The injection molding is by far the most popular process of the production of plastic parts for all kinds of applications. One of the most significant factors that influences the part quality is the polymer flow pattern．By properly guiding the polymer flow at the filling phase．some advantages could be attained

i．e．over-packing

residual stresses

non-uniform shrinkage

and warpage can be eliminated

and the creation of weld-lines and air-traps could be also minimized．A method has been proposed to improve the injection molding filling process by optimizing the injection rate．The numerical simulation for filling processing is combined with the design optimization method to find the optimum injection rate profile and to achieve a constant melt-front speed satisfying the constraint on maximum allowable shear stress．The objective function that can exhibit the flow unevenness is expressed as the sum of the difference between the melt-front speed and a reference value．Since it has great effect on the melt-front speed

the injection rates at some key points are chosen as the design variables．The constraints may limit the wall shear stress below the maximum allowable shear stress and the injection rate in a specified scope

which the injection machine can provide．The design sensitivities are evaluated with the finite difference method

and the optimization problem is solved with the sequential linear programming algorithm．The proposed method

taking under consideration the existence of a finite number of control points of a specific machine

produces a more realistic optimal solution．Finally

the success of the methodology is demonstrated by two examples．