In extrusion a screw or ram pushes a hot, softened material through a shaped opening; the profile takes its shape from the die lips and is cooled downstream. In pultrusion a puller downstream of the die drags continuous reinforcement, wetted with liquid thermoset resin, through a long heated cavity where the resin cures. The material is pulled, not pushed, and it leaves the die already rigid.
Process direction and material
| Pultrusion | Extrusion | |
|---|---|---|
| Driving force | Pulled by grippers or caterpillar belts downstream of the die | Pushed by a screw (thermoplastic) or ram (aluminum) upstream of the die |
| Material state at the die | Liquid thermoset resin on continuous fiber; cures inside the die | Molten thermoplastic or hot plastic aluminum billet; solidifies after the die |
| Material class | Thermoset composite: polyester, vinyl ester, epoxy or polyurethane with glass, carbon, aramid or basalt fiber | Thermoplastic (PVC, PE, PP, ABS, PA) or metal (aluminum, brass, copper) |
| Reinforcement | Continuous fiber along the length, plus mats and fabrics for transverse strength | None in metals; short fiber or filler at most in thermoplastics |
| Shape fixing | Chemical cure, irreversible | Cooling, reversible on reheating (thermoplastic); solidification (metal) |
| Post-die handling | Rigid at exit; cut by a flying saw | Soft at exit; needs calibration, water bath or cooling table before cutting |
| Common names | Pultruded profile, FRP, GRP, GFRP, fiberglass profile | Extruded profile, extrusion, aluminum section, PVC profile |
Continuous fiber: the structural difference
The reason the two processes exist separately is the fiber. Pultrusion exists to put unbroken glass or carbon fiber along the full length of a profile and lock it in a cured resin. That gives a profile whose axial stiffness and strength per unit weight are governed by the fiber, not the matrix, and which does not creep, corrode or conduct electricity the way the alternative materials do. Extrusion cannot carry continuous fiber: a screw would chop it and a ram would not wet it.
The same fiber is also the constraint. A pultruded profile is strongest along its length and weaker across it, so transverse loads, bolted joints and bearing surfaces have to be designed with mats, fabrics or local reinforcement. An extruded aluminum section is the same in every direction.
Tooling
| Pultrusion die (mould) | Extrusion die | |
|---|---|---|
| Form | Long, heated steel block with a full-length cavity, hard-chrome plated | Short plate or stack of plates with a shaped orifice; aluminum dies include bearing lengths and bridges for hollows |
| Length | Long enough for the resin to gel and cure during the pass; sized to profile thickness and target speed | Short; shape is set at the lips and held by downstream calibration |
| Heating | Multiple independently controlled zones; the temperature profile is a process parameter | Heated to keep melt viscosity (plastics) or at billet temperature (aluminum); not a cure control |
| Hollows | Fixed mandrel cantilevered back through the entrance | Spider or porthole die; mandrel supported by bridges in the melt stream |
| Wear | Entrance abrasion by glass fiber; chrome renewed by re-plating | Bearing wear and die line marks; nitriding and re-polishing |
| Changeover | Hours: die swap, preform rebuild, re-thread rovings, re-heat and re-tune | Minutes to an hour for plastics; longer for aluminum with die heating |
| Tooling for one profile | One die per profile; multi-cavity possible for small simple sections | One die per profile; multi-hole dies common for small sections |
Land length, entrance taper, heating-zone placement and chrome plating are explained in the pultrusion die design guide
Speed, volume and cost crossover
Extrusion is fast because nothing has to react inside the die; the limit is how quickly the profile can be cooled. Pultrusion is slow because the resin must gel and cure during the pass; the limit is residence time, so thin sections run faster than thick ones, and the figures vary by an order of magnitude between a rebar line and a thick structural channel. Specific speeds are profile-specific and are not listed here.
| Factor | Favors pultrusion | Favors extrusion |
|---|---|---|
| Volume | Medium runs where fiber performance is required; tooling is modest relative to the line | Very high volumes of commodity profiles |
| Material cost per kg | Higher raw material cost, but lower density and higher specific stiffness reduce kg per meter | Lower per kg for PVC; aluminum is price-volatile |
| Energy | Heated die and puller; no melting | Melting and holding thermoplastic or aluminum; aluminum is energy-intensive |
| Service life in corrosive, wet or electrical environments | No painting, galvanizing or insulation needed; often the deciding factor | Needs coatings or alloy selection; insulators needed in electrical use |
| Secondary operations | Cut, drill, bond; threading and welding not possible | Cut, machine, weld (aluminum), weld or solvent-bond (plastics) |
| Tooling lead time | Weeks; the die is a precision heated block | Days to weeks; faster for plastics |
The crossover is rarely about the profile alone. Pultrusion wins when the installed cost over the service life is counted: no corrosion maintenance, no electrical isolation hardware, lower handling and transport weight. Extrusion wins when the profile is a commodity, the environment is benign, or the part needs to be welded, bent or recycled as a single material.
Profiles that can only be pultruded
- Composite rebar and rock bolts: continuous glass or basalt fiber in a corrosion-free rod, used where steel would rust or where a non-magnetic, non-conductive bar is required
- Electrically insulating structural members: ladder rails, hot-stick tubes, bus-bar supports, substation structures
- Long unsupported spans in corrosive plant: grating bearer bars, handrail, cable tray, walkway stringers in chemical, wastewater and marine sites
- High-stiffness lightweight tubes and spars where carbon fiber along the length is the point: wind-blade spar caps, antenna masts, sporting goods
- Profiles with buried functional layers: a surface veil for UV, a mat for transverse strength and unidirectional rovings for bending, all in one pass
When extrusion is the right choice
- Commodity profiles at very high volume where tooling and material cost per meter dominate and the environment is benign
- Profiles that must be welded, bent after forming, anodized or recycled as a single material
- Isotropic strength requirements, where load direction is unknown or transverse loads dominate
- Very thin walls and fine detail that a fiber bundle cannot fill
- Short, variable or seasonal runs where an hour-long changeover is acceptable and a long die trial is not
Buyers weighing in-house pultrusion against buying extruded or pultruded profiles can work through the capital, tooling and staffing questions in how to start a pultrusion factory
Questions buyers ask
- What is the main difference between pultrusion and extrusion?
- Direction and material. Extrusion pushes molten thermoplastic or hot aluminum through a die and cools it afterward. Pultrusion pulls continuous fiber wetted with liquid thermoset resin through a long heated die where the resin cures, so the profile is rigid when it exits and carries unbroken fiber along its length.
- Can extrusion use fiberglass reinforcement?
- Only short or chopped fiber mixed into a thermoplastic. Continuous fiber cannot survive a screw or be wetted by a ram, so the long-fiber properties that define a pultruded profile are not available from extrusion.
- Is pultrusion slower than extrusion?
- Yes. Extrusion is limited by cooling; pultrusion is limited by the time the resin needs to gel and cure inside the die. Thin pultruded sections run much faster than thick ones, so speeds vary widely by profile and are not quoted as a single figure.
- Is a pultrusion die the same as an extrusion die?
- No. A pultrusion die is a long, heated, chrome-plated steel block with a full-length cavity and several temperature zones. An extrusion die is a short plate with a shaped orifice; the profile's shape is held by cooling and calibration downstream.
- When is pultruded fiberglass cheaper than extruded aluminum?
- Usually when installed cost over the service life is counted rather than the price per meter: corrosive, wet or electrical environments where aluminum would need coatings, isolation hardware or replacement. For commodity profiles in benign environments, aluminum or PVC extrusion is usually cheaper.
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