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  Referencia 1229
Título: Uso de paneles metálicos prefabricados en arquitectura
Título original: The use of prepainted panels in architecture
Autor: F. Sacchi, G. Botta, L. De Cet
Resumen: History of solid cladding panels

a) Early attempts (failed)
In the early 50's aluminium sheet seemed to be a most obvious choice for architectural panels used as façade closures. This idea certainly inspired architects Harrison and Abramowitz when they designed the Alcoa Headquarters buildings in 1952.
They used prefabricated panels of an AlSi5% alloy that took up a beautiful grey colour when anodized. The result was splendid to see and installation of the closure panels was greatly simplified.
In spite of such a striking accomplishment, however, solid aluminium didn't substantially appear as a façade cladding any more in the following 40 years. The reason was that the only high performance finish available at the time - anodizing - produced non-uniform finishes, and the thick (2-3 mm) sheet needed was not sufficiently flat to satisfy the requirements of modern architecture. Alcoa had performed technical miracles to overcome those problems, but in common practice solid aluminium cladding proved too expensive and was all but abandoned during the following 25 years. In the following we give an outline of the process that eventually brought up the comeback of solid aluminium in important cladding jobs as the Ferrari Wind Gallery by Arch. Renzo Piano (fig. 1) and the SNAM Towers by Arch. Kenzo Tange.

b) A successful substitute: composite sheet
A convenient substitute for solid aluminium panels was developed in the early 70's: a plastic sheet clad with thin coil coated aluminium (fig. 2).
The thick plastic core ensured perfect flatness and coil coating of the thin aluminium skin with the new PVDF products - particularly the metallized finishes imitating the real bright anodized aluminium surface - had both uniformity of aspect and good resistance to weathering.
The most frequent use of these composite sheets was in the fabrication of tray or "cassette" panels that were hung onto a structure to create a ventilated cladding.

These composite sheets required scoring on the back of bend lines before bending to shape the cladding panels: figure 3 shows the difference in bending procedures between solid aluminium and composite sheet. They also needed riveting of the corners that wouldn't be normally needed when fabricating solid aluminium sheet.
In any case, the composite sheet (marketed under various trade names as Alucobond, Reynobond, Alpolic, etc.) caused the comeback of aluminium surfaces on curtainwalls, ventilated façades and rainscreens, and so paved the way to the comeback of solid aluminium sheet once the difficulties of the latter were overcome.

c) The comeback of solid aluminium sheet As mentioned above, two main difficulties had prevented world designers from following suit after Harrison and Abramowitz's bold use of solid anodized aluminium panels in the 1952 Alcoa Buildings (fig. 4): these two drawbacks were inconsistency of colour and imperfect flatness.
Colour inconsistency had been caused by the fact that anodizing had been the only high performance finish available at the time, and the anodic films inherited their structure, colour shade and reflexes from the variable structure of their base metal. Imperfect flatness had been caused by the fact that façade panels needed thick metal (2-3 mm) which could not be stress-levelled by common equipment existing in the rolling mills at the time.
Both these defects caused designers to steer clear of solid aluminium panels, fearing customers' protests and conscious that a feeling of "well-made" object is a must for modern architecture. Alcoa had succeeded in 1952, but they had needed so many metallurgical and finishing tricks that had raised the cost beyond all reasonable limits. Obviously, the whole picture changed when uniform finishes and flat thick metal became available.
Flat, thick aluminium alloy sheet was the first improvement to come when the major rolling mills installed highly powerful stress-levelling equipment downstream of the final cold rolling passes. Approximately at that time, some major architects began using very thick cassettes, 4 and even 5 mm thick, welded at the corners and absolutely tight, to build up costly exterior claddings of important high rise buildings. The flat sheet was one achievement, and another one was the use of uniform, high performance non-anodized finishes: several layers of the new PVDF coatings, already tested on the composite sheet, were applied on the heavy welded panels.
Of course, the solid panels, welded and painted afterwards, were very expensive too, and therefore they had limited use. The real breakthrough of the solid metal cladding came about when the cladding panels could be simply fabricated from rolled aluminium, 2-3 mm thick, flat leveled and inexpensively coilcoated or coil anodized. In the following, we will describe the technology and market of this material which has become increasingly competitive with the composite sheet in the manufacture of architectural claddings.

Coilcoated solid aluminium panels

a) Special finishes
The widespread use of solid aluminium panel cladding [1] actually began when the rolled metal (mostly AlMg alloy 5005) could be prefinished in a continuous process with a coating that would withstand the punching and bending operations involved in the manufacture of panels. As stated above, if the panels should be coated after forming, the overall cost inclusive of handling and temporary protection costs would be so high that the use of solid panels would be restricted to just a few, exceptional buildings.
Therefore, even after flat leveled sheet had become available, special coilcoating procedures had to be developed before solid cladding could become a popular solution for ventilated walls and façade panels. Of course the "wet" PVDF coil coating procedure had been available since the middle 70's and had been used as the most common finish of the composite sheet. However, the thin PVDF finishes (25-35 µm) thinned dangerously down on bend corners (Fig. 5). Coil anodized sheet was also used with success, but this was prone to cracking on bend corners; however the preanodized thick (3 mm) aluminium sheet did much to bring back aluminium on architectural fronts.
The real turning point was the development of PCC (Powder Coil Coating) in the early 90's. Why was it so?: because the PCC finishes applied thick (50-70 µm) polyester coatings identical in colours and texture to the finishes currently used for architectural extruded sections used for window and façade frames since the early 70's.
Therefore, PCC sheet had a ready market in the manufacture of window complements, bay linings, shutter cases, etc. that would perfectly match the powder finishes of the extruded components.
However, the existing powder paints at the time couldn't be used for coilcoating because of their brittleness and special polyester powders had to be developed before PCC could be used to create flexible coilcoated finishes that met the requirements of panel forming without impairing the continuity of the coatings after forming.

