A window fabricator qualifying a new profile supplier is really asking one question: **will run number forty look like run number one?** Everything else — price, lead time, even certification — is secondary to that, because a lineal that drifts dimensionally between runs breaks corner joints, gasket compression, and hardware alignment across your whole production schedule.
The nine checks below turn that question into evidence. They are ordered the way a real qualification runs: paper first, factory second, product third, commerce last. We publish them knowing our own customers will use them on us — that is rather the point.
1. Legal entity and factory verification
Confirm the company you are contracting is the company that owns the factory. In China's export sector, trading companies routinely present factory photos that are not theirs. Ask for the business license (统一社会信用代码 — the unified social credit code is publicly checkable), the factory address, and whether production is in-house or subcontracted. Then verify physically: a third-party audit through SGS or Bureau Veritas costs a few hundred dollars and settles the question. A manufacturer will host the audit readily; an intermediary will negotiate about it.
2. Production capability: lines and dies
Pultrusion capacity is countable. Ask how many pultrusion lines the supplier operates, how many window-profile dies it holds, and which frame-depth series exist as standing tooling versus new-die projects. This tells you two things: whether your order competes for line time, and whether your profiles need new tooling (with its 3–6 week lead and amortization cost) or can run on existing dies.
3. Tolerance class, in writing
"Good tolerances" is not a specification. The reference standard for pultruded profile dimensional tolerance is **ASTM D3917**; the drawing should state the tolerance class and the critical dimensions it applies to — typically ±0.25 mm class on chamber-critical dimensions for window lineals. If the supplier will not put a D3917 class on the drawing, the tolerances are aspirational.
4. Material system verification
The profile's mechanical and thermal behavior lives in its material system: resin type (polyester, vinyl ester, or polyurethane), glass content, and fiber architecture. Ask for the standard datasheet values — and for coupon test reports (tensile per ASTM D638, flexural per D790) from actual production, not brochure numbers. For window profiles specifically, ask which resin runs on which series: a supplier running [polyurethane on its performance tier](/technology/polyurethane-pultrusion-windows) and polyester on the economy tier should say so plainly.
5. The certification stack
Certification tells you what has been independently tested. For window profiles and systems the stack has three levels: profile thermal characterization (EN ISO 10077-2 simulation data), unit-level type testing (EN 14351-1 for CE; NAFS — AAMA/WDMA/CSA 101/I.S.2/A440 for North America), and component certification (the PHI component certificate for passive-house work — F1's is 2491wi03, verifiable in the Passive House Institute's public database). Match the level to what you buy: profile buyers need the simulation data and can carry unit testing themselves; finished-unit buyers need the whole stack.
6. First article, paid, with a dimensional report
The single best qualification instrument is a **paid first article**: a short production run measured against the die drawing, with the dimensional report delivered before production release. Paying for it matters — it makes the exercise a contractual deliverable rather than a favor, and it entitles you to reject on evidence. Any supplier confident in its die and process will agree readily.
7. Run-to-run consistency evidence
Consistency is where fabricators have been burned, and it is evidenced, not promised. Three artifacts to require: batch **mill test certificates** for every production run; repeat dimensional reports on runs separated in time (not consecutive); and gasket-fit continuity — if the gasket channel is co-pultruded, seal fit cannot drift the way secondary-glued gaskets can. This is exactly the anxiety behind the search queries we see comparing established suppliers on "consistency" — the answer is the same for any supplier, Western or Chinese: ask for the run-separated data.
8. Finish qualification
Window lineals are architectural surfaces. The finish standard to name is **AAMA 2604 or 2605** (10-year exposure rating); the evidence is the coater's qualification report and, for dark colors, the heat-buildup discussion — dark fiberglass does not carry the warping risk dark uPVC does, but the coating system still needs the rating. Ask for finished samples in your actual RAL color, not the showroom color.
9. The commercial and logistics test
Finally, the commerce has to work as smoothly as the product: correct HS classification (fiberglass profiles under 3925.20 / 7019 — see our [DDP, tariffs and HS code guide](/resources/ddp-tariff-hs-code-guide)), genuine DDP capability with duty itemized in the quote, spare-parts and gasket supply policy, and a staged-order structure — first article, pilot order, production volumes — that lets you scale commitment with evidence.
The meta-signal
There is a tenth check hiding inside the nine: **how the supplier reacts to the checklist itself**. A manufacturer with a controlled process treats these requests as routine paperwork. Evasion on any point — the audit, the D3917 class, the paid first article, the run-separated reports — is data. In our experience the checklist does not just qualify suppliers; it sorts them faster than any factory tour.
