Extensive experimental and simulation studies have been performed on filling imbalance in geometrically balanced injection molds. Several runner systems have been applied, standard system, systems with one/two overturn elements, and system with circled element. Filling imbalance has been studied at various injection rates, mold temperatures and melt temperatures. Several polymers have been used PS, LDPE, PBT, POM, PA 6.6. In general, standard system and system with circled element caused positive imbalance which means that inner cavities were filled faster, and it was opposite for two other systems that induced negative filling imbalance which resulted in faster filling of external cavities. The experimental results indicate that injection rate, mold temperature and melt temperatures substantially affect the filling imbalance. However, filling of the cavity is strongly dependent on runners layouts geometry, and filling imbalance has never been eliminated completely. Especially, filling imbalance is most difficult to remove for high injection rates and low melt temperatures. An effect of thermo-rheological material parameters on filling imbalance has been studied. It has been observed that Cross-WLF parameters, index flow, critical shear stress (relaxation time), zero viscosity, and thermal diffusivity and heat transfer coefficient strongly affect filling imbalance. STASA QC system has been used to optimize filling imbalance. Brain Construction Algorithm (BCA) has been applied to define a response surface of the problem. Injection rate, melt temperature and mold temperature were optimized, and the global objective function was defined by the filling imbalance factor (of the highest weight), injection pressure required, molding temperature and injection time.