Printing with Metal Fill Filaments

Jackson B.

Metal Without the Foundry

Metal fill filaments, such as PLA or PETG loaded with 20–40% by weight of metal powder (iron, copper, bronze, or brass), produce prints with genuinely metallic properties: significant weight (prints are 2–3× heavier than standard PLA by volume); the ability to rust (iron fill); develop real patinas (copper fill, bronze fill); and be polished to a mirror sheen. The aesthetic possibilities are extraordinary; finished bronze fill prints can fool observers at arm's length into believing they're actual cast bronze. For sculptors, jewellery designers, award makers, and prop fabricators, these materials represent a uniquely accessible path to metal quality aesthetics from FDM printing.

Metal fill filaments are entirely different from abrasive carbon fibre or glass fibre materials in terms of their printing challenges. They print at standard temperatures and don't require exotic hardware beyond one critical requirement: a hardened nozzle.

The Abrasion Reality

Metal powder particles are significantly harder than brass nozzle material. Even the relatively soft metal powders (copper and bronze) cause measurable nozzle wear. Iron powder causes rapid wear due to its hardness relative to brass. A hardened steel nozzle is mandatory for metal fill printing. The wear manifests progressively: the nozzle orifice enlarges, line widths become inconsistent, and surface quality degrades. A hardened nozzle handles many kilograms of metal fill material without this degradation.

A larger nozzle diameter is also beneficial for metal fill: 0.5 mm or 0.6 mm, rather than 0.4 mm, reduces clog risk and allows higher flow rates with the dense, heavy material. Use with PLA base materials at standard PLA temperatures (200–220°C); this allows the metal particles not to affect the thermal profile significantly.

Post Processing for Maximum Effect

Where metal fill truly earns its name is in post processing. The process is identical to actual metalworking: sand with progressively finer grits (120 → 240 → 400 → 800 → 1200 → 2000 wet). At finer grits, the metal particles in the surface emerge as the bulk plastic matrix is sanded away. The surface begins to look and feel like metal, not painted or coated, but genuinely metallic. Polish with metal polish (Brasso, Autosol, or equivalent) to raise a shine. For iron fill, mist with water and leave it exposed for 12–24 hours to create an authentic rust patina, then seal with wax or a clear coat. For copper fill: apply liver of sulphur (from jewellery supply stores) for a beautiful, authentic antiqued copper patina, practicing the same process used by metal sculptors for centuries.

The final results, when properly finished, photograph and display magnificently and typically surprise even experienced observers who can't immediately distinguish them from cast metal objects. See our complete finishing guide for the progressive sanding technique that produces the best results and our electroplating guide for depositing actual metal layers on top of printed substrates, a complementary technique for even more convincing metallic results.