Aluminium window profiles are extruded frame, sash, mullion, bead and sill sections, almost always in 6063 or 6060 alloy, that are cut and assembled into finished windows. The right profile combines adequate wall thickness and stiffness for the wind load, a thermal break sized for the energy target, and a durable anodised or powder-coated finish. Metline supplies window extrusions to drawing, with cutting, machining and finishing on request, to fabricators and system companies worldwide.
A window frame is a small structure. It carries the weight of the glass, resists wind pressure and suction without deflecting enough to crack the glazing, keeps rain and air out at the seals, limits heat loss, and has to look the same after twenty or thirty years of sun and weather. Aluminium extrusions handle these demands well because the extrusion process can put material exactly where it is needed: chambers for stiffness, grooves for gaskets, screw ports for corner cleats, and slots for polyamide thermal-break strips, all in a single section.
Compared with timber or uPVC, an aluminium frame is far stiffer for its size. The elastic modulus of aluminium is about 69 GPa, roughly twenty times that of rigid PVC, so aluminium sections can be slimmer while carrying larger panes. That is why aluminium dominates large sliding doors, floor-to-ceiling glazing, commercial storefronts and curtain walls, and why it remains popular in residential windows where narrow sightlines are wanted.
The weakness of bare aluminium is heat flow. Its thermal conductivity is around 200 W/m·K, so a frame without a thermal break conducts heat freely and forms condensation in cold climates. Modern window systems solve this with an insulating barrier between the inner and outer aluminium shells, and much of the engineering in a window profile today goes into that barrier.
A window system is a family of extrusions that fit together. Buyers usually meet the following groups, either as a full profile system or as individual sections made to a fabricator's own die drawings:
Single-shell sections without a thermal barrier. Lower cost and simpler to fabricate, they suit warm climates, interior partitions, garages, utility buildings and shading frames where heat loss is not a design driver.
Inner and outer aluminium shells joined by glass-fibre-reinforced polyamide strips or a poured polyurethane barrier. Standard for heated or air-conditioned buildings and required by most energy codes in cold and temperate regions.
A timber window with a thin aluminium cladding on the weather side, clipped on so the wood can move. The aluminium protects the timber and gives a low-maintenance exterior finish; see our aluminium-clad window page.
The fixed perimeter section anchored to the wall opening. It carries the sash or fixed glass, holds the main weather seals and transfers wind load to the building.
The opening part of the window. Sash profiles are lighter than frames but must stay rigid enough to keep the seals compressed along their full length.
Vertical and horizontal dividers that split large openings. They carry the highest bending loads in a window, so they are deeper, thicker or reinforced, sometimes in 6005A or 6061.
External sills that shed water away from the wall, and internal or sub-sill sections that close the gap to the masonry and drain the frame.
Snap-in sections that hold the glass unit in the frame. Bead geometry is matched to glass thickness, so a system offers several bead sizes for different glazing units.
Cover strips, extension profiles, coupling sections and flashing angles that finish the joint between window and wall, or join two windows together.
Window profiles are made almost entirely from the soft 6xxx extrusion alloys. They push through complex dies at high speed, give a clean surface for anodising and gain their strength from a short ageing treatment after extrusion. The table summarises where each grade fits.
| Alloy | Where it is used | Strengths | Watch-outs |
|---|---|---|---|
| 6063 | The global default for frames, sashes, beads and trims | Excellent extrudability, fine surface, very good anodising response, good corrosion resistance | Moderate strength; deep mullions may need thicker walls or reinforcement |
| 6060 | Widely used in Europe for window and door systems | Extrudes even faster than 6063, consistent anodised appearance | Slightly lower strength than 6063 in the same temper |
| 6005A | Heavy mullions, transoms, large sliding systems, structural reinforcement | Higher strength than 6063 while still extruding well into hollow sections | Surface is less suited to bright decorative anodising |
| 6061 | Hardware parts, brackets, reinforcing bars and machined components | High strength, good machinability | Harder to extrude into thin, complex window sections; seldom used for main profiles |
| 6463 | Bright decorative trims | Takes a bright, chemically polished finish | Specialist grade; limited to visible trim |
Older literature sometimes calls 6061 the most popular window alloy. In practice almost every residential and commercial window system is extruded from 6063 or 6060; 6061 is kept for parts that are machined or heavily loaded. If a supplier proposes 6061 for thin frame sections, ask why.
