This invention relates to a method and apparatus for perforating thermoplastic sheets, for example corrugated polyolefin sheets.

Containers fo ' the preservation and transport of food¬ stuffs may be made from thermoplastic sheet material so as to be solid, light and economical. However, when used for the transport and storage of wares having high moisture content, for example fruit or vegetables, deterioration of the product may occur owing to build up of moisture within the container.

It has been proposed to provide such containers with - a plurality of holes, for example by forming them from perforated sheets, and this reduces the problem considerably.

The invention seeks to provide an improved method and apparatus for the production of perforated thermoplastic sheets.

According to the present invention there is provided an apparatus for perforating thermoplastic sheet material which comprises a platen for supporting the sheet material and means for carrying an array of heated pins so that such an array can be brought into contact with the sheet material held on the platen whereby to perforate the same.

The array of pins may be carried on a strip or strips of material capable of being heated, for example electrically. Alternatively, each needle may be individually heated. In order to prevent wholesale melting of the thermoplastic sheet when the array of pins is brought into contact

therewith, a support in the form of a layer of insulating material may be provided having a series of holes therein through which the pins project, but this may not be necessary, especially where the heating is confined to the pins. In the latter respect an internal heating unit or cartridge has been found most advantageous, for accurate temperature control.

Most advantageously, the apparatus of the invention may make use of an existing cutting and creasing apparatus; In this case the platen may be the cutting and creasing apparatus platen on which the thermoplastic sheet material is laid. The means for carrying the array of pins may be the cover carrying the cutting or creasing knives, .or the means may be attached to the corner, and the array of pins will be disposed within the _. area bound by the cutting blades. Thus the apparatus may perform a two fold action: it may simultaneously cut the thermoplastic sheet to the desired size and shape and simultaneously perforate it. Alternatively, the apparatus can be placed the stripping section of cutting and creasing machines provided with the same, and of course the apparatus can stand alone.

The invention further comprises a method for perforating a thermoplastic sheet which comprises providing an array of heated pins, bringing the same into contact with the sheet whereby to effect perforation thereof, and removing the array. The invention further includes a sheet so perforated.

The thermoplastic sheet may be formed from any suitable thermoplastic material, such as a polyolefin eg polyethylene or polypropylene. However, the sheet is preferably a multi-layer corrugated thermoplastic sheet and it is especially preferred that the sheet have three or more walls or layers. The thermoplastic material employed in the sheet may comprise partly or all recycled material

and in one aspect of the invention it is preferred that such recycled material is employed. The inclusion of the recycled material may be selectively positioned in the sheet structure, eg within the body of sheet rather than in the faces thereof.

It has been found that temperature conrol is critical to the satisfactory operation of the invention. Too cool, and rough or irregular holes may be formed and the sheet will tend to stick to the array; too hot and excessive melting or burning will take place. Surprisingly, we have found that pin tip temperatures in excess of double the melting point of the polymer sheet give best results. For preformed polyolefin sheet materials with a melting point in the region of 155°C we have found pin tip temperatures in the region of from 360 to 410°Cto be satisfactory, with around 390° being optimum. Satisfactory holes are produced from 360 ° but production speed increases until at temperatures in excess of around 410° the sheet starts burning around the hole edges.

Each pin and the underside of the array may be surface treated, for example with a release coating, and the pins are preferably specially shaped so that it does not stick to and pull the plastics material when operated at the required speed of cycle and at the necessary temperature. The avoidance of sticking is essential in order to produce holes of good quality.

The shape of the individual pins may be chosen within, wide limits. However, we have found that to obtain compromise a cylindrical main section having a tapered or conical end portion terminating in a tip which provides a lead into the conical portion.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is cross-sectional view of part of an apparatus in accordance with the present invention;

Figure 2(a) and (b) are partial plan views of arrays of pins;

Figure 3 is a perspective detail view of part of Figure 2;

Figure 4 is a plan view of the insulating and support layer;

Figure 5 is a diagrammatic sectional view of a form of pin;

Figure 6 is a diagrammatic sectional view of an array of pins as shown in figure 5; ^• and

Figure 7 is a diagrammatic plan view corresponding to figure 6.

Referring to the drawings, a thermoplastic sheet for perforation is laid on the platen of an apparatus, preferrably a modified cutting and creasing apparatus. The apparatus is provided with a cover 10 conventionally having cutting blades 12 for cutting a sheet of the required size and shape. In accordance with the invention the cover 10 is further provided with an array of pins 14 affixed thereto between the boundries delineated, by the blades 12. As can be seen more clearly from figure 2, the pins are held in a strip or strips eff material 16 which is in turn fixed to the cover 10 and which may be heated, for example electrically. The strip 16(a) (Figure 2(a)) is serpentine in configuration whereas a number of strips 16(b) (Figure 2(b)) may be affixed in parallel. Other arrangements may be employed as desired. If a strip fails it may be replaced without replacing the whole array.

