Plywood Design Fundamentals - Canadian Plywood Association

2012_april revised_48945_en_design.qxd 19/04/2012 3:04 PM Page 7
3.4 Dimensional Stability
Plywood is subject to dimensional changes in response to
fluctuations of ambient temperature and contact with liquids
or vapours. Expansion in plywoods of balanced construction
is slightly greater along the grain but much less across
the grain than for solid wood.
The dimensional change, however, is approximately the
same per unit of length in both directions because of the
averaging influence of cross-bands. Change in thickness
for all practical purposes is identical to that of solid wood in
the radial direction.
Effect of Moisture
Plywood’s cross-laminated construction provides superior
dimensional stability in the plane of the panel. The swelling
or shrinking of wood along the grain with changes in moisture
content is small, being only about 1/20 to 1/40 of that
across the grain. The tendency of individual veneers to swell
or shrink crosswise, therefore, is greatly restricted by the relative
longitudinal stability of the adjacent plies.
Expansion of a plywood panel that is free to move consists
of a uniform restrained swelling across the full width or
length, and a less restrained swelling at the edges. Edge
swelling is independent of panel size, varies with the thickness
of veneers having grain perpendicular to the direction
of expansion and, for the same veneer thickness, is about
twice as great for face plies as for inner plies.
The average coefficient of hygroscopic expansion in thickness
is about 0.003 mm per millimetre of original thickness
for each 1% change in moisture content.
Internal uniform dimension changes in a panel decrease as
the percentage of panel thickness in plies having grain perpendicular
to the direction of expansion or contraction
decreases. Standard plywood constructions of four or more
plies are well balanced and the perpendicular ply percentage,
both across the width and along the length, seldom
exceeds 60%. The average coefficient of hygroscopic
expansion (or contraction) for these constructions is about
0.0002 mm per millimetre of length or width for each 10%
change in equilibrium relative humidity; or 0.2% from ovendry
to complete saturation.
The approximate relationship between relative humidity (RH)
and equilibrium moisture content (EMC) of plywood is:
RH (%)
10
20
30
40
50
60
70
80
90
100
EMC at 24 o C (%)
2
4
6
7
9
11
13
16
20
28
In normal conditions of dry use, equilibrium relative humidity
may vary between 40 and 80%, with corresponding moisture
contents ranging from 7 to 16%. Total dimensional changes
of a 1220 mm by 2440 mm panel exposed to this change in
conditions may be expected to average about 1.0 mm
across the width and 2.0 mm along the length.
7
Effect of Temperature
Plywood expands on heating and contracts on cooling. The
rate of thermal expansion or contraction is practically independent
of the temperature range involved, but is strongly
influenced by the construction of the plywood. Thermal
changes cause wood to expand about 10 times as much
across the grain as it does parallel. By ignoring the influence
of variations in moisture content and specific gravity, the
average coefficient of linear thermal expansion a can be calculated
for a temperature change of 1 °C from the following
formulas:
For length and width: For thickness:
( ) ]
ap »
a t » 28 x 10 -6
Values of a p for various plywood constructions are presented
in Figure 2.
Changes in panel dimension (length, width or thickness) due
to a change in temperature can be calculated as:
Final dimension = Original dimension [1 + a(DT)]
where a = coefficient of thermal expansion (a p or a t).
DT= change in temperature (°C)
Coefficient of linear thermal expansion parallel or perpendicular
to face grain direction per oC (µ p x 10-6 [
)
16
14
12
10
8
6
4
2
36 1 -
40 -
1 - 0.92
0 0.0 0.2 0.4 0.6 0.8 1.0 1.2
t^ ( t )
t^
t
t^
t
Ratio of thickness of plies perpendicular to
the direction of expansion to total thickness
of panel
Figure 2. Coefficients of Linear Thermal Expansion
for Plywood of Various Constructions
certiWood Canply plywood Design Fundamentals