3D sand-printing (3DSP) has become more popular in foundry applications due to its ability to create complex gating geometries. Since filling related defects, like entrained air and bi-films, are most commonly caused by high melt velocity and turbulence, recent 3DSP research has focused on designing gating systems to reduce melt velocity and turbulence. However, there have been no reported efforts on advancements in the design of runner extensions as a method to improve casting quality, despite its tremendous impact on the initial metal flow characteristics. The ability to fabricate 3DSP molds allow for unique runner extension designs that aid in improving casting quality. This paper is the first study known to the authors that investigates novel 3D runner extension designs to determine the most effective design for reducing sand casting defects. Based on literature review and design principles developed for 3D sprue geometries, six different runner extensions were studied using Computational Fluid Dynamics (CFD) modeling for foundry pouring conditions. The designs were evaluated on their ability to reduce defects like entrained air and bubbles, as well as to prevent backflow and reflected waves. An unweighted ranking matrix and comparison matrix against the control (straight runner extension) has been established based on air entrainment, tracer, voids, and extension volume. The results showed that the by-pass principal and surge control systems are effective at reducing reflective waves and controlling the ingate flow. The novel 3D duckbill trap extension proposed in this study had the best overall performance based on a 16% reduction in entrained air and a 71% reduction in void particles in the casting volume compared to the control extension design. These results provide a framework to further optimize runner extensions, utilize the advantages of 3D Sand-Printing technology to improve mechanical strength and reduce filling defects in sandcasting.