3DPrinting

If you need to do marketably food-safe items, you're best off using your printed part to create a mold for pouring food-safe epoxy. Or at least that's quicker than having to test every print to make sure they haven't acquired contaminants inside the printer, like lead from a Chinese-made brass nozzle.

If you're not intending to sell the printed parts or any food created using them, the main thing you have to account for is the porosity of printed surfaces. There are mutiple ways of dealing with this issue: coat the printed objects, line them with something food-safe, or vapour-smoothe them.

There are filaments available that are certified as food-safe provided the printer is—here's one—but any good quality PLA (and I would expect also PVB and HIPS, since those plastics are used in food packaging—maybe others as well) shouldn't shed anything particularly dangerous. I've never heard of food-safe resin, but that doesn't guarantee that it isn't out there somewhere—I've never owned a resin printer.

The metal-impregnated PLA filaments are mostly "for the looks", but there are some that can probably be detected by various sensors ( here's one that's magnetic )—just be aware that they may not be any stronger than the binding plastic. You might be able to get actual metal parts out of an industrial laser sintering printer (a technology that uses a laser to melt powder), but those are priced way out of the range of a home hobbyist, so I don't know if anyone here has direct experience with them.

I have printed some food safe prints for a friend couple of times now. Nozzle is stainless steel, Filament is natural HIPS. Stainless & HIPS are both food safe materials (if not mixed with color pigments). After the print, you need a clean surface. HIPS can be smoothed in an Acetone-Bath. Simply put it in for 10-15 min and you'll have a glossy finish.

HIPS is very easy to print in an enclosure and the resulting product is fairly stiff and durable. It's also temp resistant up to 90-100°C. So it's dishwasher safe.

Do not inhale acetone vapor, wear respiratory masks that filter out voc like acetone vapor.

Check out Formlabs resin printers. They have special resins that may do what you need. They also have sales people on staff to walk you through everything.

They’re pretty pricey for a hobbyist (at least for this hobbyist) but a business may be ok with it.

For main application (prototyping, tools, parts, etc) you should be fine with any hoby printer. Safe bet is prusa, but it realy depends on your budget and expectations.

I dont know what exactly you want to do with your printer, I stopped being the guy saying printing is not food safe, but since you mentioned manufacturing I think you need a warning.

For food safe manufacturing, if you are doing it to make money, you will have a hard time. Its not enough to get "food safe" filament if one even exists. You need certification for whole process, at least in EU. All parts around the filament should be food safe, and you need loads of paperwork to prove its safe. There is much more, but not worth to go deep into that since FFF printing is anti food safe technology. Like others said, uneven surface, tiny gaps everywhere and thermal properties are recipe for bacteria and other unhealthy crap. Coating will not fix that. Even if its a one time use product, you could still end up with small bits of filament in food etc. Stringing and imperfections are quite normal for FFF and I dont think resin is any better for this.

If you want to play safe, get cheap printer for prototypes then order injection moulded or machined parts for production.

If you plan to use it just for yourself, I dont think its dangerous, but I still dont recommend. PLA/PETG are probably best choice since they are easiest to work with if you dont need to use it at temps above 50-60C.

There are also some extruders for chocolate and simmilar food materials, check that out.

If you find filament with metal reinforcements, abrasion will probably eat your nozzles super quick. If you need magnetic plastic, its probably better to model your parts to fit metal or magnet peace inside. Its quite common to see people stopping printer midprint, inserting magnets and then continue printing.

Also If you are worried about rust on magnets, you can paint them, dip into epoxy or maybe even powder coat.

Edit: I saw people recommending some food-safe solutions. I never tried any of these, I have no experience in food production. Im working in production company that gave up on food-safe products (stainless steel) because it was too expensive and complex (not impossible tho) to follow all required rules and law. 3D printing is just my hobby and I used to have an idea of food-safe 3D printing long time ago. Gave up on that obviously. If you still decide to continue with your idea, Im interested, keep us updated. Good luck

As Captain Flintlock said, here's something I found on Formlabs. https://formlabs.com/blog/guide-to-food-safe-3d-printing/

First of all I am not into food or GMP.
What filaments? Propably PP (if you don't need foodsafe either use PP plates or packaging tape as build plate), ASA, Nylon and if those don't cut it PEEK. I personal like FormFutura filaments as most of them print well.
To name an "unusual" filament brand: Igus. Not sure if they are food safe but they are plastics with solid lubricants. This could be interesting for moving parts but: Using injection molded skates and roller will have a higher durability compared to FDM 3D-printed. Request free samples or talk to them. Be warned they can be a pain to print.

So you've got a few main varieties of printing technology (detailed here). As for your requirements I'm not sure there's enough metal in the metallic filaments to trigger a metal detector, but this article should be a good starting point for you.

For a business, Prusa probably makes sense. You lose money when things don’t turn out right, so spending a bit more up front is the right choice.

Beware that FDM prints are full of tiny holes which are basically impossible to thoroughly clean, so they can be “food safe” for the first use, but once they’re used they may be dirty forever.

Determine how big the parts need to be. Print volume will dictate a lot and eliminate a ton of printers. I liked the Raise3D printer I had. Large volume. Fully enclosed. The nozzle will be important for food grade stuff. I've never really needed to worry about food grade, so I can't help you there, but never trust any cheapo Chinesium nozzle.