Epoxy for dummies

[Rich Holmes]

I'm a fairly new builder of LPRs. Most of my building so far has principally made use of yellow carpenter's glue for the adhesive, and it'll probably remain my preferred adhesive for a lot of my work. However I've been dissatisfied with it for some particular uses and have been doing some tests with epoxy to see if I like it better for them.

My question is, what are your best techniques for mixing and using epoxy?

I have some Devcon 5-minute epoxy and some Bob Smith 30 minute. The Devcon comes in a double syringe, which I guess is a somewhat fool-resistant system if you're reasonably careful. But it's very much more expensive than the Bob Smith epoxy... like 4 or 5 times the cost per ounce. And of course, whatever you mix up, you need to use in the next couple minutes. So either you have to waste a lot of it, or you have to mix tiny quantities of it, which seems to me to be courting large relative errors in the mixing ratio, even with the syringe.

On the other hand the Bob Smith is cheaper and in the case of the 30 minute variety gives you more time to use it after mixing. But you do have to dispense equal amounts from separate bottles. If you are mixing up a large enough quantity, measuring out equal amounts isn't too much of a problem. But what if you're just, say, installing one BT-60 coupler? The amount you need is tiny, and once again, accurately getting equal tiny amounts of resin and hardener seems challenging.

So how do you do it? Do you mix up a lot more than you need and waste most of it, or do you have tricks to accurately dispense small (say enough for one BT-60 coupler or so) quantities?

 

[cavecentral]

I mix on paper plates equal amounts (visual inspection) and mix with a bamboo skewer (cheap). For larger batches (1/4oz or more - I use a graduated mixing cup). Most places sell them. They are disposable.

https://www.hobbylinc.com/htm/buk/bukbukr8056.htm


I am not a fan of the double syringes. $$$ plus the 2 liquids are different viscosities.

 

[GregGleason]

Here is my quick take on using epoxy successfully.

1. Use nitrile gloves, not latex. Constant skin exposure to epoxy can lead to sensitization (body reacts badly). It does happen to people.

2. Get a digital scale that can weigh to the nearest gram and has a tare feature. Weighing the portions to get them to manufacturing specifications is important.

3. Mix thoroughly (and for 5 minute epoxy, quickly!). I am almost certain that I have had bad epoxy sessions because I didn't thoroughly mix the resin and hardener. I use a craft stick chucked in my drill press and let it do the job for me. An electric drill can do the same thing.

4. Epoxy gains additional strength by adding amendments. These can be milled fiber, fumed silica, chopped carbon, etc. It can be made lighter and more easily sanded by adding microballoons.

5. Use a quality epoxy (like Aeropoxy).

6. Use the right epoxy for the right job (typically for adhesion or laminating)

7. Use per the directions (when I had a question about the directions, I contacted the manufacturer for clarification)

Greg

 

[Rich Holmes]

"Visual inspection"... reliable enough? How much leeway is there in the ratio?

 

[Rich Holmes]

Get a digital scale that can weigh to the nearest gram

Given that, for a single small job like a coupler or motor mount installation, I'd say 3 to 5 grams of adhesive is plenty... it seems to me that measuring to the nearest gram would allow errors of 30 percent or so. Is that good enough? Or, again, do you mix up 20 grams and throw away 80% of it?

 

[cavecentral]

With 5 min, I was ok with visual. Sometimes I'd have 1/8 - 1/4" left in one bottle vs the other in the end, but it cured. Longer cure times I hear get stronger with better finish.

The better epoxies Greg mentioned I use for fiberglass rockets and layups. Fillers etc. Those I weigh to 0.1gms. These can take hours to fully cure.

Depends on what you are trying to do. I can build a cardboard / wood rocket pretty quick with 5 min epoxy (the wood glues work too).

 

[GregGleason]

Given that, for a single small job like a coupler or motor mount installation, I'd say 3 to 5 grams of adhesive is plenty... it seems to me that measuring to the nearest gram would allow errors of 30 percent or so. Is that good enough? Or, again, do you mix up 20 grams and throw away 80% of it?

