Plastic injection molding process is well known as the most efficient manufacturing technique for economically producing precision plastic parts with large quantities and complex geometry at low cost. This book aims to study and optimize the heat transfer process of 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 is carried out. Also, his effect on the temperature, solidifcation and shrinkage rate of the polymer is studied. The shrinkage rate distribution of the polymer materials and the temperature distribution through the mold and the polymer are also included. 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 also on the filling process is studied in this work. Finally, improvement techniques are proposed to ameliorate the filling process of the injection molding process