Metal 3D printing will be the only alternative to the manufacture of injection molds containing complex conformal cooling channels. Advantages of conformal cooling channels are well known, but processing methods are not yet effective and are becoming a hindrance to generalization. The most important issue of 3D printed molds is cost and life cycle. Printing time and metal powder prices are tens of times higher than in conventional cutting processes. In this study, a hybrid approach was adopted to solve this problem. This is an integrated build process that combines additive manufacturing with conventional CNC machining technologies. On the other hand, injection molding requires high durability because it has to withstand high-temperature and high-pressure loads of several hundred thousand cycles. However, powder bed fusion (PBF) methods are susceptible to fatigue loading because of the presence of pores in the workpiece and rough surfaces. In this study, we evaluated the usefulness of the hybrid approach by predicting the lifetime of 3D printed injection molds through fatigue numerical analysis. In order to increase the reliability of the analysis, the conditions close to the actual conditions are reflected in the numerical analysis. The finished core has a polished surface created by the cutting process and a rough surface formed by the laser. This will obviously have the different fatigue life. Also, the fatigue characteristics of the bonded surfaces by brazing are also very important factors in the life cycle.