I'm rethinking that. In practice I use a scale that will measure to 0.1 or 0.01 grams. So for really small batches I will use the 0.01 scale. I got this 0.01 scale that someone recommended on this forum:

https://www.amazon.com/dp/B003STEIYY/?tag=skimlinks_replacement-20

Greg

 

[Rich Holmes]

I got this 0.01 scale that someone recommended on this forum:

https://www.amazon.com/dp/B003STEIYY/?tag=skimlinks_replacement-20

Greg

Oooh... shiny!

(0.01 g for 15 bucks? I like it.)

 

[ArthurSull]

I was scared of epoxy at first, but then I realised it's not scary it's just glue that comes in two parts

Now I use it for everything.

 

[Davidtmp]

I'm rethinking that. In practice I use a scale that will measure to 0.1 or 0.01 grams.
Greg

I use a scale that does .1 grams. Love it. Makes the job easier than relying on the eyeball method.
I mix my epoxy in the plastic detergent cups that come on the big family sized jugs(Gain, Tide, etc), clear ones. I try to just guesstimate how much I need. If I don't use it all, oh well....but I try not to waste too much. Over time you'll get a feel for what you need. whatever I don't use, I just leave in the cup to harden. after a few days, you can squeeze the cup around the sides to loosen the epoxy, and with a push on the bottom, the epoxy will pop out, wipe out the cup and it's ready to go for more(I usually will mix more epoxy on top of the dried epoxy in the cup, so it builds up over time, and I'm talking Bob Smith stuff).

 

[troj]

"Visual inspection"... reliable enough? How much leeway is there in the ratio?

For small quantities with the Bob Smith epoxy, eyeball is good enough. I use the bottom of yogurt containers (because I use the inside to mix larger batches). Just put a blob of part A on one side, and a similar size blob of part B on the other, mix thoroughly and voila!

A scale that will measure that small of an amount with any real accuracy would cost you way more than it's worth.

-Kevin

 

[tightwad]

I normally just eyeball it. But on occassions I have used 2 syringes, one for the hardner and the other for resin. Clean-up well afterwards and reuse. The scale in the post by GregGleason could also be used for black powder in DD's, as well as measuring epoxy.

 

[K'Tesh]

I use a scale that does .1 grams. Love it. Makes the job easier than relying on the eyeball method.
I mix my epoxy in the plastic detergent cups that come on the big family sized jugs(Gain, Tide, etc), clear ones. I try to just guesstimate how much I need. If I don't use it all, oh well....but I try not to waste too much. Over time you'll get a feel for what you need. whatever I don't use, I just leave in the cup to harden. after a few days, you can squeeze the cup around the sides to loosen the epoxy, and with a push on the bottom, the epoxy will pop out, wipe out the cup and it's ready to go for more(I usually will mix more epoxy on top of the dried epoxy in the cup, so it builds up over time, and I'm talking Bob Smith stuff).

I found that instead of laundry detergent cups, the bottoms of Yoplait yogurt work wonders for the pop out and re-use mixing container.

I'm using the BSI 30 minute epoxy, and I use such small amounts, I usually just count out the drops of each liquid (yellow bottle first, then black bottle). I've yet to do a job that on a single mix needs more than 10 drops of each. I also use small wooden coffee stir sticks snagged after a presentation (they were going to toss them anyway) for my mixing.

 

[TheAviator]

I always mix epoxy by weight and have a similar scale that goes to 0.01g. It takes an extra 30 seconds to set up the scale, but then you're nearly guaranteed that the epoxy will set up correctly. I've done batches of as little as 0.5g and it works perfectly. As they say, "Measure twice, cut once."

 

[Nathan]

For places where I need a seriously strong joint I use JB Weld. For fin fillets I use low viscosity 5 minute epoxy because it self levels nicely and then hardens quickly, so I don't have to worry about it dripping down the side of the rocket after I walk away.

