Internal Fillet Help

[Kruegon]

I have some very long internal fillets to do soon. The edge of the fin tab is significantly down inside the body tube. Is there an easy and reliable way to thin the epoxy slightly to help it run the distance? I've had issues getting the BSI 30 min to run long distances easily. I don't need it too thin, maybe 20-30% thinner is all.

 

[rharshberger]

I have some very long internal fillets to do soon. The edge of the fin tab is significantly down inside the body tube. Is there an easy and reliable way to thin the epoxy slightly to help it run the distance? I've had issues getting the BSI 30 min to run long distances easily. I don't need it too thin, maybe 20-30% thinner is all.

Is it too late to do fin pockets?

 

[Kruegon]

Fin pockets? Not sure I'm familiar with this one.

 

[kcobbva]

BSI doesn't run too well. If you get RocketPoxy, it definitely runs much better and I've been a convert. I only bring BSI if I need a quick fix on something. The longer you let RocketProxy sit, it becomes stiffer and works better when you need areas you can work in. But if you need something to run the length; I'd recommend trying it out. I've not used Fin Pockets myself as Rich suggested; but you'll find a lot about that on the forum and I understand it works very well.

 

[rharshberger]

Fin pockets? Not sure I'm familiar with this one.

Basically fin pockets are internal (between mmt and airframe) "fins" on each side of the real fin, they act as dams since the CRs are already in place. Enough epoxy is put into the trough created under the fin slot so that when the fin is inserted the excess totally fills the area with a little squeeze out, basically creating a square internal fillet of any size you want. Hopefully I will be able to get the fin pckets built on my L3 tonight and I can post some photos.
I started an entire thread about them several months back.

 

[farsidius]

West 205 epoxy (105 resin + 205 hardener) runs really well for internal fillets. Even runs well when mixed with chopped carbon or fiberglass. It's all I use for my internals.

 

[michigander]

Is rear ring already on ? , if not tubing on syringe works great

 

[fyrfytr310]

Is rear ring already on ? , if not tubing on syringe works great

Ooh. I'm going to try that one.

 

[Kruegon]

Aft ring is indeed still off. it's roughly a 6-8" stretch into the BT before I hit the fin tab. And another 5-6" of run there. This is inside a 2.6" with a 29mm. It's tight but not horrible. The next build (the one I'm curious about) has about 10" into the tube before you hit the fin tab. and the tab is about 8-10" long. I haven't measured it yet. I was hoping to just thin the BSI for the current build and make life easier. I can get better running epoxy for the next build.

 

[dixontj93060]

On my Binder Exocet had to reach about 15" in. I used an 18" 3/16" dowel with a narrow "Popsicle" stick taped to the end (see: https://www.hobbylobby.com/Crafts-H...-Wood/5-1-2-Natural-Slim-Craft-Sticks/p/28584).

 

[markkoelsch]

You should use a laminating resin like the mentioned west, or Aeropoxy. They are designed to flow.

Are you going put a centering ring in place at the bottoms of the fin? Do that, drill holes in tube near fin root, and squirt laminating resin in. Hole gets sealed under external fillet.

Use some type of amendment. Chopped glass or carbon fiber work great, or while it takes a little more prep Kevlar pulp.

 

[danielsadjadi]

Is rear ring already on ? , if not tubing on syringe works great

1+

I once used these for internal fillets, they are cheap and work great. Any tube you can find around works good, but keep in mind that thick epoxy can get jammed at the throat of the syringe and push the tube out, if your not thinning the epoxy put some duct tape on the edge where the tube meets the syringe.

https://www.valleyvet.com/ct_detail...z8Ks733Ksj3oB6__RNXdKByWr5EixlZqjEaAtNm8P8HAQ

 

[Kruegon]

Well, if I could find a way to thin my epoxy, the issue would be moot. But I don't know what to use to thin it. And either no one knows what to use either, or they're not sharing lol.

 

[rharshberger]

Folks,

Just want to let you know that I anticipate having the bigger bugs fixed by tomorrow afternoon. I do have vBulletin phone support for a while, so I'm going to use that and get things straight.

Thanks for your continued patience.

