I just came across this portion of an old post by a contributor re. doing a larger and involved bent lam with epoxy:
"When I do large laminations, I measure out however many cups of resin I expect to need, then mix in however much colloidal silica to thicken it. The hardener can be measured by the mixing helper using the same cup over and over."
I believe the contributor is saying he's mixing the resin and thickener before adding the hardener. While this would seem to save much hassle in the midst of the glue-up it is a bad idea; West System warns against doing this. I know having done so on a glue-up many years ago. Finding a sample of the bend long after the fact I was shocked at how easily it came apart.
Dan, that sounds like something I posted. I don't recall if I was using WEST or system 3 at that time, but I had a crew of probably 8 friends and students who were all up to speed on vacuum bagging.
Half of them had taken two classes from me, and the others were sailing friends who had worked on boats with me.
Before they left their jobs and come to my shop, I had measured the resin into about 40 clear 16 oz solo cups before mixing in the colloidal silica, so there wasn't any doubt the ratio being thrown off by adding the silica.
Once the hardener was added, it was power mixed using a drill-press with a paint mixing paddle. I had punched a hole through a plastic coffee can lid, which was on the 1/4" mixing rod above the paddle. This kept the mixer from slinging epoxy everywhere, but took a 3rd hand to start and stop, so I had two people designated as mixers.
One would measure the hardener and pour it into the cup with resin, hand it to the other, who would bring the cup up under the plastic lid, then turn on the drill press. Then he would take the last cup, and with a stick scrape all sides and bottom while mixing to make sure there were no boundary layers on the side that hadn't gotten mixed.
He would then pass the cup to me or another friend with a roller. With these small batches, we poured the mixed epoxy onto the plys ahead of the roller as we walked from one end of these 21' x 22" wide plys. This trick is much faster than having to pick the epoxy out of a roller tray. After the first run, then we went back spreading it out to the edges, rolling sideways and diagonally.
It was in the high 90º that evening, and with all of us working hard and furious, it was almost an hour after mixing our first batch, before the bag was sealed and wrapped around the form, so I was sweating bullets, but we made it.
I have forgotten how many layers there were, but the total thickness was 2". However, I left the middle layer dry, so I could rout the tread mortice all the way through the inside half, then the tread could be inserted from the outside, which made assembly easy, rather than impossible due to the changing pitch and tapper of each tread.
Here is a link to some photos of the project. Sorry I don't have and digital shots of in progress, but I recall that after the vacuum was drawn, I figured there was 80 tons of pressure on that largest part, the outside stringer. We celebrated with a few beers after that.
If you have tried this, you must measure your ratio before adding the silica to the resin. Then mixing long and vigorous enough, scraping the sides and bottom constantly so as to not leave boundary layers of unmixed resin are mandatory.
If you are sure you've done that, and still had failure, it could be due to cold atmosphere. Heat is needed to get epoxy to kick. In cold weather, I do my mixing with the cup submerged in a hot water bath, then tent my project with a tarp and heater blowing into it.
This elliptical stair was built about 20 years ago, without any callback, so I don't think there was anything wrong with my method. If you have tried this and failed, I'd be happy to hear exactly what you did, and maybe together we can figure out what went wrong.
But for me, if I had waited to mix the resin and hardener, then add the silica, I'd probably have added 30 minutes, which probably would have been too long at that high temperature.
By the way, I only use epoxy from FGCI now. It is higher viscosity, and has a 1-1 mix ratio, which is not very sensitive about being off a little. But the best part, is the price, which is about 1/2 of the ones mentioned above.
I have a sense that a significant amount of resin stays adhered to the thickener particles in an unmixed state and thus the mix is compromised. It sort of makes sense that the 5 to 1 mix of the West System might be more sensitive to this condition than a 1 to 1 formula. Also the power mixing might help out.
I question the necessity of epoxy. I know a lot of guys think that it is an absolute must but I think it's almost never required. The only reason for ever using the stuff is if you absolutely can't get something clamped.
From the sound of your description, it sounds like you're using a vacuum bag which should provide adequate clamping.
Convince me then that I don't know what I'm talking about. Flying stairs have been built for generations without the two-part goo.
I agree. I don't do boat work, which would incline me in that direction. Urea formaldehyde mostly or Tite-Bond Extend II or III are my choices for stringer, fascia, etc bends.
I use epoxy when joinery requires gap-filling. There are photos here of a stair we did joining straight stringer sections to curved. The joinery was done with threaded rod and epoxy set into deeply drilled holes at the ends of the stringer sections. The glue-up shown is of 24" x 5/8" rods being glued and holes being filled with thickened epoxy. The dominos are mostly there for alignment.
When the poplar stringer core was complete, we veneered it with 1/16" rift white oak.
Click the link below to download the file included with this post.
Wow, very nice work Captain Dan! I built stairs too and occasionally work on boats...I still however, steer-well-clear of epoxy.
West Systems stuff was originally designed for cold-molded boat construction and other non-clamped, marine applications. In this regard there is no substitute. As I'm sure you're aware though, tightly clamped stair rail or stringer bends can squeeze the thickened epoxy out or significantly reduce its' mil thickness. This can be very bad news indeed...
I personally don't recommend learning this the hard way.
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