Randy Etken

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When joining a composite wing or fuse, what is the best mixture for strength, epoxy with just cabosil, cabosil and micro balloons 50/50, cabosil and milled glass fibers or cabosil and chopped glass ? Which would be the lightest?
Thanks
Randy

daven

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Micro balloons are the lightest, followed by cab-o-sil, followed by chopped glass, followed by milled glass.

I joined a tail yesterday Randy, and mixed a total of 30 grams of epoxy with about a 1 oz of Cabosil, and 1 1/2 - 2 oz of microballoons. Cabosil helps things from running everywhere, but you need balloons to save weight. I used about 3/4's of what I mixed up. Total Tail weight at 2.3 oz, AJ will like this tail.

I am curious to hear experts thoughts on this also.

gluedoc

I won't call myself an expert, but I do work as a professional composites engineer in aerospace. The answer to your strength question is easy, the longer the fibers, the stronger the material. There are, of course, complications. For instance, the alignment of the fibers increases the stiffness significantly, and the strength less so (that is why aligned materials like cloth and uni fabrics are stronger).

Among your choices, strength would go like this: chopped glass > > milled glass > cabosil ~ microballoons

Technically, microballoons and fumed silica (cabosil) are both thixotropic additives, and are usually not used to make things lighter.

The thing to remember is that all fillers with an aspect ratio of ~1 (e.g. spheres or spheroids) will act as voids in the epoxy and reduce its tensile and compressive strength. They will serve to increase fracture properties, but you should not be designing a model to be safe life, you should be designing it to be damage tolerant!

Sorry for the technical answer, but your question was much more complex than you realize. It has been the subject of considerable research for the last 30+ years in the composites field.

Matt

Randy Etken

My Feedback: (10)

So Matt what would you use to join a wing 10"X50" layed up in female molds. I really appreciate you answer.
Thanks
Randy

akcaj

Kinda sort of in line with this question..

I'm confused with the different types of epoxy, and what is best for what purpose..
i.e.
5 min. / 30 min. types (Devcon etc)
laminating
finishing
mas / mgs (what are these??)

purposes
i.e.
laminating (glueing balsa to balsa / balsa to ply / ply to ply)
balsa to foam
foam to foam

thanks

and so on...

gluedoc

Randy, I assume that this is a foam cored wing with either glass, carbon or balsa skins. I will also assume that you are talking about joining either 2 wing halves that each have a span of 25" and a chord of 10" (must be an aerobatic plane of some sort). I guess you could be referring to a plane that has two halves of 50" for a total span of 100" for an open class glider. My answer doesn't matter, but let me know because I am interested.

Unfortunately, the right thing to do would be to join them in the mold. You should have epoxied the foam cores together at the wing root. For this I would use epoxy with a light mixture of cabosil or microballoons (just to ensure that if the joint isn't perfect, the epoxy will stay in place). Then, I would sheet the wing as one unit and bond the sheeting on with epoxy and use the mold to ensure that I had good consolidation. No matter what the sheeting material, you will increase the strength of the wing root joint immesurably if you provide a continuous load path across that foam butt joint.

If you are joining it later (as you would do with an ARF), then the best you can do is join the halves together as I have indicated, and glass the center section using ~3-5 inch wide glass cloth that is well wet with epoxy. Make sure that you provide some means of consolidation at this point.

Matt

gluedoc

akcaj,

Great question, there are a lot of people in this hobby that don't understand what types of epoxies do what. In general, you will find several types of "common" epoxy. Typically, anything that has a time on the label that is less than 90 minutes will be cured with a thiol (a type of curing agent). It is these thiols that account for the distinctive smell of 5 minute epoxy. For this reason, 5, 15 and 30 minute epoxies are all about the same strength, but you will find that as the time increases, the material becomes less viscous.

With materials 90 minutes and over (there are epoxies that cure in 1 week!), materials are cured with a wide variety of liquid amines. These amine cured epoxies are much stronger than the thiol cured materials, and in general, the longer it takes an amine cured material to cure, the stronger it will be. Many of the repair epoxies we use in the aerospace industry can, with clever processing, be made nearly as strong as autoclave cured prepreg laminates.