Make test sample thinned with denatured alcohol, and see if it hardens fully.

 

[dixontj93060]

Well, if I could find a way to thin my epoxy, the issue would be moot. But I don't know what to use to thin it. And either no one knows what to use either, or they're not sharing lol.

Alcohol will weaken epoxy. Acetone is usually your better choice, but use sparingly--a little goes a long way. Only 5% or so whereas with alcohol you will need 3X as much to get same change in viscosity. Another option is 10 to 15 seconds in a microwave, but be very careful as epoxy absorbs heat (or microwaves) very fast. Also this will shorten your cure time, so take that into account when applying.

 

[danielsadjadi]

DO NOT THIN WITH CHEMICALS!!! Thinning epoxy especially when dealing with rockets is a dangerous, let me explain why.

Epoxy has a natural viscosity, there is no material that can thin epoxy without changing it's properties. You thin epoxy by adding a solvent, acetone, lacquer thinner, denatured alcohol, upon other thinning agents. These solvents weaken epoxy even in minute quantities and over time will dissolve, crack, and shrinken the epoxy, resulting in a damaged bond which can spontaneously break under the intense pressures the rocket will face in flight. A 5% solvent to epoxy ratio can compromise the epoxies strength by 35%. This is why thinning epoxy with chemicals is highly disregarded.

However, there is still a way to thin the epoxy without the madness. You first heat the resin and hardener SEPERATELY to 35° celsius, then mix them together while warm to create thinned epoxy. Do this in a well ventilated area or under a fumehood, since the epoxy can outgas.

Hope this helps,

Dan

 

[rharshberger]

Alcohol will weaken epoxy. Acetone is usually your better choice, but use sparingly--a little goes a long way. Another option is 10 to 15 seconds in a microwave, but be very careful as epoxy absorbs heat (or microwaves) very fast. Also this will shorten your cure time, so take that into account when applying.

Best thing is to just contact BSI for their recommendations. That being said iirc the BSI 20min epoxy is actually thin laminating resin, the 5, 15, and 30 mins are regular viscosity.

 

[dixontj93060]

However, there is still a way to thin the epoxy without the madness. You first heat the resin and hardener SEPERATELY to 35° celsius, then mix them together while warm to create thinned epoxy. Do this in a well ventilated area or under a fumehood, since the epoxy can outgas.

Yes, I should have said heat resin and hardener separately.

Regarding thinning, I wouldn't say it was "dangerous." Yes, you loose a third of the strength, but this still might meet the application need.

 

[Kruegon]

Ok guys. That was extremely helpful info. I'll be getting different epoxy for the next build. For now, I just need to get through my current build.

Thanks for the info.

 

[rharshberger]

From BSI's website

"The one question that is answered most often by the technical experts at BSI is what can epoxy be thinned with. Isopropyl alcohol that is 90-99% pure is what works best. It is available from most drug stores at a reasonable price. Rubbing alcohol contains only 70% isopropyl with the other 30% being water, which can become trapped in the cured epoxy. Denatured alcohol (which is ethanol with an additive to make it undrinkable) has also been used to thin epoxy. Acetone will thin epoxy but will sometimes change the curing characteristics. Also, epoxy thinned with acetone can have an adverse effect on white foam. Epoxies can be colored with the various tinting compounds designed for acrylic and latex paints, which are available at many hardware stores. Colored FINISH-CURE™, thinned with isopropyl alcohol, has actually been applied with a spray gun to form a beautiful final finish on several models. Of course, the immediate clean up of the spray equipment is extremely important."


https://www.bsi-inc.com/about/tips_and_tricks.html

Thanks.

 

[dixontj93060]

From BSI's website

"The one question that is answered most often by the technical experts at BSI is what can epoxy be thinned with. Isopropyl alcohol that is 90-99% pure is what works best. It is available from most drug stores at a reasonable price. Rubbing alcohol contains only 70% isopropyl with the other 30% being water, which can become trapped in the cured epoxy. Denatured alcohol (which is ethanol with an additive to make it undrinkable) has also been used to thin epoxy. Acetone will thin epoxy but will sometimes change the curing characteristics. Also, epoxy thinned with acetone can have an adverse effect on white foam. Epoxies can be colored with the various tinting compounds designed for acrylic and latex paints, which are available at many hardware stores. Colored FINISH-CURE™, thinned with isopropyl alcohol, has actually been applied with a spray gun to form a beautiful final finish on several models. Of course, the immediate clean up of the spray equipment is extremely important."


https://www.bsi-inc.com/about/tips_and_tricks.html

Thanks.

