The invention relates to a method and apparatus for shrink-wrapping containers and finds particular application in the shrink-wrapping of packs of smokable articles such as cigarettes.

It is known to shrink wrap cigarette packs and other containers in an outer film of, for example, polypropylene. During shrink wrapping, a stream of cigarette packs which are individually wrapped in a heat-shrinkable film is conveyed through a shrink tunnel where the packs are heat-treated by convection, using hot air, and/or conduction, using hot flat plates, in order to shrink the film wrappers around the packs. To increase the rate of throughput of cigarette packs through the shrink tunnel, the individually wrapped cigarette packs are typically transferred cyclically from a piling or stacking station into the shrink tunnel in superimposed pairs.

In conventional shrink wrapping apparatus, a reciprocating horizontal pusher transfers vertically stacked pairs of superimposed cigarette packs from the stacking station into the shrink tunnel, where the pairs of packs are pushed against one another to form a continuous stream. The cigarette packs are thus conveyed longitudinally through the shrink tunnel in two superimposed abutting rows with the upper faces of cigarette packs in the lower row in contact with the lower faces of cigarette packs in the upper row. The heat-shrinkable film in contact with these inaccessible faces of the cigarette packs is disadvantageously exposed to little or no heat within the shrink tunnel. To avoid uneven shrinkage of the film wrapper, which may result in the formation of creases or wrinkles and so lead to an unsightly finished pack, it is desirable for heat to be applied to the upper face and the lower face of each cigarette pack in the shrink tunnel.

US-B-6,511,405 discloses an apparatus for producing packs according to the preamble of claim 1 having an outer wrapper of improved appearance comprising a shrinking station with a top heating plate for applying heat to the upwardly directed front sides of packs in the top row and a bottom heating plate, positioned in the region between the two superimposed pack rows, for applying heat to the upwardly directed front sides of packs in the bottom row. So that they are conveyed through the shrinking station above the bottom heating plate, the packs of the top pack row are raised relative to the packs in the bottom pack row by a ramp in the region of the heating plates. Although the apparatus disclosed in US-B-6,511,405 provides access to the upwardly directed front sides of packs in the bottom row, mechanical separation of the two superimposed pack rows can result in the non-shrunken film wrappers of the packs being damaged as the packs are transferred into the shrinking station. Furthermore, there is a significant risk that the individual packs are incorrectly positioned within the shrinking station and consequently damaged packs and/or packs having unevenly shrunk film wrappers are formed.

It is an object of the present invention to provide an improved apparatus and method for shrink-wrapping a plurality of individually wrapped containers.

According to the invention there is provided an apparatus for shrink wrapping a plurality of individually wrapped containers comprising: a stacking station, a shrink tunnel, and a transfer device for transferring a pair of superimposed containers individually wrapped in a heat-shrinkable film from the stacking station to the shrink tunnel, characterised in that the transfer device is adapted to engage and separate the pair of superimposed containers and to convey the separated containers from the stacking station into the shrink tunnel.

In use, separation of the pair of superimposed containers by the transfer device, rather than by a ramp or other hard repositioning means, not only renders the upper side or face and the lower side or face of each container accessible to one or more heating devices provided in the shrink tunnel, but also reduces or avoids the risk of the containers and/or their non-shrunken film wrappers being damaged as the containers are transferred to the shrinking station.

Preferably, the transfer device is adapted to apply a vacuum to a portion of the surface of at least one of the pair of superimposed containers distant from the shrink tunnel. More preferably, the transfer device is adapted to apply a vacuum to a trailing side or face of the uppermost one of the pair of superimposed containers.

Preferably, the transfer device comprises at least one suction plate or cup for engaging a portion of the surface of at least one of the pair of superimposed containers distant from the shrink tunnel. Preferably, the at least one suction plate or cup is mounted on a pusher mounted for longitudinal movement relative to the shrink tunnel. Preferably, the pusher is mounted on a carriage on a cam track.

Preferably the shrink tunnel comprises at least one heating plate for applying heat to the film wrappers of the separated containers. More preferably, the shrink tunnel comprises an inner heating plate mounted between the separated containers for applying heat to opposed inward facing surfaces of the separated containers. Most preferably, the shrink tunnel comprises an inner heating plate mounted between the separated containers for applying heat to opposed inward facing surfaces of the separated containers and a pair of opposed, spaced apart, parallel outer heating plates for applying heat to outward facing surfaces of the separated containers.

