As rotational molding of thermoplastic parts become more and more complex, understanding the fundamental processing aspects such as powder flow becomes important for good and uniform part quality. In this work, an image analysis technique is used to determine the effect of polymer powder particle size and distribution in a biaxially rotating spherical mold. Experimental parameters such as particle diameter (d), ratio of larger over smaller particles diameters (dr), and initial powder position inside the mold and camera viewing position were studied. From the results obtained, conditions leading to homogeneity and segregation are discussed. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers