in the preceding paragraphs, standard handtools and shop equipment are utilized in the repair of bonded honeycomb structures. Standard handtools and shop equipment used in the shop include an airframe mechanic's tool kit, face shields, scissors, power shear, drill press, horizontal and vertical belt sanders, contour metal-cutting saw, and pneumatic hand drills. The general uses and maintenance of these standard tools and equipment should be famil iar to any airframe mechanic.
REPAIR MATERIALS Cleaning Solvents Before any repair is made to a bonded honeycomb structure, an area extending several inches away from the damage must be cleaned thoroughly of all paint or surface coating. This is best accomplished by the use of paint remover or MEK (methyl-ethyl-ketone) cleaning solvent, In some cases, Alconox, a powerful wetting and detergent agent, may be used for a final cleanup to remove any residue or oils remaining after application of the paint remover or the MEK cleaning solvent.
Paint removers are applied with a suitable size brush. When the paint or surface coating has loosened, it is either wiped off with a clean rag or removed with a nonabrasive scraper. Paint remover must not be allowed to enter the damaged area or be used along a bonded joint or seam because its chemical action will dissolve the bonding adhesive.
These areas should be masked and final cleanup accomplished with the MEK cleaning solvent or emery cloth. The MEK cleaning solvent and the Alconox cleaning agent may be applied with a clean sponge.
After a damaged area has been completely removed, the surrounding surface areas must be thoroughly re-cleaned. This is accomplished by the use of the MEK cleaning solvent and gauze sponges.
The MEK cleaning solvent is applied to the area with one sponge and immediately wiped off with another before it has had time to dry. This cleaning process should be continued until the surface area is lusterous in appearance and clean of any foreign matter.
To determine whether an area is completely and thoroughly clean, a water "break" test can be used.
This test is a simple application of a thin film of distilled water to the cleaned area. Any "break" of the applied thin film of distilled water will indicate that the area has not been cleaned thoroughly enough and the cleaning process must be repeated.
Safety precautions must be closely observed when working with the above solvents, especially when the work is overhead or when working in confined areas. For personal protection, rubber gloves, face shields, adequate ventilation, and respirators should be worn. A CO, fire extinguisher should be on hand or nearby and ready for use if necessary.
Primers Primers are applied to the cleaned surface areas primarily to ensure a good bond of the honeycomb repairs. The primer is applied to the cleaned surface areas with a clean gauze sponge or suitable brush.
It is recommended that the primer be applied as rapidly as possible because it will become tacky in 10 to 15 sec., and it will pull and be ruined by any further brushing. The primer will cure in approximately 1 hr. at room temperature; however, this time may be reduced by the application of controlled heat.
Adhesives and Resins Two types of adhesives presently used in the repairs of bonded honeycomb structures of some aircraft are known as the type 38 adhesive and the potting compound. The type 38 adhesive is applied to glass fabric overlay repairs, and the potting compound, as the name implies, to the potted compound (hole filling) repairs. In addition, the type 38 adhesive may be used as an alternate for the potting compound by adding micro-balloons (microscopic phenolic). The adhesives or potting compounds are prepared according to a batch mix (amount required for the repair) formula. The batch mix Accurate mixing of the adhesive ingredients by batches is considered one of the more important steps in the repair of bonded honeycomb structures.
The correct proportions of the epoxies, resins, and micro-balloons to be mixed into batches, both by weight and/or by volume, are given in the applicable section of the structural repair manual for the Core Material Fiberglass honeycomb core materials (16 in, cell size) are usually used to replace the damaged aluminum cores of the bonded honeycomb structures.
Aluminum core materials are not satisfactory for the repairs because of their flimsy and fragile structure. With this condition, it is impractical to cut the aluminum core materials accurately to the desired repair size. Fibergiass core materials are available 205
in various thicknesses and are easily and accurately double-face (two skins) and core damages. (See cut to size by the use of standard shop tools and figure 5-90.)
equipment.
Glass Fabrics Glass fabrics used in the overlay repairs to bonded honeycomb structures are manufactured from glass. The glass is spun into hibers which are in turn woven into a glass cloth with a variety of :
weaves.
Glass fabric cloth must be handled with care, stored properly, and be perfectly clean (free of any dirt, moisture, oil or other contaminants which may cause imperfect adhesion of the adhesives with which it is impregnated). Snags and sharp folds in the cloth will cause its strands to break, resulting in a local strength loss in the finished repair. Exposure to or contact with the glass fabric, dust, or particles may cause bodily itching or irritation.