Preferably, the inner heating plate is fixed, while, to reduce friction between the separated containers and the shrink tunnel, the outer heating plates are preferably adapted to move in sequence with the movement of the separated containers.

The shrink tunnel may advantageously further comprise a second pair of opposed, spaced apart, parallel heating plates, substantially perpendicular to the inner heating plate and the pair of outer heating plates, for applying heat to corresponding further surfaces of the separated containers. Where included, the second pair of opposed, spaced apart, parallel heating plates is preferably also adapted to move in sequence with the movement of the separated containers in order to reduce friction.

Any suitable known heating plates may be employed in the shrink tunnel of the apparatus of the invention, for example, heating plates heated by electric resistors. Furthermore, if desired, the apparatus of the present invention may further comprise means for, in use, supplying hot air into the shrink tunnel in order to shrink the film wrappers of the separated containers instead of or in addition to one or more heating plates.

In a preferred embodiment of the apparatus of the invention the transfer device utilizes negative pressure, applied, by way of a suction unit, to the trailing side or face of the uppermost container of a superimposed pair of containers formed in a vertical stacking station, to engage and lift the uppermost container vertically and thereby separate the pair of containers. Where the containers to be shrink-wrapped are cigarette packs, a vacuum of between about 0.6 bar and about 1 bar is preferably applied by the transfer device to the uppermost cigarette pack in order to separate the cigarette packs. The suction unit of the transfer device may, for example, comprise a suction plate including a plurality of holes or one or more suction cups through which, in use, a vacuum is drawn.

The suction unit is preferably mounted on a horizontal pusher, which in turn is mounted on a carriage on a fixed cam track. In use, as the carriage, and hence the pusher, moves toward the shrink tunnel the uppermost container is engaged by the suction plate and lifted vertically relative to the lowermost container of the pair of superimposed packs. At the same time, the lowermost container is collected by separate hinged fingers mounted on the carriage, as is known in the art, and conveyed linearly, through further movement of the carriage and pusher, at the same height towards the shrink tunnel. The transfer device may alternatively also utilize negative pressure applied by way of a suction unit to engage the lowermost container of the superimposed pair of containers.

In order to obtain a continuous process, in the preferred embodiment of the apparatus of the invention the application of negative or reduced pressure to the trailing side or face of the uppermost container is referenced to the superimposition of the pair of containers in the stacking station. To form the pair of superimposed containers, a first container wrapped in a heat-shrinkable film is preferably lifted upwards to a pre-determined height in the stacking station by a reciprocating vertical pusher, where it is held in position in a known manner, while the pusher returns to its initial position. The vertical pusher then lifts a second container wrapped in a heat-shrinkable film upwards to the same pre-determined height in the stacking station where it is held in position with the lower face or side of the first container supported on the upper face or side of the second container. In the preferred embodiment of the apparatus of the invention, the engagement and separation of the thus formed pair of superimposed containers by the transfer device takes place directly after the superimposition of the containers, during the time taken by the vertical pusher to travel back to its original position to collect a container to form the first container of a subsequent pair of superimposed containers.

Typically, medium and high speed shrink wrapping machines for cigarette packs operate with a rate of between about 400 to 600 packs per minute. Accordingly, where the preferred embodiment of the apparatus of the invention is used to shrink wrap cigarette packs, the carriage and hence horizontal pusher of the transfer device may, for example, operate at about half this rate.

To facilitate clean transfer of the superimposed pair of containers from the stacking station to the shrink tunnel by the transfer device, in the preferred embodiment of the apparatus of the invention the transfer device is also provided with a supply of compressed air to blow the container or containers off of the suction unit into the shrink tunnel.

In the shrink tunnel preferably at least the upper face of the lowermost container and the opposed bottom face of the uppermost container of the pair of separated superimposed containers are heat-treated by means of a heating device. Preferably, the heating device delivers heat through at least one central heater plate located between the pair of separated superimposed containers. More preferably, the heating device delivers heat through at least one fixed horizontal heater plate located between the pair of separated superimposed containers, which also serves as a support or steering plate for at least one of the containers. The heating device may be of any suitable known type, for example, a plate heated by electric resistors.

