includes a support for a bag spaced from the walls (3, 4; 103, 104; 203, 204; 403, 404) of the chamber so that the bag is completely enclosed within the chamber with a uniform pressure around the bag exterior; in that air extraction means comprise a fan operative to maintain a substantially constant sub-atmospheric pressure in the steam-filled enclosure around the package; and in that the fan is operative after termination of the steam generation so as to extract residual steam from the exterior of the bag before the enclosure is opened. Apparatus according to claim 11, further characterised by closing means (126, 127; 226, 227) for closing a bag within the enclosure after at least a major part of the shrinking step using sub-atmospheric pressure steam. Apparatus according to claim 11 or 12, and further characterised by a vacuum pump (138) operative to lower the residual pressure in the enclosure to a value lower than said substantially constant pressure for evacuation of the bag in said enclosure. Apparatus according to claim 11, 12 or 13, characterised in that the steam generating means (213) includes a water heater (214) and a steam control valve (228) for controlling the introduction of steam into said enclosure (203, 204). Apparatus according to claim 11, 12 or 13, characterised in that the said vacuum enclosure includes a heater (8; 131) for heating a part of the enclosure, and in that the steam generating means includes means (17; 117) for introducing water to said heated part of the enclosure for evaporation to form steam to permeate through the enclosure. Apparatus according to any one of claims 11 to 15, characterised by including a foraminous diffuser plate (10) to distribute steam uniformly throughout the vacuum enclosure for contacting the whole of the surface of a package therewithin. Apparatus according to claim 12, alone or in combination with claim 14, characterised by including a steam injection nozzle (232) for introducing steam into a bag (212) within the enclosure (203, 204), said nozzle including means (238) for communicating the interior of the bag with the interior of the enclosure around the bag for allowing escape from the bag of air and/or steam. Apparatus according to claim 11, characterised by including a continuous product support surface (342; 442) moving into and out of the enclosure (340; 403, 404) for advancing bags through the enclosure for shrinking within the enclosure. Apparatus according to claim 18, characterised by including means for moving the enclosure laterally (Fig. 6) with the continuous support surface while the chamber is in a closed configuration and for returning it countercurrent to the conveyor surface while the chamber is open, in order to effect the steam shrink operation while the enclosure and the support surface are moving synchronously in the same direction. Apparatus according to claim 18, characterised by including an inner low pressure steam-shrinking enclosure (344) and an outer air circulation enclosure (345) to permit the circulation of low pressure air around the low pressure steam-shrinking enclosure as loaded bags move through the air circulation path into the inner steam-shrinking enclosure and out of the steam-shrinking enclosure to pass once more through the air circulation path. Apparatus according to claim 20, further characterised by including respective doors (347, 348 and 340, 341) positioned between the air circulation enclosure (345) and on the one hand the ambient and on the other hand the inner steam-shrinking enclosure (344), and means for automatically opening said doors to allow product articles on said continuous product support surface to pass. Apparatus according to claim 21, characterised in that said automatic door opening means comprise photoelectric detectors and emitters (349) carried by the doors to create beams which become interrupted by the presence of an article (312) approaching the doors. Apparatus according to any one of claims 20 to 22, further characterised by including a suction line (322) to said air circulation enclosure to maintain the circulating air at subatmospheric pressure.

Inventors: GIANELLI, GIAN CAMILLO
Applicants: W.R. GRACE & CO.-CONN.
Application year: 1988
Main classification: B65B53
IPC codes: B65B31, B65B53