Beyond this, laminating resins are typically amine cured epoxies (to give good working life) that are less viscous and can be worked into fiber beds more easily. They are made less viscous by the addition of materials called reactive diluents. These serve to reduce viscosity without the use of solvents. They so reduce cured material strength, but these reductions should be neglible in the epoxy world. Finishing epoxies are similar to gel-coat materials, in that they are designed to provide a smooth finish that also protects against the environment. They are very common in the boat industry, but are not used in full scale aerospace.

Here are my opinions:

1. For gluing anything of structural importance together, I use 30 minute epoxy. It has a reasonably short cure time, and has proven strong enough for everything that I have ever done.

2. For glass and carbon laminating, I use 90 minute epoxy. This is thicker than most people can use and not damage the fiber bed, but I have a lot of experience in wet lay-up composites. For others, I might suggest a laminating resin.

3. I have said on this forum several times that you should never ever thin epoxy if you are making a laminate. The voids it creates can reduce your strength by more than 50%

4. I use 5 minute epoxy for gluing hinges and things forward of the firewall. I also thin it with acetone to brush into the fuel tank area and the firewall to provide nice fuel resistance.

I hope that answered your question.

Matt

davidfee

Matt,
I'm pretty sure Randy is talking about joining the upper and lower skins of a hollow molded wing. The skin sandwich panels (balsa or foam sheet core) are bagged into their respective molds. The spars, ribs, etc. are then installed and bonded to one of the wing skins (usually with microballoons/epoxy) as a secondary operation. After a final fit-check, the entire assy is bonded together with a bead of adhesive along the perimeter, along the spars and on the ribs... again, typically with micro/epoxy. The molds have guide pins for alignment of the upper and lower skins.

It sounds like he's asking you to suggest a potential alternate adhesive.

-David

akcaj

gluedoc +++ Doc. thanks a lot, that was a great help..

gluedoc

Randy,

If David is correct, then the situation is a bit more delicate. The fundamental problem with hollow molded wings is that you don't have a lot of area to spread load over. In the case of a foam cored wing with carbon and obechi skins, I can really screw up quite a big area of the bond and not notice the difference (simply because my loads are low in comparison to the total bonded area).

With a hollow wing, we have much lower areas to work with, and the key is to make sure that the epoxy stays where we want it (in the bonds) when we put the pieces together. This is the primary reason for microballoons, as they act solely to keep the epoxy where we put it. Interestingly enough, chopped fibers will perform this same function. They usually do too good of a job, and make the epoxy hard to work with, but more importantly, they do provide some additional strength for the epoxy.

If you are looking for a different glue chemistry (other than epoxy), there isn't much hope to offer. They are simply the strongest and stiffest glue that is out there in the modeling world. Nothing lasts quite as long and has quite as much ability to transfer load from stiff substrates such as your skins.

If you have pics of your setup, I would love to see them.

Matt

SCALECRAFT

My Feedback: (13)

Hi

This model is entirely joined together with west system epoxy resin (slow hardner) and cabosil mix, along with a glass tape as is required.

This method has been used by us in our models for over ten years and has worked well. Holding seams together, hard impact upon hard impact, never failing.

Steve

Randy Etken

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Matt please see pictures of a Quicky 500 plane call a R-200, This is a new wing mold cnc cut out of Ren board. New tail mold a little bit bigger than my old one and new plug for fuse. My question is what to use to join the parts? I have all ways used MGS epoxy with cabosil. I'm told I should try 50/50 cabosil and micro balloons it would be lighter. Or should I mix in some milled glass for strength?
Randy

diggs_74

Wow Randy, it didn't take long to get those ready.. I just saw them on Friday night and they still needed sanding.. Can't wait to see the new R-200.. Will you have one ready for the annual meeting in January??

