The invention relates to a sandwich panel and a method of manufacturing such a sandwich panel.

Sandwich panels comprising a first and a second metal profile with an in between foam core are known as are methods of manufacturing such sandwich panels.

It is found that the surface quality of the first metal profile, usually a steel profile, is particularly critical because it eventually forms the external face of the ready sandwich panel. It is common practice for manufacturers to use a primer (glue) to improve the bond between the foam core and the metal profile. Nevertheless, it is not a guaranteed solution and it remains not uncommon to find “blisters” or other related defects near the steel skin causing delamination of the metal profile from the foam at the external surface of the building at which the sandwich panel is used - particularly where the weather creates thermal cycling to open up sub-surface defects. This may lead to customer claims and costly repairs.

It is an object of the invention to provide a sandwich panel and a method of its manufacturing which to a large extent, if not entirely, tackles the above-mentioned problem.

To this end, according to the invention, a sandwich panel and a method of manufacturing such a sandwich panel are proposed in accordance with the features of one or more of the appended claims.

According to an aspect of the invention, providing the foam core is split into at least two steps, namely one step of providing a first foam layer and a further step of providing a second foam layer such that the second foam layer contacts the first foam layer after the first foam layer has risen to a predefined first distance away from the first metal profile. Accordingly a sandwich panel is provided wherein the first foam layer completely covers the first metal profile or an optional primer on the first metal profile, wherein the foam-foam interface between the first foam layer and the second foam layer is provided at a predefined distance from the first metal profile.

The invention is also embodied in a method of manufacturing a metal-foam-metal sandwich panel comprising the steps of, between a first metal profile and a second metal profile, providing optionally a primer on the first metal profile, providing a first foam layer on the optional primer and/or on the first metal profile, providing a second foam layer contacting the first foam layer, wherein the second foam layer contacts the first foam layer after the first foam layer has risen to a predefined distance away from the first metal profile which is after initial cross-linking and strengthening of the first foam layer but before the first foam layer is form-stable.

The invention may also be described as a method of manufacturing a sandwich panel comprising a foam core between a first metal profile and a second metal profile, wherein on the first metal profile an optional primer is applied, on which optional primer the foam core is applied, on which foam core the second metal profile is applied, wherein the foam core is applied in at least two separate steps involving applying a first foam layer, and subsequently applying a second foam layer and subsequently optionally applying further foam layers, all foam layers after application collectively forming the foam core, wherein the second foam layer contacts the first foam layer only after the first foam layer has reached its gel time, in order to let the first foam layer rise freely, thereby enabling the first foam layer to resist any forces that arise when the second or a further foam layer impinges on the second metal profile, and in order to achieve that if any shear forces are to cause disruptions in the foam layers the weakest spot is moved away from the first metal profile, e.g. to the foam-foam interface between the first and the second foam layer.

The foam of the first foam layer and of the second foam layer may be of the same composition. In that case the density of the first foam layer will be almost equal to but slightly lower than the density of the second foam layer because according to the invention, the first foam layer will be allowed to rise freely. The density of the foam core may be in the range of 34-50 kg/m ³ and in a specific example of a panel according to the invention, the density of the first foam layer may be 38 kg/m ³ and that of the second foam layer may be 39 kg/m ³ . In practical cases the distance of the foam-foam interface from the first metal profile will be in the order of 5-95 % and in most cases 10-90 % of the shortest distance between the first metal profile and the second metal profile.

The invention is based on the insight that the prior art defects are often caused when the top surface of the gelling foam core is compressed as it rises against the second metal profile, and that the whole foam core is, by this time, sufficiently strong to transfer forces through the foam core to the surface of the first metal profile, where friction resulting from the continuous movement of the first metal profile and the foam creates shear stresses that locally break down the foam cellular structure at or near this lower interface between the foam and the first metal profile.

By arranging that the step of providing the second foam layer on the first foam layer is executed after the first foam layer has developed to a predefined first distance away from the first metal profile, the effect is provided that the first foam layer gains more strength to resist any forces that arise when the second foam layer impinges on the second metal profile, and that if any shear forces are to cause disruptions in the first or second foam layer, that the ‘weakest’ spot is moved away from the first metal profile, e.g. to the foam-foam interface, which foam-foam interface is at least 10 mm and preferably 15 mm away from the first metal profile, and may in a practical example may be approximately 20 mm away from the first metal profile. This location for defects is found to be hugely less problematic than defects in the immediate neighbourhood of the first metal profile because such defects would not leave a visible indentation on the outer skin of the sandwich panel and also, the defects in the foam-foam interface would be insulated by the foam from the effects of thermal cycling when the sandwich panel is exposed to weather conditions on the side of a building wherein the sandwich panel is applied, such thermal cycling being known to exacerbate blistering.

Consistent with the above-mentioned considerations it is preferable that the second foam layer is provided on the first foam layer before the first foam layer has reached a second distance from the first metal profile, at which second distance the second metal profile will be provided.