Erosion and Corrosion Preventives Two coatings of preventives are applied to the bonded honeycomb structure repairs to protect the areas against erosion and corrosion. The first is two layers of zinc chromate, preferably sprayed onto the repair area. The second is two layers of aluminum pigmented Corrogard (EC843), or equal, either mable; therefore, the necessary fire precautions should be observed.
POTTED COMPOUND REPAIR Single face and core Double face and core FIGURE 5-90. Typical potted compound repair areas.
Removal of Damage Normally, no surface preparation is necessary when performing potted compound repairs. Oily or dirty surface areas to which multi-templates or pressure jigs are attached with adhesive tapes should be cleaned with any approved or recommended cleaning solvents, such as MEK.
Damages ¼ in. or less in diameter can be satisfactorily removed with a twist drill. The multi-template and a high-speed router (10,000 to 20,000
r.p.m.) should be employed in the removal of damaged areas for potted compound repairs up to 1 in.
in diameter. The amount of material removed by either method must be kept to a minimum to maintain as much of the original strength of the panel or structure as possible. Always use a face mask or protective glasses when using the router to remove damaged materials.
The following techniques, methods, and procedures are related to potted compound repairs and are typical of those used on most bonded honey.
comb structures. For all repairs, consult the applicable section of the structural repair manual. The manufacturer's procedures should always be followed.
Bonded honeycomb structure damages up to 1 in.
in diameter may be repaired by a hole-filling technique, using approved materials. The repair method is commonly known as the ipotted compound repair." It is the easiest and fastest method of repairing a damaged area of a bonded honeycomb structure. However, be sure to follow the techniques, methods, or procedures established for potted compound repairs to avoid any further damage which might result in a more complicated repair.
Potted compound repairs may be applied both to single-face (one skin) and core damages, or to After a damaged area has been completely removed and cleaned, the necessary materials for the potted compound repair (s) are prepared.
Pieces of sheet plastic materials are prepared to provide a smooth surface effect of the potted compound repair, to provide part of the reservoir for the hole (cavity) filling operation of the repair, and to hold the potting compound in place until it is completely cured. The pieces of sheet plastic to be used for any or all of the above purposes should be at least ½ in. larger in diameter than the repair hole diameter.
A thinner piece of sheet plastic material (approximately 16 in. thick) is applied to the lower (bottom) surface of the double-face repair (figure 5-91). This is done not only to give the repaired surface a smooth effect, but mainly to hold the repair surface (potting compound) in place until it is cured. The same may be applied to a single-face 206
Dixieal, sealing compound Masking tape Plexigias or acetate Drill 0.25 dia. hole Lower skin
DAMAGE TO UPPER SKIN AND CORE Duxssel, sealing compound Masking tape Plexiglas or acetate Drill 0.25 dia. hole Plexigias or acetate
DAMAGE TO BOTH SKINS AND CORE FICURE 5-91. Potted compound repairs.
repair where the work must be accomplished in an overhead position.
Sheet plastic materials ranging from ¼ in. to 16 in. in thickneas are used on the upper (top side) surface of the repair during the hole (cavity) filling procedures. A ¼-in, hole is drilled directly in the center to permit easy application of the potting compounds to the repair cavity. The hole is also countersunk, allowing a buildup of the potting compound and thus assuring that the repair cavity has been completely filled.
This piece of plastic material is also a part of the "Duxseal" dam. After the prepared pieces of sheet plastic materials are properly located and taped in place over the repair area, the Duxseal (or equal) dam is built up around the hole. (See figure 5-91.)
This dam is partially filled with the potting compound during the hole-filling operation to ensure an adequately filled repair cavity. The dam also acts as the reservoir.
Next, & sufficient batch mix of the potting compound is prepared for the repair. The cavity is filled with the potting compound, and the air bubbles are removed with a toothpick or similar tool. The air bubbles are removed to ensure that the repair cavity is solidly filled.
When the potting compound within the repair cavity is completely cured, the pieces of plastic may then be removed. Generally, these pieces of plastic can be lifted off by hand; but, if necessary, they can be pried off easily with any dull straight handtool. When the drilled top piece of plastic is removed, it will leave a broken stem protruding above the repair surface, This stem may be filed, micro- 207
shaved, or routed down to make the repair surface area smooth.
