Anisotropy / Layer adhesion (XY vs Z)

Theory

Every layer-by-layer process creates a preferred material plane. In FDM each deposited bead fuses to the previous layer by partial re-melting. The bond area is smaller than the bead cross-section, so Z tensile strength is only 60–80% of XY. SLA cross-links propagate across layer boundaries during exposure overlap, yielding near-isotropic results. SLS powder sintering is nearly isotropic at 98% XY/Z thanks to uniform thermal diffusion in the powder bed.

Application

Orient the longest load path along XY. For a bracket that will be pulled in tension, place the load axis horizontal. For thin hooks, orient the hook arm flat, not upright. In SLA/SLS the orientation penalty is negligible, so optimise instead for surface quality and support reduction.

Common mistakes

Do not assume SLA is perfectly isotropic: thin cross-sections printed vertically in SLA may peel at layer boundaries under cyclic stress. In FDM, a Z-printed bolt will strip threads before the bulk material yields.

Related terms: Build orientation decision, Surface finish vs orientation, Residual stress and stress relief (DMLS)