METHOD OF MANUFACTURING A SANDWICH PANEL

Abstract: A method of manufacturing a sandwich panel in which two rigid or semi-rigid skins are mounted in spaced, substantially parallel vertical relation with the panel edge members extending across the gap between the low edges of the skins. At least one nozzle is introduced between the upper edges of skins and a liquid foam forming material is projected laterally against the inner services of the skins, adjacent the upper edges thereof, and the liquid is allowed to flow downwardly to wet substantially the entire inner surface of each of the skins. The introduction of the liquid is continued until sufficient foam forming material has been introduced into the space between the skins to obtain the desired foam interior under the prevailing conditions. The gap between the upper edges of the skins is closed and the foam is allowed to expand and fill the space between the skins and thereafter to cure.

Description: The present invention relates to a method of manufacturing a sandwich panel having two rigid or semi-rigid skins with a foam interior between the skins. Typically such skins are formed of aluminium copper or the like filled with a foam insulation material, such as polyurethane or a similar foam. The foam material may be obtained by mixing two components, such as polyisocyanate with a mixture containing polyol. The polyol mixture itself is often obtained by adding a catalyst and blowing agent, such as freon to the polyol and further additives may be provided to suit particular needs. The mixing of the isocyanates starts the foaming reaction and therefore this mixture should only take place directly preceding the distribution into the interior of the panel. Said polyurethane foam compound obtained has reactions in three stages:- 1. There is firstly a liquid stage in which the foam forming material has a creamy nature; 2. Secondly the material goes through the expansion stage, in which the compound expands to a cellular condition; 3. Finally there is a curing stage in which the compound cures to a fully thermoset condition. Various proposals have been made for distributing the foam forming material into the space between the skins forming the outside walls of the sandwich panel, a typical example is that shown in United States Patent 3846525 which includes mounting the skins in spaced, substantially parallel vertical relation, with a panel edge member between the gap between the lower edges of said skins, introducing at least one nozzle between the upper edges of said skins and introducing a liquid foam forming material between the inner surfaces of said skins. It has been found in order to obtain sufficient bonding of the foam material to the outer skins, as well as sufficient foam density for structural strength, it is necessary to introduce the foam forming material in repetitive passes, while obtaining a series of successive layers. Such an operation is both time consuming and cumbersome. It has also been attempted to discharge the full amount of foam forming material in a single pass, but the problem then arises that the bonding of the foam material to the skins and the foam density are usually found to be totally inadequate. It is proposed, according to the present invention, to provide a method of manufacturing a sandwich panel having two rigid or semi-rigid skins with a foam interior between the skins, said method being characterised in that the liquid foam forming material is projected laterally against the inner surfaces of the skins, adjacent the upper edges thereof, allowing said liquid to flow downwardly to wet substantially the entire inner surface of each of the skins and continuing to project said liquid foam forming material until, adequate liquid has been introduced into the space between the skins to obtain the desired foam interior under the prevailing conditions, in that the gap between the upper edges of said skins is closed and in that the foam forming material is allowed to expand and fill the space between said skins. It has been found that the wetting of the inner surfaces of the skins and of a member bridging the skins at the bottom of the mould, in the liquid stage of the foam forming material guarantees the best obtainable bonding between the foam and the skins. It has also been found that distribution in this manner results in a more homogeneous cell structure, with the lowest density foam material at the very centre, and the higher density, stronger material being at the outside and being firmly bonded to the skins. Preferably the temperature prevailing is controlled so that sufficient foam forming material remains in the liquid state to reach and wet said panel edge member before the expansion stage of all of said foam forming material begins. Advantageously, also, the mixture of the foam forming material is chosen so that sufficient foam forming material remains in the liquid state to reach and wet said panel edge member, before the expansion stage of all of said foam forming material begins. It is desirable that the extent to which the nozzle projects into the space between the skins should remain substantially constant and indeed that the nozzle should be moved longitudinally of the panel skins parallel to the upper edges of these skins. While the apertures formed in the nozzle for laterally projecting the liquid may take many forms, they preferably are aligned with one another with their edges perpendicular to the inner surfaces of the skins so that the foam forming liquid is projected directly towards the confronting inner surfaces of the skin. It is also contemplated that the nozzle could take other forms and could be directed in a conical or generally divergant spray, but it is important that the upper parts of the inner surfaces of the skins should be thoroughly wetted and that the foam forming material in its liquid state should be free to run down and wet the whole of the inner surfaces of these skins. In order that the present invention may be more readily understood, the following description is given, merely by way of example, reference being made to the accompanying drawing in which:- Figure l is a schematic cross-section illustrating the carrying out of the methods of the invention; and Figure 2 is a schematic cross-section through a panel formed using the method of the invention. Referring first to Figure l there is illustrated therein a vertically extending nozzle (l) having two lateral apertures (2) the axis of which extend perpendicular to the axis of the nozzle (l). The nozzle (l) is illustrated in the drawings as being fed between the upper edges of two aluminium semi-rigid skins (3) which are bridged at the bottom by a profiled spacer member (4), the bottom edges of the skins (3) being wrapped around parallel flanges of this member (4). The skins are mounted in a mould (5) so as to be substantially vertical and parallel to one another. The skins define between them a generally rectangular vertical space (6) having a narrowed mouth (7) at the upper edges (8) of the skins (3). In carrying out the method of the invention, the nozzle is located substantially in the position shown and a foam forming mixture consisting of Polyol and Polyisocyanate in a ratio of between l:l and l:6 is used. The Poly-Isocyanate may, for example, be Polymethyl Di-Isocyanate or Poly-Isocyanurate and may include a propellant such as Freon. The mixture is fed to the nozzle at a feed pressure of between l and l0 bar and at a temperature between l5 and 30 degrees with the mould temperature between 20 and 45°C. The foam forming mixture preferably has a viscosity in the range of 200 and 20,000 centipoises and remains in the liquid stage for between 5 and 20 seconds. The expansion stage of the foam forming material is between 30 and l50 seconds. The nozzle is caused to move longitudinally, that is perpendicular to the plane of the paper so that the vertical position of the nozzle remains substantially constant and as shown in the drawing. After filling an adequate quantity of foam forming material into the space (6) so that this space will be completely full after expansion, a cover (not shown) is placed over the upper edge of the opening (7) in a similar manner to the bridging member (4). After the cover has been put in place, the foam is allowed to cure for between 3 and 25 minutes and this produces a foam density of between 40 and 70 kg per metre³ and the compression strength of the foam is between l0 and 30 N per cm² and its shared strength between 8 and 30 N per cm². Figure 2 shows the resulting product from which can be seen that the wetting of the whole outer circumference of the cavity (6) in the liquid stage of the foam forming material guarantees the best obtainable bonding between the foam (9,l0) and the skins (3). The distribution in this manner results in a more homogenous cell structure, the lowest density portion (11) being in the center. High density foam as a rule has more strength than lower density foam and the same high density is achieved at the open panel end (l2), where the mould has been closed by the cover member (not shown) during the foaming reaction.

