Well, I'll modify the previous statement somewhat...I think you will
find some data on tensile strength for various adhesives including
PVA--a quick google found several references in various journals. The
nearest I read the abstract for measured strength reduction as a
function of strain rate but in your case the rate is essentially zero.
What I doubt you'll find will be anything about the clamping time vs
strength--since the time is relatively short and it isn't nearly as
complex a chemical bonding process as the concrete example I'd expect
odds are very high that any test joints will have been cured and that
will not have been a variable in the study.
What you might find in studies for commercial applications _might_ be
some information on speeding up curing/drying time via microwave, etc.,
where production times might be a significant factor.
If I were to do any further working, it would be from the standpoint of
that as probably the driving force for the research--what can folks like
the furniture manufacturers get away with when they're gluing up reams
of stock? In general those are fully or nearly fully automated
processes and most probably use enhanced methods to shorten the times.
What, if anything, might be of use to you I've no idea.
I think I'd go back to the original thought I had--send a query to
Weldwood, et al., asking if they have any information on the subject and
the reason therefore. I've had some success w/ them in the past on some
You know, what I see in my mind's eye are micro-crystalline bonds that
are destroyed by relaxing the clamp pressure prior to a certain time.
It is as though a mechanical as well as chemical process were occuring
at the same time and that the mechanical bond was totally dependent on
the completion of the chemical reaction.
I have been told that I have too much imagination for my own good.
Hey! I got one of those 'mind-eye' thingies... mine needs to have the
batteries recharged. They have a station for that on Manitoulin
Island. (http://www.manitoulin-island.com /)
I tend to think more in terms of long helix, dreadlocks, interwoven
springs, or old fashioned telephone cords.
I don't 'see' the crystalline structures as most of those glues are
gummy and a bitch to shear(sand).
I can see crystalline structures in hide glue, because that shit gets
all cracky and brittle over time.
When in doubt, use epoxy. Uncle Lew has made me see things his way.
I have never had a WEST joint fail, and I have glued up some pretty
goofy dissimilar material joints.
I now use Weldbond for justaboot everything. http://www.weldbondusa.com /
If it's a real structural joint where the glue is a stressed member and
doesn't just hold the pieces in alignment I'd go with the old standbys,
Weldwood plastic resin glue or Weldwood resorcinol glue. Understand--they
have no gap filling capability to speak of and they cure _hard_, there is no
flex in that joint, so wood movement will over time break down the bond, but
they will not creep and the resorcinol glue is as waterproof as any adhesive
for wood gets. They're not glamorous and "modern" like the epoxies but
adhesives based on the same chemistry (but British brands, not American) put
legions of Mosquitos over the Third Reich, some of which are still flying
more than half a century later.
Please don't go talking Mozzies. A close friend of mine (best man #2
when I married Angela), eats, sleeps, dreams, Mosquitos. If he finds
out there is a thread about those birds, he'll show up and we'll never
be able to get rid of him... His aunt worked on them in Mississauga,
ON. 2 Merlins and some birch/spruce/glue. Wicked crazy plane.
You might find something in this document. Look particularly starting
around page 130.
I had searched FPL for 'pva' and 'clamp time' from this page
From what I have read and witnessed, clamps are use for the sole purpose to
hold things together while the glue dries, you could use masking tape.
Typically wood glues, PVA are quite strong after 1 hour, strong enough that
clamps should not be needed any longer. In the old days when I used
WeldWood mix with water glue I left the clamps on over night.
For the not "quite right fit" I suspect leaving work in clamps over night
may have some additional benefit "if" the joint starts to separate after 1
hour as you remove the clamps. But if the joint holds immediately it should
continue to hold. Then again it does no harm to leave the clamps on over
I sort of follow the directions on the glue (Titebond)
Clamp for 45 minutes, (I usually do something else for an hour or so), and don't
subject to stress for 24 hours..
This is in part because of confidence in the glue, but mostly because I don't
have THAT many clamps...
I leave my work in clamps for 2 hours or more, but do not work with
the glued piece until the following day. I think the 30 minute clamp
time stated on the bottle is too short. Allowing the joint to slowly
cure undisturbed will give better results, I suspect.
Most of what you're looking for is available from specific glue manufacturers
if you ask them.
Having done a little digging in years past, I can summarize (from memory),
but don't have hard facts/details to hand.
"Fully cured" time for most commonly used wood-glues is on the order of 24-48
hours, or longer.
After the glue has set up enough to provide a reasonable 'hold', there is
very minimal benefit in keeping the pressure on.
There are _minor_ benefits from keeping the clamping pressure on for somewhat
longer than the manufacturer-recommended period.
Take the manufacturer recommended clamp time, and do something like triple
it, and you're probably out in the 4-5th decimal place for the share of
ultimate strength you're giving up -- i.e., around .00005 of the 'leave it
clamped for the full curing period' strength. As this is well past the
'stronger than the wood itself' point, that small fraction contributes
-nothing- to the overall project, and you can proceed significantly faster.
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