Magnets in 3D prints: pocket sizes, gluing and press fits

Short answer

Make the pocket a few tenths of a millimeter larger than the magnet: the Gridfinity generators here use a 6.5 × 2.4 mm pocket for 6 × 2 mm magnets. Glue the magnet in with a drop of CA glue or epoxy, and check the polarity before the glue sets.

To lock a magnet in without relying on glue alone, pause the print just above the pocket, drop the magnet in and let the printer print over it. The magnet then sits on the heated bed for the rest of the print, so keep the bed below the magnet's temperature limit.

Which magnets

Almost every 3D print with magnets uses small round neodymium magnets, also sold as NdFeB or rare-earth magnets.

Magnets are named by diameter × height in millimeters. A 6 × 2 mm disc is 6 mm across and 2 mm thick. The "N" number is the grade: higher numbers are stronger for the same size. Most small discs are magnetized axially, which means one flat face is the north pole and the other the south pole. That is what you want for a pocket.

As an example, supermagnete lists its 6 × 2 mm N45 disc with a tolerance of ±0.1 mm, a nickel coating, a pull of about 740 g straight off a steel plate and a sliding force of only about 150 g. The sliding number matters more than people expect: a magnet holds much better against a pull straight away than against a sideways push. A bin on a baseplate is mostly held sideways, so the magnets keep it from shifting a little but don't lock it down.

Heat matters too. The same data sheet gives a maximum working temperature of 80 °C, and supermagnete warns that most neodymium magnets lose part of their strength for good at 80 °C. That rules out a hot car dashboard, and it is one reason to glue magnets in rather than melt them into the plastic with a soldering iron.

Neodymium magnets are meant for dry indoor use. The coating protects the metal, but once it chips the magnet can rust. For parts that live outside, cover the magnet fully with plastic.

Pocket size and clearance

A magnet pocket needs a little room, because printed holes come out smaller than designed and the magnet itself varies by about ±0.1 mm. How much room depends on how you fix the magnet:

• For glue: about 0.3–0.5 mm larger than the magnet in diameter and 0.2–0.4 mm deeper. The magnet drops in, and the glue fills the gap.
• For a press fit: start at the magnet's exact size and print a test. The plastic has to give a little, so the right size depends on your printer.
• For pause-and-insert: the pocket must be at least as deep as the magnet, with a little extra, so the top of the magnet sits below the next layer.

The pockets in the Printaro generators

The Gridfinity bin and Gridfinity baseplate generators can add magnet pockets for 6 × 2 mm magnets, four per grid cell. Each pocket is 6.5 mm across and 2.4 mm deep: 0.5 mm of room on the diameter and 0.4 mm on the depth. That is sized for glue. A magnet that is a little loose in it is normal. The Gridfinity guide explains the rest of the system.

The fridge magnet generator lets you enter your own magnet size, from 5 to 20 mm across and 1 to 5 mm thick. It adds 0.3 mm to the diameter and 0.2 mm to the depth of each pocket. Its default is a 10 × 2 mm magnet, and it places one magnet in narrow labels and two in labels 60 mm wide or more.

Measure your magnets with calipers before you print. A bag labeled 6 × 2 can hold magnets that are 5.9 or 6.1 mm across. If your printer makes small holes much too tight, see fit and tolerances.

Glue, press fit or pause-and-insert

Glue is the safest choice for most parts. CA glue (super glue) sets fast. Two-part epoxy gives you more time to fix the polarity and fills larger gaps. supermagnete recommends a magnet glue, UHU Max Repair, that stays movable for about five minutes and reaches full strength after about 24 hours. Polyethylene, polypropylene and PTFE are hard to glue, so magnets in a polypropylene print need a press fit or a printed-over pocket.

A press fit needs no glue but depends on an exact hole. Too tight and the plastic splits or the magnet won't go in. Too loose and it falls out. Push the magnet in with a flat tool, not with another magnet: two magnets snapping together can chip.

Pause-and-insert traps the magnet under a printed roof. In the slicer, add a pause at the first layer that covers the pocket. When the printer stops, drop the magnets in (check the polarity), and resume. The printer then prints over them. The fridge magnet tips suggest this. In the Gridfinity bin and the fridge magnet the pockets open on the face that lies on the bed, so the magnet stays visible on that face but can't come out the other side.

The Gridfinity baseplate is different. Its pockets open upward into each cell, with a thin 0.2 mm floor below them, so there is nothing to print over. Glue those magnets in after printing. No pause is needed, and the heated bed is not a concern for them.

For the bin and the fridge magnet, three things to watch. The magnet must sit fully below the layer height, or the nozzle hits it. Steel parts near the nozzle can pull a loose magnet out of its pocket, and a tiny drop of glue under each magnet keeps it in place. And because the pocket is open at the bed, the magnet sits on or right next to the heated bed until the print ends.

That last point decides which materials work for the bin and the fridge magnet. The 6 × 2 mm disc above has a maximum working temperature of 80 °C. A PLA bed at the 60 °C Prusa gives for PLA stays below it. A PETG bed often runs at 80–90 °C (see how to print PETG), which reaches or passes the limit. For PETG and other hot-bed materials, glue the magnets in after printing instead.

Polarity

A magnet that should pull toward steel, such as a fridge door or a steel plate, works either way round. Polarity only matters when two magnets have to find each other, as in a Gridfinity bin on a magnetic baseplate or a box lid held shut by magnets.

For two parts to attract, the faces that meet must have opposite poles. A simple way to get this right every time:

• Take all magnets from one stack, as they come out of the package.
• Mark the top face of the top magnet with a marker and slide it off the stack. Then mark the new top magnet the same way, slide it off, and repeat. Every magnet now has its mark on the same pole.
• In one part (for example every bin), glue every magnet with its marked face pointing out of the part.
• In the other part (every cell of the baseplate), glue every magnet with its marked face pointing into the pocket.
• Before the glue sets, hold the two parts together and check that they pull, not push.

Common mistakes

• One magnet turned the wrong way. It pushes instead of pulls, and in Gridfinity one bin won't sit flat. Check every pocket before the glue sets.
• Pocket too tight. The magnet won't go in, or the plastic splits around it. Add a few tenths of a millimeter.
• Pocket too shallow. The magnet sticks out, and in a pause-and-insert print the nozzle hits it.
• Magnets near heat. A part on a hot car dashboard or near a heater can lose magnet strength for good.
• Magnets that can come loose around small children. Children could swallow small magnets, and if several are swallowed they can get stuck in the intestine. supermagnete tells you to keep them away from children. Glue them in firmly or cover them with plastic.
• Pushing a magnet in with another magnet. They snap together and can chip. Use a flat tool.

Frequently asked questions

Make it

Gridfinity Bin

Create custom Gridfinity bins

Gridfinity

Gridfinity Baseplate

The grid your Gridfinity bins sit in

Gridfinity

Fridge Magnet Label

Text magnets for the fridge or whiteboard

Kitchen

Sources

1. supermagnete: disc magnet 6 × 2 mm (S-06-02-N), technical data
2. supermagnete: safety information for neodymium magnets
3. supermagnete: UHU MAX REPAIR magnet glue
4. Prusa Knowledge Base: PLA
5. Prusa Knowledge Base: PETG