Customers order extrusions in metres or pieces. You buy compound in kilos. The figure that connects the two is the weight per metre of the profile, and almost every material question on an extrusion line comes back to it: how much to issue for a job, how much to order, and whether a run is using more than it should.
The formula
kg per metre = cross-sectional area (mm²) × density (g/cm³) ÷ 1,000
It works because one metre of profile with an area of 1 mm² has a volume of 1,000 mm³, which is exactly 1 cm³. So the area in mm² is also the volume per metre in cm³. Multiply by the density to get grams per metre, and divide by 1,000 for kilos.
A worked example
Profile: glazing bead with a cross-sectional area of 60.7 mm²
Material: rigid PVC-U at about 1.4 g/cm³
Weight per metre: 60.7 × 1.4 ÷ 1,000 = 0.085 kg/m
Job: 5,750 m × 0.085 kg/m = 489 kg of material
Split by recipe (80% compound, 18% regrind, 2% masterbatch): 391 kg, 88 kg and 9.8 kg
Finding the area
If you have the profile drawing in CAD, the area property of the section gives you the figure directly. Check which drawing it is, though: the nominal profile, not the die drawing.
Weighing is better. Once the line is running steadily, cut a few accurate one-metre samples, weigh them, and take the average. That figure reflects the real die, the real draw-down and the real wall thicknesses, rather than the nominal ones. Keep weighing samples during the run too: a weight creeping up is often the first sign of wall thickness drifting.
Density: check the data sheet
Rigid PVC-U is typically around 1.4 g/cm³, but filled compounds run heavier, and flexible materials and TPEs vary widely. Always use the figure from your compound supplier’s data sheet for the grade you’re actually running.
Co-extrusions
Work out each layer separately, using that layer’s area and its own density, then add them together. That also gives you the correct split for issuing each material to the job.
Add allowances separately
The formula gives you the theoretical material in the good product. Real jobs also use material for start-up, purging, cut-offs and scrap. Add an allowance on top when issuing material, but keep the theoretical figure too, because the gap between them is your yield:
yield = good metres × kg per metre ÷ material actually used
Tracking yield job by job is one of the quickest ways to see where material is really going.
When the spreadsheet stops being enough
Mannin Systems does all of this as part of the day’s work: booked at the station, checked against the recipe, and traceable from supplier to customer. Open a job sheet with material by lot in the working demo.