The soundness of the repair can be tested by the metallic ring test. Pressure jigs may be used on the undersurface potted repairs as necessary. The repair is surface finished by the application of the recommended coatings of erosion or corrosion preventives, and a final coating of a finish of the same specifications as that of the original finish.
GLASS FABRIC CLOTH OVERLAY REPAIRS Presently, two acceptable methods of repair are being applied to the damaged skin and core materials of some aircraft bonded honeycomb structures.
One is the potted compound repair method previously discussed, and the other is the laminated glass fabric cloth overlay method applied to the various damages of honeycomb skin and core materials which exceed the repair limitations of the potted compound repair.
The differences between the two repair methods are in the techniques of removing the damaged area, preparing the damaged area for the repair, preparing and applying the repair materials, finishing and final inspection of the completed repair, and use and maintenance of the handtools and shop equipment.
Cleaning Before repairing a damaged bonded honeycomb panel or section, thoroughly clean all paint or surface coatings from a surface area extending several inches away from the damage. Basically, this is necessary to attach and secure the templates or wedge blocks to the repair area with double-backed tape. Second, thoroughly clean the area of all foreign matter to ensure a perfect adhesion of the overlay repair materials.
Effective surface cleaning is of primary importance to the success of any repair. An area that is contaminated with paint, grease, oil, wax, oxides, or such, will not take a good bond. This cannot be emphasized too strongly since the quality of the repair will be no higher than the quality of the cleaning that precedes it. Even a fingerprint will prevent a good bond, because of natural oils in the skin.
Materials such as solvents, abrasives, alkaline detergents, and chemical etches can be used for effective cleaning. One of the easiest and most effective cleaning methods known is to apply MEK to the area with a sponge and immediately wipe it away with another sponge. This procedure should be continued until a lusterous surface is obtained.
In removing paint, use caution, since paint remover will dissolve adhesives if allowed to enter the damaged area of a joint.
Removal of Damage A high-speed router in conjunction with a router support assembly, metal-cutting mill bit, and template should be used in the removal of the damaged area. (Information about the uses and maintenance of the router was discussed earlier in this chapter.)
The techniques of removing damaged honeycomb skin and core material may differ from one repair to another. Their selection depends largely on the construction features of the bonded honeycomb panels, which are primarily of either flat, tapered, or combined (flat and tapered) surface design.
Also, the location of the damaged area must be considered; that is, whether the damage occurred on the upper or lower side of the panel. Another factor that must be considered is that the honeycomb core is always installed within the panel with the cells perpendicular to the lower surface.
The techniques of preparing for and removing a damaged area on a tapered or upper surface of a panel are somewhat different from those for a flat or lower surface. Prior to the routing of a damaged area of an upper or a tapered surface, the routing template must be bridged over the repair area. This is done in such a manner that the routing template will be perpendicular to the core cells and parallel to the opposite (lower) facing. The bridge consists of two wedge blocks made of wood, at least 6 in.
long, approximately 2 in. wide, and tapered to the same degree of angle as that of the panel. The method by which the bridge is attached to the damaged area is shown in figure 5-89.
Adhesives Overlay repair adhesives consist of a type 38 batch mix. Micro-balloons are added to the resins and curing agent for "buttering" the fiber glass honeycomb core plug and cavity of the glass cloth overlay repair. The micro-balloons can also be used to control the consistency of the potted compound adhesive.
The type and location of the repair will determine the method of adhesive application. For example, a repair on an upper surface would use a low micro-balloon content and would be poured into the cavity, whereas the same repair on an under surface would use a high micro-balloon content and would 208
have to be spooned into the cavity with a spatula or putty knife. Whichever method is used, the adhesive for all repairs should be applied evenly, without trapping any air bubbles.
The type 38 adhesive will set up and bond at room temperature. If a faster bond is required, the repair area should be preheated to 130° F., the repair parts and adhesives applied, and the whole repaired area heated at the same temperature for 1 hr. to effect a complete bond.
Upon completion of the repair, test it for any separation or other flaws, using the metallic ring test.
Coro Plug Core plugs are cut slightly larger than the desired thickness and shape from a glass fabric honeycomb core material (16-in, cell size). They are sanded to the correct shape and thickness with a belt or disk sander to a tolerance of +0.010 in. of the required size.
