Sandwich panels with unidirectional core stiffeners are known for their relatively high bending stiffness at low weight, stability under compressive and shear loads and energy absorption capability. In this study, 3D printing was used to screen biocomposite sandwich panels easily and preliminarily with different unidirectional core stiffener designs. Thermomechanical pulp (TMP) fibre-reinforced poly(lactic acid) (PLA) was used in this study. A corrugated, trapezoid and arched cell structure were tested experimentally and numerically using a bimodular material model, accounting for different behaviour in tension and compression. The trapezoid structure showed the best flexural properties of the three 3D-printed sandwich beams. It was chosen to be explored further, manufacturing it by extrusion. Extrusion is a production process likely to be used in industry on a larger scale. Basic material properties of the biocomposites were obtained from injection moulded dogbone specimens. The flexural properties of the extruded panels were measured experimentally and simulated using finite element analysis. Simulations were done with a hyperelastic material model. Predictions and experiments were in adequate agreement, allowing such kind of simulation to be used for extruded biocomposite sandwich panels. © 2023 The Author(s)