Ok, i need help choosing the correct epoxy for my application...

I bought the Hobbico Avistar Select, and i was told to epoxy the wing-halves togeather.... will this epoxy work ok?

http://www2.towerhobbies.com/cgi-bin...?&I=LXC025&P=7

Sure it will work. Any 30 minutes epoxy will do, just make sure you DON'T use 5 or 10 minute epoxy, as it will not give you the required time to finish it. (and 30 minutes epoxy is stronger also)

When preparing to glue the wing halves together it is important to dry fit them first to ensure they will go together when start applying the glue. Also important is some sort of jig to hold the wing while the glue cures especially if you use the 30 minute or longer. And finally mix a large enough batch to complete the job as you don't want to race against the clock to mix a second batch while the first is curing!

I have used mainly 12 minute epoxy. It usually give enough time to do the job.

Personally, I won't even buy anything except 30 min epoxy.

Look at it this way, the 5-15-30 etc. refers to the time it takes to SET, not CURE. So if you use 5 minute epoxy, it sets in 5 minutes, but it is not HARD. Ok, so it has set, but you still need to leave the item being glued alone for 24 hrs (or at least overnight). So don't think that after your 5-minute epoxy that's holding your wing halves together has set, you can remove the dihedral blocks and set the wing aside. So as long as it must be left untouched, use the stronger 30-minute epoxy. It will give you plenty of time to work, and after the items are glued, you can add a few strategically placed drops of CA with accelerator to hold the alignment until it sets up.

OK you just peaked my interest...... what is a dihedral block ... do i have to buy one? i dont believe they sent me one.... what is that?

A dihedral block is something that gives the center section strength. It may be called something else like brace or something else as well. You don't just glue the end of the wing together. There is usually something that goes on the front and back of the spars or between them for strength. It keeps the wing from flapping.

I agree with MinnFlyer. Fast epoxy is really for field repairs, not constrution. The strength of any glue is in the ab ility to penetrate the wood as it cures. That's why the longer the cure the stronger the joint. To help this penetration process, I always perforate the surface with a lot of small punch marks from an awl, icepick, nail, etc. This allows the glue to seep into the punch marks and like mini rivets. Also, be sure to keep the joint clamped for the full 24 hour cure period or the joint may weaken.

Miniflyer gave you some good advice. Epoxy, even 5 minute, does not reach full strength for hours. Heat will accelerate the cure time (note that you can overdo the heat, use only enough to get it to around 160 to 200 degrees F.) I also recommend that you use 5 minute epoxy only to fill none structural holes, it is very poor as far as strength goes.

I have to disagree with FLYBOY here - he seems to be refering to a dihederal brace. A dihederal block is something used to prop up one wing tip to the correct angle while the center ribs and spars are glued together. It supports the wing tip in the correct position while the glue dries. Some kits include a pre-cut guide to ensure the correct alignment, others simply provide a measurement, and leave it up to you to decide how you want to hold it in position during gluing.

What do most people use for dihedral block when one is not supplied?

Give that man a cigar!

Yes, I was refering to whatever you use to block up the wingtips to hold the wing at the correct angle.

Terance, this can be anything from a block of wood to a pack of cigarettes, to a monkey wrench, as long as it works.

PMW,
I disagree strongly! 5 minute epoxy is NOT for field repairs. While 5 minute epoxy may "set" in 5 minutes, it doesn't cure for hours. Fixing anything with 5 minute epoxy and trying to fly 10 minutes later is asking for disaster.
Dennis-

I always thought that epoxy was an exothermic reaction. At least when I use expo it seems to give off heat. Heating it more would slow the cure time. I could be wrong though.
chunk

Chunk1227, think about it, if you store epoxy in a refrigerator is slows down the reaction (cure); yes heat most definately speeds up cure time.

Heat also makes epoxy very "runny". As far as I know, epoxy is exothermic (I could be wrong), but only after it has been mixed with hardener. Try mixing a batch and setting it in the fridge - it'll still cure.

Rodney - Explain this for me... I store my CA in the fridge too to extend it's shelf life, yet CA is definately an exothermic reaction. My tail feathers were smoking once because I had a pinch of sanding dust come into contact with the CA while gluing.

5-minute epoxy is for nothing. Don't even buy the stuff. Maybe we'll be blessed with it disappearing from the market.

Any field repair that needs epoxy should be done at home in your shop. Any field repair that doesn't need epoxy probably should still be done at home in your shop, but if you use epoxy at the field you're just adding unnecessary weight for a substandard repair.

The problem with 5 minute epoxy is that it's not as strong as slower-drying epoxies, yet just as heavy. It tends to turn brittle and crack within a few months to a couple years. Usually by the end of a season. It can peel out of joints. Bottom line being that it shouldn't be used anywhere in your model.

30-minute or slower is better.

I never said that. CA is great for field repairs.

I won't even allow 5-minute Epoxy in my house!

The stuff is garbage!

Don't buy it!

If you see someone using it, beat them with a stick!

Get it?

Chunk1227,

The reaction that cures epoxy is indeed exothermic. It is also true that higher temperature accelerates this reaction. There's no conflict between these two things. Same with concrete. It's kind of handy that a chemical reaction that requires heat generates some of its own.

banktoturn

All,

I keep reading here that the faster setting epoxies are weaker than the slower setting ones. Has anyone ever tested this, or is it just an urban myth? I've always used 30 minute epoxy for RC planes, just to have the working time, but I've repaired things around the house with 5 minute epoxy, and I've never seen a problem. Any data points out there?

Thanks,

banktoturn

banktoturn - I don't know about the actual strength of the epoxy. I base my conclusion on the physical properties I can observe (rubbery cure that eventually turns brittle and cracks, peeling out of joints, etc). Those things equate to weakness and are not problems with slower drying epoxies.

banktoturn,
In the case of the concrete. Concrete is exothermic and this added heat slows the curing time. For example, Hoover Dam. The engineers had to install cooling pipes throughout the layers of concrete. They claim that the concrete otherwise would still be curing today. I am no expert on epoxy, so I can't say if it holds true in epoxy cases.

I've never actually tested it, but I have had it turn yellow and brittle like CafeenMan mentioned. Nowadays I don't use any epoxy faster than 30 minute.

I also agree that any field repair requiring more than a couple drops of CA should be inspected thoroughly at home for a proper repair. Rushed field repairs often result in some damage being overlooked.

Epoxy doesn't need heat (that would be endothermic), it generates heat. In my lmited experience, heat makes my epoxy take longer to cure (I'll never do that again). How hot would I have to make it to actually accellerate it?

Chunk1227,

They do sometimes have to install cooling pipes in large pours of concrete, to avoid overly high temperatures. My understanding is that this is required to prevent overly rapid curing, not to accelerate the curing. In particular, if the curing is too fast, the excess water does not have time to get out before the curing process is done. I could be mistaken. I'll do some Googling and find out what I can. I have already done the Googling about epoxy. The Technical Information page at a manufacturer's web site was pretty specific that high temperature accelerates curing. I'll post the URL if I find it again.

banktoturn

Yes, many tests have proven this. The molecular chain formed by the slower curing epoxys is much longer.