Before a core plug is inserted or assembled into the repair area, all contacting (faying) side surfaces of the core plug and the repair area must be "buttered" with an application of adhesive.
After the core plug has been properly installed into the repair area, the excessive potting compound is removed with a plastic scraper and the surface area thoroughly cleaned with a cleaning solvent.
The core plug repair area should be cured for at least 30 min. to 1 hr. This is done to assure that the core plug is firmly in place before any further repair steps are accomplished.
Laminated Glass Cloth Overlay A laminated glass cloth overlay consists of two layers of glass fabric cloth number 181 (three lay.
ers if number 128 is used) impregnated with type 38 adhesive and sandwiched between two sheets of polyethylene film. The glass cloth layers and sheets of polyethylene film are cut larger (spproximately 4 in.) than the damage cutout. This is done to accom.
modate the cutting of the laminated overlay to correct size, allowing for the required minimum overlap of at least 1-½ in. beyond the edge of the damage cutout.
Before a laminated glass cloth overlay is applied to a repair area, the faying surface must be cleaned until no trace of foreign matter appears. After the area has been thoroughly cleaned and dried, a thin and continuous film of adhesive primer EC-776R (or equal) is applied to the faying surfaces of the area. The adhesive primer may be allowed to dry at room temperature or may be accelerated by heat at a recommended temperature.
Protective coatings to prevent erosion and corrosion should be applied in accordance with the procedures outlined in the manufacturer's structural repair manual for the specific aircraft. A control surface repair should be checked to determine whether it is within balance limits or will require the surface to be re-balanced.
ONE SKIN AND CORE REPAIR PROCEDURES Two typical methods of repairing damages to the honeycomb skin and core materials of aircraft are:
(1) Single-face repair with damage extending through the core material and to the bond line of the opposite facing; and (2) transition area repair.
When the damage to the honeycomb structure is inspected and evaluated as damage to only one skin and the core (figure 5-92), the procedures Glass cloth laminates Fiber glass Butter edges of honeycomb core honeycomb •with potting.
core compound before installing core in repair area.
14" overlap Glass cloth Butter edges of prepared laminates hole with potting compound, CORE REPLACEMENT 2071220
SECTIONAL VIEW OF REPAIR AREA FICURE 5-92. Skin and core repair.
discussed in the following paragraphs can be used.
These procedures are typical but may not apply to all types of aircraft. Consult the manufacturer's repair manual for the specific aircraft, and follow the instructions for the particular type of repair.
Removing Damaged Area A router and the applicable template should be used to remove the damaged material from the area.
The depth of the router bit is determined by gradually increasing the depth of cut until it removes all of the damaged area. If the core is only partially damaged, remove only that portion. If the entire core is damaged, remove the core down to the opposite adhesive layer.
nated sections of fiber glass cloth needed to complete the repair should be accomplished as follows:
(1) Remove surface coating from repair area.
(2) Wipe surface with clean cheesecloth moistened with MEK unil no trace of foreign material appears. Do not allow MEK to dry, but wipe it off with a clean cloth.
(3) Apply adhesive primer EC-776R (or equal) with a clean 1-in. varnish brush to faying surface area and allow to dry.
hr. at may eat not Preparing the Core Replacement be ap- The core replacement must be fabricated from m. Do fiber glass core material. If the correct thickness is ıry for not available, the replacement section may be cut to cloth size by hand sawing and/or sanding. The core plug laminates.
should be flush or within +0.010 in. of all ccom- Potting Compound paper Prepare the potting compound as follows:
(1) Select the desired mix for the repair. A hylene stiff mix is desired when making overhead an the repairs or for core plug bonding. The average mix or thin mix is desirable when sh leet making upper surface repairs. inated
(2) Add micro-balloons to the resin and mix age for 3 to 5 min.
(3) Add the curing agent to the resin and sive micro-balloon mixture. Mix for 3 to 5 usly min.
(4) Apply the potting compound to the edges of the core replacement and around the cOredges of the damaged area in the struc- flm Insertion of Core Plug Place the core plug in the repair area as follows:
(1) Insert the core plug into the repair area.
(2) Remove any excessive potting compound with a plastic scraper and clean the repair area thoroughly.
(3) Allow the core plug repair to cure for at least ½ to 1 hr. at room temperature (72° F) to assure that the core plug is firmly in place.
Application of Glass Cloth Laminates The preparation for and application of the lami- 210 sive sive ene film.
