Most recent answer: 04/26/2020
- Peter (age 76)
Maple Ridge, BC
There are two possible problems with your method, and I'm not sure which is more important.
1. It's possible that in grinding up the NdFeB the heat and mechanical stress altered the local structure, so that the aren't good ferromagnets even in each small domain. My guess is that this is not a huge problem, especially since these magnets are often made from sintered powders. If the strip is still strongly attracted to magnets, you still have magnetic domains left.
2. Assuming you still have those domains, you've now oriented them randomly. It's not easy to get a magnetic domain to switch its "permanent" orientation. You might need stronger fields than simply the NdFeB magnets themselves. I'm not sure what geometry you're using but probably just adding more magnets won't increase the fields much.
Perhaps if you heat the strip up as much as the plastic can handle just as it enters the magnetic field, that will allow the domains to reorient. here's what Wikipedia says on that "Neodymium-iron-boron magnets have high coercivity at room temperature, but as the temperature rises above 100 °C (212 °F), the coercivity decreases drastically until the Curie temperature (around 320 °C or 608 °F). " (https://en.wikipedia.org/wiki/Neodymium_magnet) Coercivity is the name for how big a field it takes to reorient the magnetic domains. Of course you then have to make sure that it passes through slowly enough that they're cooler on the way out of the magnetic field, so they stay aligned. Having a longer array of magnets would help for that. Also, you have to be careful not to heat your magnets enough to demagnetize them.
It doesn't sound easy to do well at home, but I bet you can get at least a bit more magnetization of the strip than you're getting now.
(published on 04/26/2020)