I used the two-part Arctic Silver epoxy for the first time, and tried to get the 1:1 mix right. A day later, I poked at the little blob of leftover and found that it was still rubbery rather than hard like I would have expected an epoxy to be. I gather from the instructions that it should have been hard, and not yielding to a fingernail. Is this a sign of product beyond its shelf life? If I try it again, should I use more of one part, and which part would that be - A or B?
Don’t give the leftover too much credit. It could be less mixed than the bulk of the mix, and it could be that it was thick and not curing as fast.
I have expected cracking when I bend the foil I used for mixing. No such luck. But removal of the MCPCB a year later showed the epoxy was set up hard enough to completely deform the aluminum heatsink. I don’t know how to explain the “soft feel” but I also wouldn’t worry about it. Maybe is cures better with a little heat. Many epoxies actually require heat for a proper cure.
'15 hour' JB Weld turns into '10 minute' (or less) JB Weld if you heat it up to 200*F or so. And if you accidentally mix too much it'll remain uncured for several days in the freezer.
Heat kills arctic silver epoxy.
I’ve failed at a couple attempts to add a copper heat sink to one light. I epoxied it in, then had to add more smaller copper rounds (a triple) in layers, using solder paste. The heat broke down the AA and crystalized it. The heat sink kept falling off. I could scrape off the dried AA residue with my thumbnail.
There is a limit yes 
One epoxy I’ve worked with require 100 degrees C to cure.
Soldering temp is a whole different animal than the heat needed for quicker curing. That said... I have reflowed LEDs with the star still glued down with JB Weld, and it wasn't affected not even a little bit.
Are you adding something thermally conductive to the JB Weld?
I can vouch for this too…regular clear epoxy, star was epoxied in, it wasn’t moving, heated the pill that the emitter re-flowed, plucked it off, hit the star from the back with a screwdriver, popped out ONLY after I heated it over 300C…epoxy went from clear to a yellowish brittle stuff that flaked off with scraping
My clear epoxy is 5 min quick stuff too…after 5 min it is flexible, after 30 min much harder but still flexible, after 24hrs hard as plastic, however after the 5 min set time (leave a q tip standing up in the left over on the cardboard, if I can move the object holding it up and it stays up it is setup for the most part) I usually run the light and warm things up a bit too
Nope, and for reasons I'm really tired of explaining... it works just as well, nothing works better, not even the space-age quantuminterdimensionalnanodiamond stuff that costs 100x as much... the thermal densities in flashlight applications just do not stress the thermal material to the point that the miracle properties show up in anything you can think of to measure. :angry:
JB Weld was formulated for elevated temperature up to 600 degrees F (limited) and 500 degrees F continuous.
Arctic Silver Epoxy, on the other hand is only rated to 300 degrees F.
http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm
The Arctic Silver is only suppose to meet the worst case temperature for a many devices. An emitter, for instance is only suppose to work up to 150 degrees C (300*F).
Certainly not capable of reflow temps.
So is this a reliable method of part separation for Arctic Silver?
Yes, breaking epoxy bonds with excessive heat is a recognized process in industry.
Always wondered about polyurethane for a thermal adhesive. It’s so cheap like 7 bucks for the 8x chalking guns worth.
Have a look here cheaper / DIY options that are proven to work well
'Proven' is debatable. Unfortunately, light meters (unlike the human brain) are not impressed by the placebo effect.
Link broken…
remove the “for” at the end.
Fixed.
Thanks!
Well, I tried it again using larger drops to better judge the size, and I mixed very, very thoroughly. It hardened well this time.
I had plopped the original attempt into acetone for a couple of days (within a couple of hours of noticing the hardening problem), and it dropped out readily.