Pultrusion Math

How to Calculate Pultrusion Line Metrics: Pull Speed, Fiber Volume, and Cure Dwell

The five core pultrusion calculations every process engineer runs, worked in real units from roving count to cure dwell.

Fiber volume fraction (Vf) is the anchor calculation. Compute the fiber cross-sectional area, then divide by the profile area. For a roving of TEX 2400 (2400 g/1000 m) at glass density 2.56 g/cm3, one end has area = 2400 / (1000 x 100 cm x 2.56) = 0.00938 cm2. A 3 in x 0.25 in flat bar is 4.84 cm2. To hit Vf = 0.55, you need 0.55 x 4.84 / 0.00938 = 284 ends. Add mat and veil area separately. The Fiber Volume Fraction calculator resolves this quickly once you know roving TEX and end count.

Pull speed sets everything downstream. It is bounded by cure kinetics, not the puller motor. Rule of thumb: required die dwell for a glass/polyester profile at a 0.25 in wall runs 60 to 90 seconds. With a heated die length of 36 in (3 ft), max pull speed = 36 in / 75 s = 0.48 in/s = 2.4 ft/min. Thicker sections need longer dwell because exotherm must reach the core before demold. Use the Pull Speed calculator to invert your die length and target dwell into ft/min, then verify the profile is fully gelled at exit.

Cure die dwell is simply dwell = heated die length / pull speed, expressed consistently. At 2.4 ft/min through a 3 ft die, dwell = 3 / 2.4 = 1.25 min = 75 s. The constraint is that peak exotherm must occur inside the die, typically at 60 to 75 percent of die length, so the last 25 to 40 percent completes cure and cools below the resin Tg region before pulling free. The Cure Die Dwell calculator lets you test whether a proposed speed keeps peak exotherm at, say, 24 in of a 36 in die rather than pushing it toward the face.

Resin bath consumption comes from the mass the profile carries minus fiber mass, adjusted for pickup and drag-out. If the profile weighs 0.72 lb/ft and fiber is 0.40 lb/ft, resin in the part is 0.32 lb/ft. At 2.4 ft/min that is 0.77 lb/min of reacted resin. Bath consumption runs 3 to 8 percent higher due to drippage and residue; budget 0.80 to 0.83 lb/min. The Resin Bath Consumption calculator converts ft/min and resin-per-foot into gal/hr using resin density near 9.2 lb/gal for filled polyester.

Profile weight per foot ties fiber and resin together and validates Vf. Weight/ft = fiber area x fiber density + resin area x resin density, in consistent units. For the 4.84 cm2 bar at Vf 0.55: fiber = 0.55 x 4.84 x 2.56 = 6.82 g/cm x 30.48 = 0.457 lb/ft; resin at density 1.20 = 0.45 x 4.84 x 1.20 = 2.61 g/cm = 0.175 lb/ft; total 0.632 lb/ft. The Profile Weight Per Foot calculator cross-checks scale weight against theoretical, exposing dry spots or resin-rich zones when readings drift beyond 3 percent.

Line throughput in lb/hr is pull speed x weight/ft x 60. At 2.4 ft/min x 0.632 lb/ft x 60 = 91 lb/hr per die. A multi-cavity die running four 0.632 lb/ft profiles yields 364 lb/hr. Convert to pieces by dividing linear feet per hour (144 ft/hr here) by cut length; a 12 ft part gives 12 pieces/hr per cavity. The Line Throughput calculator handles cavity count and cut length so you can compare a slower thick profile against a faster thin one on a mass and piece basis simultaneously.

Where inputs come from matters. Roving TEX and end count come off the creel setup sheet, densities from the resin and glass data sheets (glass 2.54 to 2.60, polyester 1.10 to 1.30 filled), and die length from the tooling drawing. Measure actual pull speed with an encoder on the puller, not the setpoint, because slippage of 2 to 5 percent is common. Always reconcile theoretical weight/ft against a cut-and-weigh sample every shift; a 5 percent gap usually means a lost roving end or a bath viscosity change altering pickup.

Published 2026-07-01.