Fibers visible on the surface of glass-fiber reinforced materials; a matte and rough surface. It arises because the plastic-to-fiber shrinkage difference is 200:1.
Glass fiber marks are the reinforcing fibers coming out onto the part surface and becoming visible. They leave light-colored, feather-like or line-like marks on the surface and feel rough to the touch. They appear when the distribution of fiber and polymer at the surface is disturbed; the cause is mostly the flow behavior and the mold surface temperature.
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|---|
| Surface Delamination | With delamination the surface layer lifts off like a skin. With glass fiber marks the surface is intact; only the fibers are visible. |
| Flow Lines | A flow line is a difference in color and gloss. A glass fiber mark is a physical roughness that can be felt to the touch. |
On a cold mold surface the polymer solidifies quickly and cannot flow enough to cover the fibers. Raising the mold temperature allows a thin, polymer-rich layer to form at the surface — it is the single most effective correction for this defect.
Yes. At the weld line the fibers line up perpendicular to the flow direction; the mark becomes pronounced and the strength of that area falls. In load-bearing glass-fiber parts the position of the line should be planned in the design; see weld line.