A manifold is judged by what it does not do: it must not leak, must not trap residue and must not restrict flow. All three depend on how the internal passages and sealing faces are machined.
Cross-drilled and intersecting channels need clean transitions and controlled burrs — a burr at a passage junction is both a flow restriction and a contamination trap.
Flatness and surface finish on sealing faces decide whether an O-ring or gasket seals. They are machined for flatness and verified, not just cut and hoped for.
Aluminium and stainless steel for structural and hydraulic service; PEEK, PVDF and PTFE where chemical resistance or cleanliness rules, including microfluidic manifolds with narrow channels.