The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

98 Core Boatbuilding Techniques
In sandwich construction, fiber reinforced
resin serves as the structural skin of a
part. Separating two structural skins
with a core material will improve the
composite’s stiffness and reduce
weight, much like the web of an I-beam
separates the flanges. Cores can be of
plywood, end-grain balsa, foam or
honeycomb. Normally, the core is
much lighter than the skin material.
Like most materials, different cores
have advantages and disadvantages.
Loads tend to concentrate at the load
bearing skin/core interface, so lowstrength
cores are prone to shear
failure. Cores are low in density and tend to insulate the laminate.
This can be detrimental as heat concentrates on the skin, softening
the resin or causing adhesion problems between the skin and core.
To facilitate assembly around curved surfaces, cores are often gridscored.
As the core passes around curves, the grid score opens. If
not filled, the open score may act as a conduit for moisture passage.
Figure 11-11 Core types for sandwich construction.
epoxy will exotherm easier in a lamination with foam
than with other materials especially in a heavy application
or very thick glue line.
Interlaminate application rates of WEST SYSTEM epoxy
vary a great deal. Tests on samples of wood hull laminate
show an average rate of 12.8 square feet per pound
(2.7kg/m2 ) of epoxy per glue line. <strong>The</strong> application rate
of a medium-density mixture of epoxy and filler,
applied with a notched plastic spreader to bond core
materials to plywood skins, is about 18 square feet per
pound (3.7kg/m2 ) of epoxy. A rate of approximately 28
square feet per pound (5.7kg/m2 ) can be expected for
mixtures applied over coated surfaces. Factors determining
these rates include surface condition of the
substrate, pot life of the epoxy, ambient temperature,
work habits, size of the job, and quality of clamping
pressure.
Bonding Core Materials
Different core materials require slightly different bonding
techniques. End-grain wood and foam absorb epoxy at
unusually high rates, and allowances must be made for
this in order to guarantee good joint strength. <strong>The</strong>
cells of resin-impregnated paper honeycomb bond
to wood in a very atypical pattern of small fillets.
Normal laminating procedures are sufficient, however,
when your core material is low-density wood in flat
grain dimension.
Large quantities of WEST SYSTEM epoxy will penetrate
end-grain wood. When too much epoxy is absorbed
by a wood core, there will not be enough adhesive for
a strong bond with a skin, and the joint will be epoxystarved.
To solve this problem, we recommend a twostep
application procedure. First, use a foam roller to
apply unthickened epoxy to the surfaces to be bonded.
Your coating should be adequate but not overabundant.
Next, apply a mixture of epoxy and filler, in thick syrup
consistency, to the core. Use enough of the mixture to
fill any voids between the core and the surrounding
skins.
This adhesive mixture may be too thick to apply evenly
with a foam roller, so use a notched spreader like the
WEST SYSTEM 809 Notched Spreader shown in Figure
11-12 to apply even, controlled amounts of the thicker
adhesive, especially over large areas. You may also
modify an 808 Plastic Spreader by using a chisel, knife
or bandsaw to cut notches in the edge. Experiment to
Figure 11-12 <strong>The</strong> 809 Notched Spreader has three different
notch sizes to apply epoxy at different rates.