How to read a blow-moulded chair spec sheet: three lines tell you the truth

Put five blow-moulded chair quotes side by side and the spec sheets read almost identically: HDPE, steel frame, a load figure, a colour list. The prices, meanwhile, can spread by thirty percent. That gap is not magic and it is usually not margin — it lives in three lines most buyers skim past. Having written a lot of these sheets ourselves, here is how to read one like someone who has to honour it.

Wall thickness: ask where, not just how much

A blow-moulded seat is a hollow, double-walled part, and the wall number on the sheet is a nominal — the thickness the process aims for on the broad, flat faces. The physics of blow moulding means the material stretches more where the mould is deepest, so corners, edges and deep draws run thinner than the nominal, sometimes by a third. Two chairs both sold as “2.5 mm wall” can carry very different walls at the seat corner, which is exactly where a stacked chair takes its knocks.

So the question that separates a real spec from a brochure is not “what is the wall” but “what is the minimum wall, and where is it.” We measure ours at the corners with an ultrasonic gauge, and if you cut a sample seat in half — which we encourage on first orders — you will see the story of the whole process in that cross-section. A supplier who will not discuss minimum wall is telling you something.

Weight: the one line you can check with a luggage scale

Here is the buyer-friendly truth about blow moulding: the plastic that went into the chair is the weight of the chair. There is no upholstery to hide behind. If two “identical” folding chairs differ by 300 grams, the lighter one has thinner walls, a smaller parison or more foaming agent — there is nowhere else for the missing material to have gone. That makes unit weight the single most verifiable line on the sheet: a €15 luggage scale audits it in your own warehouse, no lab required.

Use it in both directions. When you approve the golden sample, weigh it and write the weight into the order with a sensible band of a few percent. When the container lands, weigh five chairs from different cartons. A production run that comes in consistently light against the approved sample weight has been thinned, and the wall discussion above tells you where from. We put the weight on our sheets for our blow-moulded range precisely because it keeps both sides honest across reorders.

Resin grade: where the price difference actually hides

The sheet says HDPE. The price depends on which HDPE. Blow-moulding grades are higher molecular weight than injection grades — that is what buys the impact toughness and stress-crack resistance the process is known for — and within blow grades, virgin resin with a proper UV and pigment package costs meaningfully more than a recipe stretched with regrind. Regrind is not a scandal; we have written a whole note on where recycled HDPE belongs. But every cycle through the machine shortens the polymer chains a little, and a chair quietly running high regrind is a chair that will crack earlier under impact, in a way no showroom inspection can see.

What to ask for: the resin maker and grade name on the order, the regrind percentage in writing, and the UV package by name if the chairs live outdoors — our UV stabilization note covers that side. A supplier who specifies “HDPE” and nothing else has kept the most expensive decision on the sheet to themselves.

What the sheet will not tell you

Three things never appear on a spec sheet and matter anyway. The pinch line — the seam where the mould closed — should be tidy and fully welded; a ragged pinch is a future split. Flash trimming should leave no sharp edges where hands stack the chairs. And the frame interface — where the steel meets the plastic — should locate firmly without rattling, because that joint, not the moulding, is where a cheap chair announces itself. None of these cost much to do right; all of them are invisible in a PDF. This is the case for cutting up one sample chair from every new supplier, ourselves included — ten minutes with a saw answers questions a month of emails will not, and a supplier who objects to the saw has answered a question too.

The trade-off, stated plainly

A heavier wall and a virgin blow-grade resin make a chair that costs more per unit and slightly more per container, because resin is most of the cost of a plastic chair and weight is resin. For a gentle-duty indoor chair, paying for a premium recipe is money your end user never feels — a tidy injection-moulded stacking chair may serve better anyway. For a rental fleet or an outdoor venue, the heavier, better-resined chair survives years longer, and the spec-sheet lines above are how you make sure you are actually buying it. We build both recipes to BIFMA/EN test methods and third-party testing can be arranged per order — the test is on the finished chair, which is the only place the truth of a spec sheet shows up.

Send us your duty cycle and destination market through the contact form and we will quote with the wall, weight and resin lines filled in rather than left vague — or read how a custom run works on the OEM / ODM page.