Typical composition limits for the two main grades, from EN 573-3 and the Aluminum Association registrations:
| Element (wt %) | 6063 | 6060 |
|---|---|---|
| Silicon (Si) | 0.20 – 0.60 | 0.30 – 0.60 |
| Magnesium (Mg) | 0.45 – 0.90 | 0.35 – 0.60 |
| Iron (Fe) | ≤ 0.35 | 0.10 – 0.30 |
| Copper (Cu) | ≤ 0.10 | ≤ 0.10 |
| Manganese (Mn) | ≤ 0.10 | ≤ 0.10 |
| Chromium (Cr) | ≤ 0.10 | ≤ 0.05 |
| Zinc (Zn) | ≤ 0.10 | ≤ 0.15 |
| Titanium (Ti) | ≤ 0.10 | ≤ 0.10 |
| Aluminium | Remainder | Remainder |
Minimum mechanical properties for extruded profiles to EN 755-2 (wall thickness up to 10 mm for 6063; values fall slightly for thicker walls):
| Alloy and temper | Tensile strength Rm min | 0.2 % proof Rp0.2 min | Typical use |
|---|---|---|---|
| 6063-T5 | 175 MPa | 130 MPa | Most frame, sash and bead profiles; cooled at the press and aged |
| 6063-T6 | 215 MPa | 170 MPa | Profiles needing more strength; solution treated and aged |
| 6063-T66 | 245 MPa | 200 MPa | Controlled-chemistry high-strength version for larger spans |
| 6060-T66 (≤ 3 mm wall) | 215 MPa | 160 MPa | Common European window temper |
| 6005A-T6 (≤ 5 mm wall) | 270 MPa | 225 MPa | Heavy mullions and structural members |
Values are specified minimums summarised from EN 755-2; always design to the figures of the edition named in your contract. The US equivalent for 6063 extrusions is ASTM B221.
Wall thickness is the question buyers ask most, and the honest answer is that thickness alone does not make a good window. What matters is the stiffness of the whole section (its moment of inertia) against the wind load on the opening, plus enough metal in the right places for screws, cleats and hardware to hold.
| Profile | Typical wall thickness | Why |
|---|---|---|
| Residential window frames and sashes | 1.4 – 2.0 mm | Enough for screw ports and hardware fixings at moderate wind loads |
| Commercial windows and large vents | 1.8 – 2.5 mm | Higher wind pressures, heavier glass units and more frequent operation |
| Sliding door and lift-slide frames | 2.0 – 3.0 mm | Track loads from heavy sashes and wear at rollers |
| Mullions and transoms | 2.0 – 3.5 mm or reinforced | Highest bending moments; depth is often more effective than thickness |
| Glazing beads and trims | 1.0 – 1.4 mm | Non-structural; must snap in without permanent deformation |
Many national building codes set a minimum wall thickness for the main load-bearing members of windows and doors, and these minimums differ by country and change over time. Check the code that applies to your project and put the figure on the purchase order, together with the thickness tolerance (EN 12020-2 for precision 6060/6063 profiles, or ASTM B221 / ANSI H35.2 for US orders). A profile quoted as "2.0 mm" that arrives at the lower tolerance limit may not meet the code.
For tall or wide openings, deflection usually governs before stress. A common limit is span/175 or span/200 for mullions under design wind load, and some glass manufacturers set an absolute limit, such as 15 mm, to protect the edge seal of insulating glass. Increasing depth is the most efficient way to gain stiffness: doubling the depth of a box section raises its moment of inertia roughly eight times, while doubling the wall only doubles it. Where depth is fixed, steel or high-strength aluminium inserts are slid into the mullion chamber.
A thermal break interrupts the aluminium path between the inside and outside of the frame. Two technologies dominate:
Glass-fibre-reinforced polyamide strips, typically PA66 with 25 % glass fibre (PA66 GF25), are rolled into knurled grooves in the two aluminium shells. The polyamide conducts around 0.3 W/m·K, several hundred times less than aluminium, and its thermal expansion is close to aluminium's. Strip depth ranges from about 14 mm to more than 40 mm; deeper strips and foam inserts give lower U-values. The composite is tested for shear and transverse strength under EN 14024.
A two-part polyurethane is poured into a channel in the profile, cures, and the aluminium bridge beneath it is then cut away. Common in North American systems. It is economical for high volumes but gives a narrower barrier than deep polyamide strips, so it is used more for moderate thermal targets.
Cheaper unreinforced PVC barriers exist, but PVC has lower heat resistance and a much higher expansion than aluminium. It softens at temperatures reached by powder curing and dark frames in sun, so glass-reinforced polyamide is the safer specification.
