Injection molding represents the most important process for producing precision plastic parts with large quantities, complex gemoetry and low cost. Cooling process during injection molding is of great importance for plastic produced by injection molding because it affects extensively on the cycle time and the productivity, the accuracy and the quality of final product. This book aims to study and optimize the cooling process during the injection molding to obtain optimum product quality at low cost and highly production rate. The effect of cooling system on the heat transfer through the injection molding process, the cooling and solidification of the polymer, the shrinkage rate of the polymer material and the temperature distribution through the mold and the polymer is carried out. Moreover, the effect of different process parameters (polymer properties, injection temperature, filling time, coolant flow rate, gate location, etc.) on the cooling and solidification of the polymer material and on the filling process is studied. Also, improvement techniques are proposed to improve the filling and cooling process of the plastic during injection molding.