smooth on and lamination creep

[Kevin Brooker]

I have built a few bows and am very familiar with composite lamination (vac bag, resin infusion etc.) and have a project requiring laminating a few pieces of wood in order to create a tighter radius.

The parts are 4 layers of 3/16" maple 1 1/4" wide and 9" long bent to approximately an 11" radius. The finished part will be subject to a lot of vibration and the choice of smooth-on was based upon the success of using it with bows.

For the lamination in question I weigh up the resin and hardener according to the directions and apply a thin layer to both gluing surfaces. During the clamp up excess resin squeezes out from between the parts so I know there is good wetout and the parts are not resin starved.

After glue-up the parts are placed into a greenhouse which gets to approximately 120F and they remain in the greenhouse under clamp for 48 or so hours.

This morning I went to begin machining the parts to make up the final shape and noticed the pieces looked a bit flat so I placed them back into the molds and sure enough, they are significantly straighter. The parts hold the clamped shape upon removal and  there are varying amounts of creep in each part.

If the smooth-on requires a hatbox cure to fuly set and remain tough enough to  hold I can do this. I didn't want to run it when the temps seem hot enough to cure and I have had success curing bows at greenhouse temps and have never had any creeping issues.

By comparison, parts made using Tightbond 3 have help up fine but the dried glue is a bit brittle and my hope was the smooth-on will be a better and more durable bond.

Have any of you experienced lamination creep? Any other suggestions or adhesives or ideas of how I might be misusing the smooth-on?

[kennym]

Hmmm, I always get a little springback , but figured because you were bending the lams and they wanted to bend back strait.

I think everyone would agree if you lay a bow with curves back on the form it won't quite fit.

Hmmm, now you got me thinkin.....

[Kevin Brooker]

The relaxing is well beyond acceptable springback. Just for grins I bent a few of them in the reverse direction (similar to how a recurve might bend) and the glue failed in shear enabling me to pull the plys apart. The adhesive sheared and not the wood. I suspect mixing errors  might play a part but am now unsure of the smooth-on.

[monterey]

How old was the glue?

[jess stuart]

You want to be sure and mix it very well.  I mix for a minimum of five minutes.   ( Timed minutes.)  Did you rough up the gluing surfaces with sandpaper or similar?

[Kevin Brooker]

The glue was purchased this past winter and was sealed up until three weeks ago when I began the project. The wood came straight out of the surface planer and has a nice tooth. The adhesive remains on the wood but failed in shear. There is no wood tearout just a rough surface of glue. The failure is in the adhesive. When I reverse flexed the pieces it is easy to watch the individual plys slide past each other. There is no sudden failure which leads me to believe the mixing was off either in my technique of mixing or measuring. If I messed it up (fully winning to believe it was me) it is hard to believe I made the same error over 5 layups. Entirely possible.

To enable moving the project forward (figuring out the machining and setting up jigs) I have laid up more pieces and used Tightbond 3/TB2 and some West epoxy I've had great success with when using carbon and glass for non-archery projects.

Thanks for the help and suggestions and please keep the advice coming .

I've been measuring the hardener/resin with a scale using the ratios on the label. How sensitive or not is the mix ratio? I know too much resin will leave many epoxies a bit soft and this is what the failures look like; the resin never hardened off. Some resins won't fully cure unless a temp profile is reached and my understanding is the heat box just speeds the cure but does it also bring the smooth-on to full cure?

[bamboo]

try one lay up with 2a-1b [by volume]---cure at room temp--then heat--following instructions
 
 http://www.smooth-on.com/tb/files/EA40.pdf

---good luck

[kennym]

You def don't want extra hardener. Had a guy call me trying to figure why his EA-40 wasn't setting up. Left a message , but when I got back to him, he had called the company and extra hardener is a no no. Won't set.

Have had a well known bowyer tell me he mixes as Mike said always.

[Kevin Brooker]

Thanks. I've downloaded the .pdf and will try the 2a/1b ratio and cure schedule and see what happens.

[bamboo]

it gives you better heat resistance
as well as stronger bond

[Stagmitis]

Kevin there is definately a "Creep" factor with smooth on.  Most bowyers with R/D or recurve designs will see some springback right out of the form but typically no further. In a hill style which has a much higher core to glass ratio glue creep is enhanced and causes further stretch. There is a loss of backset when strung for a period of time (hysterisis) which recovers again once the bow is at rest.  
You may want to try  Unibond 800 (The new Urac 185) for wood to wood lams. I think there is less creep than smooth on , fills gaps better and certainly not as "brittle" as Titebond 3, and much, much cheaper!

[Overspined]

You need to measure by volume not weight. They weigh different amounts.  I just eyeball it, always making sure at least 1:1 or more resin than hardener. Never had an issue. My guess is you're not getting enough of the right stuff.

[Overspined]

Ha just saw the same answer. Never mind..

[Kevin Brooker]

Thanks. The ratios by weight are 83:100 which does eyeball out to 1:1 by volume. Small quantities are difficult to mix by eye so I weight it. Until now it has worked. I did just order a quart of Unibond 800 to see how it works. Ill keep you guys updated and thanks again for the help

[mikkekeswick]

Nothing wrong with weighing it. 83:100.
Ea40 is a superb expoxy and simply wouldn't fail if everything was ok. Yours must be out of date. EA40 lasts for roughly one year unopened. The problem with shelf lives is that you don't know how long it sat on the suppliers shelf before you bought it.

[Kevin Brooker]

The Unibond 800 arrive yesterday and it was easy to mix and spread. Another piece is currently in the clamps and I'll fill you guys in on how it works. If this piece fails too then I need to rethink what I am trying to do. I've also reconfigured my hoybox to handle the parts and allow a higher temp cure with either the Smooth-On or Unibond.