 

[JeromeK99]

For gluing small parts for LPR, if I use epoxy, I'll just run equal length beads next to each other on a piece of paper or cardboard scrap. Mix with a stick and use it right away. If I know I'll need more I use small epoxy mixing cups found at hobby stores and mix by volume (gram lines on cup). Usually what is left over will pop right out when cured and I can use it again. Eyeballs are ok for small amounts. Never had a problem....


Jerome

 

[Jmhepworth]

I use small disposable condiment cups for small amounts and just eyeball the 1:1. For larger amounts, when I want a really strong bond, or when the ratio isn't 1:1, I use a digital scale.

 

[jderimig]

For those mixing by weight be aware that mass ratiois different than volume ratio.

 

[Rich Holmes]

For those mixing by weight be aware that mass ratiois different than volume ratio.

Yep. But I quote from https://www.rcgroups.com/forums/showthread.php?t=994056:

I wrote Bob Smith Industries about weighing instead of measuring their epoxies, rather quickly I got a very nice reply.

Yes, I use BSI epoxies, my local hobby shop carries it (I like to support brick and mortar stores), and they work rather well for this hobby

The correct ratio for weighing is 1.1:1, as the resin is 10% heavier than the hardener. But in reality there is a built in "fudge factor" with their epoxy, so you can just go 1:1, as per Charlee Smith of BSI without any discernible differences.

So, not from the horse's mouth but at one remove, this says 10% errors are OK.

I'll still probably pick up one of those scales; at $15, why not? I'm sure I'll find lots of uses.

 

[Onebadhawk]

You're paying way too much for epoxy because you're buying cheap epoxy......
Your 100% correct in looking at it as price per ounce......

Aeropoxy structural from Giant Leap Rocketry.....
or
Rocketpoxy from Wildman Hobbies.........

You're done,,,, that's it,,,,you don't need anything else....

It may sting a bit when you order it,,,,,
but you'd already have it if you didn't already use the home cheapo epoxy............

Most important.............
It's 10 times more important that you mix epoxy thoroughly
then it is that you get exactly correct 50/50 proportions of part A and B

Eyeballing it on a piece of cardboard for a mixing tray with a popsicle stick is absolutely fine........

But mix it thoroughly...........

Teddy

 

[Rich Holmes]

I don't need a quart of epoxy. Or at least, I don't expect to use anywhere near that much in the next couple years.

 

[cavecentral]

I like the 5 min for the smaller applications. The gallon of US composites is cheaper per ounce than the 9oz sets of BSI. Just takes so long to cure that it is not worth it on a smaller rocket unless it was built for some extreme flight. Especially if the rocket is cardboard / wood.

 

[CarVac]

I don't need a quart of epoxy. Or at least, I don't expect to use anywhere near that much in the next couple years.

You can get a pint of Aeropoxy from Giant Leap for the price of 3 little bottles of BSI epoxy.

It lasts years, and it's so much easier to use you will never want to go back.

 

[Cyril's Scribe]

I personally use West Systems Epoxy with the pre-measured pumps, this is a fairly expensive system, but I work on marine stuff (sailboats mostly, got to love teak and varnish) so it wasn't exactly something I was not familiar with. I was told by West Systems that there is a plus of minus twenty percent leeway with most epoxy products. This does not include some of the very expensive epoxies like Cotronics, but those are for heat resistance for Mach two plus birds and are extremely expensive. So you can eye ball almost all small batches, also, be aware that with epoxy, whatever you use it to glue together will not come apart. The material around it will fail before the epoxy!

 

[rheil]

Also second West Systems... get a quart and you'll be surprised how often you use it. Also JB Weld is great for heat sensitive joints like engine retainer. There are large tubes or get some at Home depot.

Also I think the key is mix, mix, mix... and be fairly accurate on the ratio.

 

[troj]

I don't need a quart of epoxy. Or at least, I don't expect to use anywhere near that much in the next couple years.

If you're only using small amounts, stick with what you're buying. If you find yourself buying a new set of bottles every couple of months, then it's worth switching to something else.

-Kevin

 

[Rich Holmes]

You can get a pint of Aeropoxy from Giant Leap for the price of 3 little bottles of BSI epoxy.

