Why 6063-T6 is the benchmark alloy for window profiles
6063 is a heat-treatable Al-Mg-Si alloy. In T6 temper it delivers tensile strength ≥205 MPa, proof stress ≥170 MPa and elongation ≥8%. Its real advantage is extrudability: multi-chamber, thin-wall and complex cross-sections (still within EN 12020 and GB/T 8478 tolerances), which makes isotherm alignment and pressure-equalised drainage possible, plus excellent response to anodising and powder coating.
Specify main load-bearing wall thickness ≥1.8 mm (≥2.0 mm for doors); this is the first line of defence for oversized sashes and high-rise wind loads. A common buying mistake is checking the alloy designation while ignoring wall thickness and chamber design.
Why the thermal break decides everything
- Thermal conductivity ≈0.30 W/(m·K) — roughly 1/700 that of aluminium; the physical basis of the thermal break.
- Linear expansion ≈2.5×10⁻⁵ /K — close to aluminium (≈2.3×10⁻⁵ /K), so the composite does not crack or de-bond through thermal cycling. This is the fundamental reason it outperforms PVC.
- Tensile strength ≥80 MPa; heat deflection temperature ≥240 ℃ — survives knurling, roll-crimping and powder-coat curing at 200 ℃.
- Width and geometry — 14.8–64 mm I, C and multi-chamber shapes; with isotherm alignment, each 10 mm of effective break width improves frame Uf by roughly 0.1–0.2 W/(m²·K).
Why glass is the biggest single variable
Glazing covers 70–85% of the window area, so centre-of-glass Ug dominates the whole-window result. A 12–16 mm cavity filled with argon (initial fill ≥90%, annual leakage <1%) plus a Low-E coating gives Ug of 1.1–1.4 W/(m²·K) with double silver; triple glazing with two Low-E coatings reaches 0.6–0.8 W/(m²·K). Switching to a warm-edge spacer (TPS / composite foam) cuts the linear Psi-value by about 0.04–0.07 W/(m·K) and lowers whole-window U by a further 0.1–0.2 while reducing edge condensation — the detail premium clients actually notice.
From material to quotation logic
Translate materials into three sentences the buyer understands: strength — how large can I build and what wind load will it take; thermal — will it pass inspection and how much heating cost does it save; durability — who pays twenty years from now. Itemise quotes across five axes: profile wall thickness, thermal break width, glazing build-up, finishing class and hardware brand. Visible specification difference is the only way out of price war.
FAQ
Q: Why must a thermal break use PA66 GF25 instead of PVC?
PVC has a very different coefficient of expansion from aluminium and a low heat deflection temperature (around 70–80 ℃), so it shrinks and cracks under thermal cycling and powder-coat curing temperatures, collapsing the break and causing condensation and leakage. PA66 GF25 matches aluminium’s expansion behaviour, withstands ≥240 ℃ and delivers ≥80 MPa tensile strength, giving a service life matching the profile. It is the standard material under EN 14024 and GB/T 23615.
Q: Which surface should the Low-E coating be on, and when do we use double or triple silver?
In heating-dominated climates the coating is normally placed on surface #3 (counting from outside) to admit solar gain, giving higher SHGC around 0.45–0.55. In cooling-dominated or high-irradiation climates it goes to surface #2 to reject solar radiation, SHGC 0.25–0.35. Double silver balances low U-value with good light transmission and is the best value in most climates; triple silver suits façade-grade projects needing both aggressive solar control and high daylight.
Q: Which finish should be specified for coastal or poolside projects?
For high salt or humidity, specify PVDF fluoropolymer (resin content ≥70%, AAMA 2605) or AA20–25 anodising, with stainless steel fixings. For normal urban environments Qualicoat-approved polyester powder at 60–80 μm is sufficient; check the 1 000 h acetic acid salt spray (AASS) result and under-creep corrosion width.
Q: Does argon filling really work, and does it leak out?
Argon has lower thermal conductivity than air and improves Ug by roughly 0.2–0.3 W/(m²·K) while suppressing cavity convection. Quality units use dual sealing (butyl plus polysulphide or silicone structural sealant), start at ≥90% argon fill and lose less than 1% per year, retaining most of the gas after 25 years — a long-term performance investment rather than a marketing claim.