Randy Etken

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Need to build new wing to finish fuse plug. Should have one flying by mid December.
Randy

daven

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Looking forward to seeing a wing Randy, if you can perfect it, I can sell my molds

Troy-RCU

My Feedback: (1)

A 50/50 micro/cabosil mix is plenty sufficient, you can even do a 25/75 if you want it lighter. Pure micro mix tends to be brittle but adding a little cabosil keeps it "wet" and sticky. Too much cabosil and it's heavy, not enough and it seems too dry. Chopped fibers is strong but heavy. Better for hard points in measured amounts. If you build your shear webs and inner structure with close enough tolerances, you shouldn't need to rely on the filler mix to keep things strong. Less filler, more carbon!!

ptxman

My Feedback: (2)

Some builders of comparable racing models are adding a bit of foaming agent in with the thickened resin mixture for this specific application. One I know for sure is an MGS product. Since you are already familiar with the resin you could probably source the agent. As I understand it, a few drops basically starts a mild micro bubbling action. You lay in a bead & it grows to some increased volume proportioante to teh amount of agent. Its not crazy liek expanding foam, Im guessing maybe less than 2x. I havent used it myself but have seen samples inside some flying surfaces, seem slike a good thing but only if you are seeking to pinch the last grams. Of course, where there is now bulk porosity there is now less stength. Ive also seen beads of macrobaloon (only) slurry & cant imaginge that would b etoo much heavier.

Nice molds BTW. Where did you get them milled if you dont mind my asking & what is the specific grade of tooling board?

Randy Etken

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With the new weight of 3.75 pounds I will not have much trouble staying close. I would be afraid the epoxy would lose some strength adding a foaming agent, but I'm interested, will look in to it.
Molds were milled by a friend who is retired from the business and just purchased a new cnc machine. Took 60 hours total to design and cut. A regular machine shop charges $100 on hour. This would be to rich for my blood. He did give me a deal which was much appraicated.
The tooling board is from www.epoxi.com under foundries, red board, #65. Problem now is there plant burned 3 week ago so need new source.

troposcuba

hey gluedoc, not tryin to hijack the thread here, but this question seemed in line with the discussion going on here. i am an aircraft structural maintainer in the USAF. we use a lot of epoxies (obviously) doing repairs to composite structures. one of the common ones we use cures in about a week at room temp or about 2 hours at 160*F. so just out of curiosity, does heat curing it affect the end strength?

GR7Racer

I've been looking for this "foaming agent" that is referred to here in this thread. I've seen it in use on close outs and it works well to insure a gap is filled. Post-crash inspection shows that the cured mix is still very strong. I've seen it used by RnR on their sailplanes and other kits.

Does anybody have a link to a company that sells this foaming agent? Is it a system that has it's own resin and hardener, or is it a stand alone agent that can be added to any resin/hardener mix?

Thanks for the help!

ptxman

My Feedback: (2)

From what I understand its a standalone liquid that is mixed in small proportions to the epoxy mix & causes teh foaming action. It is an MGS product, or at least thats the one I heard about. Im not usre about its useability in other resin systems.

The MGS dealer here in Canada is Airheart Distributing, email is [email protected]
When I bought some resin off them in this past summer they either had it in stock or thought they could get it in. Dont know about quantity sizes.

In the USA I would try CST as they carry MGS resin. If they dont have it, you can usually ask nice & convince them it would be worthile to carry. Their business is built on dividing up volumes & selling to modeler type guys.

You also mentioned RnR, maybe you can sweet talk them into selling you some.

I do know that Ruud who operates Composite models (in the Netherlands) uses it & will sell it. We discussed it by email earlier this year & he was willing to ship in small qty's. He builds F5D racers, so very similar application. His website is here & email contact within.

http://home.planet.nl/~zandv055/frameNL.htm

Super D

Glue DOC, I'm surprised you use regular epoxies. I have heard that industrial epoxies are much stronger. From what I understand industrial epoxies have weird mixing ratios such as 30:1, and that our hobby epoxies are use extenders to create the 1:1 ratios. The extenders supposedly weaken the epoxies to some degree.
Here is an epoxy I was going to try some of this, as I have heard it was very strong. Sure normal hobby epoxies work for most applications but for a 2 meter pattern plane that will see 3-400 flights in a season, I'd pay more for the piece of mind.
[link=http://www.westmarine.com/webapp/wcs/stores/servlet/categoryg/10001/-1/10001/674/11160/4/man/asc/0/grid]West Marine epoxies[/link]

Also I know of some adding this as well for strength and weight savings.
[link=http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&classNum=165&subdeptNum=11160&storeNum=4&productId=28508]additive[/link]

Just wondering what your 2 cents are, Thanks Dylan

T_om

Then you heard wrong.

There are only a couple of companies in the US that make epoxy resin.

These resins are formulated for different hardnesses temperature ranges and applications, but are well documented as to their physical properties.

Tom