Desirably the second foam layer is provided on the first foam layer after initial cross-linking and strengthening of the first foam layer but before the first foam layer is form-stable. In practice this form stability will be reached around the time that foam experts call “gel time”. Gel time means that the core of the foam is not in a liquid state anymore, but is becoming a rubberlike solid.

Suitably the second foam layer is provided on the first foam layer within a 0 - 10 s (seconds) time slot following the initial cross-linking and strengthening of the first foam layer.

It is possible to avoid using a primer, but it may in certain circumstances be advantageous that between the first foam layer and the first metal profile a primer is provided. Instead of a regular gluing primer a less costly substitute may be applied such that any shear stresses are properly relieved in the intermediate foam-foam interface rather than near the first metal profile. The primer may be foamable to provide higher temperatures for the polymerising foam and therefore increase its cohesion. The primer may contain flame retardant additives to provide improved performance in case of fire. The primer typically has a thickness of 0,5 to 1 ,0 mm.

Other advantages of applying two (or more) foam layers between the first metal profile at the second metal profile can be itemised as follows.

Allowing the (quality critical) first foam layer to form as a “free rise” foam means it is not significantly affected by being constrained within a restricted space. Further it is possible that improved thermal and/or mechanical properties can be provided to the sandwich panel by a suitable selection of foams. Also, the method of manufacturing the sandwich panel which could be extended to having three or more foam layers lends itself to tailoring the properties of the respective foam layers through the applied thicknesses - for example using more fire retardant (and relatively costly) chemicals near the interface between the first metal profile and the first foam layer, where the sandwich panel is more likely to be exposed to fire, and less of those costly chemicals in the midsection where thermal properties and low cost are more important. It is possible that the foam layers between the first metal profile and the second metal profile comprise individually different additives, preferably selected from the group comprising solid powder additives, electronic devices and sensors.

It is further possible that a small foam density reduction can be applied because of better foam structure resulting from more ‘free rising’ of the first foam layer and the potential for better control of the volume of liquid applied for the second foam layer. This relates to the distance that the second foam layer must ‘rise’ until it meets the second metal profile, the liquid application therefore may get reduced and precision of application can be improved. Foam is an expensive component of panels and small reductions in density are accordingly valuable.

The invention will hereinafter be further elucidated with reference to a schematic drawing of an exemplary embodiment of a method of manufacturing a sandwich panel according to the prior art and according to the invention that is not limiting as to the appended claims.

In the drawing:

Figure 1a/1b shows a method of manufacturing a sandwich panel according to the prior art; and

Figure 2 shows a method of manufacturing a sandwich panel according to the invention.

Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts.

In figure 1a a schematic side view is shown of manufacturing a sandwich panel according to the prior art, wherein the sandwich panel comprises a bottom sheet 1 , a top sheet 2, a single insulating foam layer 3 which is provided on the bottom sheet 1 by a series of foam nozzles 4 (figure 1b showing the nozzles according to the line X -X in figure 1a), and optionally a primer nozzle 5 for applying a gluing primer on the bottom sheet 1. As the schematic drawing of figure 1 a shows the single foam layer 3 is provided on the bottom sheet 1 , and rises up to the underside of the top sheet 2. When the foam 3 impinges on said top sheet 2, it rolls back, and forces are transmitted from this upper side through the foam 3 down to the bottom sheet 1 , where shear forces between sheet 1 and foam 3 occur that cause damage to the foam cell structure.

Making now reference to figure 2, the method of manufacturing a sandwich panel according to the invention is schematically shown wherein two rows of foam outlets 4a, 4b are applied. With the first row of foam outlets 4a a first foam layer 3a is applied to the bottom sheet 1. With the second row of foam outlets 4b a second foam layer 3b is applied to the first foam layer 3a, such that the first foam layer 3a completely covers the first metal profile or bottom sheet 1 and a foam-foam interface between the first foam layer 3a and the second foam layer 3b is provided at a predefined location between and distant from the first metal profile or bottom sheet 1 and the second metal profile or top sheet 2. It is also possible, although not necessary, that with the primer nozzle 5 a primer is provided to the first metal profile or bottom sheet 1. This primer may be foamable. The interface between the first foam layer 3a and the second foam layer 3b is provided such that the first foam layer 3a has risen to a predefined first distance away from the first metal profile or bottom sheet 1, before the second foam layer 3b is provided on the first foam layer 3a. At that time initial cross-linking and strengthening of the first foam layer 3a has started but the first foam layer 3a is not yet form-stable. Preferably the second foam layer 3b is provided on the first foam layer 3a within a 0 - 10 s time slot following the initial cross-linking and strengthening of the first foam layer 3a.

Although the invention has been discussed in the foregoing with reference to the method of the prior art and an exemplary embodiment of the method of the invention, the invention is not restricted to this particular embodiment which can be varied in many ways without departing from the invention. The discussed exemplary embodiment shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiment is merely intended to explain the wording of the appended claims without intent to limit the claims to this exemplary embodiment. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using this exemplary embodiment.