3 min read
Polyethylene is bought by weight and used by the piece. The conversion between them is where a quotation quietly stops being comparable, and it goes wrong in two specific places: the number of walls, and the core.
The roll
Ask for it. A 1 kg core on a 15 kg roll is a 7% price rise nobody wrote down.
The specification
Leave empty for continuous tubing or film.
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A sack is a tube and has two walls; a film is a single web. That factor of two is the most common error in this calculation — the same 600 mm at 30 µm weighs 33 g per metre as tubing and 17 g as film.
Want this as a PDF? The answer above is yours either way. The PDF is the working written out — the figures you entered, the result, and the assumptions behind it — for a colleague or a purchase requisition.
One relation, run in either direction
grams per metre = walls × width (cm) × thickness (cm) × 100 × density
Densities: LDPE and LLDPE 0.92 g/cm³, HDPE 0.95, polypropylene 0.905.
Walls: the factor of two
A sack is a tube. The film goes round, so one metre of 600 mm tubing contains two metres of 600 mm film. A stretch film or a sheet is a single web.
| 600 mm at 30 µm | Grams per metre | Metres on a 14.1 kg roll |
|---|---|---|
| Tubing — two walls | 33.1 g | 426 m |
| Film — one wall | 16.6 g | 851 m |
Get that wrong and every figure downstream is out by exactly half, in a direction that always looks plausible. It is the most common error in comparing a sack quotation against a film one, and it is why two suppliers can appear to be a factor of two apart on price when they are quoting the same product.
The core
Rolls are quoted gross. A 0.9 kg core on a 15 kg roll is 6% of what you are weighing, and at €1.85 a kilo that is €1.67 a roll for cardboard.
That is not an argument against cores — a roll needs one. It is an argument for asking what it weighs, because two quotations at the same price per kilo with different cores are not the same price, and the difference appears nowhere on either document.
The calculator flags it whenever the core is 5% or more of the gross weight.
Worked, so you can check it
A 15 kg roll of 600 mm LDPE tubing at 30 µm, 0.9 kg core, made into 900 mm sacks at €1.85 a kilo:
| Film on the roll | 14.10 kg |
| Grams per metre | 33.1 g |
| Metres per roll | 426 m |
| Weight of one sack | 29.8 g |
| Sacks per roll | 473 |
| Cost per sack | €0.0551 |
| Sacks per tonne | 33,548 |
That last line is the one to keep. Sacks are sold per thousand, per case, per roll and per tonne, and the only way to compare four quotations written four ways is to reduce them all to a cost per sack.
What it is for, day to day
- Checking a delivery. Weigh a roll, subtract the core, and see whether the metres are there. A roll 8% light is not a rounding error.
- Comparing a price per tonne against a price per thousand. Both are normal in this trade and neither is comparable to the other without this arithmetic.
- Costing a specification change before you ask for it. Dropping a sack from 40 µm to 35 is a 12.5% saving on polymer, and this tells you what that is per sack before the quotation arrives.
What we quote
Our quotations give the roll weight net of the core, the core weight separately, the metres, the piece count, and the price per thousand pieces as well as per tonne. It is more numbers than a quotation usually carries, and the reason is that any one of them alone can be made to look like a better deal.