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
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(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.
FIGURE 3-17. HANGING OF ACRYLIC SHEETS
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c. Small forming jobs, such as landing light covers, may be heated in a kitchen baking oven. Infrared heat lamps may be used if they are arranged on 7-or 8-inch centers and enough of them are used in a bank to heat the sheet evenly. Place the lamps about 18 inches from the material.
d. Never use hot water or steam directly on the plastic to heat it, because this will likely cause the acrylic to become milky or cloudy.
3-22. FORMS. Heated acrylic plastic will mold with almost no pressure, so the forms used can be of very simple construction. Forms made of pressed wood, plywood, or plaster are adequate to form simple curves, but reinforced plastic or plaster may be needed to shape complex or compound curves.
a. Since hot plastic conforms to any waviness or unevenness, the form used must be completely smooth. To ensure this, sand the form and cover it with soft cloth such as outing flannel or billiard felt.
b. The mold should be large enough to extend beyond the trim line of the part, and provisions should be made for holding the hot plastic snug against the mold as it cools.
c. A mold can be made for a complex part by using the damaged part itself. If the part is broken, tape the pieces together, wax or grease the inside so the plaster will not stick to it, and support the entire part in sand. Fill the part with plaster and allow it to harden, and then remove it from the mold. Smooth out any roughness and cover it with soft cloth. It is now ready to use to form the new part.
3-23. FORMING METHODS. (See Table 3-1.)
a. Simple Curve Forming. Heat the plastic material to the recommended temperature, remove it from the heat source, and carefully drape it over the prepared form. Carefully press the hot plastic to the form and either hold or clamp the sheet in place until it cools. This process may take from ten minutes to one-half hour. Do not force-cool it.
b. Compound-Curve Forming. This type of forming is normally used for such parts as canopies or complex wingtip light covers, and it requires a great deal of specialized equipment.
There are four commonly used methods, each having its advantages and disadvantages.
c. Stretch Forming. Preheated acrylic sheets are stretched mechanically over the form in much the same way as is done with the simple curved piece. Special care must be taken to preserve uniform thickness of the material, since some parts will have to stretch more than others.
d. Male And Female Die Forming. This requires expensive matching male and female dies. The heated plastic sheet is placed between the dies which are then mated. When the plastic cools, the dies are opened.
e. Vacuum-Forming Without Forms. Many aircraft canopies are formed by this method.
In this process a clamp with an opening of the desired shape is placed over a vacuum box and the
Par 3-23 Page 3-23 heated sheet of plastic is clamped in place. When the air in the box is evacuated, the outside air pressure will force the hot plastic through the opening and form the concave canopy. It is the surface tension of the plastic that shapes the canopy.
f. Vacuum-Forming With A Female Form. If the shape needed is other than that which would be formed by surface tension, a female mold, or form must be used. It is placed below the plastic sheet and the vacuum pump is connected. When air from the form is evacuated, the outside air pressure will force the hot plastic sheet into the mold and fill it.
g. Sawing and Drilling.
(1) Several types of saws can be used with transparent plastics, however circular saws are the best for straight cuts. The blades should be hollow-ground or have some set to prevent binding. After the teeth are set, they should be side-dressed to produce a smooth edge on the cut.
Band saws are recommended for cutting flat acrylic sheets when the cuts must be curved or where the sheet is cut to a rough dimension to be trimmed later. Close control of size and shape may be obtained by band sawing a piece to within 1/16 inch of the desired size, as marked by a scribed line on the plastic, and then sanding it to the correct size with a drum or belt sander.
TABLE 3-1. TYPICAL TEMPERATURES FOR FORMING ACRYLIC SHEETS Thickness of sheet (in.)
0.125 0.250 0.125 0.250 Type of forming Regular acrylic plastic.
MIL-P-6886 Heat-resistant acrylic plastic, MIL-P-5425, and craze-resistant acrylic plastic, MIL-PRF-8184 ºC ºF ºC ºF ºC ºF ºC ºF Simple curve 113 235 110 230 135 275 135 275 Stretch forming (dry mold cover) 140 284 135 275 160 320 150 302 Male and female forming 140 284 135 275 180 356 170 338 Vacuum forming without form 140 284 135 275 150 302 145 293 Vacuum forming with female form 145 293 140 284 180 356 170 338
(2) Unlike soft metal, acrylic plastic is a very poor conductor of heat. Make provisions for removing the heat when drilling. Deep holes need cooling, and a water-soluble cutting oil is a satisfactory coolant since it has no tendency to attack the plastic.
(a) The drill used on acrylics must be carefully ground and free from nicks and burrs that would affect the surface finish. Grind the drill with a greater included angle than
Par 3-23 Page 3-24 would be used for soft metal. The rake angle should be zero in order to scrape, not cut.
(See Figure 3-18.)
FIGURE 3-18. DRILL HAVING AN INCLUDED ANGLE OF APPROXIMATELY 150º,
USED TO DRILL ACRYLIC PLASTICS (
b) The patented Unibit (see Figure 3-19) is good for drilling small holes in aircraft windshields and windows. It can cut holes from 1/8- to 1/2-inch in 1/32-inch increments and produces good smooth holes with no stress cracks around their edges.
FIGURE 3-19. UNIBIT DRILL FOR DRILLING ACRYLIC PLASTICS
h. P olymerizable Cements. Polymerizable cements are those in which a catalyst is added to an already thick monomerpolymer syrup to promote rapid hardening. Cement PS-30 and Weld-On 40 are polymerizable cements of this type. They are suitable for cementing all types of Plexiglas acrylic cast sheet and parts molded from Plexiglas molding pellets. At room temperature, the cements harden (polymerize) in the container in about 45 minutes after mixing the components. They will harden more rapidly at higher temperatures. The cement joints are usually hard enough for handling within 4 hours after assembly. The joints may be machined within 4 hours after assembly, but it is better to wait 24 hours.
(1) PS-30 and Weld-On 40 joints retain excellent appearance and color stability after outdoor exposure. These cements produce clear, transparent joints and should be used when the color and appearance of the joints are important.