Par 3-3 Page 3-10 FIGURE 3-9. SAMPLE BAGGING LAYUP CROSS-SECTION 3-
4. SAMPLE BAGGING AND CURING PROCESS. Figure 3-9 shows a typical bagging arrangement for a localized repair in which patch plies of prepreg are cured with a layer of adhesive, and a heating blanket is used to supply heat.
a. The materials used for most bonded repairs require elevated temperatures and pressure, during their cure, to develop full strength. The following paragraphs describe the operations required to enclose the repair in a vacuum bag. When the part to be cured can be placed in an autoclave, additional pressure and heat can be applied. For cured-in-place parts, vacuum pressure and portable heat blankets usually suffice.
(1) When selecting materials, especially the parting agent film, the temperature at which the repair is to be cured must be known. Polyvinyl alcohol (PVA) film is ideal when the bonding temperature does not exceed 250 °F. PVA film has very high tear resistance and may be heat-sealed effectively. When the bond temperature is not above 180 °F, polyvinyl chloride film can be used. For temperatures up to 450 °F, a polyvinyl fluoride film is used. These three types of films are available in a variety of weights and widths. Most nylon bagging films are used for temperatures up to 400 °F.
(2) When all repair details are in place and ready to be cured, they are enclosed in a bag of plastic film or thin rubber. Air is removed from the bag by a vacuum source so that atmospheric pressure exerts a pressure on the repair as it is cured.
(3) To provide a path to draw off the air initially inside the bag, layers of fiberglass cloth or similar noncontaminating materials, known as breather plies, are placed inside the bag.
When prepreg is being cured as part of the repair, it is sometimes necessary to bleed off excess resin. To do this, layers of fiberglass cloth or similar materials known as bleeder plies are placed
Par 3-4 Page 3-11 over the prepreg. Some repairs have been made with a net resin prepreg that does not require bleeding, and therefore does not require bleeder plies. Porous separator plies or film are used between the prepreg and the bleeder and nonporous separator plies or film are used between the bleeder and breather plies to control the flow of resin.
(4) Small parts may be envelope bagged (i.e., enclosing the entire part in the bag).
Larger parts with localized repairs can be bagged by sealing the surface completely around the repair areas with sealing tape and applying the bagging material to the sealing tape.
CAUTION: The whole panel must be vacuum bagged to prevent delamination in sandwich skins when using an oven or autoclave. Contoured parts must be restrained with tooling to prevent warpage.
b. When the heat for curing the repair is provided by a heat blanket, the blanket can be either inside or outside the vacuum bag. However, the blanket should be covered to minimize heat loss, and the blanket should be separated from direct contact with most bagging materials by layers of fiberglass cloth.
(1) This will prevent localized overheating that could damage the bag. It is sometimes helpful to place a thin aluminum sheet under the heating blanket to minimize localized heating.
A thin rubber blanket can help smooth the surface of the material being cured. A pressure plate should be used when two or more heat blankets are applied to the same repair.
NOTE: Understanding that various resins behave differently during cure, the choice of bagging arrangements will often vary with the material being cured.
(2) The procedure for the bagging arrangement is as follows:
(a) Place a peel ply over the patch material to provide a surface finish for subsequent bonding or painting if not previously accomplished. Place a layer of porous separator cloth over the patch, extending beyond the prepreg and the adhesive. Smooth to avoid wrinkles.
(b) With the patch material in place, place the end of the thermocouple wire next to the edge of the prepreg. Tape the wire to the structure inside the bag with heat-resistant tape. The tape should not be in contact with the prepreg or the adhesive.
(c) Place bleeder plies as shown, extending 2 to 3 inches beyond the patch. The number of bleeder plies needed will vary with the type of resin and the resin content required.
(d) Place a layer of nonporous parting film over the bleeder plies, cut 1 inch smaller than the bleeder plies. This layer is intended to stop resin flow from bleeder plies into breather plies while still providing an airflow path when vacuum is applied.
