Each of the six balls in this set is rotational molded. This process converts liquid vinyl, rotating inside a heated mold, into a seamless inflatable ball. Through this process, we are able to duplicate the feel, texture and weight of our best rubber playground balls, while eliminating the inherent problems of shape distortion, seam breakage and air leakage. Sold in a set of six, one of each color. 8-1/2" in diameter.