(10) Place and center one layer of the glass fabric cloth over the adhesive-covered area of the sheet of polyethylene film.
(11) Pour an adequate amount of type 38 adhesive over the first layer of glass fabric cloth to cover and penetrate its entire area. Spread the adhesive evenly over the area with a plastic scraper.
(12) Apply a second layer of glass fabric cloth in the same manner as the first layer.
(13) Apply a sufficient amount of type 38 adhesive over the second layer of glass fabric cloth in the same manner as over the first layer.
(14) Place and center the second sheet of polyethylene film over the layers of adhesive-impregnated glass fabric cloth.
(15) With a plastic scraper, work out all the air bubbles towards the edges of the laminated overlays. Turn the laminated overlay as necessary in working out the air bubbles.
(16) With a pair of scissors, cut the sandwiched polyethlyene-laminated overlay with MEK cleaning solvent. Take care to prevent any of the cleaning solvent from entering the bond of the repair area.
(27) Allow the laminated overlay repair to cure for at least 12 hrs. at room temperature (72° F.) before the final sheet of polyethylene film is removed.
Transition Area Repair Some bonded honeycomb panels are constructed of a doubler separating an upper and lower skin and sectioned into bays of honeycomb core material. (See figure 5-93.) The edge section of a bay _Flange - Transition By area ½ in. to ¾ in. larger than the actual size of the laminated overlay.
(17) Place and center the sandwiched polyethylene-laminated overlay between the two Doubler disk templates that were previously prepared for the repair.
(18) With a pair of scissors, carefully cut the Transition laminated overlay around the edge of the area disk templates.
(19) Remove the disk templates from the sandwiched polyethylene-laminated overlay. Bav area
(20) Peel off one sheet of polyethylene film from the sandwiched laminated overlay.
Discard the polyethylene film.
(21) Place the laminated overlay with the exposed adhesive side down and in position over the repair surface area.
(22) Remove the remaining sheet of polyethylene film from the top side of the lami- CRUSHED EDGE DESIGN nated overlay. Discard the polyethylene film. Flange area
(23) Cut another sheet of polyethylene film ¾ in. to 1 in. larger than the laminated Transition overlay. area
(24) Place and center this sheet of polyethyl: Bay area ene film over the laminated overlay positioned over the repair surface area.
(25) With a smooth plastic scraper, sweep out any excess resin or air bubbles that may be present within the laminated overlay.
This step is of utmost importance to the overall quality of the repair. Therefore, this step should be accomplished with the greatest of care and patience.
(26) Clean the surrounding area of the repair
FORMED RING EDGE DESIGN FicURE 5-93. Typical bonded honeycomb panel bay construction.
area in which the honeycomb structure joins the laminated area of the panel or section is known as the transition area. Effective repairs to the transltion areas are particularly essential because of the local transferring of the streases.
The preparation of the repair materials and the assembly and curing of the core plug are basically the same as for the bonded honeycomb skin and core repairs. However, because of the, shape and contour of a transition area, especially at the corners of a bay, give special attention to the cutting and shaping of the glass fabric honeycomb core material.
In this repair, four layers of impregnated glass cloth, number 181, are preferred for the overlays.
The preference for glass cloth number 181 is because of its flexibility and ease of application, particularly when making repairs to a corner of a bay where a compound (double) contour is encountered.
Glass cloth laminates Fiber glass honeycomb core Repair Procedures The steps to be followed in the repair of a transition area are as follows:
(1) Outline the repair to a circular shape (not to exceed 2 in. in diameter) that will encircle the damaged area.
(2) Using a router, remove the damaged area down to the opposite adhesive layer.
Depth of the router cut is determined by gradually increasing the depth of the cut until the adhesive layer is reached.
(3) Fabricate a fiber glass honeycomb core to replace the damaged core section. The correct thickness and contour of the transition area may be obtained by hand sawing and/or sanding. The core plug must be shaped to fit flush or within +0.010 in.
(4) Prepare the potting compound.
(5) Butter the edges of the fiber glass honey-
• 2.0" Max. →
2.0" Max. (typ.)
Butter edges and bottom of prepared hole with potting compound before Butter edges of honeycomb installing honeycomb core. core with potting compound before installing core in repair area.
FIGURE 5-94. Transition area repair.
comb core plug with potting compound.
(See figure 5-91.)
(6) Install the buttered core plug in the repair area.
