Glue for a built up fin

[grandcross]

I'm building a fin set for my Saturn IB rocket that goes from quite thick to a knife edge. I plan on using a birch plywood core, with balsa on either side sanded to shape, and finally covered with a layer of carbon fiber. It is a high power model, but I have quite a large margin for fin flutter, so that's really not a concern.

So my question is this. Is there any advantage to using epoxy over wood glue for attaching the balsa to the plywood core? Wouldn't the wood glue be lighter (a concern) and adequate to the task? The outer carbon fiber layer would of course be using epoxy.

 

[dixontj93060]

My only concern with wood glue is water-base and it warping the balsa. Since you are covering with CF you really only need a temporary bond. Have you considered using spray adhesive like 3M 77? Makes it easy to apply and reposition if necessary.

 

[MCriscione]

I don't have any experience with these kinds of build-ups, but I'm interested to learn the answer. I would have thought that you'd want a good strong bond between your core materials. Fully cured wood glue should be plenty strong and won't trap any more moisture than is already present in the wood itself.

 

[grandcross]

My only concern with wood glue is water-base and it warping the balsa. Since you are covering with CF you really only need a temporary bond. Have you considered using spray adhesive like 3M 77? Makes it easy to apply and reposition if necessary.

I'm not really worried about warping. It will be under pressure as it sets, and sanded into its final shape after. I would be worried about a temporary bond though. Even if fully laminated, having unbonded parts seems to me an invitation to warping and vibration. I have nothing to back this up though so knowledgeable opinions are welcome.

 

[rharshberger]

I would use the wood glue for the wood to wood bonds as it is stronger than material its bonding. Wood glue really wont add much moisture if used properly, and the plywood is pretty stable against warpage. A little weight will make sure everything stays flat.

 

[Woody's Workshop]

Tite Bond II is stronger than either wood you are using. It also gets into the wood unlike epoxy which is a surface adhesion only.
Laminating it should be fun.

 

[dixontj93060]

I'm not really worried about warping. It will be under pressure as it sets, and sanded into its final shape after. I would be worried about a temporary bond though. Even if fully laminated, having unbonded parts seems to me an invitation to warping and vibration. I have nothing to back this up though so knowledgeable opinions are welcome.

If you use the right laminating epoxy it will go through carbon fiber and sucked right into the balsa. I'm also assuming the exposed edges of the fin will be the birch, not the balsa so you will get epoxy creeping in all around the edges of the balsa. Believe me, nothing is going to be moving after the CF layer sets. Heck I use 3M 77 to apply fiberglass before laminating, does not affect adhesion at all. And the 3M 77 will add "zero" weight while any other internal adhesives will (if aft weight is a concern to you?). And if you are really worried just use 3M 90 instead. BTW, be careful how much pressure you apply to the balsa sheets. I have used end grain balsa in cutouts with a plywood frame and it works fine as no direct pressure is applied to the balsa, but having it as a full layer has it completely exposed to being nicked/smashed until the CF cures.

 

[markkoelsch]

I'm building a fin set for my Saturn IB rocket that goes from quite thick to a knife edge. I plan on using a birch plywood core, with balsa on either side sanded to shape, and finally covered with a layer of carbon fiber. It is a high power model, but I have quite a large margin for fin flutter, so that's really not a concern.

So my question is this. Is there any advantage to using epoxy over wood glue for attaching the balsa to the plywood core? Wouldn't the wood glue be lighter (a concern) and adequate to the task? The outer carbon fiber layer would of course be using epoxy.

I think that using wood glue is the way to go. Wood to wood should be doe with wood glue. I like Titebond. Make sure that the parts mate well, put a layer of glue over the entire mating surface, put the part together, and clamp. You want enough glue that you squeeze some out of the joint. The glue joint, once set, should be stronger than the wood itself.

 

[tomsteve]

ya might want to glue up a test piece to see if the wood glue causes any warpage.
if theres no warpage, id have no problem using titebond II. i use it in furhiture building all the time. after trying quite a few different brands, i stuck with titebond II a couple years ago.
i did a little test a few years back with tb II to see what the strength was. well, not like labratory pressure gages and all, but just glueing 2- 1 by 4 by 12" long pieces of pine and 2 of oak together in a lap joint- 3" lap- then breaking them apart. kt wasnt the glue that failed. there were wood fibers from one piece completely covering tne lapped area on tbe other on both the oak and pine.