Void formation in short-fiber thermoplastic composites

Voids, or bubbles, are formed in short‐fiber thermoplastic composites because of entrapment of air in the compounding and melt flow processing steps and as a result of uneven shrinkage due to temperature gradients involved in the solidification step by cooling. The experimental results suggest that bubble nucleation takes place at fiber ends and their volume content depends on processing conditions. The shape and distribution of the voids were studied by optical examination of longitudinal sections of extrudates, using standard metallographic polishing technique and reflected light microscopy. The void volume fraction of extrudates was found to increase with increasing extrusion rate, temperature, fiber concentration, and fiber length, and with a decreasing draw ratio. The void content in fiber‐glass‐reinforced injection moldings is low, ≈︁1% by volume; however, by heating these moldings, the contained pressurized bubbles expand and significant void volume fractions are observed.