PLYWOOD

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Plywood is made from an odd number of
constructional veneers bonded face to face with
the grain running in alternate directions
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It shrinks, warps, twists and swells less than
ordinary wood and has great resistance to
splitting at the ends
Plywood's main advantage is that by gluing
together an odd number of layers of veneer, a
material that is lightweight and workable, yet
rigid and strong can be obtained
plywood can be manufactured in sheets far wider
than the trees from which it was made.
High uniform strength - wood is 25-45 times
stronger along the grain than across the grain.
Crossing the adjacent sheets tends to equalise
the strength in all directions.
Ease of fabrication of curved surfaces
One of the important aspects in the manufacture
of plywood is that it results in the
conservation of timber by the elimination of the
waste.
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The manufacture of plywood can be divided into
two main stages The production of veneers The
assembly and pressing of veneers into plywood
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In general however most veneers wether rotary cut
or sliced, are cut to size and squared by a dry
clipper, edge jointed, taped or spliced and
graded into cores, cross bands, or face veneers
prior to being assembled for gluing and
pressing.
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Roller coaters
Curtain coatersThese apply adhesive in a thin
film or curtain of adhesive to one side of the
veneer as it passes through the coater.
spray coaters
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Urea formaldehyde glues are extensively used for
interior and intermediate grade bonding
Phenol-formaldehyde glues are standard for
exterior bonds
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Fillers may be used to assist in reducing
penetration of the glues into the wood, increase
its viscosity at the critical time before
setting, and tend to fill small cavities between
veneers, thus preventing starved joints. Fillers
are usually finely ground inert minerals such as
clay or wood/grain flour finer than 180 mesh.
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The pre-press (see Pic 1) which is unheated
operates at pressure slightly below that of the
hot-press. Pre-pressing can takes up to 50 sheet
at a time, with a pressing time of 3 to 4
minutes
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Hot pressing is carried out in a hydraulic press
incorporating multiple heated Platens between
which each individual panel assembly is subjected
to heat and pressure. Hot press pressure for
softwood pine plywood usually range between 1242
to 1380k Pa. Platen temperatures of around 120
degree Celsius are used and pressing times for
panels 3.5 and 22 mm are 2.75 and 9.5 minutes
respectively
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In hot pressing a considerable amount but not all
of the water in the glue line escapes in the
form of steam. Moreover, hot pressed panels are
often dipped or sprayed with water immediately
after removal from the press in order to restore
the moisture in the face plies. Consequentially,
a conditioning or redrying period is necessary
and hot pressed plywood is usually hot stacked
in solid piles for periods ranging from one or
two hours to overnight. In this way the glue
joints attain full cure and the moisture content
of the panels is reduced and equalised through
out the panels before further processing is
carried out