Pultrusion in brief
Pultrusion makes constant-section FRP profiles — angles, channels, I-sections, tubes, rods, gratings bars — continuously. Glass roving and mat are pulled from creels, impregnated with resin in a bath or injection box, drawn through a shaped steel die heated in zones, and cured on the way through. Pullers grip the cured profile downstream and a saw cuts it to length.
Everything about the resin is set by that die. The profile has to cure in the short time it spends in the hot zone, while still flowing and wetting at the die entrance.
Specified by hot-die gel time
That is why the 2026 catalogue specifies SPR 401 PT by a hot-die gel time — 40 to 80 seconds at 130–140 °C typical — rather than by room-temperature working time. It also gives an ambient QC gel time of 15–25 minutes at 25 °C, which is a laboratory consistency check, not a production condition.
Its typical viscosity is 300–600 cPs at 25 °C, specific gravity 1.10–1.14 and styrene 36–42% by weight, and it is non-thixotropic so that it wets roving readily.
A heat-activated cure, not MEKP and cobalt
Laminating grades cure at room temperature with MEKP and a cobalt accelerator. A pultrusion resin must stay stable in the bath for hours and then cure in seconds when heated, which calls for peroxides that activate at die temperature. The catalogue’s production cure for SPR 401 PT is a heat-activated peroxide blend at a total dosage of 1.0–2.0%, added separately during processing. Production MEKP is 0%; production cobalt is 0%.
The blend, die temperature profile, pull speed and filler loading are line-specific, which is why the catalogue calls for trials on the production line before commercial runs.
Terephthalic and high-strength
SPR 401 PT belongs to the terephthalic SPR 401 series, and the catalogue describes the pultrusion grade as high-strength. That is a qualitative description: no numerical mechanical values are published for the resin, and a profile’s strength depends heavily on fibre content and architecture. Profile performance is established by testing the profile.