For a pipe manufacturer, the selling price is set per length or per kilogram, but the cost is driven by material. Each size and pressure class has a standard weight per metre that meets the required wall thickness. Running above it gives material away; running below it risks failing tests and losing trust.
Where the target comes from
The standard weight per metre follows from the outside diameter, the wall thickness required for the pressure class and the material's density. Most plants set the target close to the minimum wall allowed by the standard, plus a safety margin agreed by quality and production.
How to measure it
- Weigh sample lengths at regular intervals each shift and at every size change
- Record the line, size, class, shift and operator with each reading
- Compare each reading with the standard for that size and class
- Check wall thickness and outside diameter at the same time
Why it drifts
Weight drifts upwards for understandable reasons: operators prefer a safety margin, the line is set heavy after a start-up and never brought back, haul-off speed and extruder output are not balanced, or die settings vary around the circumference so the thinnest point sets the whole pipe's weight.
Turning it into savings
Once actual weight per metre is recorded every shift, multiply the over-weight by the metres produced and the material cost. That figure, shown per line and shift, usually gets attention quickly. Set a target band per size, review it daily, and make bringing the line back to target part of the start-up routine.
Do not forget quality
Reducing weight must never take the wall below the minimum. Keep dimension checks and the tests required by the standard linked to each production batch, and hold any batch that fails.
A worked example
Take an illustrative line producing 600 kg of pipe an hour that runs 2% heavier than the standard weight per metre. It uses 12 kg of extra material every hour, nearly 290 kg a day on a three-shift operation, all shipped without payment. Over a month that is several tonnes of material from one line. Your own figures will differ, but the arithmetic is the same.
PVC and HDPE differ
In PVC pipe the compound formulation sets the material's density, so a change in filler level changes the weight per metre for the same dimensions; track both together. In HDPE pipe, coils and straight lengths are sold by length but made by weight, so weight per metre is the link between what you sell and what you consume, and the black masterbatch dosing has to be controlled as well.
Tools that help
- Gravimetric dosing that records material throughput per hour
- Regular weighing of sample lengths at fixed intervals
- Wall thickness checks around the circumference to centre the die
- Line speed and output recorded together, so weight per metre can be calculated continuously
Make it a daily number
The plants that control weight best review it every day, per line and size, next to output and wastage. When over-weight costs a visible amount of money each morning, it gets fixed.
Formezy compares actual weight per metre with the standard for every size and class and records batch tests. See PVC pipe ERP software and the pipe process flows.
