Obtain the specified core material for the repair and cut a replacement plug to fit the hole. The replacement core should be slightly thicker than the original core to allow approximately 3/8-inch above the top skin. Allow approximately 1/8-inch gap between original core and core plug. Align the core ribbon directions when sizing the core plug fit the hole. (See Figure 3).
Clean the repair area with an approved solvent (e.g. MEK, acetone, or other approved solvent). If possible, dry the repair area in an oven or use a heat blanket at 180 ±10 degrees F for 2 to 24 hours until all moisture is evaporated. This is difficult to determine in most cases, so, as a precaution, two hours is the recommended minimum dry time.
Moisture is a major concern in sandwich structures since the presence of water vapors during a cure can cause porosity and voids in the adhesive matrix.
Performing the cure
Wrap the core plug or cavity wall with foaming core splice adhesive and insert the plug into the hole. Cure the foaming adhesive per the manufacturer's recommendations. Curing involves heating the foaming core splice adhesive to the cure temperature that activates the foaming agent in the adhesive. Foaming action expands the adhesive from one to five times its original thickness to fill the open cells of both original core and core plug, which bonds the plug in place. After the core is bonded in place, it is machined to trim the excess core height to near flush with the surface by using a core cutter router bit and router holder. The router holder will stabilize the tool during cutting operations. Allow a slight oversize so as not to cut into the top ply of material.
Finish sand by hand with 220-grit or finer abrasive paper on a flat wooden block. Vacuum the repair area to remove any sanding debris. Clean with clean, lint-free cloth and approved solvent. Allow to air dry.
Patch must match
Fabricate a replacement patch from either prepreg or wet layup with liquid adhesive. Patch must match the original surface ply orientations. (Note that patch fabrication procedures would require another article on its own). As a minimum, the same number of plies as the parent structure must be used in the patch (See Figure 4 ).
The patch is usually cured in-place using a silicone rubber heat blanket and a Hot Bonder/Controller set to precisely apply power to the heat blanket at a rate that provides accurate cure temperature over the specified time. Vacuum is drawn over the repair through use of a vacuum bag made up of the materials discussed earlier. Maximum vacuum is drawn for maximum compaction pressure.
Precise control of the ramp-up, dwell and ramp-down temperature over a specific period of time is required to obtain maximum strength and stiffness of the patch. Hot bonders offer virtually worry-free control for most repairs, unless there are underlying heat sinks that can create cool spots. The technician must possess skills and knowledge of heat management techniques to be able to counteract these problems. This is another place where the "artisan" must be present as several methods are available to assist in heat sink problems if they are a factor. The repair area may be augmented by a second heat blanket, hot air or heat lamps, provided each is controlled properly and not allowed to overheat the structure. Some hot bonders also provide finite control of these accessories.
Special considerations for curing
The cure is extremely important to the success of the repair. If the resin is heated too quickly, air and volatiles may be trapped inside the resin, causing porosity and thus producing a weak repair. Another factor that may affect the repair is the lay-up sequence and construction of the ancillary materials in the vacuum bag. Bleeder material and release film porosity may affect the repair and should be given careful consideration. Always ensure the bag is built properly for efficient curing.
Once the patch has been cured, the repair is subjected to another NDI test to verify the repair is of acceptable quality. Then the area is prepped for finishing before the part may be returned to service.
Since the technician must have an acute understanding of all the factors involved in repairing ACM parts, it is highly recommended they attend one of the various schools that can provide the skills and knowledge needed to prepare the sheet metal mechanic to become the "composite artisan."