The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 11 – Laminating and Bonding Techniques 99
Figure 11-13 Applying thickened epoxy with a modified
plastic spreader. Cut notches in the edge, sized and spaced
to suit the desired rate of application.
find the notch depth and frequency that will deposit the
correct amount of epoxy/filler mix on your wood
as in Figure 11-13.
Foam cores absorb slightly less epoxy than end-grain
wood, and their absorption rates are very predictable.
When the large pores on the foam’s surface are filled,
the rest of the epoxy/filler blend will fill gaps between
the core and laminate skin. Since the object is to make
a bond which is at least as strong as the foam but is
lightweight, use low-density, gap-filling filler; even if the
mixture is heavily loaded with filler, the bond will be
stronger than the foam. To be sure that all voids are
filled on more porous foams, use additional low-density
additive to make a thicker adhesive mixture. <strong>The</strong>
notched squeegee will help force thick adhesive into
foam pores.
Paper honeycomb presents an unusual bonding situation.
Because it is made up of cells ranging from 1 ⁄4" to 1 ⁄2" (6-
12mm) in diameter, this type of core is actually joined
to only a small percentage of the total area of the
surface skins. Many little fillets form when honeycomb
is bonded in lamination. <strong>The</strong> adhesive “gets legs” as it
is applied to the core and, due to its surface tension,
travels up the cell walls a distance of up to 1 ⁄8" (2mm),
although 1 ⁄16" (1mm) is normal. This process results in a
bond that is far stronger than the paper honeycomb.
Take steps to be sure that these little fillets develop. First,
use a foam roller to wet out the edges of the honeycomb
with unthickened epoxy. Don’t try to deposit quantities
of epoxy on the edges as you do this—the goal is to
saturate cell wall edges in the same way that you apply
a first penetrating coat of epoxy to wood. <strong>The</strong>n, still
using the foam roller, apply a thick coat of unthickened
epoxy to the surfaces of the structural skins. If your
skins are precoated, slightly less epoxy is required. In
this case, follow the directions in Chapter 15 to prepare
the coated surfaces for bonding. To avoid the formation
of high spots, assemble your lamination before the
epoxy on the honeycomb has a chance to set up.
If the fit between a honeycomb core and plywood skins
is not close or you suspect that your clamping pressure
may be inadequate, you may wish to modify this technique.
Coat your core with unthickened epoxy and then
coat your wood with a mixture of epoxy and low-density
filler. <strong>The</strong> viscosity of this mixture depends on the fit
between core and skins. Use more filler if you have a
rough fit, less if your surfaces present fewer gaps. If
you know that there is perfect contact between your core
and wood, you may need no additive, provided that you
apply the epoxy in sufficient quantity.
When you use a honeycomb core, be careful to avoid
dry bonds within the laminate. This is usually a result
of the wood’s absorbing too much epoxy. If the skins
absorb most of the epoxy you apply, there will not be
enough epoxy on the surface to develop the fillet bond
needed for maximum strength. One way to prevent this
is to coat the wood, wait an hour or so until the epoxy
is tacky, and then apply a fresh cot and bond the
honeycomb to the skins.
Whatever the core material and adhesive mixture used,
structural strength will be only as good as the quality
of the bond that is developed between the components.
We suggest that you make a sample laminate of the
combination that you intend to use and then destruction
test it so that you can assess the quality of the
bond. This is a simple and quick way to determine both
the ideal mixture and the amount of adhesive needed
for a given surface to ensure adequate bonding. Once
you have tested several laminates with a given type of
core material, you will have gained enough experience
to be able to determine the adequate amount of adhesive
for a given surface without having to test each laminate.
To perform a destruction test, make two samples, each
about 3" wide � 10" long (75mm � 250mm). Place
two 2" � 2" � 4" (50mm � 50mm � 100mm) blocks