(e) If a pressure plate is used, place it over the previous separator ply. The plate is frequently perforated with small holes to permit airflow to the breather plies. Bleeder plies may be necessary when using a pressure plate.
Par 3-4 Page 3-12 NOTE: Pressure on the repair will be reduced if the pressure plate does not conform to the repair.
(f) Place the heat blanket over the assembly, making sure it extends 3 to 4 inches beyond the material to be cured.
(g) One or more thermocouples should be in contact with the heat blanket to monitor its temperature. Additional thermocouples should be placed near the curing repair to monitor the temperature of the curing resin.
(h) When using a heat blanket as the heat source, four to six layers of fiberglass surface breather or the equivalent should be used over the heat blanket. This will insulate and prevent damage to the nylon bagging film. Ensure that the breather plies are in contact with the bleeder plies so that an air passage exists.
(i) Place a bead of sealing tape against the parent material around the edge of the breather plies. Seal the thermocouple wires to prevent vacuum leakage.
NOTE: Two layers of sealing tape may be required in order to provide a good seal.
(j) Cover with a suitable vacuum bag, smoothed to minimize wrinkles. Press the bag firmly onto the sealing tape to obtain an air-tight seal. Place pleats in vacuum bag to allow the bag material to stretch.
(k) Install two vacuum probes or sniffers through openings cut in the bag. One will be used for the vacuum gauge and the other will be connected to the vacuum source. The vacuum probe must sit on the breather plies, but must not touch the patch or adhesive.
NOTE: Place the vacuum gauge on the opposite side of the vacuum port, where applicable. Do not place vacuum probes near repair area.
(l) Connect the vacuum source and smooth the bag by hand pressure as the air is removed. Check for leaks and reseal as necessary. A minimum vacuum of 22 inches of mercury is required.
(m) Place insulating material over the vacuum bag to prevent heat loss.
(n) Apply power to the heat blanket and control its temperature as specified for the material being cured.
(o) Observe cure time requirements established by the product manufacturer.
3.5–3-9. [RESERVED.]
Par 3-18 Page 3-13 SECTION 2. METALLIC SANDWICH SECONDARY STRUCTURE REPAIRS
3-10. REPAIRS TO METALLIC SANDWICH SECONDARY STRUCTURE. Magnesium, titanium, or stainless steel facings require special procedures that are not included in the following methods of repair. Aluminum alloys such as 7075-T6, 2024-T3, and 2014-T6 are commonly used for the repair of facings for sandwich structural parts having aluminum facings.
For maximum corrosion resistance, use only clad aluminum for repairs to clad aluminum alloy facings.
a. Dents, scratches, or fractures, not exceeding 1/4 inch in largest dimension in aluminum facings, may be repaired with a suitable filler such as viscous epoxy resin. Dents that are delaminated shall not be filled but repaired. Thoroughly clean the repair area with fine sandpaper and acetone before applying the filler. After the resin has partially cured, remove any excess resin with a sharp plastic scraper. When the resin has completely cured, sand to the original contour. If the damage included a fracture, reclean the area around the filled hole and apply a surface patch.
b. Fractures or punctures in one facing and partial damage to the core of an aluminum-faced laminate may be repaired by several different methods. The technique used will depend upon the size of the damage, the strength required, and the aerodynamic loads of the area involved. If the repair requires aerodynamic smoothness, the facing surrounding the repair core cavity may have to be step cut to one-half its thickness. This can be done by using a router with an end mill bit and a template.
c. Damage that extends completely through the core and both facings may be repaired using the same general techniques as those used for repairing fiberglass laminates when both facings are accessible.
d. After locating the extent of the total damaged area by tapping or other nondestructive test methods, remove the damaged facing and that portion of the core material that is also affected. The depth to which the core must be removed will depend upon the type of core material and the method of repair. The replacement core material must be the same material and core cell size as the original. Fabricate core material to shape, keeping the same core ribbon or grain direction. When a substitution is permissible, wood or glass-fabric honey-comb cores are sometimes used in the repair of aluminum honeycomb cores, as they are generally easier to shape. Typical types of core replacements are shown in Figure 3-10. Resin fills can be used to replace the core and facing where smaller core damage exists. Phenolic microballoons, low-density insulating materials, and/or other ingredients are added to lower the density and give greater flexibility.
