In the world of construction, we're accustomed to thinking in right angles. Square or rectangular windows, straight lines, pure geometry. But what happens when architecture demands curves?
Whether you're restoring a neoclassical building in the heart of Athens, constructing a traditional island home, or your architect has designed a striking round porthole for the stairwell, right angles simply won't do. You need Arched, Circular or Asymmetrical windows.
But aluminium and uPVC leave the extrusion plant as rigid, straight bars 6 metres long. How can a hard, thermally broken metal profile or a multi-chamber plastic be bent into a perfect semicircle without cracking, crushing or losing its insulation? Let's enter the inner sanctum of specialist fabrication and discover the art of profile bending.
Not every aluminium fabricator can produce such windows. It requires specialised, expensive machinery (bending machines) and extensive experience. The method differs radically depending on the material.
Aluminium is bent "cold" through mechanical pressure. The straight bar passes through a machine with three heavy-duty rollers. The rollers press the profile millimetre by millimetre, passing it back and forth dozens of times, gradually increasing the curvature until the desired radius is achieved.
Modern aluminium isn't solid. It's hollow with a plastic (polyamide) thermal break in the middle. If you simply press it, the chambers collapse ("crush") and the polyamide snaps. The solution: before bending, the chambers are filled with special sand, flexible nylon inserts or resins. These keep the interior "full" during bending. Once shaped, the filling is removed!
Synthetic plastic can't be bent cold - it would simply snap. Here the process resembles... pastry-making.
The uPVC profile is placed inside a special hot-air oven (or a glycerine bath) at 130–150°C. At this temperature, the plastic softens and becomes pliable - like a piece of cooked pasta.
Technicians quickly remove the hot profile and place it onto a pre-shaped wooden or metal mould with the exact required curve. They clamp it to the mould and let it cool. When it freezes, it retains the curved shape permanently.
Windows with special shapes can be grouped into three main categories.
The classic window with a rectangular base topped by a semicircular (or oval) arch. Often, for cost and sealing reasons, the lower part is openable while the upper (curved) portion remains fixed.
Fully round windows. They lend a unique nautical or contemporary character. They can be manufactured as fixed units or with a centre-pivot mechanism for ventilation.
Here we deal not with curves but with "unusual" angles. Widely used in loft conversions (to follow the roof pitch) or beneath staircases. The difficulty isn't bending but rather cutting and welding at acute or obtuse angles that deviate from the standard 90° or 45°.
If you've received a quote for an arched window, you may have been surprised to see it costs up to 2 to 3 times more than a rectangular window of the same dimensions. The price is entirely justified.
Each arched window is unique. A bespoke mould (jig) must be fabricated to the exact measurements of your particular opening.
During bending, the profile ends are destroyed and discarded. To produce a 2-metre arch, you may need to "sacrifice" 4 metres of material.
Cutting the double-glazed unit to a perfect curve requires CNC robotic cutting machines. In addition, tilt-and-turn hardware must use special "articulated" fittings to follow the curved profile.
Arched and circular windows are not mass-produced commodities. They are works of technical excellence, tailored to your architecture. Whether restoring a listed heritage building or adding a romantic, organic touch to your new home, proper profile bending guarantees that you won't sacrifice thermal insulation or weatherproofing on the altar of design.
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