The direction-dependent mechanical properties of FDM parts, where strength along the print axis (Z) is significantly lower than in the XY-plane.
FDM parts are inherently anisotropic because layers bond by thermal diffusion rather than chemical fusion. The XY-plane strength (along filament roads) can reach 70-90% of bulk material strength. Z-axis strength (perpendicular to layers) is typically 20-50% of bulk. This anisotropy is analogous to wood grain: the part is much stronger in the extrusion direction than across layers. Crystalline polymers (nylon, PEEK) show more pronounced anisotropy than amorphous ones (PLA, ABS) due to crystallite formation at layer interfaces.
Orient critical features so that primary loads run in the XY-plane rather than Z-axis. For tensile-loaded bolts or pegs, orient them parallel to layers. For bending loads, orient the neutral axis along the Z-direction. Use higher temperatures, lower fan speed, and pressure advance calibration to maximise interlayer bonding. Post-annealing crystalline polymers (PETG, nylon) below their Tg can improve Z-strength by 10-20%.
FEA simulation using bulk material properties overestimates FDM part strength by 2-5x in the Z-direction: always apply anisotropy correction factors. Changing print orientation to solve anisotropy may worsen surface quality or support volume. Filled filaments (carbon fibre, glass fibre) improve XY-plane strength but often worsen Z-axis adhesion due to poor fibre-matrix bonding at layer interfaces.
Related terms: Layer Adhesion, Part Cooling, Wall / Perimeter
| Field | 3D printing, mechanics, engineering |
|---|---|
| Also called | layer anisotropy, FDM anisotropy, directional strength |
| Source | EDi Brain: tech/fdm |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert