Aft Landing Gear Bulkhead

[GuinnessGuy74]

Trying to finish up Chapter 4, but ran into a snag. I was doing the aft face of the aft LGB, and was laying up the 3 extra plies of Uni along the ears and hardpoints. On the second ply, I think I used too much heat when wetting them out because the epoxy started to become stiffer. When I would squeegee, it would turn whitish, almost like hot glue. It seemed like it was wet out just fine, so I applied the last layer and mixed up a quick batch of fresh epoxy and finished. Peel ply and plastic, squeegeed out a bit of excess and weighed it down flat.

 

When I removed the peel ply 3 days later, this is what I saw (attached picture). White streaks, perfectly along the uni fibers. I find it hard to believe this is air trapped, as they are too perfect. It didn’t look dry to me while I was putting the last layer on, if anything there was too much epoxy on there. Is it possible the white is from the epoxy getting stiff and when I spread it with the squeegee it stayed that color (like the above hot glue reference)? Or did I really make it too dry. Maybe the stiffer epoxy did not wet out the glass? I am not sure what I should do now. Is it possible to heat this up, remove the glass and redo it? Do I need to redo the whole part? This is on BOTH the right and left sides of the bulkhead. I appreciate the help...

 

[Neverquit]

First, relax, have a Guiness. You're learning early of your mistakes and giving yourself good answers.

 

The white streaks, as discribed somewhat in the plans are areas where the epoxy didn't penetrate. You're correct in assuming probably from overheating. This can happen from re-heating hardened epoxy as well. Get out your heat gun, peel the top plies off. Sand it down with 30 grit and re-apply the two layers.

 

You don't need to heat the epoxy too much. The idea is to make it thinner in consistency. 90-100 degree is great. Small plies like this don't need a lot of heat attention. You'll find the heat gun/hair dryer comes in more handy when you're doing real big layups and need to move out the wrinkles.

 

Another thing, don't over heat the plies with the heat gun after peel ply. The dacron will shrink a bit and cause plies to buckle or joggle.

 

You're on the right track. Keep er' going.

[steve]

dido,

[Edge 513]

Was said regarding using the heat, to be careful when using the heat gun. Maybe just vernacular, but a decent wattage hairdryer provides plenty of heat without buying and using a huge wattage heat GUN. Thats a LOT of heat capability that might get you in trouble if on a higher setting. I offered to commandeer my daughters Conair hairdryer and buy her a newer fangled one a couple years ago...she was delighted..and we both got the tool we needed! Just my .02

[GuinnessGuy74]

Thanks guys! I SHOULD have realized that once the epoxy started to get difficult to move around, I should have peeled the plies off and started over.

 

Edge, you are right on track. I was using a heat gun! I use it on low, aiming it over my shoulder trying to keep it a good 3 ft away from the layup. I am sure I had it close a few time and thats when I noticed it getting stiffer.

 

I will do what Neverquit says and heat and peel. Shouldn't be too bad. Better to ask questions than continue on and never learn.

 

Thanks for all the advice. 40 hours down - 2960 to go!!

 

Jim

[TMann]

On the second ply, I think I used too much heat when wetting them out because the epoxy started to become stiffer.

Sounds more like the hardener than the heat. If you are not in a rush for the part, use a slower hardener. Typically the epoxy thins out with heat. It does not get thicker/stiffer.

 

I use a heat gun as well but if there is any possibility of hitting nearby foam, I either protect those areas with foil or defer to the blow dryer.

 

Also, try hitting your epoxy cup with that heat periodically to keep it thin.

Works for me.

[GuinnessGuy74]

TMann,

 

I too use some heat on the cup to keep it thin. I didn't mean to imply that the epoxy got stiffer right away with the heat, but i think some aggressive heating lead to a shorter working time. I usually use about 60:40 slow to hard, and mix up batches of 100 grams to start with. I found in my first layups with 50:50, since i was being extremely sure of everything, it started to gel. So I bumped it down to 60:40 and have been happy so far. Once I get to the sides, I will obviously need all slow.

 

Great advice everyone. I appreciate the support. Back to building tonight!

 

Jim

[Marc Zeitlin]

Sounds more like the hardener than the heat. If you are not in a rush for the part, use a slower hardener. Typically the epoxy thins out with heat. It does not get thicker/stiffer.

If you hit the epoxy with a heat gun (rather than a hair dryer) for too long, you can EASILY get the epoxy hot enough, even in a thin layer, to "kick" and become very thick. I've had it happen a couple of times even with a hair dryer, if I'm not careful.

 

Also, try hitting your epoxy cup with that heat periodically to keep it thin. Works for me.

I do this as well (on low, or else the hair dryer will blow the epoxy out of the cup and all over the place - ask me how I know) but again, be VERY careful not to overheat the epoxy - the cure rate increases by a factor of two for every 10 degrees C increase (did I get that right this time?), so your pot life goes to crap if you overheat the cup - it'll get stringy and unspreadable (and burn your hand as it exotherms).

[Jason Heath]

I find it hard to believe this is air trapped, as they are too perfect.

But that's exactly what your seeing.

 

I had the same thing happen during my first attempt with UNI so I played with epoxy temperatures ranging from room temp epoxy to hot boxed epoxy at 110 degrees, with and without a heat gun or hair dryer. I could always duplicate both positive and negative results and found that my application and inspection technic had everything to do with the outcome.

 

Epoxy temperature is certainly important but how to use a brush, roller, squeegee, flashlight and your inspection of each layer are equally important.

[Big Steve]

This must be an MGS epoxy thing. I use eze 84 and a heat gun on everything I build. I have never ran out of time to do a layup except on My nose Chapt.13(I did the nose and the canopy at the same time. big mistake) and that is because it took 12 hours on a warm day to do. I just brush the epoxy on the glass run the heat gun over it then go back with the heat gun while squeezing I have never had it go hard on me. Of course my brown parts are not as pretty as those blue parts. STeve build on

[Edge 513]

Of course my brown parts are not as pretty as those blue parts. STeve build on

Brown= not pretty??? Just think of your parts as having gone to the beach and got a dark "tan"...=)

[Big Steve]

Man would it be nice to go to the beach and get a tan right now. It is 2 degrees outside right now. I am tired of being cold Steve build on

[Cozy1200]

Man would it be nice to go to the beach and get a tan right now. It is 2 degrees outside right now. I am tired of being cold Steve build on

Not to be cruel or anything, but Melbourne, Australia is currently starting a projected 4 days straight of 104F weather. It hasn't done that in something like 50 years.

[GuinnessGuy74]

104F? I hope thats a dry heat

 

I did the forward side of the bulkhead last night. I took my time, only used the heat gun 2 times and was dilligent in my inspection process between all 8 layers. This morning I took a look before work and everything looks great.

 

Lessons have been learned. Time to move on to Chapter 5....

 

Thanks everyone!

Jim

[Neverquit]

If you hit the epoxy with a heat gun (rather than a hair dryer) for too long, you can EASILY get the epoxy hot enough, even in a thin layer, to "kick" and become very thick. I've had it happen a couple of times even with a hair dryer, if I'm not careful.

 

I've made some great modern art, some still attached to the floor using too much heat.