This invention relates to the cooling of sheet material and in particular relates to the cooling of plastics sheet material following the formation thereof and to a calibration plate there or.

In the production of thermoplastic sheet material with a hollow profiled structure, for example a corrugated or I-beam structure, a calibration unit is used to cool the sheet following extrusion. A traditional calibration plate is illustrated in section in figure 2 of the accompanying drawings, from which it can be seen that the plate contains passageways through which chilled water is passed in order to effect cooling. This stage in the process represents something of a bottle-neck as the speed of throughput is restricted for two main reasons. These are, firstly, that there is friction between the surface of the sheet being cooled and the calibration plate or plates; and, secondly, the cooling effect is relatively slow owing to the indirect transmission of heat from the sheet surface to the chilled water through the material of the calibration plate itself.

The present invention seeks to provide a calibration plate for the production of thermoplastic sheet material improved in the above respects.

According to the present invention there is provided a calibration plate for the cooling of thermoplastic sheet material which comprises a plurality of passageways for chilled fluid characterised in that at least part of the or each passageway is open to the operative surface of the plate.

As with traditional calibration plates, vacuum passages may also be provided to draw a vacuum and maintain the sheet to be cooled in close contact with the plate during the cooling operation. The calibration plate of the invention therefore

allows chilled fluid, in particular chilled water, into direct contact with the sheet surface. This has two beneficial effects: firstly, it lubricates the sheet surface, cutting down the friction; and, secondly, with direct contact between the cooling medium and the sheet surface the cooling effect is much more rapid and marked. The calibration plate of the invention therefore enables much higher throughput speeds to be achieved, speeding up the entire process of the production of the sheet material.

The invention is applicable to the production of various kinds of thermoplastic sheet material, but is particularly useful in the production of alveolate sheet material such as corrugated thermoplastic sheet or I-beam structure thermoplastic sheet, particularly useful in the subseguent preparation of boxes and cartons among other uses. The sheet materials may be of any suitable thermoplastic material but polyolefin materials are preferred.

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

Figure 1 is a diagrammatic perspective view of a calibration unit for sheet material;

Figure 2 is a section on line A-A of figure 1 illustrating a traditional calibration plate; and

Figure 3 is a section on line A-A of figure 1 of a calibration plate according to the invention.

Referring to the drawings, it can be seen from figure 1 that an extruded thermoplastic corrugated sheet 10 is moved between an upper calibration plate 12 and a lower calibration plate 14 in order to cool it. The sheet is moved by a series of motorised rubber coated rollers (not shown).

Conventionally, the calibration plates 12,14 have the internal structure similar to that shown in figure 2. That is, there are vacuum passages 16 for holding the sheet material 10 in close contact with the calibration plate and passageways 18 through which chilled fluid, such as water, is passed to effect cooling.

In accordance with the invention, however, as illustrated in figure 3, a calibration plate 12 comprises vacuum passages 16 as before but in this case the passageways 18 carrying the chilled fluid such as water are not totally enclosed within the body of the plate 12 but are open at the surface 20 of the plate.

In use, the surface of the sheet 10 in effect provides the missing wall of each passageway 18 but the water flow, being in direct contact with the sheet 10, ensures a rapid cooling effect. Moreover, sufficient water spreads across the whole surface of the sheet 10 to provide a lubricating effect. Excess water is sucked up through the vacuum passageways 16.

More than one calibrator 12,14 can used as reguired by the sheet thickness and the intrinsic heat capacity of the material of the sheet. The calibrators can be fixed or movable according to the thickness of the sheet material and the production speed to improve heat dispersion on the surface, especially if the the material is not a good heat conductor and therefore needs extra time to disperse heat from the core of the sheet to the surface.

The system of the invention provides a very much more effective and rapid means of cooling a thermoplastic sheet and removes the "bottle neck" effect of previous calibration plates, enabling much more rapid production to be achieved.