Summary

Some metals can be directly cast into 3D printed ABS molds.

Instructions

I won't talk about metal casting practices or safety because this is not the place for long, boring instructions which can so easily be found elsewhere. If you lack the necessary skills or knowledge, study and learn before attempting to cast metal. My aim here is to illustrate things specific to this technique.

While struggling to build a plaster mold a couple weeks ago, it occurred to me that an ABS printed mold might be able to contain small amounts of some lower melting point metals. I began testing things..
First attempt was with pewter M.P. 466F. Then lead (M.P. 624F) and then an unknown flavor of Zamak (the family of zinc die casting alloys, mp ~775F)

I have no idea if anyone else has already done this. Didn't see anything around here or with a short search. But if so then credit to them. There may be nothing new under the sun but this is all new to me.
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A pyrometer will probably be required. Some multimeters have a "Temperature" function and include a chromel-alumel wire lead.
I also have a non-contact IR thermometer but didn't try it. It would measure the surface temperature but cannot be immersed in the melt and I think it would be less accurate. Still, some trial and error might overcome that.
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Zinc:
Zinc pour was at 770F or slightly less. It began to solidify at about 760F. 780F was too hot and melted some of the mold in the 0.10 inch thin test-print mold (See Files). The too-hot mold got real soft and bulged outward in spots, but nothing melted through...

Generally speaking, there are numerous reasons you want the metal to cool fast but plastic is a good heat insulator. Compared to steel molds, the zinc solidifies very "slowly" on contact with the plastic mold walls, and stays pretty hot for a long while. So, zinc casts suffered significant shrinkage and were weak and brittle with the resultant large crystal formation.

Zinc permanent mold die casters can "pour" it cooler and almost slushy, like at 700-725F, because it is forced into the mold. But for gravity casting the metal must be very liquid or it won't pour at all.

Lead:
Tried pouring some lead. Pure lead was poured at around 600F but this approaches the point of being difficult to work with. I used a lead caster's pot (Palmer 400W Hot-Pot) commonly used for casting fishing weights and bullets. Max temp on this pot is near 900F.

Lead is cheap, soft, dull and ugly. It doesn't pick up detail like pewter.
I don't know why anyone might waste their time casting lead. 3D printed molds deserve better than that.
Pewter might be more expensive but is far superior in every other way.

Pewter:
Pewter, OTOH, with its lower melting point of 466F (Formula R-92: 92% tin, 8% antimony) offered no difficulties whatsoever, even in thicker sections. Achieved excellent detail. And it can be melted on a laboratory hotplate in a stainless steel cup.

Remove mold plastic with sharp nippers but be careful not to damage your parts. I would not use fire or heat or brute force to break out the mold. Pewter bends, softens and melts too easily. I used acetone to dissolve any ABS pieces which couldn't be safely removed with nippers.

Acquisition of pewter: Thrift store hunting or buy it.
Various heavy, cast decorative items are very often pewter. Zinc predominates in mass produced items. Pewter items are often stamped somewhere on the bottom with the word "Pewter".

Metal suppliers sell ingots. My ingots weigh about 4 lbs each. Many years ago, just for kicks I walked into a local Rotometals distributor and bought some fusible alloy and a couple ingots of R-92 pewter (92% tin, 8% antimony) to make a chess set... which never happened. I recall because they didn't know me and wouldn't take a business check so I had to go out and get some cash. I didn't touch the pewter until now.
I see Amazon also sells pewter.

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Having said all that, just forget about lead and zinc.. seriously. Those were just experiments to see what was possible. In the following days I plan to try aluminum and pour something at perhaps 1100 F, just to explore the limits. It might be possible to pour aluminum in very thin sections.. Ya never know.

Go get some Pewter. It's the best casting material by far. Make jewelry, rings or medallions or a million other things with the least amount of trouble and the greatest beauty and detail reproduction. It is pretty strong, easily tooled and it polishes to a bright silver shine.
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Mold design:
The toughest part of all this is mold design. You are working in reverse and creating a negative. You are designing an empty cavity. Boolean operations are an indispensable tool here, imo, but they can't think for you.

The cavity cannot have ANY support inside of it. You don't want to pour metal around some hidden, stray ABS support or your finished pewter piece will contain plastic within it. And you can't get into the mold and remove the support. So, use cores and / or multiple mold parts to avoid printing any internal support.
Most people have no idea what I just said but once you try to design some fancy mold cavity it will dawn on you.. guaranteed.

A 2 piece mold for a flat plaque or pendant shouldn't pose any difficulties but a more complicated design may require significant thought and careful planning.

The two 0.10 inch thick test plate mold prints took 60 min, 35grams ABS with Afinia defaults but with minimum fill. Strength is not a factor here and a thick fill is wasted filament and takes time (and / or more acetone) to remove.

Pewter cast in ABS mold