The present invention concerns double-walled panels of trans¬ parent synthetic material for use in greenhouses and for similar purposes. By double-walled panels made of transparent synthetic material we understand at least two parallel, sepa¬ rated sheets which are interconnected by means of ribs, and where the open ends of the sheets are closed by longitudinal flanges formed by the external surface of each sheet. It is common to say acrylic panels if the panels are made of poly- acryl material, or carbonate panels if the panels are of a polycarbonate material. In this text, these sheets will consistently be referred to as double-walled panels to draw attention to the internal construction of the sheet.

In recent years it has become more and more common arti¬ ficially to supply carbon dioxide to greenhouses to stimulate plant growth.

However, to have the full benefit of this procedure, the greenhouse cannot be ventilated. On hot sunny days, however, this will unfortunately result in a leaf temperature which exceeds the optimal one for plants, whereby the growth will be reduced. Consequently, the carbon dioxide cannot be utilized effectively in this way.

It is therefore known to fit vents in the roof of the green¬ houses and to employ some kind of blind to prevent heat buil¬ ding up in it. However, if the vents in the roof are opened, the supply of carbon dioxide must be cut off. If an attempt is made to reduce the temperature in the greenhouse by shading the light surface, so much light will be removed that this will inhibit the growth of the plants. B TRE--U

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In other words, this is a vicious circle as either costs increase or growth is inhibited.

Several methods of solving the above problem have been suggested. One of these methods, which is relevant to the present invention, is to use a liquid medium, e.g. water, to filter off the infrared rays in sunlight.

The Norwegian Patent No. 118.047 describes a construction for use with a flat greenhouse roof, where a layer of water covers the transparent roofing material. However, greenhouses are not usually built with flat roofs in countries with snow fall in the winter. The said construction is therefore not suitable in these countries.

If double-walled panels of transparent synthetic material are used for greenhouses with sloping roofs, these make suitable sun filters when the channels between the ribs are filled with a liquid medium. Such use of double-walled panels is mentioned in the West German Patents Nos. 2,500,015; 2,637,914; 2,719,273 and 2,850,264, in which the open ends of the panel are closed by manifold tubes or manifold boxes supplying the liquid.

However, in practice it is extremely difficult to seal properly the connections between the manifold pipe and the open ended panel. The purpose of the present invention is therefore to provide a construction which eliminates the considerable weaknesses experienced by the use of manifold tubes or manifold boxes according to the above-mentioned West German patents.

Surprisingly, it has proved possible to close the open ends of the double-walled panels by exposing them first to heat treatment and then to compression, whereby the subject of the invention, the manifold tube becomes an integrated part of the panel itself.

According to the invention the said construction is pre¬ ferably achieved in the following way that the narrow rib walls terminating the open ends of the double-walled panels are recessed or otherwise removed for a certain distance (a) inwards of the panel, yet leaving intact at least the two outermost rib walls which constitute the outward edges of the panel. Then the open ends of the panel are heated using normal appliances and methods, for instance by using hot air or infrared radiation, at a temperature suitable for the material after which the open heated ends are pressed together over a certain width (b). As a result a permanent and sealed connection is formed externally, while the recesses in the rib walls secures an open internal connection between the channels. Hereby the liquid medium is distributed over the entire cross-section of the sheet. The inlet and outlet for the liquid are arranged in the sheet in a per se familiar way, e.g. by fitting connecting sockets in the outer enclosing surfaces of the sheet itself.

The invention is characterized in accordance with the claims below.

For a further understanding of the invention reference is made to the accompanying drawings, of which

Fig. 1 illustrates an open-ended double-walled panel viewed obliquely from above.

Fig. 2 illustrates the same panel with rib walls removed.

Fig. 3a and 3b illustrate the panel after the open ends have been closed.

Fig. 4 illustrates a cross-section of the panel along line A-A in Fig. 2, while

Fig. 5 illustrates a section along the B-B line in Fig. 3 after the open ends of the panel have been closed.

Fig. 1 shows a double-walled panel of transparent synthetic material, where the open ends of the panel, forward (1) and rear (2), can be seen with the rib walls (3) and the channels (4). The sheet is restricted or defined by an upper surface (5) and a lower surface (6) which is not seen, a lateral surface or edge (7) and a lateral surface or edge (8) not seen. The rib walls (3) are indicated by broken lines on the upper surface (5) .

In Fig. 2 the rib walls (3) are removed inwards in the panel for length (a). Therefore the rib walls (3) are not visible in the front of the figure, but they are indicated as broken lines on the upper surface (5). The rib walls (3) are removed by means of known techniques, e.g. cutting. If cutting is used, this can be done with a sharp-angled incision in the rib wall or it may be rounded off. A rounded incision is pre¬ ferred due to the risk of fracturing when sharp angles are used.

In Fjg. 3a the same panel is illustrated with the previous open sheet parts (1 and 2) closed. The closing (9) is illu¬ strated as a flange running transversely in relation to the panel. It is possible to close the surfaces (5 and 6) in such a way that the flange (9) is centered equally distant from surface (5) and surface (6) or the flange (9) may form a direct elongation of the surface (5) or surface (6). The flange (9) shall have a width (b) which is smaller than the length (a) of the cutouts (10).

Fig. 4 illustrates a section of the forward opening (1) of the sheet along the line A-A in Fig. 2. The forward opening (1) is closed by the upper surface sheet (5) and the lower surface sheet (6). The lateral surface (8), which is part of the rib walls (3), is seen. The rib walls (3) are shown recessed along a length (a) with cutouts (10) completed with a round incision (11).

Fig. 5 shows a cross-section of the same panel along line B-B in Fig. 3a where the closure or flange (9) connects the upper surface (5) and the lower surface (6). The cutout (10) now forms holes (12) in the rib walls (3) through which the liquid medium may be distributed over the whole panel.

With the present closure of the upper and the lower sheet it has been possible to arrive at a simple and above all a practicable construction. The construction also makes possible for the panel to be used as roofing material for greenhouses without having to alter the construction of the greenhouse.