Nowadays, many fields such as transportation, are more and more involved to elaborate materials which can reduce the fire risk. The standard approach to design flame retardant materials consists in incorporating in the plastic some fillers known as flame retardants, but they are usually efficient at high loading, thus reducing the mechanical properties of the polymer. Moreover, the synthesis of new additives, very efficient at low loading and of low toxicity is hard and usually expensive. In this work, a novel approach is considered. Instead of changing the material formulation, the design and the layout of materials are modified to reach optimized fire protection performances. On the contrary to standard polymer shaping processes which do not easily allow designing sophisticated shapes without using a complex mold [1], additive manufacturing appears as a very flexible technique. Among all 3D printing techniques, Fused Deposition Modelling (FDM) has the best quality to cost ratio, but the range of filaments commercially available is limited and costly. However, in some specific 3D printing processes, no filaments are necessary. Polymers pellets feed directly the printing nozzle, allowing investigating many polymeric matrices with no commercial limitation [2]. In this work, innovative flame retarded sandwich structures using 3D printing were created with two skins completely filled and a partially filled grid patterned core. In the first step, different grid patterned cores were prepared by varying the infill density (either 30 or 50 %) as well as the FR additive used in the ethylene vinyl acetate (EVA), (aluminum tri-hydroxide (ATH) and Expandable graphite) (Figure 2). In a second step, biphasic materials were designed: pores of the sandwich core (composed on EVA and 30 wt.-% of ATH) were filled with air, water and potassium carbonate (Figure 2). A full characterization and explanation of the fire properties of these original 3D designs was performed. It revealed that new light design with potassium carbonate in liquid phase inside the core material reached very good flame retardant properties, with a fast flame extinguishment and strong HRR reduction. This work is a proof of concept of the potential of FDM technology to design new flame retarded materials, offering a way to make safer materials at low cost.