Experimental and theoretical studies were performed on filling imbalance in geometrically balanced injection molds. Balancing the melt flow between cavities was investigated using several different runner systems at various operating conditions. Experiments indicate that injection rate, mold, and melt temperatures substantially affect the filling imbalance. It is strongly dependent on runners layouts geometry, and it has never been eliminated completely. It is most difficult to remove for high injection rates and low melt temperatures. Standard element geometry and circled element geometry cause positive imbalance which means that inner cavities fills faster, and it is opposite for one/two overturn element geometries which induce negative imbalance. A special modeling procedure is required to simulate properly the imbalance. This includes inertia effects, geometrical modeling of the nozzle where the imbalance starts, 3D tetrahedron meshing with minimum 12 layers. Simulations were consistent with experiment, however, when the imbalance increased, the discrepancies between simulation and experiment also increased. It can be stated that filling imbalance problem is still unsolved. There are serious thermo-rheological aspects to explain for better understanding this phenomenon. Trends in modeling of injection molding are presented and new concepts solving the problem are discussed including simulation/optimization approach and a novel concept of global modeling of injection molding process. POLYM. ENG. SCI., 59:233-245, 2019. (c) 2018 Society of Plastics Engineers