(7) Prepare glass fabric adhesive.
(8) Prepare four glass fabric laminates-the first, sufficient in size to adequately cover the damaged area with no overlap; the others, 0.25 in. on all sides. The lamì- nates should not extend over fasteners, If the laminates do extend over fasteners or fastener holes, cut around locations as shown in figure 5-94.
(9) Apply the four glass cloth laminates to the repair area as shown in the section view of the repair illustrated in figure
5-94.
(10) Allow the repair to cure properly.
(11) Apply the necessary erosion and corrosion preventives as specified.
PLASTICS According to their chemical sources, plastics may be classified into four general groups: (1) Natural resins, (2) synthetic resins, (3) protein plastics, and (4) cellulose plastics.
Natural resins include such materials as shellac, pitch, amber, asphalt, and resin. These materials require fillers when molded.
Synthetic resins are made from petroleum, glycerol, indene, calcium-cyanamide, benzene, urea, ethylene, phenol, and formaldehyde. Products made from synthetic resins include acrylic plastics, nylon, vinyl, styrene, polyethylene, urea-formaldehyde, and others.
Protein plastics are manufactured from a variety of agricultural products. Sources included are peanuts, cashews, milk, coffee beans, and soy beans.
The cellulose plastics are the oldest of the group and include celluloid. Other plastics which fall into this class are acetate, nitrate, ethyl cellulose, butyrate, and propionate.
Nearly all of the early-day plastics were molded.
However, today a large percentage of the plastics we know and use are cast, machined, rolled, laminated, or formed by other methods.
TRANSPARENT PLASTICS Two types of transparent plastics used in aircraft windows, canopies, and similar transparent enclosures are thermoplastic and thermosetting materials.
Thermoplastic materials are originally hard but become soft and pliable when exposed to heat.
When pliable, the plastic can be molded; and, as it cools, it will retain the molded shape. When heated again and allowed to cool without being restrained, the plastic will return to its original shape. This process can be repeated many times without damage to the material unless the specified heat ranges are exceeded.
Thermosetting plastics are molded and allowed to cool and set in the desired shape. No amount of reheating will cause them to become pliable and workable. Once formed, they retain that shape and cannot be re-molded or re-shaped.
Each of these types of transparent plastics is available in monolithic or laminated forms. Monolithic plastic sheets are made in single solid uniform sheets. Laminated plastic sheets are made from transparent plastic face sheets bonded by an inner layer of material, usually polyvinyl butyral.
Optical Considerations Optical qualities of the transparent material used in aircraft enclosures must be as good as those of the best quality plate glass. The ability to locate other aircraft in flight, positive depth perception necessary to land safely, all require a medium which can readily be molded into streamlined shapes and yet remain free from distortion of any kind. Such a medium must also be simple to maintain and repair.
In addition to their ease of fabrication and maintenance, plastics have other characteristics which make them better than glass for use in transparent enclosures. Plastics break in large dull-edged pieces; they have low water absorption and they do not readily fatigue-crack from vibration. But on the other hand, although they are nonconducters of electricity, they become highly electrostatic when polished.
Plastics do not possess the surface hardness of glass, so they are more easily scratched. Since scratches will impair vision, care must be used in servicing an aircraft. Specific procedures to avoid damaging the transparent plastic parts are discussed elsewhere in this chapter. Some general rules to follow are:
(1) Handle transparent plastic materials only with clean cotton gloves.
(2) Never use harmful liquids as cleaning agents, i.e., naphtha, gasoline, etc.
(3) Follow rigidly the applicable instructions for fabrication, repair, installation, and maintenance.
(4) Avoid operations which might scratch or distort the plastic surface. Be careful not to scratch the plastic with finger rings or other sharp objects.
The cellulose acetate plastics have a yellowish tint when viewed from the edge, and they are softer Both acrylic and cellulose acetate base plastics Identification have characteristic odors, especially when heated or craft can be determined by the MIL specification The identity of transparent plastics used on air- burned. Burning a small sample and comparing its odor to that of a known sample is a very reliable number on the part. Common MIL numbers and the method of identification. The acrylic odor is fairly type of material are as follows (fig. 5-95) : pleasant, but acetate is very repugnant. Acrylic plastic burns with a steady, clear flame, whereas Specifications Type Material Edge Color acetate burns with a sputtering flame and dark smoke.
