A platen assembly for use with an extrusion-based 3D printer includes a grid assembly comprising at least a 4×2 framework of interlocked perpendicular x direction beams and y direction beams, providing a substantially planar upper surface and a bottom surface. The platen assembly includes a platen comprising a thin metal sheet supported on the upper surface of the grid assembly and secured to the grid assembly such that the top surface provides a substantially flat build surface. The x direction beams, the y direction beams and the platen are constructed of substantially a same thermal expansion properties, and wherein the build surface of the platen has a build surface area of at least 400 square inches and maintains its flatness to within a flatness tolerance of 0.020 inches over a temperature range of at least 20 C-300 C.