Surface roughness at the melt/gas transition sites of gas-assist injection molded thermoplastic composites

Gas assist injection molding has proven itself a breakthrough technology in molding thermoplastic composites. However, there are still unsolved problems that confound the overall success of this technique. This report was to study the surface roughness phenomenon occurring at the melt/gas transition sites of gas assist injection molded composite parts. The material used was 35% glass-fiber filled Nylon-6 composite. Experiments were carried out on an 80-ton injection molding machine equipped with a pressure-controlled nitrogen-gas injection unit. A spiral mold was used for all experiments. After molding, a roughness meter was used to measure the surface quality at the melt/gas transition sites of the parts. Various processing variables were studied in terms of their influence on the surface roughness of molded composites: melt temperature, mold temperature, melt filling speed, short-shot size, gas pressure, and gas injection delay time. A scanning electronic microscope was also used to characterize the surface roughness phenomenon of molded parts. It was found that the roughness at the transition sites was mainly caused by the fiber exposure of molded composites. Experimental investigation of a gas assist injection molding problem can help our understanding of the formation mechanism of surface roughness at the melt/gas transition sites, so that steps can be taken to improve the surface quality of molded parts.