Laminate Gluing Pressure

[Delwin D Fandrich]

----- Original Message ----- 
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: <pianotech@ptg.org>
Sent: August 26, 2003 6:50 AM
Subject: Laminate Gluing Pressure


> I am toying with the idea of laminating spruce ribs with Titebond and
providing clamping pressure with the vacuum bagging technique. Of concern
to me is the relatively low pressure attainable with vacuum bagging. With a
perfect vacuum, you will only get atmospheric pressure - 14.7 psi. Del
Fandrich has reported good bonding with 40 to 50 psi and the Titebond web
site recommends over 100 psi. Clearly, less than 15 psi is reason for
concern. However, Ron Overs reports that he is using vacuum bagging to
manufacture his laminated soundboard panels. I presume atmospheric pressure
on the east coast of Australia is about the same here in Florida. How does
one reason all this out - vacuum bag a rib and then watch it for 75 years?
>
> Terry Farrell



By understanding the characteristics of the wood you are working with, the
characteristics of the adhesives you are working with, and the structural
demands of the physical structure you are assembling. There are a number of
factors at play here-- 

First, I would not trust the clamping pressures generated by vacuum-bagging
to adequately form any structural joint having a relatively small contact
area. We also are experimenting with vacuum-bagging to build up laminated
soundboards, but these cannot really be called structural--the soundboard
panel does not carry and significant load. Also consider the large area
involved. On the other hand I know of at least one piano maker who tried
vacuum bagging ribs (against my advice) and gave it up after the associated
problems became overwhelming (which were consistent with my advice).

Second, the recommendations given by adhesives manufacturers are very
generalized and are usually based on tests using hardwoods and they assume
certain shop and material standards and practices. Spruce is not a
hardwood. Nor do we follow industry practices as we assemble piano
soundboards. The adhesive problems we encountered with our
rib-to-soundboard tests when using higher pressures was primarily related
to "dry" joints. Basically when the clamping pressure got above the
(approximately) 60 lb (27 kgf) range there was too much glue squeeze-out
and too much of the adhesive solvent (usually water) was forced into the
very porous earlywood leaving the adhesive without enough solvent to
properly cure. We talk of glue "drying" but it is actually a curing
process. Water is necessary for the glue liqueur to fully cure. An adhesive
can be dry but not cured--there will be a considerable difference in
bonding strength. (And let's not bring epoxy into this discussion--it's a
whole other technology.)

Finally, while this problem showed up in our tests, which were conducted
with the wood samples dried to 4% to 4.5% MC, it may not be a problem with
wood samples at some higher and much more reasonable MC. Regardless of the
clamping pressures, gluing up wood samples at this MC is problematic.
Within the woodworking industry it is understood and accepted that the
ideal MC for most wood processing operations--including adhesive
bonding--is around 7% to 8% MC. The practice of gluing up any wood
structures at 4% MC would be enough to get most woodworking plant managers
fired.

Del