The present invention relates to thermo-formed containers particularly, but not exclusively, to thermo-formed plastics containers suitable for transporting and displaying fresh produce such as fresh fruit or salad.

Background to the Invention A common form of such containers comprises first and second thermo- formed shells that are adapted to engage with one another around their margins. In order to provide ventilation, to stop the food produce from spoiling too quickly, it is common practice to punch ventilation holes in the base and/or side wall of at least one of the shells.

A disadvantage of using such a punching operation to produce ventilation apertures is that the resulting plastics punchings need to be collected and may on occasion be left inside the container to become a foreign body in the food product.

Summary of the Invention According to one aspect of the invention a method of producing one or more ventilation apertures in a thermo-formed container comprises piercing a wall of the container with a piercing tool that does not separate and remove a fragment of plastics from the wall, and allowing or causing the pierced opening to be enlarged by deformation of the adjacent plastics material.

When the thermo-formed material is pierced, some reversion of the plastics can take place whereby the pierced opening enlarges through the resulting redistribution of internal stresses in the thermo-formed plastics.

Whilst it may be possible to use a heated wire as a piercing tool we prefer to use at least one blade which is moved relative to the container wall in order to slice a portion of the wall so as to create a slit in the wall. The resulting slit is then enlarged, preferably by partial reversion of the plastics.

The blade is preferably heated to a temperature above ambient to assist in the piercing operation, but preferably not to a temperature at which the plastics contacted by the blade would melt, since if the plastics were to melt this could clog up the blade, and could produce undesirable debris.

Each blade is preferably in the form of a rotating disc.

A plurality of such discs is preferably mounted on a common drive shaft, the drive shaft incorporating heating means for heating the discs.

The heating means preferably comprises a fluid passage extending through the drive shaft for receiving a warm fluid, such as water or oil to maintain the discs warm.

The piercing tool may be arranged to engage with the side wall of a shell of the container and/or the basal wall of the shell.

The piercing tool may be arranged to be moved into engagement with the container shell, or the container shell may be urged into engagement with the piercing tool.

Preferably a plurality of container shells are thermo-formed from a ribbon of plastics sheet material and the connected thermo-formed container shells are transferred to a piercing station at which the connected shells are brought into engagement with the piercing tools, and the action required to urge the connected shells into engagement with the piercing tools may be brought about by cutting dies which separate the shells from one another.

When the container shell wall is provided with a plurality of substantially parallel flutes in order to strengthen the wall, the apertures are preferably slits positioned intermediate the flutes and extending substantially parallel to the flutes.

When discs are employed, the discs may conveniently be positioned to engage simultaneously with the side walls of two adjacent container shells in the connected ribbon of container shells.

It is common practice, with fruit punnets for example, to place a cushion, such as bubble sheet, in the base of the container shell. Such a cushion may be adhesively fixed to the base. Such a cushion may conveniently be located in position in the shell prior to the step of piercing the side wall of the container.

According to a second aspect of the invention we provide apparatus for creating ventilation apertures in a wall of a thermo-formed container shell, the apparatus comprising loading means for bringing a shell to the piercing station, a piercing tool located at that station and actuation means to urge a container shell relatively towards the piercing tool, heating means for heating the piercing tool to a temperature above ambient that is less than the melt temperature of the plastics material of

the shell, and unloading means for removing the pierced shell from the piercing station.

According to a third aspect of the invention we provide a thermo-formed container shell provided in at least one wall thereof with one or more ventilation apertures, the aperture having been produced by piercing of the wall with a piercing means followed by opening up of the pierced region without removal of material.

The invention will now be further described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings In the drawings: Figure 1 is a side elevation of a soft fruit punnet provided in the side walls with ventilation holes created by a method in accordance with one aspect of the invention, Figure 2 is an enlarged local cross-section on the line 2-2 of Figure 1 to show how the margins of a slit cut in the tray have deformed by partial reversion of the thermo-formed tray wall, Figure 3 is a schematic perspective view of ganged cutting discs for producing four ventilation apertures, similar to the apertures of Figures 1 and 2, in the wall of a thermo-formed tray, Figure 4 is a schematic side elevation of a portion of a ribbon of thermo-formed trays, similar to the tray of Figure 1, and showing a single cutter disc at a piercing station being used to produce slits in the adjacent side walls of successive trays in the ribbon, and

Figure 5 is a partial side view of a soft fruit punnet similar to that of Figure 1 but in which the ventilation holes have been created by the method of the invention and so as to extend further up the side walls.

Description of Preferred Embodiments With reference to the drawings, Figure 1 shows a side elevation of a substantially conventional thermo-formed tray 1 that has been provided with ventilation apertures 3 of elongated arrow-head shape in the sloping side walls 4 of the tray. The tray has been thermo-formed from a ribbon of thermoplastics sheet by a well-known thermo-forming technique, such as vacuum thermo-forming. The ventilation apertures 3 have been produced by a subsequent procedure in accordance with the invention which does not involve the creation of plastics debris, as is inevitably associated with known punching techniques.

The apertures 3 have been produced by urging a warm rotary cutter assembly, similar to the cutter assembly shown in Figure 3 but with five cutting discs instead of the four discs of Figure 3, against the lower side wall 4. The discs slice vertical slits in the lower side wall 4. The cutting action does not involve the removal of any plastics material but is simply a piercing operation. Once pierced, however, the margins 7 of the slits retract by partial reversion of the thermo-formed sheet, due to the presence of residual stresses in the side wall material.

As shown in Figure 2 the material of the margins 7 tends to bend inwardly of the tray. This is enhanced in the embodiment of Figures 1, 2 due to the fact that the slits extend through the rounded basal corner 8, where the side wall 4 connects with the base wall 9, and increased stress

relief is achieved by a local reduction in the circumferential length of the material of the corner 8.

As shown in Figure 1, the ventilation apertures 3 have been positioned between the vertically-extending flutes 11 in the side wall, so as not significantly to affect the strength of the side wall 4.

Figure 4 shows how slits, and thereby ventilation apertures, can be produced in adjacent trays 1 of a ribbon 10 of connected thermo-formed trays by a single rotary cutter assembly 5 at a piercing station where the trays are supported on horizontal support 12.

The piercing operation may conveniently be performed substantially simultaneously with the cutting operation performed on connecting webs 13 to separate the trays from one another in the ribbon of thermo- formed trays.

As shown in Figure 3, the common drive shaft 14 of the cutters 6 is hollow, and warm fluid, such as oil or water, is passed through the bore 15 of shaft 14 to maintain the discs 6 at a temperature above ambient.

In trials of the invention satisfactory piercing of a PVC thermo-formed tray was achieved with cutting discs at a temperature of 135°C. This temperature was achieved by employing an electrical cartridge heater within the bore 15 of shaft 14.

With reference to Figure 2, a cushion 14 of bubble sheet material may be mounted on the base 9 of the tray prior to the piercing operation. It does not matter that the cutting discs 6 may pierce the margin of the cushion 14.

In the modified punnet of Figure 5, the ventilation apertures 3'have been created so as to extend further up the side wall of the punnet than in the punnet of Figure 1. This has been achieved simply by increasing the relative vertical displacement of the cutter assembly 5 relative to the thermo-formed trays 19 when creating the apertures 3. Alternatively, it is possible to produce ventilation apertures 3", as shown also in Figure 5, that are remote from the base 9, by bringing a cutting disc against the side wall 4.