Moldable Iron Powder

Interesting! What is it called, and is there a web reference?

Don't know about the power, but I've used these people for sample cores. Fast service and good product. They might be able to find you some powder.

Searching on "magnetic paint" on the Home Depot or Rust-Oleum sites should get you there.

Hi Tim, There are a lot of iron filled epoxies out there. Two names that come to mind immediately are Moglice and Devcon. I recall Dave Trumper at MIT had a nice mag-lev demo that used a photocell as feedback.

-- Jeff Lowe

TTim Wescott wrote:

Firstly, why do you think it's an iron-powder paint? Nickel, cobalt, hematite, are all ferromagnetic (and both hematite and some kinds of nickel are used in paint).

Secondly, if you want custom magnetics on the cheap, you can get soft iron wire and make toroids quite easily; no powders required. Small cores, untwist a coathanger. Large cores, soft iron wire from building-supply places is reasonably priced (five pound rolls are common, for the ferroconcrete trades).

It isn't terribly practical to make magnetic parts from low-density mixes of slightly magnetizable particles in a plastic matrix, because the size of the core gets larger, then you need more wire, which has to be thicker, so the aperture has to be larger, then the core has to be larger... And, if you COULD make it work, the plastic core would age and change values more than a solid slab of iron or a sintered bunch of powder or a milled/fired ceramic. It'd also be more likely to catch fire or outgas toxins than the 'standard' cores.

There's uses for magnetic paint, though; like conductive paint, it blocks EM radiation. You could, theoretically, paint the right kind of bullseye pattern on your house siding over the primer, and under the finish coat, to make a kind of hologram mirror (stealthy satellite dish construction, the homeowner's association will never know).

Parenthetically, common hematite-based black paints aren't ALL ferromagnetic; the particle size in the pigment is critical, very small particles don't hold a magnetization like a large crystal of the pure material would.

How about a ping-pong ball into a chemical silvering mirror solution and then electroplate iron onto it in a simple ferrous plating bath?

Yes. Although the raw material when dry requires a bit of careful handling preferably in an inert atmosphere. Finely divided iron powder intended for magnetic crack detection is usually supplied as a suspension mixed in kerosene. In the air as a dust it can be pyrophoric.

It is fun to paint some on a magnetic card strip you no longer need.

You might be better off with finely ground magnetite. That is easier to handle and doesn't go woof. This stuff is in vogue these days for fancy magnetoresponsive goops in various esoteric devices. You can make it do some of the T2 type flow then rigid things with a bit of effort - except it is dirty black gunge and gets everywhere instead of a nice mirror finish like in the movie... and a bit fanciful in some articles by the proponents of the technology. eg

Someone like Alfa or an NDT specialist supplier will sell you the stuff but whether you will think the price is reasonable is harder to answer.

Regards, Martin Brown

Thanks Jeff. That gave me the keywords I needed.

Goodness but it's expensive stuff, at least from McMaster. Gotta think about this again, maybe.

I assumed, but yes it could be some other magnetic material.

I considered that, but for the shapes I want it'd be hard to get what I wanted without considerable hand work.

I distinctly said that my air gaps are huge, and furthermore that I checked this with a FEA program that shows I don't need huge permeabilities. I also said that specific values aren't critical. Are you saying that of all the iron-powder-in-plastic materials that you're personally familiar with there's none with relative permeabilities in the mid 100's?

If you could give me a list of the brands that you're familiar with I'll go check specifications myself, thanks.

It's either soft iron, or it needs a strong field to polarize it. How is it used? (IIRC, the permeability of ceramic magnets id low, so they support large gaps too.)

Jerry

From what I found searching, it's soft. that is, magnets are attracted to it, but it's not attractive on its own.

Jerry

Right. I'm looking for an armature material for an oddball electromagnet

-- big, light (well, relatively), and capable of supporting a significant AC component without wasting it all as heat.

Devcon A is the original epoxy filled with a high percentage of "steel" of some sort. It was around in the 60's and is still available:

There are now others, turned up with search on "iron filled epoxy":

None of which discuss magnetic properties, but you could try calling the mfgrs.

Some years ago, we were designing bias tees for 10 GB/s laser transceivers and stumbled across those conical coils made by Piconics. A colleage of mine was so delighted that he wouldn't wait for samples and decided to build a few over the weekend. On monday he showed up with some coils that looked _ugly_ but the bias tee was flat to 20 GHz.

He never told us how he did it exactly, but I think he ground some cores in a mortar and added epoxy glue.

We finally found a combination of cheap 0402 devices that worked nicely, too.

regards, Gerhard

Reminds me of a friend, who decided to do his own roofing so that he could make sure that the spacing of the nails would not interfere with the satellite dish in the attic. He had the truss designed specially too, to give him the room and metal free window to the south...

Charlie

If you consider that the relative permeability is pretty close to the ratio of total gap to total path through the magnetic material, where the total gap includes all the gaps between the magnetic particles, you quickly see that the overall permeability of any practical mix of magnetic particles and plastic is going to have a permeability of around

Argh.

Need to keep thinking about this, then.

Err... but packing of spheres yields packing fractions of 0.523 and up, and the 'gap' fraction would be the cube root of that, 0.803 and up. This isn't a killer effect.

You should be able to get fine iron shot (used in some toner cartridges) which isn't a bad fire hazard, and mix into Bondo (auto body putty, known to be compatible with/bond to iron) and form some shapes easily enough.