Mark -
Keeping the magnets from actually touching the fender would indeed reduce the strain on the glue, but it would also reduce the overall grip strength to the tractor. I'm
greedy -- I want grip strength
and a permanent bond between cup and magnet. /w3tcompact/icons/smile.gif
You're absolutely right about roughing up the surface of the plastic before gluing, and I have done so on my latest attempt.
As if I don't have any
real work to do today, I talked to Aladdin Industries, who make the Extreme Gulp cup upon which the Chalkley Cup is based.
They tell me the cup is made of polypropylene, and they know of no adhesive that bonds with that material. They themselves use "spin welding" and "sonic welding" to attach the various cup parts, including the handle. Both of these processes essentially "melt" the plastic together in a true welding fashion. Of course that only works for joining pieces of polypropylene together,
not to steel.
They laughed when I mentioned epoxy, and sure enough, my epoxy package has a disclaimer that it will
not stick to polypropylene. They had no first-hand knowledge of Goop, and they conceded that it "might" work well enough for hobby use, but certainly would not be reliable enough for commercial use.
My web search so far has turned up
one product which claims to bond polypropylene to itself and just about anything else. It's called
Nbond, which, although it is a 2-part epoxy, is unlike all others in that it is specifically designed to adhere to polypropylene and polyethylene, claims the manufacturer.
Although it's not cheap ($11 for 50ml), it's not unreasonable, and I will probably give it a try. If I can't find it locally I will order it off their web page.
My wife says I "obsess" over trivial pursuits, such as sticking a magnet to a plastic cup. To me it's almost like a game -- the challenge ain't over 'til it's
solved. /w3tcompact/icons/laugh.gif