Thermoplastic These plastics can also be identified by the appli- MIL-P-6886 Regular Acrylic Practically clear cation of acetone and zinc chloride. Rub an area of MIL-P-5425 Heat Resistant Practically clear MIL-P-8184 Craze Resistant Slightly yellow the plastic with a solution of acetone, where it will Thermosetting not interfere with vision. Then blow on the area. If MIL-P-8257 Polyester Blue-green the plastic is acrylic, it will turn white; if it is Laminated acetate, it will soften but will not change color. A MIL-P-7524 Laminated Practically clear drop of zinc chloride placed on acetate base plastic MIL-P-5425 will turn the plastic milky, but will have no effect MIL-P-25374 Laminated MIL-P-8184 Slightly yellow on acrylic plastic.
Distinguishing Base Name Features Acrylate Plexiglas Absence of color STORAGE AND PROTECTION Lucite Greater Transparent plastics will soften and deform when Perspex (British) Greater stiffness transparency high temperatures must be avoided. Plastic sheets heated sufficiently. Therefore, storage areas having should be kept away from heating coils, radiators, Cellulose Acetate Fibestos Slightly yellow or hot water or steam pipes. Storage should be in a tint cool, dry location away from solvent fumes (such as Lumarith Greater flexibility may exist near paint spray and paint storage Plastacele Lower areas). Paper-masked transparent plastic sheets transparency should be kept indoors. Direct rays of the sun will Nixonite Softer accelerate deterioration of the masking paper adhe- FIcURE 5-95. Characteristics of plastics. moval is difficult. sive, causing it to cling to the plastic so that re- If the parts are not marked, the information in the following paragraphs will help to identify the material.
Transparent plastic enclosures and plate glass enclosures can be distinguished from each other by lightly tapping the material with a small blunt instrument. Plastic will resound with a dull or soft sound, whereas plate glass will resound with a metallic sound or ring.
Very few of the transparent plastics are color clear when viewed from the edge; some are practically clear, while others have a slight yellowish tint, or a bluish or blue-green tint.
Plastic sheets should be stored, with the masking paper in place, in bins which are tilted at approximately a 10° angle from the vertical to prevent buckling. If it is necessary to store sheets horizontally, care should be taken to prevent chips and dirt from getting between the sheets. Stacks should not be over 18 in, high, and the smaller sheets should be stacked on the larger ones to avoid unsupported overhang.
Masking paper should be left on the plastic sheet as long as possible. Care should be used to avoid scratches and gouges which may be caused by sliding sheets against one another or across rough or dirty tables.
Formed sections should be stored so that they are amply supported and there is no tendency for them to lose their shape. Vertical nesting should be avoided. Protect formed parts from temperatures higher than 49° C. (120° F.). Protection from scratches can be provided by applying a protective coating, i.e., masking paper, untreated builders' paper, posterboard, or similar material.
If masking paper adhesive deteriorates through long or improper storage, making removal of the paper difficult, moisten the paper with aliphatic naphtha. This will loosen the adhesive. Sheets so treated should be washed immediately with clear water.
Aliphatic naphtha is highly volatile and flammable. Use extreme care when applying this solvent.
Do not use gasoline, alcohol, kerosene, benzene, xylene, ketones (including acetone, carbon tetra chloride, fire extinguisher, or deicing fluids), lacquer thinners, aromatic hydrocarbons, ethers, glass cleaning compounds, or other unapproved solvents on transparent acrylic plastics to remove masking paper or other foreign material, because they will soften or craze the plastic surface.
When it is necessary to remove masking paper from the plastic sheet during fabrication, the surface should be re-masked as soon as possible. Either replace the original paper on relatively flat parts, or apply a protective coating on curved parts.
Certain protective spray coatings are available for formed parts. The thickness of the coating should be a minimum of 0.009 in. A layer of cheesecloth should be embedded in the coating at the time of application to assist in the removal of the masking spray. Coatings which remain on formed parts longer than 12 to 18 months become difficult to remove. Under no circumstances should transparent plastic, or formed parts coated with this material, be stored outdoors where it will be subject to direct sunlight for longer than 4 months.
To remove spray masking from the plastic, peel it off or lift a corner of the film and flow a jet of compreased air under it. If the film is too thin to be removed as a continuous film, apply a fresh coating of the compound, reinforced with a layer of cheesecloth, to obtain a thicker film. Allow to dry. Soaking the coated part, using a clean cloth saturated with water at room temperature, will help soften the film so that it can then be peeled off by hand. In no case should a solvent be used.
