Consider this example: Looking at a unidirectional laminate from the Z direction (top view), we see the fibers stabilize the laminate in the X-axis. So resin shrinkage will be exhibited in the Z-axis and the Y-axis. In a biaxial laminate, also viewed from the Z direction, we see the laminate is stabilized in the X-axis and Y-axis. Shrinkage will be exhibited mostly in the Z direction. However, a biaxial laminate contains two plies of rovings – unidirectional rovings (0 degree or UD rovings) on bottom and 90 degree rovings on top. The common nomenclature for this is (90, 0). This is not a balanced laminate, and it will warp in a predictable way. To eliminate this warpage, a balanced laminate must be created – for instance, (0, 90, 0) or (0, 90, 90, 0).


The effect of fiber orientation dramatically affects mechanical and structural properties, so getting this right is critical. Smaller bundle tex usually improves tensile strength and impact strength, but gives up tear strength. Better fiber dispersion often results in lower glass contents and lower tear strengths, but improves tensile strength and cosmetics. Smaller filament diameter is often credited with better tensile strength, fatigue resistance and impact strength, although this is more noticeable in a thermoplastic matrix than thermoset. It is also more noticeable in high glass content composites.

good and bad surface cosmetics

Side-by-side examples of good and bad surface cosmetics.

Two more fiber properties worth mentioning are fiber length and sizing. Laminates made with continuous fibers are stronger than those made with chopped fibers. However, the advantage drops off in the fiber length range of 2 to 4 inches depending on fiber type, resin matrix and property being measured. Typically, processing issues make this a moot point.

Next to orientation and fiber volume percent, the choice of sizing has the biggest effect on laminate properties. The chemical composition of the sizing determines whether the resin sticks to the reinforcement and whether the load transfer occurs properly in the composite. This thin layer of chemistry applied to the glass, which makes the reinforcement processable in a variety of molding techniques, also facilitates the bond to the selected resin matrix. While the topic of sizing is vast and warrants its own column, the sizing effect must be mentioned when discussing mechanical and structural performance. Nearly all mechanical properties are affected by sizing.

Wet-out and Wet-through

For all glass fiber reinforcements, wet-out and wet-through are affected by filament diameter, bundle tex and fiber dispersion. Wet-out describes the wetting of individual filaments and bundles of glass fibers. Wet-through describes the ability of the resin to permeate through multiple layers of reinforcement.