Put a pultruded FRP I-beam next to a molded FRP grating panel and the difference is immediately visible even without knowing how either was made. The I-beam has a smooth, resin-rich surface with visible longitudinal fiber orientation. The grating panel has a textured surface with a mesh pattern and visible cross-bars. These two products represent the endpoints of FRP manufacturing: continuous pultrusion for linear profiles, and open molding for planar structures. The differences in their properties are not cosmetic — they are direct consequences of how the glass fibers are oriented and compacted during production.
Fiber Architecture: The Performance Origin
The fundamental distinction between pultruded and molded FRP is the orientation of the load-bearing glass fibers. In a pultruded profile, roughly 70–80% of the glass (by weight) is aligned in the longitudinal direction — the continuous rovings that were pulled through the die. This produces maximum strength and stiffness along the length of the profile, but significantly lower properties across the width and through the thickness. The result is a material that is highly orthotropic: strong in one direction, weaker in the other two.
In a molded grating panel, the glass fibers are laid in the mold in two perpendicular directions, forming a grid. The fiber distribution is approximately equal in both in-plane directions, giving the panel bi-directional strength and stiffness. The trade-off: the fiber volume fraction is lower (25–40% vs. 50–65% for pultrusion) and the fibers are not as straight and well-compacted, so the absolute mechanical properties in any one direction are lower than a pultruded section of equivalent thickness loaded in its strong direction.
Mechanical Property Comparison
| Property | Pultruded (longitudinal) | Molded (in-plane) |
|---|---|---|
| Tensile strength | 200–400 MPa | 70–100 MPa |
| Tensile modulus | 17–28 GPa | 7–14 GPa |
| Flexural strength | 200–350 MPa | 100–170 MPa |
| Fiber volume fraction | 50–65% | 25–40% |
| Density | 1.7–1.9 g/cm³ | 1.5–1.7 g/cm³ |
Product Geometry: What Each Process Can Produce
Pultrusion is limited to constant cross-section shapes. The die defines the profile, and anything that can be pulled through that die can be manufactured — I-beams, channels, angles, square and round tubes, flat bar, solid rod, and custom profiles with hollow sections and complex flanges. Length is limited only by transportation and handling; 12 m lengths are standard, and longer is possible. Open molding is limited by mold size, not cross-section. A single molded grating panel can be 1 m × 4 m with integral cross-bars, stiffening ribs, and edge details. Tank covers, dome structures, and large housing components that would be impossible to pultrude are routine open-molding products.
Limitations and Defects
Pultruded profiles can exhibit surface cracking at the die exit if the pulling speed is too high or the resin cure is uneven. Molded products can have resin-rich areas at the intersection of glass bundles, which are locally weaker and more brittle. Neither process is immune to quality variation; the difference is that pultrusion defects tend to be continuous along the length of a profile, while molding defects are localized to specific areas of the panel.
For pultruded products, see FRP Structural Profiles. For molded grating, see Molded FRP Grating.