Composite Joints

[mithrandir]

OK... Long over due for this thread. Lots of pictures and not a lot of verbage.....

I have intended for some time to write a thread on the proper way to build a composite joint. This procedure is suitable for any type joint you may encounter. It is appropriate for a wooden plywood Fire wall being bonded to a wooden engine box, or to attach a Composite bulkhead inside a fiberglass fuselage. The basic steps are the same.

This example is the proper means of bonding a Cowling Ring into the top cowl of a QQ120 Yak.

As a first step, we ZAP'd the plywood ring into the correct location. If you use Zip-Kicker, allow 15 or 20 minutes after use to allow the kicker to fully evaporate. Zip-Kicker can affect an epoxy bond if it is not permitted to fully evaporate.

Once the ring is tacked into place with the ZAP, you need to prepare your work area.
Use a table you won't mind getting slimed.
First step is to use some sheet plastic as a protective layer and a "Carrier" for the Carbon Strips. (Two layers) ...A trash bag works great. Use some masking tape to hold it down on your table surface. Get it taped down as smooth as possible. Apply the tape at the 12 and 6, then 3 and 9 o'clock positions first. Pull the plastic a little tight and then tape down the corners. (Taping the corners first doesn't work as well). We want to have two layers taped down, one directly over the other.

Next take your pieces of carbon (Fiberglass will work the exact same way and will actually go around corners easier then carbon)... and place them on top of the plastic. Use something like a Credit Card, or Magnetic Hotel Card as a squeegee and make sure you have at least 3 pairs of gloves. Surgical gloves work the best!
You will also need some Cotton Flox (or micro ballons... flox is better though)... two mixing cups, a couple mixing sticks, Sacrificial Scissors, several paper towels and some slow curing epoxy. (30 minute epoxy is too fast for most jobs like this). Might as well get a quart of acetone handy for cleanup when we are all done!

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[mithrandir]

Be sure to thouroughly sand the bonding region. What we are doing is called a "Wet Tie". So we sand the wood ring and a region about two inches wide along the ring-cowl joint. If you fail to sand this region, Delam is likely to occur.

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The next image shows what our work area looks like before we get started. Material is layed out, Squeegee is ready, Scissors ready and Slow Cure ZAP Brand Two Part Epoxy and gloves and mixing cups.... all ready to go!

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The next images show what Cotton Flox looks like.. it is simply very short fibers of cotton.. it serves as a filler or to make the Epoxy a bit pasty so that it will not run or flow. Being a fiber, it is a little tougher then Micro Ballons would be.


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Ok... lets Glove Up!!

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[mithrandir]

Now we are ready to mix up some epoxy! I like the ZAP Slow "Finishing Resin" epoxy for these tasks because it is two part, so simple to measure/mix.
Mix up more then you think you will need.

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It seems easier to me if I dispense both the resin and hardener simultaneously... you can see in the cup if you are not squeezing out equal amounts.

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Be sure to thouroughly mix. This step is likely the number one reason epoxy bonds are low quality. You must be sure to scrape the sides and the bottom of the cup! Once you are done mixing..... Mix it s'more!

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OK.. this is gonna sound wierd, but poor off a little bit into a second cup. In the second cup we will add the Cotton Flox to make a paste.

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[mithrandir]

Now pour some of the resin onto the cloth. Try to pour such that it is distributed evenly and not allowed to puddle in one spot.

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Now use the Squeegee to spread out the resin. Don't press down too hard at first, give the resin some time to soak thru the cloth to the plastic sheet underneath. Once it is mostly soaked thru, then you can press a little harder to spread the epoxy out. If you are doing large areas of this sort of work a good rule of thumb is to weigh all the cloth you wet out and mix that same weight of resin for the cloth. (50-50 ratio of cloth - to - resin).



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[mithrandir]

The next step is to paint straight resin onto the bonding regions of the fiberglass cowl and wooden Cowl-Ring. The purpose of this is to make sure that every little nook and cranny gets wetted out. Especially with Carbon Fabric... once it goes down, you can not see what is happening underneath... so this light coating of resin is a bit of extra insurance that the joint won't be dry and prone to delamination.

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Next we mix up some Cotton Flox in the resin batch we set aside earlier. Add enuff flox so that the consistancy is between Toothpaste and Peanut butter...... CREAMY!

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[mithrandir]

Now lets take that Paste-ee-Poxy and create a fillet in the corner formed by the ring and the cowl..... A tongue depressor or popsickle stick works good for mixing and scraping into the corner the epoxy. A fillet radius a little smaller then a Dime is a good size. Be sure to scrape off all unused paste.


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[mithrandir]

Now lets go back to the Carbon we wetted out and cut it into strips about 1.5 inches wide... notice I have cut the carbon initially such that the stright edges are at a "Plus'r'Minus 45 degree angle" to the weave. I cut it off the roll at a 45 degree angle. This 45 degree bias (or orientation) will make a stronger joint as well as being much easier to get the carbon to take the compound curve.

Also notice... This is IMPORTANT!!!!... I cut the carbon and ONE Layer of the plastic together!!! If you miss this... you will be hosed... The carbon tape is on a 45 degree bias, and without the plastic layer I cut with it, it would pull and distort....

BE SURE TO CUT THE CARBON WITH ONE LAYER OF PLASTIC AS A BACKING!!!!

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[mithrandir]

So now we have several strips of Carbon, about an inch'n'a'half wide with the plastic still attached... See how easy it is to handle?

See how easy it is to fold in half????

Why do you s'pose I would want to fold it in half?????


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[mithrandir]

Well... I fold it in half to make laying it in the corner MUCH EASIER!!! I try to align the folded edge along the corner.... Smooth it out a little bit with the plastic still on. And once it is smoothed out, I carefully peel the plastic off.
See how nicely the carbon lays into that corner!?!?!?

Peel the plastic off slowly so things don't get mussed up!

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[mithrandir]

Next use that acid brush to make sure the carbon is pressed down and all air bubbles removed....

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Hey... that went so easy... lets lay down the other strip!!!
Start out by folding the Carbon tape in half... and lay'r down!

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[mithrandir]

Now carefully peel back the plastic and repeat the process of "Stippling" the bubbles out from under the carbon with the brush....

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[mithrandir]

These types of joints are prone to delaming at the ends... so lets add an additional layer of carbon right at each end of the ring.... Same process... just little pieces....


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And now that we are done... lets use that Acetone to clean the scissors....

Be carefull... a long time ago, with my gloves still on, I was cleaning my scissors with an acetone soaked rag... and the blades cut right thru the rag and drove Acetone right into the resulting cut!!!!

OUCH!!! Truly a conscioussness raising experiance!!!

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[mithrandir]

So... lets look a little more closely at our work....

You can clearly see the 45 degree bias to the bidirectional cloth. This orientation made this job a slam dunk. If we had strips of carbon at a "Zero-90" orientation, it would have been nearly impossible to have wrinkle free results..

And the small fillet of Spooge we put in the corner means we didn't have to spend the next two hours constantly trying to push the cloth into the corner... Zero-90 would lift up out of the corner... even with the spooge fillet.

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And That is how to do a composite joint....

Highlights are... clean, uncontaminated bonding surfaces, straight resin painted into all joints and taped surfaces, cloth at a 45 degree bias... don't cut your hand with an acetone soaked rag!!!

I hope this has helped yuh'all and makes sense to you.

Mith