A pultrusion line pulls continuous glass fiber (glass fibre), carbon fiber or other reinforcement through a resin impregnation stage, a preformer and a heated steel die, where the thermoset resin cures. The cured FRP or GRP profile exits at full strength, is gripped by the pulling mechanism and cut to length. Everything the operator adjusts sits between the creel and the saw.
This page does not publish line settings. The factory records its own line parameters for every die trial and production run, and will share dated measurements for a comparable profile on request. A number quoted without the profile, resin and fiber it was measured on is not a parameter; it is a guess.
The parameter set
Die temperature profile and why there are three zones
The heated die is divided into independently controlled zones along its length. Three is the common minimum because the resin has to do three different things in sequence. In the first zone it must stay liquid and get hot enough to drop in viscosity, so it wets the fiber fully and lets trapped air escape backward. In the second zone it gels: the viscosity rises sharply and the profile takes the shape of the cavity. In the third zone the gelled profile cures to a degree high enough to hold dimension after exit.
The pultrusion die temperature setpoints are therefore not three numbers to copy; they are a way of placing the gel point in the middle part of the die. Too hot too early and the resin gels in the entrance taper, where it builds up and eventually seizes the line. Too cool at the end and the profile leaves soft, shrinks after exit and warps. Thick sections need the hot zones further back because heat takes longer to reach the core; carbon fiber conducts heat inward faster than glass and usually wants a cooler first zone.
Line speed versus cure
Pultrusion line speed and degree of cure are the same variable seen from two sides. At a fixed die length, speed sets the residence time: double the speed and every element of the profile spends half as long in each zone. To keep the gel point in place the zone temperatures must rise, and there is a ceiling where the surface cures and chars before the core has gelled.
Speed is also bounded by resin chemistry. A reactive resin with a short gel time allows high speed on thin sections but is unforgiving on thick ones; a slow resin tolerates thick sections but caps throughput. This is why speed ranges published for pultrusion vary by an order of magnitude: thin rods and rebar run fast, thick structural channels run slow, and both are correct.
Resin bath versus injection
In an open resin bath the dry fiber passes through a trough of catalyzed resin and is wiped to the right pickup by guide plates before the die. It is simple, tolerant and suits most polyester and vinyl ester work. Its weaknesses are styrene emission, resin pot life (the whole bath ages together) and limited control of fiber volume fraction, which is set by how hard the preformer wipes.
Injection impregnation replaces the bath with a closed chamber attached to the die entrance, into which resin is pumped under pressure. It cuts emissions, lets fast-reacting resins such as polyurethane be mixed just before use, and controls resin pickup by metering rather than wiping. The cost is a more complex entrance block, a metering and mixing unit, and sensitivity to fiber packing: if the dry preform is too dense, resin cannot penetrate to the core at the pressure available.
Fiber tension and the creel
Each roving is pulled from a bobbin on the creel through guide eyes to the preformer. Tension must be enough to keep the roving straight and in its lane in the section, and even across all rovings. Uneven tension is the quiet cause of bow and twist: a roving under high tension sits deeper in the resin, is straighter after cure and pulls the profile toward its side. Bobbins that are nearly empty deliver different tension than full ones, so creel layout and tensioning devices matter more than they look.
Pull force
Pull force is the sum of the friction of compacting wet fiber in the taper, the viscous drag of the resin in the liquid zone, the adhesion peak at the gel point and the sliding friction of the cured profile against the land. It is an output of the process rather than a setpoint, and a very useful one: a steady rise over a shift means resin build-up or die wear, a sudden spike means a roving has bunched or a mat has folded.
Machine pull capacity sets the ceiling. The clamp force must exceed the pull force by a margin so the profile does not slip in the grippers, and the gripper pads must be shaped to the profile so the force is spread without crushing it. Reciprocating hydraulic pullers and caterpillar (crawler) pullers handle this differently; the machine pages explain which suits which section.
Cut-off
A flying saw travels with the profile while cutting, so the line never stops. Cut length tolerance depends on the encoder and the saw's acceleration; cut quality depends on blade type, feed and whether the profile is fully cured and cool at the saw. A profile that is still warm at the saw tears at the cut edge and drifts in length as it cools.
