Magnetic Catches & Closures

Catches and closures are where the magnet is asked to hold rather than to rotate: a door catch pulling a steel plate shut, a curtain tieback closing around the fabric, a latch keeping a cabinet or an access panel closed. The magnet is never the whole mechanism — it is one half of a magnetic circuit whose other half is the steel counterpart it pulls against, and whose working distance is the small air gap between them. That gap is what the specification has to be written around.

Pull force is written at the gap, not in free space

A magnet quoted in free space does not describe a catch, because a catch works across a gap. Pull force rises steeply as the gap closes and falls steeply as it opens, so the useful number is the force the catch must hold at the gap it actually sees, against the steel part it actually pulls. Stating that number — the required pull at the working gap, against the counterpart — is what turns a vague request into a magnet that can be specified.

• The pull force depends on the magnet, the steel counterpart, the gap and the contact area together, so it is a circuit value confirmed per project, not a number read off a magnet on its own.
• A steel cup or pot concentrates the flux on one face and shortens the circuit, which is why catch magnets are so often pot magnets rather than bare blocks.

The magnet is brittle, the mounting is not optional

Ferrite in particular is a ceramic: it holds a useful field cheaply, but it chips and cracks under impact, and a door catch takes impact every time the door closes. The standard answer is to carry the magnet in a steel cup or housing that both concentrates the flux and takes the mechanical load, so the magnet itself never meets the strike plate. Mounting is therefore not an afterthought — it is part of the magnetic and mechanical design at the same time.

Corrosion follows the room, not the magnet

A curtain tieback in a bathroom, or a catch on an exterior gate, sits in humidity that a catch on an interior door does not. Sintered ferrite is normally supplied uncoated and is generally corrosion-resistant; NdFeB needs a protective coating, and that coating is selected against the actual exposure. Whether an uncoated part is acceptable is confirmed for the project against where the part lives.

Where the material choice lands

Catches and closures sit almost entirely on one of two answers. Where the volume allows it and the pull requirement is modest, strontium ferrite does the job uncoated and at the lowest cost. Where the same pull has to come out of a smaller envelope, or the catch must feel firm across a wider gap, sintered NdFeB delivers several times the flux in the same volume, at a higher material cost and with surface protection that is mandatory. The decision is a pull-at-the-gap and volume question, confirmed per project.

Recommended forms

Pot and cup magnets with a steel shell are the established form for catches, latches and closures; bare ferrite blocks and discs suit assemblies where the customer’s own housing carries the load. Sintered NdFeB discs or blocks in a cup cover the higher-pull, smaller-envelope cases. In every case the form follows the mounting and the pull target rather than the other way round.