46 Chetwynd St., Loganholme, Queensland, 4129, Australia
P07 3806 1944 (+617 3806 1944) F07 3209 7711 (+617 3209 7711)
Marine & Exterior Plywood Glue Bond Inspection.
Plywood is a wonderful material, but like all things man made,
occasionally things can be less than perfect. No matter where in the world
plywood is manufactured and even with the most sophisticated
quality systems and procedures the mills cannot achieve 100% pass
rate in their bond ratings. The Engineered Wood Products Association of
Australasia (EWPAA) requires that glue bond samples be taken daily from a
production run and that all but a very small percentage must be of the
highest strength. Please note that this means that they do allow a small
number of poor glue bond strength samples to occur in a batch of plywood
and not to reject the batch for that reason.
Mills that produce
plywood to a standard must, under that standard, conduct ongoing testing
and third party auditing to maintain their stamp of approval. However due
to the fact that sheets which have areas with low bond strength or no
areas with no bond are hard, and sometimes impossible to detect without
destructive testing, these sheets will inevitably find their way into the
market. We use the term delamination to describe this fault.
Plywood intended for truly life and death situations, such as aircraft
applications can be non destructively inspected and certified sheet by
sheet, using ultrasonics, at considerable cost and delay.
rest of us, we have to adopt simpler methods. The first thing to
understand is that this doesnít happen very often. The second thing is
that you canít tell by just looking at the sheet. The third thing is that
it is just as likely in a top of the quality range sheet of AS2272 hoop
pine marine ply as it is in a sheet of cheap imported almost BS1088 marine
ply. The fourth thing to understand is that it might only be a small part
of the sheet which is affected.
So what to do.
sheet up as per normal. Take an off cut, throw it into a bucket of water.
Leave it there for 24 hours, then put it in the sun. Do that a couple of
times and if it is still intact, then there is no problem with the glue
bonds in that off cut. You could also boil the off cut for a few hours to
see what happens.
Please understand that these water tests will
not damage a good glue bond. If it comes apart it is because the glue
bond was unsatisfactory. The water test simply highlights the unglued
areas that were already present, which were impossible to see.
Admittedly, this only tests the off cut, not the whole sheet, how-ever our
experience is that this will detect almost all of the bond failure
situations. Most of the bond faults that we have seen cover most or all of
the sheet. You would get further assurance by testing other off cuts from
the sheet. They donít need to be big pieces, 2Ē x3Ē (50mm x 75mm) is more
than big enough. Alternatively you can water test the actual piece of
plywood that will ultimately be used. Please remember to air dry the
pieces flat, and remember, water testing may cause some grain to rise
which can be sanded smooth again once dry.
If a sheet of plywood
does show signs of trouble, just give us a ring and we will replace the
sheet without question.
Please call Boatcraft Pacific
07 3806 1944 ( +617 3806 1944) for how to contact your nearest reseller.
Copyright Boatcraft Pacific Pty.