I don't really use hobby epoxy much. Here are the numbers from West System. Acetone leads to lower viscosity while retaining more strength than thinning with alcohol or lacquer thinner at every dilution percentage up to 5% (they don't recommend more than 5%).

 

[danielsadjadi]

From BSI's website

"The one question that is answered most often by the technical experts at BSI is what can epoxy be thinned with. Isopropyl alcohol that is 90-99% pure is what works best. It is available from most drug stores at a reasonable price. Rubbing alcohol contains only 70% isopropyl with the other 30% being water, which can become trapped in the cured epoxy. Denatured alcohol (which is ethanol with an additive to make it undrinkable) has also been used to thin epoxy. Acetone will thin epoxy but will sometimes change the curing characteristics. Also, epoxy thinned with acetone can have an adverse effect on white foam. Epoxies can be colored with the various tinting compounds designed for acrylic and latex paints, which are available at many hardware stores. Colored FINISH-CURE™, thinned with isopropyl alcohol, has actually been applied with a spray gun to form a beautiful final finish on several models. Of course, the immediate clean up of the spray equipment is extremely important."


https://www.bsi-inc.com/about/tips_and_tricks.html

Thanks.

You shouldn't use thinner with epoxy, it degrades the bond and structure.


Denatured alchohol is ethanol (C2H4OH) with methanol (CH20H) added to make it non-drinkable

Isopropyl (C3H70H) is a different alchohol, however both work equally bad in epoxy.

Methanol does not react with epoxy, the molecular structure of both chemicals make it so similar that one cannot compare to the other in terms of performance. But why take the risk? So much easier to heat it up.

 

[danielsadjadi]

I don't really use hobby epoxy much. Here are the numbers from West System. Acetone leads to lower viscosity while retaining more strength than thinning with alcohol or lacquer thinner at every dilution percentage up to 5% (they don't recommend more than 5%).

View attachment 285224 View attachment 285225

The compressive strength of all the thinners are pretty much cut in half with a dilution percentage of 5%, which is the amount you need to effectively thin epoxy.

 

[dixontj93060]

The compressive strength of all the thinners are pretty much cut in half with a dilution percentage of 5%, which is the amount you need to effectively thin epoxy.

Actually a little over a third for acetone, but yes, a half for alcohol.

 

[danielsadjadi]

It isn't usually the pressure that the bond is dealing with that breaks it (although that is a big factor), it is the formation of cracks, shrinkage of the epoxy, among others physical characteristics that the thinner changes. For example, holes and cracks in lacquer thinner occur MUCH more frequently than in other thinners like alcohol.

 

[dixontj93060]

It isn't usually the pressure that the bond is dealing with that breaks it (although that is a big factor), it is the formation of cracks, shrinkage of the epoxy, among others physical characteristics that the thinner changes. For example, holes and cracks in lacquer thinner occur MUCH more frequently than in other thinners like alcohol.

I agree. By far heat is the best option.

 

[o1d_dude]

What about just injecting the internal fillets through holes drilled into body tube along the fin root?

Crazy Jim swears by it. I have used it as well with great results.

 

[dixontj93060]

What about just injecting the internal fillets through holes drilled into body tube along the fin root?

Crazy Jim swears by it. I have used it as well with great results.

Totally agree, but BSI 30-minute (which is what he's working with) just doesn't flow well.

 

[fyrwrxz]

Just wondering why nobody has suggested foaming...

 

[Kruegon]

Just wondering why nobody has suggested foaming...

Not looking to learn something new like that this time. I've heard about it, but never seen it in action. It's definitely something I'm interested in learning for a future build.

Plus I'm already mid work on the epoxy fillets.