In order to prevent the heated strips 16 from coming into contact with the thermoplastic sheet, a support and insulating layer 18 is provided. If desired the cutting blades could also be attached to this sheet. As can be seen more clearly from Figure 4 the latter comprises a series of aligned holes 20 each of which may receive a needle 14. It is preferred that the layer 18 be provided with sufficient holes 20 for receiving the maximum desired number of pins 14 but that arrays of pins of lesser number may be employed leaving some of the holes 20 redundant. This facilitates a change of design in that the array can be modified by changing the strip 16 without needing to change the support layer 18' also. This is particularly useful when the sheet is to be used for food containers as ^■ the requirements for ventilation differ greatly between different kinds of ,e.g., fruit and vegetables..

Whilst the cover 10 is illustrated as flat, it will be appreciated that certain cutting and creasing machinery incorporates other configurations, for example a cylindrical roller carrying knives which is rolled accross the sheet held on the platen. The array of pins 14 can equally be affixed to such an apparatus.

While the above embodiment has been described referrring to "pins" 14, this does not mean that the perforating means heed necessarily be conical or cylindrical. Thus the pins 14 could be replaced by knives or implements of other cross-sectional shapes. The word "pins" should be construed accordingly.

Referring now to figures 5 to 7, a preferred form of pin 14 is illustrated. As can be seen from figure 5, the bulk of the pin is cylindrical in shape but the end portion is conical terminating with an "onion-shaped" tip 15. Other shapes may be employed, e.g. conical end portion terminating with a spike, but the former shape has been

found to meet the necessary requirements of clean perforation followed by clean separation of the pin from the plastics material sheet when the array is moved vertically. Each pin is provided with a central heating unit or cartridge 22 having wiring 24 enabling it to be connected into an electrical circuit. Each pin is therefore directly heated and preferably each pin is wired in series with a number of other pins heating units to give a circuit which may be controlled -by means of its own thermocouple or pyrometer. A number of such circuits will be provided each of which may be controlled individually so as to give maximum flexibility in the heating of individual arrays of pins. This also increases ease of replacement of arrays and facilities selection of a particular configuration of pins to suit a given end-use.

As can be seen more clearly from figure 6, each pin may be set into a plate 25 which may optionally be covered above and below with sheets of insulating material 26,28 respectively. Moreover each pin 14 may be fixed into the plate 25 by various methods. One method of fixing the pins into the plate would be to provide the pins with a screw thread and a corresponding thread in the plate. Alternatively, the pins may be glued or welded in place.

As can be seen from figure 7 the .wires 24 from a number of heating units 22 within an array of pins 14 may be connected in parallel to a circuit 30. Each circuit may be provided with its own temperature controller (not shown) e.g. a pyrometer of thermocouple. The upper portion of figure 7 illustrates two such rows of pins being connected to a circuit 32.

By directly heating each pin the most constant and consistent temperature at the pin tip is obtained. The device illustrated may be used with individual cut sheets and may preferably operate in a cycle whereby a sheet is

placed below the array of pins, the array brought downwardly to the sheet in an accurate vertical movement in order to ensure precise and regular hole size, and the array thereafter being retracted so as to cleanly remove the pins from the sheet. The perforated sheet is then moved and replaced with an unperforated sheet and the cycle repeated. The platen remains stationary during this process, and very accurate holes as regards size, shape and distribution can therefore be obtained.

As mentioned above the size and number of pins in a given area may be varied. This is restricted only by reference to the need to secure the pins in the plate 25 and the pin size. Subject to these limits the device can be equipped with any density of pins 14 and, if desired, with pins of different sizes. This together with the precise temperature control and the precise control of the arrays* entry to the sheet to be perforated, allows sheets to be perforated in a controlled manner according to the exact requirements of humidity and passage of air in the finished box made from the perforated sheets.

The plate 25 carrying the pins can be made either of metal or plastics material and the pins will normally be of metal in order to produce the necessary mechanical and thermal properties.

The invention therefore provides simple and economical apparatus and method for introducing perforations of the size and disposition into sheet material, for example for use in packaging. It would be appreciated, of course, that the uses of the sheet material are not so-limited and the invention may be employed to produce thermoplastic sheet material for any end use desired.