This section refers to two types of bonding. Two-step
bonding is the preferred method for most situations because it promotes maximum epoxy
penetration into the bonding surface and prevents resin-starved joints. Single-step
bonding can be used when joints have minimal loads and excess absorption into porous
surfaces is not a problem. In both cases, epoxy bonds best when it is worked into the
surface with a roller or brush.
Before mixing epoxy, check all parts to be bonded for
proper fit and surface preparation, gather all the clamps and tools necessary for the
operation, and cover any areas that need protection from spills.
Note: The term bonding as used here and
other WEST SYSTEM literature refers to structural adhesion or gluing of components, not
the electrical bonding
Single-step bonding is applying the thickened epoxy directly to both bonding surfaces without first wetting out the surfaces with neat
resin/hardener. We recommend that you thicken the epoxy no more than is necessary to bridge gaps in the joint (the thinner the mixture, the more it can penetrate the surface) and that you do not use this method for highly-loaded joints or for bonding end grain or other porous surfaces.
The term "laminating" refers to the process of bonding numbers of relatively thin layers, like plywood, veneers, fabrics or core material to create a composite. A composite may be any number of layers of the same material or combinations of different materials. Methods of epoxy application and clamping will differ depending on what you are laminating.
Because of large surface areas and limitations of wet lay-up time, roller application is the most common method for applying epoxy. A faster method for large surfaces is to simply pour the resin/hardener mixture onto the middle of
the panel and spread the mixture evenly over the surface with a plastic spreader. Apply thickened mixtures with an 809 Notched Spreader .
Using staples or screws is the most common method of clamping when you laminate a solid material to a solid substrate. An even distribution of weights will work when you are laminating a solid material to a base that will not hold staples or screws, such as a foam or honeycomb core material.
Vacuum bagging is the ideal clamping method for laminating a wide range of materials. Through the use of a vacuum pump and plastic sheeting, the atmosphere is used to apply perfectly even clamping pressure over all areas of a panel regardless of the size, shape or number of layers.
Primary bonding relies on the chemical linking of adhesive layers such as the wet lay-up of fiberglass laminate in a mold. All the layers of adhesive
cure together in a single fused layer. Epoxy applied over partially cured epoxy will
chemically link with it and is a primary bond. The ability to chemically link diminishes
as the epoxy cures and it becomes a secondary bond.
Secondary bonding relies on the mechanical linking of an adhesive to a material or cured
epoxy surface. The adhesive must "key" into pores or scratches in the surface-a
microscopic version of a dovetail joint. Proper surface preparation provides a texture
that will help lock the cured epoxy to the surface.
Any method of clamping is suitable as long as the parts to
be joined are held so that movement will not occur. Common methods include spring clamps,
"C" clamps and adjustable bar clamps, heavy rubber bands cut from inner tubes,
nylon-reinforced packaging tape, applying weights, and vacuum bagging. When placing clamps
near epoxy-covered areas, cover clamp pads with duct tape, or use polyethylene sheeting or
release fabric under the clamps so they don't inadvertently bond to the surface. Staples,
nails or drywall screws are often used where conventional clamps will not work. Any
fasteners left in should be of a non-corroding alloy such as bronze. In some cases the
thickened epoxy or gravity will hold parts in position without clamps.
Bonding with fillets
A fillet (fil'it) is a cove-shaped application of thickened
epoxy that bridges an inside corner joint. It is excellent for bonding parts because it
increases the surface area of the bond and serves as a structural brace. All joints that
will be covered with fiberglass cloth will require a fillet to support the cloth at the
inside corner of the joint.
The procedure for bonding with fillets is the same as
normal bonding except that instead of removing the squeezed-out thickened epoxy after the
components are clamped in position, you shape it into a fillet. For larger fillets, add
thickened mixture to the joint as soon as the bonding operation is complete, before the
bonding mixture is fully cured, or any time after the final cure and sanding of exposed
epoxy in the fillet area.
1. Bond parts as described in Bonding.
|2. Shape and smooth the
squeezed-out thick epoxy into a fillet by drawing a rounded filleting tool (mixing stick)
along the joint, dragging excess material ahead of the tool and leaving a smooth
cove-shaped fillet bordered on each side by a clean margin. Some excess filleting material
will remain outside of the margin (Figure 13). Use the excess material to re-fill any
voids. Smooth the fillet until you are satisfied with its appearance. A mixing stick will
leave a fillet with about a 3/8" radius. For larger fillets, an 808 Plastic Squeegee,
cut to shape or bent to the desired radius, works well.
Apply additional thickened epoxy to fill voids or make larger fillets. Apply the
mixture along the joint line with the rounded mixing stick, using enough mixture to create
the desired size of fillet. For longer or multiple fillets, empty caulking gun cartridges
or disposable cake decorating bags can be used. Cut the plastic tip to lay a bead of
thickened epoxy large enough for the desired fillet size. Heavy duty, sealable food
storage bags with one corner cut off may also be used.
|3. Clean up the remaining excess
material outside of the margin by using a sharpened mixing stick or a putty knife (Figure
14). Fiberglass cloth or tape may be applied over the fillet area before the fillet has
cured (or after the fillet is cured and sanded).
4. Sand smooth with 80-grit sandpaper after the
fillet has fully cured. Wipe the surface clean of any dust and apply several coats of
resin/hardener over the entire fillet area before final finishing.
For information on bonding in cold weather, see Cold Weather Bonding - West System