A method for injection molding a thermoplastic composition that contains a polyarylene sulfide and a boron-containing nucleating agent is provided. By selectively controlling certain aspects of the polyarylene sulfide and nucleating agent, as well as the particular manner in which they are combined, the crystallization properties of the resulting thermoplastic composition can be significantly improved. This allows the “cooling time” during a molding cycle to be substantially reduced while still achieving the same degree of crystallization. The cooling time can be represented by the “normalized cooling ratio”, which is determined by dividing the total cooling time by the average thickness of the molded part.