An Empirical Study of the Surface Blush in Gas-Assisted Injection Molded Parts

Abstract Gas-assisted injection molding of thermoplastic materials has become an important process in industry, because of its capability of producing parts that have thick and thin sections with a good structured rigidity. However, there are some unsolved problems that confound the overall success of this technique. Surface blush of the molded parts caused by glossy difference of the materials is one of them. An L'18 experimental matrix design based on the Taguchi method was conducted to investigate the surface blush of gas-assisted injection molded thermoplastics. The material used was black-pigmented polypropylene. Experiments were carried out on a 4.1 oz reciprocating injection-molding machine equipped with a high-pressure nitrogen-gas injection unit. A plate cavity with gas channels of different geometry was used to mold the parts. After molding, the surface quality of the parts was characterized by a gloss-meter. For the factors selected in the main experiments, mold temperature and gas pressure were found to be the principal factors affecting the surface quality of gas-assisted injection molded parts. The surface glossy difference increases with the content of the pigment in the materials. In addition, a trapezoidal gas channel of small width was found to mold the parts with the least glossy difference. An aluminum mold helps reducing the surface blush in molded parts.