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vacuum forming machine of a known type comprise a vacuum chamber arranged to receive at least one unsealed product package and operable to perform a vacuum sealing operation on the at least one product package. Typically the product packagesare products such as food stuff arranged in a bag formed by a heat-shrinkable film. After loading and closing the vacuum chamber, the vacuum sealing operation normally comprises vacuumisation, sealing the mouth of the vacuumised bags, and reintroducingair into the chamber. Then the chamber is opened and the vacuum chamber is unloaded. The product packages may then be conveyed to a heat-shrinking unit, typically a hot water tunnel or a dip tank. The vacuumisation step typically takes at least 20-30 seconds which is mostly wasted time in the overall packaging process. During this time, the only step which can be taken is to prepare the next product packages for loading into the vacuumchamber, for example by conveying them onto an in-feed conveyor. Accordingly, the vacuum packing machine causes a bottle-neck in the overall packaging process.According to the present invention, there is provided a vacuum forming machine for performing a vacuum sealing operation on product packages, comprising a vertical stack of vacuum chambers each arranged to receive at least one unsealed productpackage and operable to perform an independent vacuum sealing operation on the at least one product package. The provision of more than one vacuum chamber in the vacuum forming machine allows respective vacuum chambers to perform a vacuum sealing operation while another vacuum chamber is being loaded and/or unloaded. Therefore, the machine mayminimize the wasted time in the vacuum packaging process. Consequently, the present invention can increase through-put and increase productivity of a packaging line including the machine. Furthermore, by arranging the vacuum chambers in a verticalstack, this increase in productivity may be achieved without significantly increasing the floor area of the vacuum forming machine. The extra vacuum chambers only increase the height of the machine. This is a significant advantage in manufacturingplants where increasing the footprint of the vacuum forming machine would create real problems but where there is normally space to increase the height of the machine. Preferably, the vacuum forming machine further comprises a conveyor arrangement operable to load and unload a selective vacuum chamber with the at least one product package, the machine being operable to operate the respective vacuum chambersto perform the vacuum sealing operation while the conveyor arrangement is operated to load and unload another vacuum chamber.The conveyor arrangement can automatically load and unload selected vacuum chambers. Operation of one or more of the vacuum chambers while the conveyor arrangement is loading and unloading another vacuum chamber reduces the amount of timewasted, thereby increasing through-put and increasing productivity of a packaging line including the achine. Preferably, the machine is operable to operate the conveyor arrangement to load and unload the vacuum chambers in a cyclical sequence and synchronously to operate the respective vacuum chambers to perform the vacuum sealing operation on the atleast one product packages after loading. Such a cyclical operation allows the machine to be utilised in an automatic continuous packaging line. It is desirable that the number of vacuum chambers is sufficient relative to the duration of the vacuum sealing operation to allow theconveyor arrangement to operate continuously because this minimises the amount of wasted time. Time wastage can be reduced further by designing the conveyor arrangement to load and unload the vacuum chambers more rapidly. The described embodimentsinclude particularly suitable conveyor arrangements as follows.Preferably, the conveyor arrangement includes at least one in-feed conveyor operable to load a selected vacuum chamber with the at least one product package. Preferably, the conveyor arrangement includes at least one out-feed conveyor operable to unload a selected vacuum chamber with the at least one product package, although as an alternative the in-feed conveyor may be operable in reverse to unloada selected vacuum chamber.Provision of separate in-feed and out-feed conveyors allows the loading and unloading to occur simultaneously, preferably with the in-feed and out-feed conveyors being linked by an internal conveyor in each vacuum.Preferably, the at least one in-feed conveyor and/or the at least one out-feed conveyor are vertically movable to select the vacuum chamber to be loaded. Additionally or alternatively, the plurality of vacuum chambers are movable togetherrelative to the conveyor arrangement to select the vacuum chamber to be loaded and unloaded. The conveyor arrangement may include a plurality of in-feed conveyors and/or out-feed conveyors which are movable together. In this case, the vacuum chambers are preferably have a regular spacing and the in-feed conveyors and/or out-feedconveyors have a relative spacing equal to the spacing between the vacuum chambers. This allows more than one vacuum chamber to be loaded and/or unloaded Desirably, the vacuum chambers each have a sealing bar arranged along a side of the respective vacuum chamber for sealing the at least one product packages, preferably extending along the internal conveyor. This prevents the sealing bar fromhindering loading and unloading improves the automatic operation of the machine because the product packages always have the same orientation. Advantageously, the vacuum chambers and/or the in-feed conveyors and/or the out-feed conveyors have a modular construction. This allows the modular parts to be added and removed in order to assemble the machine with a variable number of theparts in order to provide a productivity and cost appropriate to the particular packaging line in which the machine is used. Thus, this modular construction increases the flexibility of the machine and allows it to be used in different packaging lines. This flexibility is particularly advantageous with the vacuum chambers being arranged in a vertical stack because the productivity of the machine may be altered whilst covering the same floor space within the manufacturing plant because only the heightof the machine is altered. Advantageously, each vacuum chamber comprises at least two parts which are relatively vertically movable to open and close the vacuum chamber. This construction for the vacuum chambers is advantageous because it allows for a simple machinedesign, lower manufacturing costs and simple servicing and maintenance operations as compared to a vacuum chambers which are open and closed by the provision of doors. In order that the present invention may be better understood, the followingdescription of preferred embodiments is given by way of non-limitative example with reference to the accompanying drawings. FIG. 1 is a top plan view of a vacuum forming machine 1 which is an embodiment of the present invention arranged in a packaging line 13 constituted by a series of conveyors. At a bagging section 14, products are bagged in heat-shrinkable filmbags, or alternatively in small pouches made from thin films, and arranged on line 13 as product packages 2. A vacuum forming machine 1 performs a vacuum sealing operation on the product packages 2 which are then output back onto the packaging lines13 which conveys them through a shrink tunnel 15 to perform a heat-shrinking operation. The product packages 2 move continuously through the shrink tunnel 15 which is advantageous over heat-shrinking of products in batches where it is difficult toobtain uniform shrinking of the packaging around each product as a result of contact or proximity between the various product packages 2.
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