Pull capacity, clamp type, zone count and saw type are machine-level choices. Compare the hydraulic and caterpillar models on the pultrusion machines overview
Parameter table: what each controls, too high, too low
The table is written as physics, not as settings. Ranges are profile-specific and are not given here.
| Parameter | What it controls | If too high | If too low |
|---|---|---|---|
| Zone 1 (preheat) temperature | Resin viscosity drop, wet-out, air release | Premature gel in the taper; resin build-up; rising pull force | Poor wet-out; voids; dry spots on the surface |
| Zone 2 (gel) temperature | Position of the gel point in the land | Gel point moves toward entrance; surface cures before core; cracking | Gel point moves toward exit; soft profile; dimensional drift |
| Zone 3 (cure) temperature | Degree of cure at exit | Surface scorching, blistering, color change | Undercure; low Barcol hardness; post-exit shrinkage and warp |
| Line speed | Residence time; throughput | Undercure at exit; higher pull force; loss of tolerance | Over-cure; lower throughput; higher cost per meter |
| Resin pickup / fiber volume | Mechanical properties; surface finish; shrinkage | Resin-rich profile, surface marks, high shrinkage, sloughing | Dry fiber at the surface; voids; very high pull force |
| Fiber tension | Fiber straightness and lane position | Roving breakage; high pull force; fiber wash at the entrance | Rovings wander; wavy fibers; bow and twist |
| Pull force (measured) | Health indicator for die and process | Slippage, surface tearing, machine overload | Normal unless suddenly dropping, which indicates slip or breakage |
| Cut-off temperature | Edge quality and length accuracy | Torn cut edges; length drift as the profile cools | Not a problem; cooler is better at the saw |
How parameters interact: speed × temperature × thickness
The three variables that lock together are line speed, die temperature profile and profile wall thickness. Thickness is fixed by the design; speed and temperature are the two levers. For a given resin, the operator's job is to find the speed at which the gel point sits in the right part of the land and the exit degree of cure is acceptable, then hold the zone profile that keeps it there.
- Increase speed: the gel point moves toward the exit. Compensate by raising zone 2 and zone 3, within the limit where the surface starts to scorch.
- Increase thickness: the core lags the surface. Compensate by lowering speed and shifting heat toward later zones so the surface does not cure long before the core.
- Change from glass to carbon fiber: heat reaches the core faster. Lower zone 1 to avoid gelling at the mouth, then re-tune.
- Change resin reactivity: a faster resin shifts the gel point toward the entrance at the same settings; lower zone 1 and consider raising speed.
- Add filler or low-profile additive: viscosity and exotherm change; re-tune from a conservative, slower start.
Open versus closed sections
Closed tubes and boxes shrink symmetrically against the mandrel and hold shape. Angles, channels and I-beams shrink unevenly and spring in, and the amount of spring-in increases with speed because more of the cure happens after exit. For open sections the parameter window is narrower, the die is often cut with compensated angles, and a cooled shaping fixture after the die may be part of the process rather than an afterthought.
What a parameter record should contain
Whether you keep your own log or ask a supplier for one, a parameter record is only useful if it names what it was measured on. The record the factory keeps for each die trial includes the fields below. On request, a dated record for a profile comparable to yours is shared as part of the quotation discussion.
- Profile drawing number and the die serial it ran in
- Resin system, filler, initiator scheme and batch
- Reinforcement schedule: roving tex and count, mat type and weight, veil
- Zone setpoints and actual readings for each zone, with thermocouple positions
- Line speed, measured pull force (steady and peak), clamp pressure
- Exit temperature of the profile and cut-off temperature
- Dimensional and Barcol hardness results against the agreed tolerance
- Defects observed, with photographs
Speed and resin pickup are also the two largest drivers of unit cost. How they feed into a per-meter model is covered in the pultrusion cost per meter guide
Questions buyers ask
- What are the main pultrusion process parameters?
- Die temperature profile (usually three or more zones), line speed, resin pickup or fiber volume fraction, fiber tension at the creel, pull force and cut-off length. Temperature and speed are the two levers an operator adjusts; the rest are set by the die, the reinforcement schedule and the machine.
- Why does a pultrusion die have three heating zones?
- The resin must do three things in sequence: thin out and wet the fiber, gel into the cavity shape, then cure hard enough to hold dimension at exit. Independent zones let the operator place the gel point in the middle of the die rather than at the entrance or exit.
- What limits pultrusion line speed?
- Residence time. At a fixed die length, faster pulling means less time in each zone, so the temperatures must rise until the surface would scorch before the core cures. Resin reactivity and profile thickness set where that ceiling is, which is why thin rods run much faster than thick structural sections.
- What is the difference between resin bath and resin injection pultrusion?
- An open bath wets the fiber in a trough and relies on the preformer to wipe off excess resin. Injection pumps metered resin into a closed chamber at the die entrance, which lowers emissions, suits fast-reacting resins such as polyurethane and controls fiber volume by metering rather than wiping.
- Does the factory publish its line settings?
- Not on this site. The factory records parameters for every die trial and production run and will share a dated record for a profile comparable to yours on request, with the resin, reinforcement and die it was measured on, so the numbers can be interpreted correctly.
Ready to move from reading to a scoped quotation?
Request a dated parameter record for a comparable profile