Note: this is step 1 of a print / sinter / polish process.
This is a quick overview for how to print metal filaments in a "normal" 3D printer: Colorfab (Brassfil, Bronzefil, Copperfil), Proto-Pasta (composite SS, Iron, and Copper), and Virtual Foundry (Filamet in bronze, copper, cluminum) sell filaments with at least 50% metal content. Other "metal" filament with less content can be printed like normal PLA (or whatever base plastic) and is therefore less challenging.
Cost: this is not cheap. Colorfab and Proto-Pasta run about $80/kg, or roughly 4x the cost of decent PLA. Virtual Foundry's product is $200/kg, or ~10x the cost of decent PLA (and 2.5x the cost of Proto-Pasta, my go-to brand). I often run tests in cheaper plastic before switching over to the more expensive metal-filled materials.
Nozzle: use a big one. I've had lots of success with 0.50, but at least one material (Iron from Virtual Foundry) seems too big for it. Colorfab's materials, all Proto-Pasta, and brass/bronze/copper from Virtual Foundry all work great at 0.50. The nozzle should also be a hardened steel unit: bronze particles are harder than a brass nozzle, and will clean it out fast. Even the harder nozzles need to be replaced, I tend to run a couple per year. Keep in mind a nozzle is about $10, and a kilo of filament can run $80-$200 (depending on brand etc) so it doesn't make sense to ruin prints to save a few percent on a nozzle...
Bed: Filamet works best on a clean glass bed, heated to at least 50c. The material sticks REALLY well, and needs some encouragement to lift up when the bed cools.
The 50% products (Colorfab and ProtoPasta) like a little PVA, in addition to heat.
Layers: 0.1mm works great. Period. Larger is fine too. The *first* layer is pretty important, particularly if you're polishing, as gaps can really stand out when the material is polished.
Temps: the metal powder holds heat better than plastic, and it takes longer to heat up and to cool down. You can either run hot and fast, and cool and slow... but too cool+fast will clog, too hot+slow will puddle (top layer makes previous layers melt). Also pay attention to any automatic slowdown for small layers. GENERALLY I've had success around 190 and slow (30mm/s), but a friend runs like 210c at 60 mm/s on the same printer & nozzle. YMMV.
If the feed slows while heat is up, the material can debind inside the nozzle (burn out all the PLA and leave mostly powder). If / when this happens, you have to clean the nozzle or replace it. The jam won't clear with more heat & pressure, and it's pretty hard to get the block of powder back out.
Speeds: related to temp. Generally I like to let the printer run slowly and relatively cool, to avoid puddling and risk of clogging the nozzle.
Infill: doesn't matter. Do what you need. Generally I want something that looks like metal to feel like metal, so I keep them at (or very near) 100% infill.
Support: matters a whole lot. Don't be too stingy here: a metal print is harder to repair, and you'll be post-processing it anyway.
I'll write another article about finishing.