Adding magnets to a 3D print can be very useful in a design, but there are some things that can trip you up if you’re not aware of them. In a recent video by [Lost in Tech] some of the essentials are covered, including why you shouldn’t get magnets near most extruder nozzles or the printing bed.
The easiest method is of course to add magnets in after printing, using friction fit with or without ribs, or with a dab of glue. Here making sure that the magnet stays in place is the trick, as you do not want the magnet to get lost or end up in the tummy of a curious pet or toddler.
Things get spicy when you’re talking about adding magnets during the printing process, as some extruders are made of a ferromagnetic material and thus a magnet will happily stick to said nozzle if it’s not pure brass or similar. As seen in the video even some purported ‘brass’ nozzles aren’t pure enough to not be significantly ferromagnetic.
Another issue is that of heat, which is something that magnets generally do not like much. Using magnets like you’d use heat inserts for bolts is a recipe for disaster, as the heat from a soldering iron will demagnetize the magnet, which for the typical magnet is less than 200°C. At least this should mean that the magnet stuck to your extruder nozzle will eventually fall off by itself after it demagnetizes.
With the bed of the typical FDM printer these days you’re talking about magnetically attached plates, with the underlying heated bed using a Halbach array configuration as is typical of flat magnets, yet with the gotcha that these aren’t typically real Halbach arrays, but knock-offs with simply alternating north-south pole magnets. As it turns out, these types of magnetic arrays can be disturbed by another magnet, such as a powerful neodymium magnet near said printing bed, flipping polarity in a way that cannot be easily undone.
You can still install magnets during printing, but it’s recommended to use something like side-insertion, where the extruder nozzle cannot pull out a magnet. Regardless of your approach, it’s good to know of the risks with ferromagnetic nozzles, the magnetic bed and treating magnets like they’re just heat inserts. While you can get higher-temperature magnets, many of the same issues still remain here.
Embedding magnets in prints is abusing them in the first place. Magnets like short air gaps, so you’re pretty much wasting your high grade neodymium magnets by separating them with millimeters of plastic. If you want two parts to stick to each other by magnets, something like magnetic tape on both surfaces would perform better with less stray fields around where you don’t want it.