Preferably, the shrink tunnel further comprises heating plates that move in sequence with the separated containers, that is, for example, vertically spaced apart, parallel top and bottom heating plates for heat treating the upper surface of the uppermost container and the lower surface of the lowermost container, respectively, that index up and down in time with the movement of containers directly underneath or over them respectively. This advantageously reduces friction which otherwise would occur if the containers were pushed through a shrink tunnel with static heating plates. The shrink tunnel of the preferred embodiment of the apparatus of the invention may further comprise two horizontally spaced apart, parallel side heating plates, perpendicular to the top and bottom plates, that move in sequence with the pair of separated superimposed containers for heating the side faces thereof.

The height of the stacking station of the preferred embodiment of the apparatus of the present invention is preferably increased by about the height of one container compared to conventional stacking stations to about the height of three containers. Preferably the apparatus of the invention further comprises a pre-sealing unit mounted in a lower portion of the stacking station for, in use, point sealing opposed folded ends of the unshrunken film wrapper around the lowermost of the three containers therein. Preferably, the pre-sealing unit comprises a pair of opposed hollow, reciprocating pneumatic cylinders mounted in opposite sides of the stacking station, which, in use, are heated with hot air. Where the apparatus of the invention is used to shrink wrap cigarette packs, the pneumatic cylinders are preferably positioned to spot heat two points on opposed sides of the lowermost pack where, due to the manner in which heat-shrinkable film is conventionally folded around cigarette packs, five thicknesses of film are folded.

Preferably, the apparatus of the present invention further comprises a choke detection device mounted in the stacking station. In a preferred embodiment, the choke detection device comprises a pair of opposed spring-loaded fingers hingedly mounted on either side of the upper end or exit of the stacking station at right angles thereto. In use, accidental insertion of an additional container into the stacking station forces the spring loaded fingers to pivot upwardly against the sides of the stacking station thereby generating, by way of a sensor, a signal for the apparatus to stop. In addition to detecting additional containers in the stacking station that could jam or choke up the apparatus, the spring-loaded fingers also advantageously reduce bouncing of the individually wrapped containers as they are fed into the stacking station.

In a preferred embodiment of the invention, one side of the stacking station of the apparatus of the invention is hinged to the remainder thereof in order to allow easy access to the interior stacking station to, for example, remove accidentally inserted additional containers.

According to the invention there is also provided a method for shrink wrapping a plurality of individually wrapped containers comprising: stacking at least two containers individually wrapped in a heat shrinkable film in a stacking station to obtain a pair of superimposed containers, applying a vacuum to at least one of the pair of superimposed containers to separate the superimposed pair of containers, and transferring the separated containers into a shrink tunnel.

Preferably, the method comprises applying a vacuum to a trailing side or face of an uppermost one of the pair of superimposed containers. Where the containers to be shrink-wrapped are cigarette packs, the method preferably comprises a vacuum of between about 0.6 bar and about 1 bar.

Preferably, the method of the invention further comprises applying heat to opposed inward facing surfaces of the separated containers in the shrink tunnel through a heating plate mounted between the separated containers.

The apparatus and process according to the invention are explained in more detail below, by way of example only, with reference to the accompanying drawing in which:

• Figure 1 shows apparatus according to the invention comprises a stacking station 100 including a vertical stack of three individually wrapped cigarette packs 10, 11, and 12. Cigarette pack 10 is separated from pack 11 by the application of vacuum through suction plate 110 integrated into horizontal pusher 120. Packs 10 and 11 are pushed forward by horizontal pusher 120 into a longitudinal shrink tunnel 200 comprising a central heating plate 210. Arrow 98 indicates the upward movement of pack 10. Arrow 99 indicates the downstream movement of packs 10 and 11.

The method and apparatus according to the invention find particular application in the production of packs or smokable articles such as cigarettes, especially in the production of hinge-lid cigarette packs having rounded and/or bevelled edges.

It will be appreciated that existing conventional shrink wrapping apparatus comprising a stacking station and a shrink tunnel may advantageously be readily retrofitted by inclusion of a suitable transfer device in order to produce apparatus according to the present invention.