Claims: A method of manufacturing a sandwich panel having two rigid or semi-rigid skins (3) with a foam interior between the skins, said method comprising mounting the skins in spaced, substantially parallel vertical relation, with a panel edge member (4) between the gap between the lower edges of said skins, introducing at least one nozzle (1) between the upper edges of said skins and introducing liquid foam forming material downwardly between the inner surfaces of said skins, characterised in that the liquid foam forming material is projected laterally against the inner surfaces of the skins, adjacent the upper edges thereof, allowing said liquid to flow downwardly to wet substantially the entire inner surface of each of the skins and continuing to project said liquid foam forming material until adequate liquid has been introduced into the space between the skins to obtain the desired foam interior under the prevailing conditions, in that the gap between the upper edges of said skins is closed and in that the foam forming material is allowed to expand and fill the space between said skins. A method according to claim 1, characterised in that the temperature prevailing is controlled so that sufficient foam forming material remains in the liquid state to reach and wet said panel edge member before the expansion stage of all of said foam forming material begins. A method according to claim 1 or 2, characterised in that the mixture of said foam forming material is chosen so that sufficient foam forming material remains in the liquid state to reach and wet said panel edge member, before the expansion stage of all of said foam forming material begins. a method according to claim 1, 2 or 3, characterised in that the extent to which said nozzle projects into the space between said skins remains substantially constant. A method according to any preceding claim, characterised in that the nozzle is moved longitudinally of the panel skins parallel to the upper edges thereof. A method according to any preceding claim, characterised in that the nozzle has two laterally directed apertures (2) whereby the foam forming liquid is projected directly towards the confronting inner surfaces of the skins.

Inventors: DE BESTE, WILLEM HENDRIK
Applicants: HUNTER DOUGLAS INDUSTRIES B.V.
Application year: 1987
Main classification: B29C67
IPC codes: B29C39, B29C44, B32B5, E04C2, B29L31, B29K105