Encapsulating Epoxy

[Bruce Johnson]

Yes, my experience in the electronics lab with the potting epoxy in the squirt tubes with the spiral mixing nozzles is the same as Joe's. They were very wasteful of the epoxy (which was expensive!) and they weren't reliable. Maybe 5 or 10 percent of the time, the epoxy wouldn't cure properly. I remember asking my boss why we couldn't use a marine epoxy like West Systems, and he said he'd love to, but it wasn't certified under UL or the electrical code. It's not that it would be any real fire hazard, but it just hadn't been certified for use on electrical equipment.

I've never yet seen any shrinkage or aging problems with the West Systems. I've got some parts that I've cast in it that are about 21 years old, and no real change in appearance or dimensions that I can see. I've also got a pair of bass bodies (a long story in itself) which I hung outside in direct So Cal sunlight for 26 months. They used West Systems for the structural seams and filling the grain of the ash. It never shrunk or cracked.

There are only two conditions where I've ever had any problems with it not curing:
1.) When the Hardener has gone bad from age. It absorbs moisture and turns dark amber and gets thicker. If it looks like that, it's risky to use.
2.) Very small quantities. The smallest quantity that I mix is 6ml (5ml resin & 1ml of hardener). That works fine, but smaller than that, and it often won't cure no matter how carefully you mix it.

I use small plastic 5ml and 1ml measuring spoons to measure the mix, stir it slowly for 30 sec to a minute, and pour it in. Nothing fancier than that. I've been doing that for more than 20 years, with no real failures other than the two cases I mentioned above. Most of my castings aren't that fussy about a few tiny bubbles, by design. On the parts that I cast that are more visual, I just take more care to stir the mix slowly, like at 1 revolution per second for two minutes. I haven't found the need to use a vacuum chamber so far.

[Hugh Evans]

My main experience with static mixing tips has been with two part polyurethanes. As long as the system is properly designed the mix ratio should be perfect and the amount of mixing possible with such tips is outstanding. I will add that for larger cartridges it is much more common to see pneumatic guns in manufacturing. All static mixers I have ever worked with are considered to be single use devices, and they can be purchased separately in bulk.

[David Schwab]

Yes, but there is one missing step - the little sample in the cup, cured at 180 F before using the batch.

I always do something equivalent, or at least keep the mixing board, so I can tell if I've a bad batch.

Even if I'm not making stuff that will go to the Moon.

Well I can do that, but since I'm working from 3 liter bottles of the stuff, I'd imagine I'd only need to do the test once?

[David Schwab]

I've never yet seen any shrinkage or aging problems with the West Systems. I've got some parts that I've cast in it that are about 21 years old, and no real change in appearance or dimensions that I can see. I've also got a pair of bass bodies (a long story in itself) which I hung outside in direct So Cal sunlight for 26 months. They used West Systems for the structural seams and filling the grain of the ash. It never shrunk or cracked.

There are only two conditions where I've ever had any problems with it not curing:
1.) When the Hardener has gone bad from age. It absorbs moisture and turns dark amber and gets thicker. If it looks like that, it's risky to use.
2.) Very small quantities. The smallest quantity that I mix is 6ml (5ml resin & 1ml of hardener). That works fine, but smaller than that, and it often won't cure no matter how carefully you mix it.

I've never had any problems with it either, and we even have a batch of it at the workshop that has to be several years old and it still cured just fine.

I also agree about too small of a quantity. I mix up 150 ml at a time.

[Hugh Evans]

Keep in mind that not all epoxies like to cure at 180°F, although that is a fairly safe average figure for most. The recommended curing temperature for an epoxy is generally very close to the glass transition temperature (Tg), which allows unreacted end groups to wiggle around and form as many bonds as possible. Also, if you want to buy some time while testing a mixed batch, it's perfectly fine to refrigerate the epoxy both before and after mixing to dramatically slow the reaction. A cheap mini-fridge and old toaster oven (or new one that has been repurposed as a reflow oven) would be ideal for the operation.

[Joe Gwinn]

Well I can do that, but since I'm working from 3 liter bottles of the stuff, I'd imagine I'd only need to do the test once?

Well, it can go bad, so I'd mix a bit too much every so often and do a test in parallel. The mark is if the cured sample is still sticky when felt with a clean dry finger.

[Sheldon Dingwall]

That's some good information guys, thanks. We are currently using a polyurethane from Epoxies etc. with a static mixing system. It's been working good so far and the guys love the convenience and lack of mess. We were using G2 epoxy and we'd get failures out in the field - maybe one out of every 200-300 pickups. The failures were usually cold temperature related. I.E. the pickup worked fine before a flight, failed sometime during.

[Bruce Johnson]

That's some good information guys, thanks. We are currently using a polyurethane from Epoxies etc. with a static mixing system. It's been working good so far and the guys love the convenience and lack of mess. We were using G2 epoxy and we'd get failures out in the field - maybe one out of every 200-300 pickups. The failures were usually cold temperature related. I.E. the pickup worked fine before a flight, failed sometime during.

Sheldon;
When you had those field failures, was there an obvious physical problem, like shrinkage or cracking? I've never done any cold temp testing on mine. Maybe I should stick a junk pickup in the freezer for a few months, and see what happens?

[Joe Gwinn]

Sheldon;
When you had those field failures, was there an obvious physical problem, like shrinkage or cracking? I've never done any cold temp testing on mine. Maybe I should stick a junk pickup in the freezer for a few months, and see what happens?

This is classic, usually caused by mismatched temperature coefficients of linear expansion.

The standard test is not soaking in the cold, it's alternating hot and cold, with enough soak time at each temperature so the item achieves uniform temperature throughout. For something like a pickup, this will take hours. One can tell by measuring the DC resistance - when the resistance no longer changes, uniform temperature has been achieved.

The solution for electronics is often a buffer coat of some kind, to isolate the stress-sensitive component from the epoxy. For instance, glass packaged diodes would be coated with urethane rubber before potting in rigid epoxy. For pickups, a wax dip would work.

See 1010.8 in MIL-STD-883 - Wikipedia, the free encyclopedia

For small components,. MIL-STD-883 has one plunging the component in liquid nitrogen then hot oil in alternation, a very severe test. For pickups, we will need to be a bit more gentle.

[Bruce Johnson]

Yes, I might try putting a pickup in the freezer for a day, then out in direct sunlight for a day, back and forth like that for a week or two. For one of our guitars that would (hopefully!!) represent extremes beyond what they would ever see. It might be interesting for our education.

Because my pickups have a perforated brass shell cast into the epoxy on the outer surface, plus a thin ebony cap on top, any real problems with differential expansion rates should be obvious on the exterior.

[David Schwab]

I haven't had any pickups fail yet. A small number do fail right after being potted, but if they make it past that they seem to be fine.

[Sheldon Dingwall]

Thanks Joe, we've done freezer tests but not cycling freezer tests.

Dave, we leave ours a week after potting, then test before installing. We lose the odd one during that week and roughly 1/2% still fail in the field. No data on the urethane yet, we've only used it on 50-75 so far.

[Joe Gwinn]

Yes, I might try putting a pickup in the freezer for a day, then out in direct sunlight for a day, back and forth like that for a week or two. For one of our guitars that would (hopefully!!) represent extremes beyond what they would ever see. It might be interesting for our education.

Because my pickups have a perforated brass shell cast into the epoxy on the outer surface, plus a thin ebony cap on top, any real problems with differential expansion rates should be obvious on the exterior.

That sounds reasonable. I would use a toaster oven set to warm (110 F or so) instead of the daylight, for consistency, and because a guitar in a closed car in the sun can easily get to 150 F.