Re: [ULF-1] Epoxy bonding

From: Member 1
Date: 2018-06-13 00:43 UTC-05:00
Message: #0616
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.