First, three little bottles (I assume you mean three of each component) of BSI is 27 ounces, a lot more than a pint.

Second, where on the GLR site is this pint of Aeropoxy? All I see is 1 quart (pint A + pint B) for $23.99. And yes, 1 quart = 32 oz for $23.99 is cheaper than 9 oz for $9.99 (price of BSI at local hobby store), but not overwhelmingly so, and it's no bargain if I never use that much. If all I ever make are LPRs, and if I use wood glue mostly, epoxy only for the places I don't like using wood glue, I could end up leaving half a quart to my son in my will. Of course I might at some point decide to switch over to all epoxy, or to building HPRs. Naturally at that point I'd be looking at stuff coming in larger quantities. But not now.

It lasts years, and it's so much easier to use you will never want to go back.

Easier to use is significant. But how is it easier?

 

[CarVac]

First, three little bottles (I assume you mean three of each component) of BSI is 27 ounces, a lot more than a pint.

Second, where on the GLR site is this pint of Aeropoxy? All I see is 1 quart (pint A + pint B) for $23.99. And yes, 1 quart = 32 oz for $23.99 is cheaper than 9 oz for $9.99 (price of BSI at local hobby store), but not overwhelmingly so, and it's no bargain if I never use that much. If all I ever make are LPRs, and if I use wood glue mostly, epoxy only for the places I don't like using wood glue, I could end up leaving half a quart to my son in my will. Of course I might at some point decide to switch over to all epoxy, or to building HPRs. Naturally at that point I'd be looking at stuff coming in larger quantities. But not now.

Easier to use is significant. But how is it easier?

Yes, I meant a pint of each component.

Unlike laminating epoxies which are thin and syrupy, or Bob Smith epoxy which is moderately thick and syrupy, Aeropoxy ES6209 is more like a gel: you can spread it thin easily, but if you form a small fillet it will tend to stay put. I can easily pick up a sizeable glob on the end of a popsicle stick and keep it in a nice ball indefinitely by flipping it over now and then without it dripping off. This lets you work faster by grabbing larger amounts at once (prolly not an issue for you) but more importantly it reduces drips.

It soaks into wood and paper products really well, strengthening the material surrounding the joint. No other epoxy I've seen does this.

It doesn't smell nearly as bad as Bob Smith epoxies.

 

[Rich Holmes]

Thanks, good information I hadn't seen before.

 

[Onebadhawk]

Yes, I meant a pint of each component.

Unlike laminating epoxies which are thin and syrupy, or Bob Smith epoxy which is moderately thick and syrupy, Aeropoxy ES6209 is more like a gel: you can spread it thin easily, but if you form a small fillet it will tend to stay put. I can easily pick up a sizeable glob on the end of a popsicle stick and keep it in a nice ball indefinitely by flipping it over now and then without it dripping off. This lets you work faster by grabbing larger amounts at once (prolly not an issue for you) but more importantly it reduces drips.

It soaks into wood and paper products really well, strengthening the material surrounding the joint. No other epoxy I've seen does this.

It doesn't smell nearly as bad as Bob Smith epoxies.

Absolutly,,,,,,,,,,,,100%

Rich,,,,,,,,,
Please understand,,,,,,,,we've all been where you are,,,,,,,,,
it just wasn't that long ago I thought to myself,,,,,,,
Theres no way I'm shelling out that kind of money for epoxy............
It's just a simple fact,,,,,,,
Unless your actually planning on leaving rocketry,,,,,,,,,,
you'll save yourself some money by buying better quality epoxy..........
and,,,, you'll be using better quality epoxy..............
It's really kinda funny,,,,,,everything your saying,,I said..........
you'll laugh your rear end of if you print this thread and pin it up over your work spot............

And by the way,,,,,, you can see I'm an advocate of good quality epoxy,,,,,,,,,
slow set,,,, the longer it takes to set,,, the longer it has to soak in,,,,, thats where you get strength from.........

However,,,,,,,

I believe on porous paper and unfinished wood,,,,,
I think the yellow carpenters glue your using is strongest................

Teddy