Araldite failure?

I've had repeated failure when fixing small (3mm diameter) rare earth magnets to tufnol; the magnets simply pop out of a glue matrix (which stays
firmly fixed to the tufnol).
I've done the obvious - careful cleaning of all surfaces prior to fixing with meths.
Anyone else seen this?
BugBear
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bugbear wrote:

Not tried it, but epoxy won't play nicely with a number of surfaces. Try using silicone sealant or adhesive.
For those purposes I generally use glass (aquarium) sealant. If tufnol will stand it, try hot glue?
--
Old Nick

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On Mon, 3 Sep 2012 19:47:52 +0100, David Littlewood

Samarium cobalt magnets are better in that regard. I had some custom ones made when I designed some 4 position stirring plates with heating jackets for 220 C. Samarium cobalt can go to 300 C. No commercial heated stirring plate I could find went above 200 C. They mostly use neodymium magnets I believe.
Pete Keillor
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If the surface is greasy use white sprit or petrol to clean it; meths is a poor solvent for grease. Roughening the surface to give batter key will also help.
Cliff Coggin.
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I don't know the chemistry of rare earth magnets, but I know that aluminium forms an oxide film within seconds of being cleaned, the most effective way of bonding with araldite is to mix the Araldite, spread it on the aluminium, and abrade it with coarse sandpaper. Then the araldite bonds with core material, and it is a strong bond. Rare earth may be like it.
But 3mm is small!
Michael Bell
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On 09/03/12 12:03, bugbear wrote:

It may be the setting characteristics of the epoxy, but more likely something on the surface of the magnets. You could use a dremel to grind steps into the magnet at the glue point, but tedious if you have a lot to do...
Regards,
Chris
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On Mon, 03 Sep 2012 13:03:07 +0100, bugbear

Rare-earth magnets are invariably plated. Usually with Nickel. They corrode rather quickly if the plating is removed, so don't go down the path of abrading them. You could try setting them a bit more deeply into the holes and _covering_ them with the Araldite. That way it'll be a mechanical hold, rather than just an adhesive one.
Regards Mark Rand RTFM
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On 03/09/12 13:03, bugbear wrote:

I'm a bit surprised the glue sticks to the tufnol so well... but then there are lots of different tufnols.
Most rare earth magnets are plated with nickel, and are very smooth and shiny. The smooth surface is not very good for epoxy adhesion, and some of the additives, especially stabilisers for the nickel plating baths, also prevent good adhesion.
Some people may tell you to scratch or sand the surface, but the nickel coating is usually pretty thin, and if you go through it then it will most likely lead to corrosion problems.
Meths is not the right stuff to clean magnets or tufnol (or anything else - save it for lighting Tilley lamps and other heating tasks) - degrease with isopropyl alcohol. You can also degrease with acetone, but it has to be real acetone and not nail varnish remover, which has added grease to replenish the grease it takes off your nails.
You could also immerse the magnets in 4% citric acid and ~ 0.1% common salt in water for 3 minutes, rinse, immerse in 3% washing soda for three minutes, rinse twice in deionised water, let dry. Don't let the magnets dry in between immersions, and don't touch them with fingers afterwards.
You can also dry them on kitchen paper or loo roll, but don't touch the kitchen or loo roll first - seriously, sufficient traces can get from your hands to the kitchen roll and then on to the magnets to prevent best adhesion.
The treated magnets should be a bit less shiny, and look very slightly blanched or have a faint bloom (in a strong light - don't worry if you can't see it).
While Araldite do make some good glues, the sort of epoxy which comes in two small tubes or a double syringe on a card for a few quid is not amongst them.
A really good slow-setting epoxy adhesive - anything which says fast or rapid or 5 minute is either very expensive or it's sacrificing adhesion and final strength for quick setting time - like Devcon 2-ton, or even Araldite professional (about 12-18 for two tubes on a card, but Devcon 2-ton is better value), or one of the structural epoxies, is an altogether different beast and will give much better results.
Also, epoxy probably isn't the best kind of glue for this either - a methacrylate adhesive would be better. Try bondloc 3295, 7.66 for 28 ml at
http://www.ebay.co.uk/itm/Bondloc-Structural-Adhesive-Products-Full-Range-/140758975405?pt=LH_DefaultDomain_3&var=&hash=item20c5e383ad
or loctite 326
http://www.ebay.co.uk/itm/Loctite-326-50ml-Structural-Adhesive-Fast-Curing-/140843246364?pt=LH_DefaultDomain_3&hash=item20cae9631c
plus 7649 activator
http://www.ebay.co.uk/itm/LOCTITE-142479-ACTIVATOR-7649-150ML-/251143220076?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3a794d6b6c
which are considerably more expensive, but which are actually made for gluing magnets and plated surfaces.
I might be getting some suitable glue in in about 3 or 4 weeks, if you are not in a hurry. Remind me then if you still need some.
-- Peter Fairbrother
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