Thermal performance is expressed as a U-value in W/m²·K: the lower the number, the less heat passes through. The frame value (Uf) is calculated to EN ISO 10077-2; the whole-window value (Uw) combines frame, glass (Ug) and the linear loss at the glass edge (ψ) under EN ISO 10077-1. In North America the equivalent ratings are calculated and certified through NFRC procedures.
| Frame construction | Indicative Uf (W/m²·K) | Typical market |
|---|---|---|
| Non-insulated aluminium | 5.5 – 7.0 | Warm climates, interior and utility use |
| Basic thermal break (narrow strip or PU pour) | 3.0 – 4.0 | Mild climates, older energy codes |
| Standard polyamide thermal break | 2.0 – 3.0 | Most residential and commercial projects in temperate regions |
| High-insulation systems with deep strips and foam | 1.0 – 1.6 | Low-energy and Passivhaus-level buildings |
Indicative ranges only. The certified value depends on system depth, strip geometry, gaskets and glass rebate, and is published by the system designer for each section.
Glass covers most of the window area, so glazing choice matters as much as the frame. Low-emissivity (low-E) glass carries a microscopically thin metal-oxide coating that reflects long-wave heat back into the room while letting daylight through. A double-glazed unit with one low-E surface and argon fill reaches a Ug of about 1.0 – 1.1 W/m²·K, compared with about 2.8 for plain double glazing; triple glazing with two low-E coatings reaches 0.5 – 0.7. Solar-control low-E coatings also reduce cooling load in hot climates. Pair the glass with a "warm-edge" spacer to cut the edge loss that otherwise concentrates condensation at the bottom of the pane.
The finish sets the appearance and most of the service life of a window. Specify the coating system by standard, not just by colour, because two coatings of the same RAL shade can perform very differently outdoors.
| Finish | What it is | Typical specification | Best for |
|---|---|---|---|
| Anodising | Electrochemical growth of a hard oxide layer, clear or coloured (bronze, black, champagne) | EN ISO 7599 / Qualanod; class 20 (≥ 20 µm) for exterior, class 25 (≥ 25 µm) in coastal or industrial air | Metallic look, very high abrasion resistance, no peeling |
| Polyester powder coating | Electrostatically applied powder cured to a continuous film | Qualicoat Class 1 (standard), Class 2 (high durability) or Class 3 (super durable); film usually 60 – 80 µm; AAMA 2603/2604 in North America | Unlimited RAL colours, textures and gloss levels |
| PVDF (fluorocarbon) coating | Liquid-applied fluoropolymer, usually two or three coats | AAMA 2605; 70 % PVDF resin systems | High-rise and landmark facades needing long colour and gloss retention |
| Wood-grain transfer | A printed film sublimated into a powder base coat | Over a Qualicoat-approved base powder | Timber appearance without timber maintenance |
| Electrophoretic (e-coat) | Clear or tinted acrylic deposited over an anodic film | Thin, very uniform film | Added protection and gloss over anodising |
Popular powder colours for windows include RAL 9016 traffic white, RAL 9010 pure white, RAL 7016 anthracite grey, RAL 9005 jet black (often in matt or fine texture), RAL 7035 light grey, RAL 1015 light ivory and RAL 8017 chocolate brown. Dark colours absorb more solar heat, so in hot climates check that the thermal-break material and gaskets are rated for the frame temperatures reached.
Thermal-break profiles can be finished in two colours, for example anthracite outside and white inside, by coating the two shells separately before the polyamide strips are rolled in. Pretreatment matters as much as the topcoat: chromate-free conversion coatings are now normal, and for coastal sites a "Seaside" class pretreatment with deeper etching gives better resistance to filiform corrosion under the paint. Our surface treatment overview compares these processes in more detail.
Quality checks worth asking for on every order: a mill certificate (EN 10204 3.1) showing chemistry and mechanical results; dimensional reports for critical features such as gasket grooves, thermal-break slots and screw ports; coating thickness and adhesion results to the named standard; and, for thermal-break profiles, shear strength results to EN 14024. Webster hardness is a quick production check for 6063 temper; a reading in the low teens for T5 is a common acceptance level, but agree the figure with your supplier.
Profiles are supplied in mill lengths (often 5.8 or 6.0 m) or cut to size. Frames and sashes are mitre-cut at 45° on double-head saws; mullions are notched and end-milled to fit.
Drainage slots, hardware holes, lock pockets and handle bores are machined on copy routers or CNC profile centres. Machining before finishing avoids damaging the coating.
High-volume holes and slots are punched with dedicated tools, faster and cheaper than milling for repeated features.
Most window corners are joined with aluminium corner cleats that are crimped or pinned into the chambers, with corner glue for a watertight joint. Welding is rare for coated profiles because heat damages the finish.
Arched and curved windows are roll-bent from straight profiles. Bending is easier in softer tempers and before coating, or uses profiles designed for bending.
Gaskets, glazing beads, hinges, multipoint locks and handles are fitted last. Hardware from established suppliers is designed around specific profile grooves, so confirm compatibility early.
