There is a version of heat deflection in the quotation and a different one in the goods. The gap between them is a tolerance, and it is where the money is.
What heat deflection actually controls in PVC slats
Slats in a sun-facing window reach 60 C+ behind glass.
That makes it a design decision rather than a purchasing one, which is why changing it after the product is fixed costs more than changing the material.
For PVC blinds and slats, the working range is >= 65 C. Inside that band the choice is commercial; outside it, the product changes category and so does the price.
What >= 65 C does not tell you
| Position | Value | Effect on PVC slats | Where it shows up |
|---|---|---|---|
| Floor | >= 65 C | Lowest cost, least margin for process variation | Yellowing within a season |
| Working middle | >= 65 C | Predictable performance at a normal price | Nothing to negotiate, nothing to explain |
| Ceiling | >= 65 C | Best performance, highest cost | Cost the end product may not recover |
Specifying the ceiling is only justified when the end product carries a warranty that would be expensive to honour. Otherwise it is cost without a customer-visible benefit.
The figure that decides the outcome is not heat deflection itself but the tolerance around it. A supplier who quotes >= 65 C without a tolerance has not quoted a specification, and the first lot will define the tolerance for you.
How to measure heat deflection
The reference method here is Fire performance, run to EN 13501-1, with an acceptance of B-s1,d0 where specified. The method is the easy part; sampling and conditioning are where reports diverge.
Measure on the goods as delivered, not on the greige or the pre-finished material. The figure that matters is the one in the product your customer receives.
Record the measured value rather than pass or fail. A marginal pass on heat deflection tells you the process is running at the edge, which is the information you need before the next lot rather than after it.
Heat deflection does not stand alone
Heat deflection moves with colour (white / woodgrain / solid colours) and with fire performance (available to EN 13501 B-s1). Tightening one usually costs money in another, and a supplier who accepts every tolerance without comment has either not understood the interaction or is planning to substitute.
When you change heat deflection, re-check the two neighbours before you approve the sample. Most of the samples we re-make are caused by a change in one figure that was not followed through.
The clause to write for heat deflection
The line below is the version we can quote against without inventing anything. Heat deflection appears with a tolerance and a test, not as a standalone number.
PVC venetian blind, 50 mm slat, 2.0 mm wall, UV stabilised, QUV 300 h dE <= 3, HDT >= 65 C, co-extruded woodgrain, cordless to EN 13120
Write it once and reuse it. The buyers who avoid claims are not stricter than everyone else; they send the same well-written clause every time.
When heat deflection goes wrong
Yellowing. White slats turn cream within a year in a sunny window.
The cause is usually insufficient or wrong uv stabiliser; often tio2 grade rather than a true uv package.
The test that catches it: QUV 300 h, measure dE and yellowing index.
What to specify instead: Specify the stabiliser system and require the QUV report; do not accept ‘UV treated’.
None of this is visible in a quotation. It becomes visible in a retained sample and a lot record, which is why the paperwork matters more than the price on this parameter.
Four questions about heat deflection, before you order
- What tolerance do you hold on heat deflection, and what did your last three lots measure against it?
- Which method do you use for fire performance, and which laboratory signs the report?
- When heat deflection falls outside >= 65 C, do you rework, downgrade, or ship and hope?
- Which of colour or fire performance would you move first to hit a lower price?
The answers are more useful than a certificate, because they show whether the process is controlled or merely inspected. Full ranges for PVC slats are on the product page, and the measurement regime is described on the quality control page.
Questions buyers ask before ordering
Will white PVC yellow in a sunny window?
Not if it is properly stabilised. Ask for a QUV 300 h report with dE <= 3. Without that report, assume it will yellow.
Is PVC allowed in the EU?
Yes for window coverings, but the compound must comply with REACH and heavy-metal limits, and some buyers restrict PVC on brand policy grounds. We supply both and will tell you which is more common in your market.
Failure modes worth a clause in the purchase order
Claims rarely come from a batch that was obviously wrong. They come from a uv stabiliser figure that sat inside the quoted band and outside what the product needed.
| Failure | What triggers it | Test to demand on the production lot |
|---|---|---|
| Heat warping | Heat deflection temperature below the in-window temperature. | Yellowing — QUV 300 h; acceptance dE <= 3 |
| Cold brittleness | High filler content (calcium carbonate) to cut cost. | Heat deflection — ISO 75 / 60 C load test; acceptance no visible warp |
The other two, with acceptance numbers attached, are in the full failure analysis for PVC slats. A supplier who cannot name the mode he sees most often is describing a catalogue, not a process.
Seven lines that turn a quotation into a specification
Most post-delivery arguments are not about quality. They are about a document that never said which figure would be measured, on which instrument, at whose cost.
- Yellowing resistance (QUV 300-500 h, dE <= 3): state where in the band this order sits. The middle of the band is not what a tolerance is for.
- Heat deflection: the figure you will accept inside the >= 65 C band, the tolerance on it, and the method used to measure it.
- Colour (white / woodgrain / solid colours): name the number, not the word. If the answer only repeats the band, ask which point inside it is being offered for this order.
- Fire performance: available to EN 13501 B-s1 is the market band. Fix the figure you will test at goods-in, and the instrument you will test it with.
- Price validity: the period during which the quoted figure holds, and what happens to it when the raw-material index moves.
- Remedy: rework, replacement or credit, and the point at which each applies.
- Sample retention: one production sample per lot stays with the mill for twelve months. Claims are settled against it, not against memory.
Need heat deflection specified and tested against your end use?
Send a window schedule or a single set size and we will return a quotation with the loading plan, the safety standard that applies to your market and the lead time for the colour you need. Catalogue and current price list on request.
Related reading
Catalogue and current price list · Neutral packing and private label · Consolidation, documents and lead times
