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Bonding

This section refers to two types of structural 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.

Two-step bonding

1. Wet-out bonding surfaces- Apply an unthickened resin/hardener mixture to the surfaces to be joined. Wet out small or tight areas with a disposable brush. Wet out larger areas with a foam roller or by spreading the resin/hardener mixture evenly over the surface with a plastic spreader. You may proceed with step two immediately or any time before the wet-out coat becomes tack free.

2. Apply thickened epoxy to one bonding surface. Modify the resin/hardener mixture by stirring in the appropriate filler until it becomes thick enough to bridge  any gaps between the mating surfaces and to prevent "resin-starved" joints. Apply enough of the mixture to one of the surfaces, so that a small amount will squeeze out when the surfaces are joined together with a force equivalent to a firm hand grip.

Thickened  epoxy can be applied immediately over the wet-out surface or any time before the wet-out is no longer tacky. For most small bonding operations, add the filler to the resin/hardener mixture remaining in the batch that was used for the wet-out. Mix enough resin/hardener for both steps. Add the filler quickly after the surface is wet-out and allow for a shorter working life of the mixture.

3. Clamp components. Attach clamps as necessary to hold the components in place. Use just enough clamping pressure to squeeze a small amount of the epoxy mixture from the joint, indicating that the epoxy is making good contact with both mating surfaces. Avoid using too much clamping pressure, which can squeeze all of the epoxy mixture out of the joint.

4. Remove or shape excess adhesive that squeezes out of the joint as soon as the joint is secured with clamps.

Single-step 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,especially when bonding end grain or other porous surfaces.

Laminating

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 a 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 foam or honeycomb core material.

Vacuum bagging is a specialised 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.

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 fibreglass 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 bonding except that instead of removing the squeezed-out thickened epoxy after the components are clamped into 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 becomes tack free 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. Use the excess material to refill any voids. Smooth the fillet until you are satisfied with its appearance. A mixing stick will leave a fillet with about a 9.3mm radius. For larger fillets, an 808 Plastic Spreader, 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 mixing stick or a putty knife. Fibreglass 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.

 

BONDING FASTENERS & HARDWARE

Installing screws and other threaded fasteners with WEST SYSTEM epoxy dramatically improves load carrying capacity by spreading the fastener's load into a greater area of the substrate. There are several methods or levels of hardware bonding depending on the loads on the hardware.

Basic fastener bonding

For improved pullout strength and waterproof connections, the easiest  fastener bonding method is to simply wet out stripped fastener holes and new pilot holes before installing the screws. Epoxy penetrates the fibre around the hole, effectively increasing the fastener diameter. Epoxy also provides a stronger interface with the fastener threads than wood fibre and keeps out water.

1. Wet out a standard-size pilot hole. Work the mixture well into the hole with a pipe cleaner or syringe. Thicken a second coat of epoxy as necessary for stripped or oversized holes.

2. Insert the fastener in the hole and allow the epoxy to cure.

 Advanced fastener bonding

For greater strength and stability, drill oversized holes to increase the exposed substrate area and the amount of epoxy around the fastener.

1. Drill oversized holes 2/3-3/4 the depth of the fastener. The hole diameter is 6.3mm larger  than the fastener diameter.

2. Drill a normal sized pilot hole at the bottom of the oversized hole to the full length of the fastener. The normal sized pilot hole serves to hold or clamp the hardware in position until the epoxy cures. If the fastener/hardware can be clamped by other means, the oversized hole can be extended to the end of the fastener.

3. Wet out the holes and the fastener with epoxy. Allow the epoxy to thoroughly soak into the exposed end grain of the wood.                                                                                           

4. Fill the hole with thickened epoxy/adhesive filler. Use high-density 403 Microfibre Blend.

5. Install the fasteners with just enough force to hold the hardware in place. Allow the epoxy to cure thoroughly before applying load to the hardware.

Bonding hardware

Bonding hardware goes a step beyond bonding the fasteners only. By bonding the hardware base directly to the surface you further increase hardware load capacity and provide a solid bearing surface for the hardware. It also seals the wood underneath, and is a stronger, longer lasting attachment than bonding the fasteners only. It is especially useful to mount hardware on curved, uneven or unlevel surfaces.

1. Prepare the mounting surface and the hardware base for good adhesion.

2. Wet out the oversized hole with epoxy. Allow the epoxy to soak into the exposed end grain of the wood(as with faster bonding).

3. Coat the bottom contact surface of the hardware with unthickened epoxy. Wire brush or sand the wet epoxy into the surface with 50-grit sandpaper.

4. Inject a non-sagging epoxy/403 Microfibre Blend mixture into the hole. Use enough mixture so there are no voids in the hole after inserting the fastener. Coat the bottom of the hardware and the fastener threads with thickened epoxy.

5. Place the hardware in position. Insert and tighten fasteners until a small amount of the mixture squeezes out of the joint.

6. Remove excess epoxy or shape into a fillet. Allow the epoxy to cure at least 24 hours before applying load to the hardware. Allow more time in cool weather.

Casting a base

Use the thickened epoxy to cast a base under the hardware when mounting hardware to a curved or uneven surface, or mounting hardware at an angle to the surface.

1.  Prepare the fasteners, holes, substrate and base as described above.

2. Bond small blocks to the substrate to support the base at the desired height and position.

3.  Apply enough thickened epoxy to cover the blocks.  If the gap between the base and the surface is over 12.7mm, fill the gap in two separate layers to avoid exotherm.

4. Place the hardware in position, resting on the blocks and install the fasteners.

5. Smooth the excess epoxy into the desired fillet shape around the base Allow the epoxy to cure fully before loading.  Protect exposed epoxy from UV.

Bonding Studs

Bond threaded rods or studs into the substrate (instead of bolts or screws) and attach the hardware with nuts.  This variation is appropriate for many engine, motor or machine installations.  Coat the base with wax/mold release to make the hardware removable.  Although the hardware is not "bonded" to the substrate, the epoxy still provides a bearing surface that perfectly matches and supports the base of the hardware.

1. Prepare the studs/threaded rods by waxing the upper ends (above the surface) and cleaning the lower ends (below the surface).

2. Place a nut and washer on the studs, wet out the lower ends and push them into the epoxy filled holes.  Allow the epoxy to cure thoroughly before attaching hardware and tightening the nuts.

Removing Fasteners

If you know that you will want to remove the fastener, you can coat the threads with wax or mold release (contaminating the surface enough to prevent a good bond).

Remove a permanently bonded fastener by applying heat to the head of the fastener with a soldering iron or propane torch.  Use a heat shield to protect the surrounding area.  Heat will travel down the fastener, softening the epoxy in contact with it.  At about 121^oC the epoxy should soften enough to allow the fastener to be backed out.  Allow more time for heat to travel down longer or larger diameter fasteners.