FIGURE 3-10. TYPICAL TYPES OF CORE REPLACEMENT A. REPLACEMENT WITH CORE PLUG OF SAME
OR OTHER TYPES OF CORE MATERIAL
ORIGINAL HONEYCOMB CORE PLUG ROUTED AREA
CORE EPOXY-RESIN /ADHESIVE B. RESIN FILL
EPOXY-RESIN FILL ROUTED AREA
ORIGINAL PARTIAL OR COMPLETE
HONEYCOMB DEPTH OF CORE CORE C. RESIN FILL, UNDERCUT CORE
ROUTED AREA
EPOXY-RESIN FILL PARTIAL OR COMPLETE /DEPTH OF CORE
ORIGINAL
HONEYCOMB CORE UNDERCUT CORE D. GLASS-CLOTH-RESIN LAMINATE, UNDERCUT CORE
GLASS-CLOTH
RESIN LAMINATE HONEYÇOMB ORIGINAL CORE ROUTED AREA UNDERCUT CORE E. REPLACEMENT WITH CORE PLUG OF SAME
TYPE OF SANDWICH CONSTRUCTION
ORIGINAL COMPLETE SANDWICH ROUTED AREA
HONEYCOMB CORE PLUG EPOXY-RESIN
CORE /ADHESIVE Par 3-18 Page 3-14
Par 3-18 Page 3-15
e. For the repair of larger holes in which it is inconvenient to use a face patch because of aerodynamic smoothness requirements in that area, both the core and facing are sometimes replaced with glass-fiber fabric discs and resin. Undercut the core, as shown in Figure 3-11, in order to obtain a better bonding of the fill with the facing. Fill the core cavity with accurately shaped resin-saturated glass cloth discs, and press each ply down to remove any air bubbles.
Special care should be taken that the final plies fit well against the underside of the top facing.
When the core cavity is filled, close the cutout in the facing with resin impregnated glass fiber fabric discs that have been precut to size.
FIGURE 3-11. TYPICAL UNDERCUT CORE MATERIAL CAVITY
f. O verlap repairs, typically called scab patches, have a long history of use in repairing aircraft structures. These repairs simply cover the damaged area with patch material. Overlap repairs can be bonded and/or mechanically fastened. Figure 3-12 shows a cross-section of a typical bonded and mechanically fastened repair. The damaged area may or may not need to be filled.
FIGURE 3-12. CROSS-SECTION OF BONDED AND BOLTED OVERLAP REPAIRS (
1) Bonded overlap repairs work well on most structures. The overlap repair consists of a solid patch material such as metal, precured laminates, prepreg or wet layup material co-cured in place.
(2) Bolt or blind rivet. Mechanically fastened, bolted, or blind rivet repairs are primarily used for thick structures. Primary concerns include bolt/rivet spacing, fastener diameter, number of fasteners, and sealant type.
g. Core potting is the process of filling the core cutout with a curable paste filler material.
If the damage is sustained in an area with an already-potted core, the replacement core should
Par 3-18 Page 3-16 also be potted. In other cases, if the honeycomb or foam core is damaged, it may be potted rather than plugged if the damaged area is small (1 inch or less). Remove the face sheet with a power router, using a router template to prevent injury to undamaged face skin. The router may be adjusted to remove one of the face skins only, a face skin and part of the core, a face skin and all of the core, or both the face skins and the core. (See Figure 3-13.)
FIGURE 3-13. HONEYCOMB CORE REMOVAL
(1) I t may be necessary when routing a tapered section such as an aileron to use a wedge-shaped block between the routing template and the upper surface. This will allow the router to cut the core material parallel with the lower surface. (See Figure 3-14.)
