Epoxy for injected fillets

I just finished up my first fiberglass rocket, and rather than using injected fillets, I left the aft centering ring off and used a thin dowel to poke epoxy up in it and spread the fillets. It worked okay, but it made a big mess and I think I ended up using a lot more epoxy than I needed to.

So I want to try injected fillets on my next build. I'm not sure what kind of epoxy to use though. Right now I have some Bob Smith 5 minute epoxy, and rocketpoxy. I worry the 5 minute stuff won't be strong enough, and I worry that the rocketpoxy is too thick to spread out once it's inside the body tube.

What do you use for injected fillets?

West System with chopped carbon.

US Composites 635 (3:1) with milled fiber. US Composites is great epoxy and not nearly as expensive as some of the others. I use a Monoject 412 irrigation syringe bought from a local Ranch & Home supply, I cut the tip down to about 1/8" or so and drill holes just large enough to insert.

I am down to the last dregs of my cheapie aeromarine quart and a half kit.

Most of the quart was used in saucer molds and wrapping a couple of tubes.

Was thinking of going for a gallon next but wanted 'the good stuff' if i am going to load up.

Proline 4100 is available again..how does that match up against the us composites and aero poxy or there are two upgraded aeromarine epoxies one looks structural and the other is slightly thicker than there cheapie stuff.

I looked at the viscosity and shear strength from the data sheets and not seeing a stand out. Had used proline before and was amazed how clear it ..but how much better is it?

Kenny

KenRico said:

I am down to the last dregs of my cheapie aeromarine quart and a half kit.

Most of the quart was used in saucer molds and wrapping a couple of tubes.

Was thinking of going for a gallon next but wanted 'the good stuff' if i am going to load up.

Proline 4100 is available again..how does that match up against the us composites and aero poxy or there are two upgraded aeromarine epoxies one looks structural and the other is slightly thicker than there cheapie stuff.

I looked at the viscosity and shear strength from the data sheets and not seeing a stand out. Had used proline before and was amazed how clear it ..but how much betty is it?

Kenny

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In my experience with US Composites 635 is a very clear resin mainly designed for laminating,however with additives I use it everywhere, its definitely less expensive than Proline or Aeropoxy. Aeropoxy, Proline, and West are "premium" epoxies and you pay for that, that being said they have great reputations in the hobby and probably have an edge in performance too, if I were building an all FG rocket I might considered spending the extra money for them, but as long as I am building paper and wood, or FG wrapped paper and wood, there is zero (IMO) reason to use a more expensive epoxy. 635 is also easy to work with and it only $71.00/gallon.

jeff2space said:

West System with chopped carbon.

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Chopped carbon be prepared for a learning curve. Use too thin of epoxy and it just comes out with furballs and may not be distributed evenly. Too thick an epoxy and you sure has heck am not going to be able to inject the mess anywhere. I have my doubts about the additive strength of randomly oriented fibers. If there is too much chopped stuff in there, I believe the bond will be substantially weakened. I suspect there may be a "narrow" window where a given amount of chopped stuff adds strength. Too little and added work for no gain. Add too much and the bond is weaker.

Where one sees the most strength is where folks laminate fiberglass, or kevlar socks or carbon fiber mat in layers and rotate the orientation of the fibers in each layer. Do vacuum bagging and holee molee that is strong stuff.

I've injected "neat" Proline 4500 and Cotronics 4525B and 4525IP without trouble, without chopped fibers and am satisifed with the results. It is easy to get into trouble if working in tight spaces with a small diameter syringe and tubing if injecting CF gunk.
Get a clog and you are in deep doggy do do. Can lose your whole batch. That's why I only inject the "expensive" stuff neat. If you can use a large diameter syringe and tubing you can use more viscous "stuff".

If you are going to mix CF with your epoxy, try on EXternal fillets first as you will have the room to work with it and reshape it. Better yet, do small practice batches and slather it on "practice pieces" of plywood and see how it turns out.

Actually, I think surface prep is more important than whatever anyone can add to the epoxy. Clean with solvent. Use as course a grit of sandpaper you can get away with on the surfaces to increase the surface area of adhesion and clean again. Then go for it.

I used to use PowerPoxy Weld (out of production, NOT J&B Weld different stuff) and would hit it with a heat gun after 5 minutes or so. It would re-liquify for a short period of time and that would allow it to REALLY soak in. Of course it
would also accelerate the curing so it would start to set up quicker. I discovered after I did an autopsy on a crashed irrepairable fincan, I could NOT snap a fin fillet off of the plywood fin after I chopped it out of a cardboard rocket body.
I had to go at it with a screw driver to pry the bead off the plywood. Now that's adhesion!

Now some epoxies you hit with a heat gun and they will just set up quick and not re-liquify. I discovered Proline 4500 will reliquify so I am very happy I can use that technique with that product. Heat on 4525 if used has to be tried early on after application otherwise it will just accelerate the setup without having liquifaction first.

The ideal fillet adhesive is one that goes where you put it, stays where you want it and doesn't stay "runny" for hours and hours.

The PowerPoxy I could pull the tape after 15 to 20 minutes and if the tape interface bead "wasn't pretty" I could usually hit it a second time with the heat gun and get it to reflow and look nicer. Kurt