b) Objections against precoated metal panels
Some objections have been raised against the use of precoated metal on rainscreens and façades. Figure 6 sums up the positive answers that have been confirmed in real practice. The main objections were a) that precoated panels have unpainted, bare cut borders liable to corrosion, and b) that the precoated "cassette" panels cannot be made perfectly watertight by welding.
Both objections denote scarce knowledge of aluminium physico-chemical properties: bare aluminium quickly passivates itself by contact with atmospheric oxygen (coatings are applied for aesthetical purposes only), and claddings can be designed to collect and flush away any rainwater that may have penetrated through the unwelded cassette corners. The only real danger would be the permanence of stagnant water, which is avoided by proper design. On the other hand, the use of solid metal instead of plastic-Al composites has evident advantages if fire hazard regulations are considered.

c) Recent developments - new finishes

While PCC (Powder Coil Coating) marked the breakthrough of solid metal panels in façade cladding, new finishes have been developed in order to widen the range of colours, textures and weathering performance available to designers. Recent advancements are:
a) The "Bright" coatings where metallic powder is dispersed in a PVDF layer, and a clear PVDF top layer is applied. PVDF has the best behaviour in critical high-UV exposure and can be guaranteed for over 20 years without discoloration. Recent plants - like the one in figure 6b - can apply these bright PVDF coatings on the thick (2.5-3 mm) metal used to fabricate large panels;
b) The "Multiple" superdurable PCC coatings, where a clear UV resistant top coating is applied onto a polyester powder coat, so raising the expected weather performance to over 20 years;
c) The "Iridium" coatings that apply a refractive product sandwiched between two thin PVDF layers, so causing the reflected light to vary according to day hours and the season.
Figure 7 recapitulates the conventional and new finishes now available for solid aluminium panels in an almost unlimited range of colours.

Technology of solid aluminium cladding

a) Panel Fabrication
Solid aluminium panels can be fabricated and installed much in the same way that has been used for composite sheet. The "cassette" design (fig. 8) is by far the most frequently used. When using solid aluminium, it is not necessary to score the back of the sheet along the bend lines (fig. 3) and the folds can be easily made with a brake press: this will result in radiused bends, which normally are not objected to.
And of course sharp bends can be made on solid sheet too, provided that the back is scored the same way as it is done with composite sheet.

However, there seems to be no point in favour of such practice, considering that a façade cladding is always observed from a distance. In any case, even if sharp bends should be requested, the solid sheet can be scored more lightly than the composites, leaving up to 1.5 mm of sound metal in the bottom of the scored lines, i.e. 3 times the metal remaining in a composite panel. When a good amount of solid metal is left on the bend lines, the panels can be folded according to the 90° pattern which is simpler and stronger than the 45° riveted joints necessarily used for composites.
Solid panels can be easily worked automatically on machines like the one in figure 9. Some of these are able to do all the cutting, punching and scoring and eventually fold the sides delivering panels ready for installation.

b) Cladding installation Once the panels have been fabricated, the rainscreen and ventilated wall design requires that they be attached to a bearing structure attached to the primary wall, frequently with the introduction of insulating panels against the primary wall. An elementary design is shown in figure 10.
Recent proprietary systems, readily available from a dealer's stock, permit quick installation and easy alignment of the panels. Technical handbooks as the one in Bibl. (2) provide detailed instructions about such critical points as junctions to the roofing and windows, curved panelling, etc.


Summing up what said above:
a) Solid aluminium sheet has been coming back as a preferred cladding material since the early 90's, when thick (2-3 mm), flat sheet began to be available and inexpensive high performance coil coatings began to be applied.
b) The advent of solid sheet benefited from the pre-existing technology of composite panels, that had been imposed for many years when sufficiently flat thick aluminium sheet had not been economically available.
c) Actually, the comeback of solid sheet began with coil anodized sheet in the late 80's, but it became a fast growing tendency since when the more versatile and flexible coil painting (either powder or more recently wet PVDF, or still more recently a combination of powder and wet coatings) came into the picture.
d) Acceptance of solid aluminium cladding panels has been rapidly increasing since the designers realized that neither the cut borders of the panels need protection, nor the tray panels need to be watertight: these are consequence of the physico-chemical self-passivating nature of the aluminium metal and its alloys such as the AlMg1 alloy normally used for coil coated thick sheet.
e) Present world consumption of coilcoated solid panel sheet is estimated at 5,000,000 sq m but is expected to grow at fast pace while gradually replacing other materials in the total world market of approximately 40,000,000 sq m.


1) L. Fenzi, F. Sacchi - "Solid aluminium panels for exterior wall cladding" - ICBEST '97 Proceedings pp.261-267 - Centre for Window & Cladding Technology, University of Bath (U.K.)
2) "Mirasystem A: A learner's guide to ventilated wall cladding by the hung cassette design" - SEC, Bergamo (Italy), 1996
Idioma: Inglés     País: Rusia     Año: 2005
Tipo: Artículo
Área: Productos > Productos metálicos para la construcción > Otros productos
Subsectores de aplicación: · Fabricación de productos metálicos, excepto maquinaria y equipo
Entidad: Otefal Russia
Fuente: Aluplanet
Localización en fuente: articles on aluminium
Descriptores: aluminium; prepainted panels; architecture; fachadas ventiladas
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