Metline can supply profiles cut to length and machined to drawing, with matching accessories, so the fabricator receives parts ready for assembly. Because tolerances on gasket and hardware grooves govern how well a window seals, send the hardware and gasket references with your enquiry.
Two groups of standards apply: those for the aluminium extrusion itself, and those for the finished window. The profile supplier works to the first group; the window system designer and fabricator test the finished product to the second.
| Area | Europe / international | North America and others |
|---|---|---|
| Alloy chemistry | EN 573-3 | Aluminum Association registrations, ASTM B221 |
| Profile mechanical properties | EN 755-2 | ASTM B221 |
| Dimensional tolerances | EN 12020-2 (6060/6063 precision profiles), EN 755-9 | ANSI H35.2 |
| Thermal-break composites | EN 14024 | AAMA 505, AAMA TIR-A8 |
| Anodising | EN ISO 7599, Qualanod | AAMA 611 |
| Organic coatings | EN 12206-1, Qualicoat | AAMA 2603, 2604, 2605 |
| Finished window product | EN 14351-1 with EN 12207 (air), EN 12208 (water), EN 12210 (wind) | AAMA/WDMA/CSA 101/I.S.2/A440 (NAFS); AS 2047 in Australia |
| Thermal transmittance | EN ISO 10077-1 and -2 | NFRC 100 |
Under EN 14351-1 a window is classified, for example, as air permeability class 4, watertightness class 9A and wind resistance class C4. These classes come from tests on complete windows, so a profile alone cannot "meet" them; the profile must be part of a tested system. When you buy profiles to an existing system design, ask for the test reports that relate to the exact sections being supplied.
| Factor | Aluminium (thermal break) | uPVC | Timber |
|---|---|---|---|
| Stiffness and strength | High; large panes and slim frames | Low; needs steel reinforcement in larger frames | Moderate; frames are bulkier |
| Sightlines | Narrowest | Wide | Wide |
| Thermal performance | Good to very good with modern breaks | Very good, inherently low conductivity | Very good |
| Thermal expansion | About 23 µm/m·K | About 70 – 80 µm/m·K | Low along the grain, moisture movement across it |
| Fire behaviour | Non-combustible metal | Combustible; softens with heat | Combustible |
| Maintenance | Washing only | Washing; may discolour over long periods | Periodic repainting or oiling |
| Service life | Long; finishes covered by long coating warranties | Medium | Long if maintained |
| Recyclability | Fully recyclable with high scrap value | Recyclable but less often recovered | Biodegradable, reusable |
| Initial cost | Medium to high | Lowest | Medium to high |
uPVC is the budget choice for standard residential windows. Aluminium wins where size, slenderness, durability and appearance matter, and in commercial work where fire performance and structural loads govern. Timber is chosen for heritage or warmth of appearance; aluminium-clad timber combines a timber interior with an aluminium exterior.
Aluminium windows perform well near the sea if the finish and detailing are right. Salt deposits hold moisture on the surface, so the coating is the first line of defence and the aluminium alloy itself is the second.
6063 is the standard choice worldwide, with 6060 widely used in Europe. Both extrude into thin, complex hollow sections with a fine surface that anodises and powder-coats well. 6005A is used for heavily loaded mullions and large sliding systems that need more strength.
Residential window frames and sashes are typically 1.4 – 2.0 mm, commercial windows 1.8 – 2.5 mm and sliding door frames 2.0 – 3.0 mm. The governing figure is the minimum set by your local building code and the stiffness needed for the design wind load, so state the minimum and the tolerance on the order.
T5 profiles are cooled at the extrusion press and then aged; T6 profiles are fully solution heat-treated before ageing. T6 is about 20 – 25 % stronger by specified minimum (215 MPa versus 175 MPa tensile under EN 755-2). Most window sections are T5; T6 or T66 is used for larger spans.
A thermal break is an insulating barrier, usually glass-reinforced polyamide strips, that separates the inner and outer aluminium shells. It cuts heat flow through the frame by roughly half or more and prevents condensation on the inside. You need one for any heated or air-conditioned building in temperate or cold climates, and most energy codes require it.
Yes, provided the finish is specified for marine air: anodising of at least 25 µm or a powder coating over a Seaside-class pretreatment, stainless steel fixings, good drainage and regular rinsing with fresh water.
Yes. Send the die drawing or a sample section with the alloy, temper, finish and quantity. New dies are made to the drawing, and sample lengths are approved before production.
Yes. Metline supplies window extrusions worldwide from Mumbai, packed in export crates with interleaving or film protection and mill certificates, on EXW, FOB, CIF or DAP terms.
Send your profile drawings, alloy, finish and quantities. Our team will review the specification and reply with a delivered quotation.
