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Bonding to Metal and Other Substrates

As part of composite processing, it becomes necessary to bond pieces of composite laminates to metal and other various substrates. Processing requires multiple steps to achieve a successful connection or bond between an array of components. In general, adhere to the following main steps for metals/wood to composite surfaces:

Surface preparation for secondary bonding of composites can start in with the laminate cured or during the last layup sequence of the composite.

(Optional) During layup: An option for surface prep as the final layer of composite fabric is placed upon the tool, a thin sacrificial layer of woven fiberglass may be applied on top the existing plys in which the secondary bonding process will occur after cure. This is not mandatory to the process but will mitigate the chances of fiber damage to the underlying plys during surface prep during secondary bonding.

In addition to fiberglass, there are options for “peel ply” to be placed on top of the final ply’s surface to provide a consistent, non-gloss surface finish to the cured composite. This will provide a better surface for bonding and mitigate excess surface resin on the composite during cure. Peel plys will work on most any composite curing method.

Once the composite component is stripped for the mold, and edges have been de-flashed of sharp edges, and trimmed to needed dimensions for bonding, best practices are to clean the composite of any potential foreign debris or oils accumulated during any of the above processes.

Remove all contaminants from the repair area and ensure a clean working surface for secondary bonding. Surface preparation process is the most important piece of the process. It is the used to increase the surface area of the 2 bonding components and maximize its potential for a lasting bond. Simply cleaning the area will not allow for the adhesive to “grab” unto the parts’ surfaces during the cure process.

Items Needed: 220-400 grit sandpaper, isopropyl alcohol (cleaning), lint free cloth, latex/nitrile gloves
(Using a 60 or 80 grit sandpaper will likely be too abrasive and cause fiber damage to underlying plys on composite component) On metal components it is not as important that the higher grit is used.

Note: If a shop vac is handy it is advised to use during sanding process to keep the area clean of debris as resin is sanded away.

Place the 2 components together in which they will be placed together in the future, mark area and place masking tape or Flashbreaker tape (~.100″) beyond surfaces. Sand planned bonded areas of both surfaces lightly. If bonding with a carbon fiber composite, sand until the surface starts to blacken, as the CF plys will be exposed. DO NOT go beyond this. For Aramid and Fiberglass, it is difficult to tell when sanding into the fibers. With Kevlar, if a frayed like surface is evident, Kevlar fabric layers have been sanded into. As a best practice take any gloss or “flash” from the cured composite surface and sand until a dulled surface is achieved when dry.

Perform a “water-break test” of the prepped area by using a spray bottle of water. Spray the water on a non-prepped area of the part. Notice that the water will bead up, this is not an area that would see a bond but will provide what a failing water break test would look like. Spray water onto the prepped area, notice a single, well defined line of the prepped surface or no line? The part is likely ready to apply adhesive for secondary bonding. Not understanding how this process works or what it does.

Wipe surface using a lint free cloth drenched in alcohol until clean. Best practice is to fold the cloth into four sections and wipe once across the surface on each clean section until no dust or loose resin is picked up.

Apply masking tape to any areas that will not want to have adhesive on it. This is only for general cleanup as some adhesives will run and may cause run out to be taken care of by sanding means after bond.

Secondary Bonding

Note: If bonding to wood or using wood as a bonding substrate, DO NOT use treated lumber or plywood. It will not bond sufficiently to the composite. Non-Treated Wood can be used in secondary bonding successfully. Use of “Coosa Board”, a high-density polyurethane foam with reinforced layers of fiberglass, would be an alternative option to wood.

An unlimited array of resins and adhesives are used for many secondary bonding and substrate surfaces. For many simple at home fixes, epoxy-based systems will bond to nearly any surface. However, structural or toughened adhesives are recommended for load bearing structures. Structural adhesives are proven to eliminate the need for fasteners on an array of applications. For gap filling or curved / complex surfaces, a thixotropic adhesive will stick to surfaces better than standard “runny” adhesives. It is up to part design for researching specifics from each manufacturer. Each manufacturer may have specific adhesives that work best when used for certain substrates, temperature needs, and strength requirements. Commonly used aerospace and general adhesives can be found in the tools needed section.

Often, different adhesives will achieve needed results based on budget, final use, temperature and structural needs. Note: pot life / working life is important to note in these processes. Allow for needed application time. It is advised to wait to mix until surfaces are ready for application.

Wearing clean nitrile/chemical gloves, measure out needed amount of adhesive of both Part A & B in different measuring cups using a scale. Do not yet mix until ready to bond. (Mechanical / additional pressure for bonding means, if used, should also be in a state that is ready to use) Application of specific adhesive may vary but are mostly similar in mixing a needed amount of part A to B (per volume) Use tongue depressors or paint stick to mix adhesive properly. Apply adhesive to both bonding surfaces using a light brush.

After adhesive is placed onto both surfaces, place parts together. Dependent on how the part is bonded, use of any additional applied pressure is advised, whether mechanical or by gravity. Some options include vacuum bagging, mechanical pressure (such as clamps) or simply placing a sandbag (weighted bag) on top the mating surfaces separated by a release film or equivalent. This will help the bond achieve the goal of keeping substrates together for its designed purpose. Without added pressure, the bond may not seal properly or to needed quality specifications.

Cure adhesive to manufacturers specification. Note: some oven cured adhesives may exceed the service temperature for some room temp cured epoxies or plastic resins. Detail in adhesive selection is critical to the process of secondary bonding.

Once cured, remove any masking and clean excess areas with alcohol and / or perform light wet sanding as needed. Use care to not cause any fiber damage.

With ABS plastic bonding, the process is a little less straight forward. ABS plastic is almost like bonding to a release agent. It is not best practice to do this however, it is chemically possible to bond to it and achieve satisfactory results. Many of the epoxy-based systems will work for bonding various substrates to plastic.

Degrease the plastic surface to remove loose contaminants from the surface. Most commonly this can be achieved by using a lint free cloth soaked in isopropyl alcohol. Best practice is to fold the cloth into four sections and wipe once across the surface on each clean section until no dust or loose resin is picked up.

Lightly abrade the surface with 220 grit sandpaper. The goal is to achieve a slightly roughened surface without causing any damage to the plastic. After abrading the plastic, remove all contaminants using a lint free cloth soaked in acetone or isopropyl alcohol.

ABS Flame Treatment: Using a flame from a torch or other gas flame, expose the plastic to the flame for a few seconds to oxidize the surface and to react with the surface of the ABS resulting in a higher surface energy for bonding. Care should be used to not allow the surface to be charred or to deform or warp the plastic.
Allow the surface to cool and apply chosen epoxy-based adhesive as shown in secondary bonding procedure above.

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