One of the problems encountered in fluid-assisted injection-molded parts is the gas or water "fingering" phenomenon, in which gas (water) bubbles penetrate nonuniformly into the core of the parts and form finger-shape branches. Severe fingerings can lead to significant reductions in part stiffness. This study investigated the fingering phenomenon in fluid-assisted injection-molded disk parts. Experiments were carried out on a reciprocating injection-molding machine equipped with gas- and water-injection units. The material used was virgin polypropylene. A disk cavity with two different thicknesses was used for all experiments. The effects of various processing parameters on the fingering were examined. It was found that the melt short shot size and mold temperature were the principal parameters affecting the formation of part fingerings. In addition, the formation mechanism of part fingerings has also been proposed to better understand the formation of part fingerings. It has been shown that the fluid-assisted filling process is an unstable system by nature. Any small perturbation by material viscosity or by temperature gradient can trigger the unbalance of gas (water) penetrations in the parts and result in fingerings. POLYM. ENG. SCL, 49:2257-2263, 2009. (C) 2009 Society of Plastics Engineers