A spool of bronze-filled PLA is mostly bronze. Loadings run from about 40% up to 80% by weight, and what remains is polylactic acid acting purely as a binder. The result is a filament that behaves nothing like the polymer it is built on: it is cold to the touch, it does not feel like plastic in the hand, and a finished part rings faintly when you tap it.
It is also, by a wide margin, the heaviest and shortest-lasting filament you can buy, and the one most likely to wreck a cost estimate.
The density surprise, in metres
Metal-filled grades run from 2.0 to 4.0 g/cm³ against PLA's 1.24 — the widest and highest density band on this site. Two consequences follow immediately.
A spool holds far less. One kilogram of 1.75 mm filament at 3.2 g/cm³ is 129.92 m. The same kilogram of plain PLA is 335.28 m. You are buying roughly a third of the length for a higher price, which is the single biggest reason people run out mid-print on a model they have printed successfully before.
The same model weighs two and a half times as much. A part that consumes 200 g of plain PLA needs the same volume of the filled grade: at 3.2 g/cm³ that mass is 158% higher, and where the PLA version drew 67.06 m off the spool the bronze version draws only 25.98 m for its 200 g. If your slicer profile still carries a PLA density, every gram figure and every price it produces is wrong by that factor. Fix it in the profile and check it against the length and weight converter.
Finishing is the entire point
Straight off the plate a metal-filled print looks dull and slightly gritty, and people conclude they have been sold sparkly plastic. The material only becomes what it was bought for after finishing, and the finishing works because the particles at the surface are genuinely metal.
- Sand progressively, then burnish with fine steel wool or a brass brush. Each pass exposes and smears more metal across the surface, and the shift from grey-brown to a real metallic lustre happens quite suddenly.
- Patinate it chemically. Bronze and copper fills respond to the same treatments as solid metal — a liver-of-sulphur bath darkens them, salt and vinegar over a few days brings up verdigris. The green is real copper corrosion, not paint.
- Rust the iron grades on purpose. Iron-filled PLA left damp with a little salt water develops genuine surface rust, which is a finish nothing else on a desktop machine can produce.
- Seal it afterwards with wax or lacquer, or the patina keeps developing.
Steel and stainless-filled grades are the least rewarding here: the particles are harder, they polish less readily, and the result reads as dark grey rather than as steel.
It is very weak, and that is not a defect to tune out
Tensile strength lands between 10 and 30 MPa — the lowest figures on this site, below every unfilled polymer here. Metal powder is a filler, not a reinforcement: the particles are roughly spherical, they bond poorly to the matrix, and each one interrupts the polymer's continuity. Displacing most of the plastic with something that carries no load makes a weak part.
So this is a decorative material with a structural ceiling, and heavy decorative parts have their own failure mode: the mass itself can peel a tall print off the plate or shear a thin section under its own weight. Keep the centre of gravity low and the walls generous.
Feeding a heavy, sluggish melt
Heat is the one thing that does not change. Since the binder is polylactic acid and the metal powder simply rides along in it, the temperatures are PLA's — 195 to 225 °C at the nozzle, 45 to 60 °C on the bed. Everything mechanical is different:
- Hardened steel nozzle, no exceptions, and 0.6 mm as a practical minimum. Metal powder is more abrasive than carbon fibre and it will hollow a brass nozzle at a rate you can measure in hours.
- Print slowly. The dense melt is heavy and sluggish, and it does not respond crisply to acceleration changes.
- Expect settling. Filament that has sat on a shelf for a long time can extrude unevenly at the start of a print as the loading distribution recovers.
- Drying is genuinely optional; metal powder is not hygroscopic and the PLA binder tolerates humidity as usual.
What metal-filled PLA is genuinely bad at
- Load-bearing anything. See the tensile figure.
- Nozzles, gears and PTFE liners — it consumes all three.
- Small nozzles and fine detail.
- Heat and sunlight, exactly as plain PLA, since the binder's glass transition is unchanged.
- Long prints unattended, because partial clogs from agglomerated powder are common.
- Cost. $45 to $100 per kilogram, for a kilogram that goes a third as far.
What it is not
It is not metal printing. There is no sintering step, no furnace and no densification; the finished object is a plastic part carrying metal powder, and it has the strength of the plastic, not the metal. Filaments that genuinely become metal exist — they are debound and sintered in a kiln, they shrink dramatically doing it, and they are a different process entirely.
Used for what it is — medals, coins and tokens, jewellery, ornaments, sculpture, weighted bases, replica hardware and props — it does something no other filament can. Used as a structural material, it is the weakest thing on the shelf.