Abstract: A method of forming a ball core is disclosed. A flowable homogeneous mass of a thermoplastic resin and a blowing agent is injected into the spherical cavity of a mold under temperature and pressure conditions sufficient to activate the blowing agent. The mold is cooled to 32.degree. to 65.degree. F. so as to form a skin on the surface of the mass. The mass is held within the mold cavity for a sufficient time so that the skin has adequate strength to prevent its rupture by blowing agent contained within the mass, upon removal of the mass from the mold cavity. The mass is cooled at an exterior temperature of 32.degree. to 65.degree. F. A ball core for baseballs and softballs is also disclosed.