Extrusion Math

How to Calculate Extrusion Output, Line Speed, and Resin Usage for Pipe, Film, and Profile

The core extrusion formulas worked end to end: converting screw RPM to kg/h, matching haul-off to output, resin mass per foot, residence time, and die swell allowance.

Start with volumetric output, because everything downstream is metered against it. A single-screw extruder pumps roughly Q = k x N x rho, where N is screw RPM and k is the specific output constant in cm3/rev. A 90 mm line running 24:1 L/D typically delivers 8 to 12 kg/h per RPM-equivalent depending on channel depth. For HDPE at 0.95 g/cm3, a k of 42 cm3/rev at 70 RPM gives 42 x 70 x 0.95 / 1000 = 2.79 kg/min, or 167 kg/h. The Extrusion Output Rate and Screw RPM Throughput calculators do this conversion and back-solve RPM from a target mass rate.

Line speed is the bridge between mass output and part geometry. For pipe, mass balance says output (kg/h) = cross-section area (m2) x wall density (kg/m3) x line speed (m/h). A 110 mm SDR 11 pipe has a 10 mm wall, so annular area is pi/4 x (0.110^2 - 0.090^2) = 0.00314 m2. At 950 kg/m3 and 167 kg/h, line speed = 167 / (0.00314 x 950) = 56 m/h, about 0.93 m/min. Line Speed By Wall Thickness inverts this so you set wall and read the speed the melt pump must sustain.

Resin mass per unit length is the estimating and metering backbone. Per meter of that same pipe: 0.00314 m2 x 950 kg/m3 = 2.98 kg/m, or 0.91 kg/ft. For blown film it is simpler: mass per m2 = thickness (m) x density (kg/m3), so a 50 micron LDPE film is 0.00005 x 920 = 0.046 kg/m2. Resin Usage Per Foot returns these figures and lets you add a color or additive letdown ratio, for example 2% masterbatch, so the metered feed matches the natural resin draw.

Melt residence time tells you whether heat-sensitive resins will degrade. Approximate it as t = filled barrel volume x fill fraction / volumetric throughput. A 90 mm barrel at 24 L/D holds roughly pi/4 x 9^2 x (24 x 9) = 137,000 cm3 of bore; at maybe 45% fill and 2,940 cm3/min output, t = 0.45 x 137,000 / 2,940 = 21 minutes at low RPM, dropping near 8 minutes at high RPM. Melt Residence Time flags when PVC or PET sits long enough to yellow or hydrolyze, generally a concern past 6 to 10 minutes for rigid PVC.

Die swell must be built into tooling and take-off before you cut a single die. Extrudate diameter expands as it exits, commonly 5% to 40% depending on shear rate and melt elasticity, with LDPE and rigid PVC on the high end. If a profile must finish 20.0 mm wide and your resin swells 15%, the die land is cut to 20.0 / 1.15 = 17.4 mm and haul-off is tuned to draw the balance. Die Swell Allowance converts a target dimension and a measured swell ratio into a die opening, so you are not chasing dimensions by trial cuts.

Cooling and haul-off close the loop between thermal reality and mechanical speed. Vacuum tank dwell must hold the part until the wall centerline drops below the resin's freeze point; a 10 mm PE wall needs roughly 8 to 12 seconds per mm of wall in the sizing and spray tanks, so 80 to 120 seconds of submerged length at line speed sets tank length. At 0.93 m/min, 100 seconds means 1.55 m of effective cooling. Cooling Tank Dwell sizes this, and Haul-Off Speed matches caterpillar RPM to line speed so you neither stretch nor buckle the profile.

Tie the numbers together with a mass-balance check before every run. Screw output, resin-per-foot, and line speed are three views of one equation, so compute any two and the third must agree within a few percent. If Screw RPM Throughput says 167 kg/h but Line Speed By Wall Thickness only supports 140 kg/h at your target wall, you will over-thicken the wall or gain surge. Resolve it by lowering RPM, raising haul-off, or accepting a heavier wall, then re-run Melt Residence Time to confirm you did not push residence past the resin's thermal limit.

Published 2026-07-01.