The two manufacturing processes that produce the vast majority of industrial FRP products — pultrusion and open molding — are fundamentally different in how they place glass fiber, how they introduce resin, and what they can produce. Understanding these differences helps an engineer understand why a pultruded I-beam behaves differently from a molded grating panel, even when the resin system is identical.
Pultrusion
Pultrusion is a continuous process. Glass fiber rovings, continuous strand mat, and surface veil are pulled from creels through a resin bath where they are saturated, then through a heated steel die that cures the composite into a constant cross-section shape. The pulling is continuous — a pultrusion line can run 24 hours a day, producing profiles that are cut to length as they emerge from the die. The key characteristics of pultruded products flow directly from this process: high fiber volume fraction (50–65%) because the die compacts the fibers; unidirectional fiber orientation (the rovings run the length of the profile); excellent longitudinal mechanical properties (tensile strength 200–400 MPa, modulus 17–28 GPa); and relatively modest transverse properties (modulus 7–10 GPa) because only the continuous strand mat carries load across the fiber direction. Pultruded shapes — I-beams, channels, angles, square and round tubes, flat bar, solid rod — are the workhorse structural elements of FRP framing, walkway support, and cable tray systems.
Open Molding
Open molding covers the family of processes — hand lay-up, spray-up, and the automated variants — in which glass fiber reinforcement is placed into or onto an open mold and resin is applied to saturate the fiber. The mold defines the final shape, and curing occurs at ambient temperature or with mild heat. Molded FRP grating is produced this way: continuous glass fiber rovings are laid into a grid mold in two perpendicular directions, saturated with resin, and cured. The result is a monolithic panel with integral cross-bars and a bi-directional fiber architecture — roughly equal strength in both in-plane directions. Fiber volume fraction is lower than pultrusion — 25–40% — and the mechanical properties reflect this, but the process can produce shapes that pultrusion cannot: full grating panels up to 1 m wide, tank covers with integral stiffening ribs, and complex housing structures.
Process Selection: What Drives the Choice
The choice between pultrusion and open molding is driven primarily by the product geometry. Constant cross-section, long, straight elements → pultrusion. Large, one-piece, complex shapes → open molding. There is overlap — some grating products are now made by assembling pultruded bars into panels — but the fundamental dividing line remains: pultrusion for linear profiles, open molding for planar and shell-type structures. The resin systems available are similar across both processes (isophthalic polyester, vinyl ester, phenolic), though the curing chemistry is adjusted for the process speed — pultrusion resins cure in seconds in the heated die, while open molding resins have a pot life measured in minutes to hours.
For the full range of pultruded structural profiles, see FRP Structural Profiles. For molded grating configurations, see FRP Grating.