FIGURE 3-14. REMOVING HONEYCOMB CORE FROM A TAPERED CONTROL
SURFACE
Par 3-18 Page 3-17
(2) Select the appropriate potting adhesive as recommended by the manufacturer. Mix a sufficient quantity of filler to fill the hole and add microballoons if they are needed to serve as a filler. When the resin and filler are thoroughly mixed according to the manufacturer’s recommendations, pour the mixture into the hole filling all of the cells, then work out all of the bubbles with a toothpick. If performing an overlay repair, fill the core cavity to slightly above the part’s surface. If performing a flush repair, fill the core cavity to slightly above the original core.
(3) Cure the compound according to the manufacturer’s directions. Trim the top of the cured potting compound flush with the surface, for the type of repair you are performing.
h. A core plug repair replaces damaged core material with a shaped piece of similar core material.
(1) Complete removal of core material to the opposite face generally requires some hand-cutting with a core knife. Figure 3-15 shows core material being removed with a core knife.
The core can be peeled away from the skin bond using duckbill pliers. Sanding is then required to remove irregular accumulations of adhesive from the undamaged inner face. Remove only enough adhesive to produce a smooth finish.
FIGURE 3-15. REMOVAL OF CORE WITH CORE KNIFE C AUTION: Care should be used when peeling core material from thin skin sandwich face sheets, because the skin can be damaged by pulling on the core.
(2) A core plug should be fabricated as follows. Select the core density. Cell size and ribbon direction and thickness should be at least the same as that used in the original construction. Trim the sides of the plug to a loose fit in the routed cavity. Trim the plug height so the top of the plug sits .001 inch higher than the level of the original surface. The core will compress and set during cure thereby requiring the extra height. Carefully remove the trimmed core plug from the machined cavity. Use a vacuum cleaning device to remove any dust or
Par 3-18 Page 3-18 particles remaining on the core plug or in the repair area. Clean the core plug by rinsing with an approved solvent and wrap the plug in a clean polyethylene bag until needed for assembly.
CAUTION: When handling film adhesives, prepreg fabrics, or parts with prepared surfaces, latex gloves must be worn.
(3) Using a film adhesive, the core plug should be installed as follows. Select the appropriate adhesive film. Cut one disk of adhesive to the same shape and size as the perimeter of the repair cutout if the repair extends through the entire core thickness. Cut two disks if a partial depth core repair is being made. Cut one strip of core splice adhesive to wrap around the core perimeter to its full depth. For a partial depth core repair, also cut out a fiberglass or aluminum disk, again matching the size and the shape of the repair cutout. Figure 3-16 shows the details of a partial depth core repair. Preassemble the pieces.
FIGURE 3-16. DETAILS OF CORE REPAIR
Par 3-18 Page 3-19
(4) Wipe the bottom and sides of the cutout area with solvent. Allow the area to dry.
Insert the core plug assembly with splice adhesive applied to the perimeter into the core cutout.
Ensure the core plug ribbon direction matches that of the parent core. In the case of a partial depth core repair, the plug and disk may be cured to save time. Some restraining method, such as vacuum bagging, may be desirable over the core splice adhesive as this material usually foams up and out during cure. Cure the adhesive according to the manufacturer’s directions and allow the area to cool.
(5) Trim the top of the cured core plug flush with either the original core or the mold line, depending on the type of laminate repair to be performed. Proceed with laminate repair.
3-11. FINISHING. The type of finish coating applied to a metallic sandwich repair will normally be determined by the exposed material and the application of the part or assembly. Rain erosion of plastic parts, the need for electrical or dielectric properties, and/or the necessity for anti-corrosion coatings must be considered when the choice of finish is made. Plastic-faced parts such as radomes are finished primarily for rain erosion while aluminum or other metal-faced laminates are finished for corrosion protection. For coatings to perform their function properly, it is essential that they be applied to surfaces that are clean, free of voids, and smooth. The edges of all parts not protected by a bonding of aluminum or glass-fabric laminate must be sealed to reduce the rate of moisture absorption.
3-12. –3-17. [RESERVED.]
