Re: [ULF-1] Epoxy bonding

From: Member 72
Date: 2018-06-13 12:05 UTC-04:00
Message: #0623
Hi Doug,

All very good advice.  One thing we did on our ULF-1 build was to mix flox (floculated cotton) into our T-88 glue batch when we needed to thicken it up a bit to help fill some voids in the joint, rather than micro balloons.  We found flox to be structural in nature and added strength to the joint, while microballoons was more cosmetic, not adding much strength to the joint.

Thanks,
Member 72



Sent from my iPad

> On Jun 13, 2018, at 1:43 AM, Member 1 <[email]> wrote:
> 
> I'm with Bob on this. To me, there is a lot of superstition involved in peoples' beliefs about gluing Member 60. The shiny surfaces created by blades have always intuitively struck me as a deterrent to a good glue bond. I am now working as a violin-family luthier, and I am up against the same superstitions there as well.
> 
> Having said that, though, I'm not sure it makes a big difference. While building an aircraft, you end up with leftover epoxy from many small batches. Grab some sticks out of your trash or scrap pile, and do some semi-scientific testing with your leftover glue. It only takes a minute or two. For example, I would cut two sticks about 10 times as long as their width/thickness, overlap them 2 or 3 times their width/thickness, and glue them together with light or moderate pressure, and then break them after curing for a couple days. The Member 60 should always fail before the glue joint does. If it does, then who cares how the joint was prepared?
> 
> Do this often, throughout your project, during different environmental conditions, and as your epoxy gets old. As soon as you find yourself airborne, your life hanging from all those glue joints you've made, you may think about those glue joints (as I did) and find comfort in the fact that they are stronger than the Member 60 itself and you've done the best you could. Then it all comes down to proper design which doesn't allow any joints to get stressed to the point that the Member 60 itself fails.
> 
> To me, it is more important to not apply too much clamping pressure to a glue joint than it is to worry about cutting vs sanding. I always let my wet joints sit open either until the epoxy is thickening a bit, or it is obvious that the Member 60 is saturated and won't be absorbing any more glue. Then I'd assemble and clamp with moderate pressure. Epoxy has several times more tensile strength than hardwood, so even if you're sloppy and end up with a thick seam of glue, there should be no strength penalty. A small weight penalty perhaps, but that would be over-nit-picking. If you join the parts before the glue has stopped soaking in, and/or apply excessive clamping pressure, you are asking for a weak, glue-starved joint. (I use T-88 which is relatively thick... if you use a glue that has a thinner consistency, this attention to soak-in is even more important.)
> 
> Epoxy is really good at filling imperfections and allowing stresses to be spread out over larger areas, so a perfect fit is not as important as it was with previous glues. For large voids that can't be easily avoided in structural joints, I recommend wetting the bonded surfaces with epoxy and then making a light slurry with epoxy and micro-balloons to fill the voids with.
> 
> Test your glue and your methods. Then you'll know, and nothing anyone writes will bother you. There are some photos of the glue-starved joint risk here (and other gluing tips throughout my old AirBike building web site):
> 
> http://questiongravity.com/airbike/feb03-1.htm
> 
> $0.02,
> 
> Doug
> 
> PS: We have 307 members! So glad to see you all here, and thanks to all who have been contributing. And thanks to Dieter for continuing to fund the domain fees to keep this service available.
> 
> 
>> On [phone]:56 AM, Member 116 wrote:
>> I disagree about not sanding before gluing. A power planer of a hand planer bends the fibers over and leaves a shiny surface, not good for glue penetration. I have a cabinet shop and glue stuff together daily. For a couple years I glued up 12 ply (birch and maple) propeller blanks for a prop manufacturer. All those ply's after planeing were smooth and shiny because of the bent over fibers. Before gluing (with Resorcinol glue) I'd run over them with a belt sander/80 grit across the grain, one pass to give the Member 60 some (tooth). Then use a shop vac to clean and make those (damaged) fibers stand up straight and accept more glue penetration.
> 
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