Fail of The Week — Accidental Demagnetization

There’s a trick in the world of plastic enclosures. The threaded insert is a small cylinder of metal with threads on the inside and a rough edge on the outside. To make a plastic part with a hole for securely connecting bolts that can be repeatedly screwed without destroying the plastic, you take the threaded insert and press it (usually with the help of a soldering iron to heat the insert)  into a hole that’s slightly smaller than the insert. The heat melts the plastic a little bit and allows for the insert to go inside. Then when it cools the insert is snugly inside the plastic, and you can attach circuit boards or other plastic parts using a bolt without stripping the screw or the insert. We’ve seen Hackaday’s [Joshua Vasquez] installing threaded inserts with an iron, as well as in a few other projects.

This trick is neat. And I’ve now proven that it does not work with neodymium magnets.

It happened while working on a new product. I’m using a plastic part as a rotating cover. In one position it covers two holes. In the other position, a magnet embedded in the plastic moves over a magnetic reed switch, supplying power to the microcontroller and turning the device on. It’s a slick way to turn on/off the device without a visible mechanical switch. Unfortunately, the magnets kept failing to trigger the switch. Eventually I discovered that the magnets were losing their magnetism when I was trying to press fit them into the plastic with the aid of a soldering iron. This was not a simple problem to troubleshoot.

We covered the basics of magnets nearly two years ago, and there is a specific property of magnets that tripped me up. Once heated up, magnets can lose their strength. For some metals these temperatures are pretty high. For neodymium, it happens to be very low.

Working Temp, Irreversible Loss, and Curie Temp

The max working temperature of a magnet is the temperature below which there should be no loss of strength, and for neodymium magnets this varies by grade of neodymium. The temperature for N-type comes in at 80°C, while AH-type is workable up to 230°C.  Since ABS extrusion happens around 230°C, the temperature needed to insert a magnet will almost certainly be above the max for the magnet. Fortunately, the max working temperature is only for reversible loss. Once the magnet cools again, it should be at roughly the same strength, though prolonged heat can permanently reduce the strength.

The next level of damage is irreversible loss, which happens above the max working temperature but below the Curie temperature. In this region of heat, the magnet loses its strength, and that loss is maintained even after it is cooled. The only way to repair the magnet is to put it in a strong enough external magnetic field, but who wants to add an extra step of recovering the magnet after inserting it?

The Curie temperature is where the permanent damage takes place. Above this temperature, the magnet is done. For a neodymium magnet, the Curie temperature is 310°C for N-type, ranging up to 350°C for AH-type.

Magnet type	Max. working temperature	Curie temperature
Neodymium N	80°C	310°C
Neodymium M	100°C	340°C
Neodymium H	120°C	340°C
Neodymium SH	150°C	340°C
Neodymium UH	180°C	350°C
Neodymium EH	200°C	350°C
Neodymium AH	230°C	350°C
Nickel		354°C
Iron		770°C
Cobalt		1127°C

In theory, then, it may be possible to heat up a magnet to snuggle into some ABS and only suffer some reversible loss of strength, but if you aren’t careful and have your iron set to max, you’ll destroy the magnet permanently. In the future I’m not going to risk it. I’ll be 3D printing my holes slightly larger than the magnet and using an adhesive.

This lesson only cost me a buck in lost magnets and some time. May my shame and failure bring you success.

Filed under: Engineering, Fail of the Week