Par 3-18 Page 3-20 SECTION 3. TRANSPARENT PLASTICS
3-18. GENERAL. Plastics cover a broad field of organic synthetic resin and may be divided into two main classifications – thermoplastics and thermosetting plastics.
a. Thermoplastics. Thermoplastics may be softened by heat and can be dissolved in various organic solvents. Two kinds of transparent thermoplastic materials are commonly employed in windows, canopies, etc. These materials are known as acrylic plastics and cellulose acetate plastics.
(1) Cellulose acetate was used in the past but since it is dimensionally unstable and turns yellow after it has been installed for a time, it has just about passed from the scene and is not considered an acceptable substitute for acrylic.
(2) Acrylic plastics are known by the trade names of Lucite or Plexiglas and by the British as Perspex and meet the military specifications of MIL-P-5425 for regular acrylic, and MIL-PRF-8184 for craze-resistant acrylic.
b. Thermosetting Plastics. These plastics do not soften appreciably under heat but may char and blister at temperatures of 240 to 260 ºC (400 to 500 ºF). Most of the molded products of synthetic resin composition, such as phenolic, urea-formaldehyde, and melamine formaldehyde resins, belong to the thermosetting group. Once the plastic becomes hard, additional heat will not change it back into a liquid as it would with a thermoplastic.
3-19. STORAGE AND HANDLING. Because transparent thermoplastic sheets soften and deform when they are heated, they must be stored where the temperature will never become excessive. Store them in a cool, dry location away from heating coils, radiators, or steam pipes, and away from such fumes as are found in paint spray booths or paint storage areas.
a. Paper-masked transparent sheets must be kept out of the direct rays of the sun, because sunlight will accelerate deterioration of the adhesive, causing it to bond to the plastic and making it difficult to remove.
b. Plastic sheets should be stored with the masking paper in place, in bins that are tilted at a ten-degree angle from the vertical. This will prevent their buckling. If the sheets are stored horizontally, take care to avoid getting dirt and chips between them. Stacks of sheets must never be over 18 inches high, with the smallest sheets stacked on top of the larger ones so there will be no unsupported overhang. Leave the masking paper on the sheets as long as possible, and take care not to scratch or gouge the sheets by sliding them against each other or across rough or dirty tables.
c. Formed sections should be stored with ample support so they will not lose their shape.
Vertical nesting should be avoided. Protect formed parts from temperatures higher than 120 ºF (49 ºC), and leave their protective coating in place until they are installed on the aircraft.
3-20. FORMING PROCEDURES AND TECHNIQUES. Transparent acrylic plastics get soft and pliable when they are heated to their forming temperatures and can be formed to almost any shape. When they cool, they retain the shape to which they were formed. Acrylic plastic may be
Par 3-20 Page 3-21 cold-bent into a single curvature if the material is thin and the bending radius is at least 180 times the thickness of the sheet. Cold bending beyond these limits will impose so much stress on the surface of the plastic that tiny fissures or cracks, called crazing, will form.
3-21. HEATING. Before heating any transparent plastic material, remove all of the masking paper and adhesive from the sheet. If the sheet is dusty or dirty, wash it with clean water and rinse it well. Dry the sheet thoroughly by blotting it with soft absorbent paper towels.
NOTE: Wear cotton gloves when handling the plastic to eliminate finger marks on the soft surface.
a. For the best results when hot-forming acrylics, use the temperatures recommended by the manufacturer .A forced-air oven should be used—one that is capable of operating over a temperature range of 120 to 374 ºF (49 to 190 ºC). If the part gets too hot during the forming process, bubbles may form on the surface and impair the optical qualities of the sheet.
b. For uniform heating, it is best to hang the sheets vertically by grasping them by their edges with spring clips and suspending the clips in a rack. (See Figure 3-17.) If the piece is too small to hold with clips, or if there is not enough trim area, lay the sheets on shelves or racks covered with soft felt or flannel. Be sure there is enough open space to allow the air to circulate around the sheet and heat it evenly.