Extreme care must be used to avoid scratching the surface of the plastic. Tools must never be used in removing the film because of the danger of scratching the plastic.
FORMING PLASTICS Transparent plastics become soft and pliable when heated to their respective forming temperatures. They can then be formed to almost any shape; and, on cooling, the material retains the shape to which it was formed, except for a small contraction. It is not desirable to cold form compound curvature transparent plastics (that is, to spring them into a curved frame without heating).
Transparent plastics may be cold bent (single curvature) if the material is thin and the radius of curvature to which it is cold bent is at least 180 times the thickness of the sheet. For example, an 18 in. length of transparent plastic, 0.250 in. thick, should not be deflected more than ¾ inch. Cold bending beyond these limits may eventually result in tiny fissures, called crazing, sppearing on the surface of the plastic because of stresses being imposed beyond those recommended for continuous loading. For hot forming, transparent plastics should be maintained at the proper temperature recommended by the manufacturer.
Fabricating Processes The fabrication of transparent plastics can be compared generally to that of wood or soft metal.
Good craftsmanship, suitable equipment, and proper design are no less essential to the successful fabrication of transparent plastics than to that of other materials worked by similar methods. Light to medium woodworking equipment with minor modifications is satisfactory, but heavy-duty machines which are less apt to vibrate are better.
Where extreme accuracy is not required, the 0/01 work can be laid out by penciling the cutting lines directly on the masking paper. For close tolerances, however, it is advisable to scribe layout lines directly on the surface of the plastic. Use straightedges or layout templates according to the requirements of the job. If the masking paper is removed before scribing, it should be replaced to within about ¼ in. of the scribed markings before the piece is cut.
Layout templates may be of plastic sheeting to which suitable handles can be cemented. Sharp edges or rough spots in such templates should be carefully rounded or smoothed. In the case of metal templates, it is good practice to cement thin flannel over the contact surfaces.
Cutting Scribing and edge sanding is the cutting method most generally used on flat sections or two-dimensional curved pieces, The sheet is first cut to approximate shape on a band saw, using a scribed line as a guide and cutting approximately 18 in.
oversize, Use disk sanders when removing material from straightedges and outside curves. Use drum or belt sanders for inside curved edges. When sanding irregular shapes or larger pieces which are awkward to manipulate around a fixed machine, use an air-driven sander or small electric hand sander.
Drilling For the sake of both accuracy and safety, hold work in suitably designed clamps or fixtures. The twist drills commonly used for soft metals can be used successfully for transparent plastics if ordinary care is observed. However, the best results can be obtained if drills are re-pointed with the following in mind:
(1) The drill should be carefully ground free of nicks and burrs which would affect surface finish.
(2) It is particularly important that the cutting edge be dubbed off to zero rake angle.
(3) The length of the cutting edge (and hence the width of the lip) can be reduced by increasing the included angle of the drill.
(See figure 5-96.)
-• Cutting direction Margin Lip clearance Dubbed-off to angle zero rakr angle Included tip angle 140° Slow spiral-polished flutes FIGURE 5-96. Drill for acrylic plastics.
Use drills with slow-spiral polished flutes. Flutes should be as wide as possible. The best lubricant and coolant for drilling plastics is a water-soluble cutting oil. For drilling shallow or medium depth holes, no coolant is needed. For deep holes, of course, a coolant is necessary.
Cleaner, more transparent deep holes can be produced by first drilling a pilot hole a little more than half the diameter of the final hole, filling this pilot hole with a wax stick, then redrilling to the final diameter. If the pilot hole is drilled all the way through, the Plexiglas must be backed with wood to close the hole and make the wax stick effective. The wax lubricates the cut and supports and expels the chips during drilling. In clear Plexiglas the resulting hole is cleaner, smoother, and more transparent than holes produced by other methods.
Large diameter holes can be cut with hollow-end mills, hole saws, fly cutters or trepanning tools.
The cutters of the latter should be ground to zero rake angle and adequate back clearance, just as lathe tools are ground. All these tools can be used in the standard vertical spindle drill press or in flexible shaft or portable hand drills.
In general, the speed at which Plexiglas sheets can be drilled depends largely on the quality of the equipment used. Plexiglas can be drilled at the highest speed at which the drill will not "wobble" sufficiently to affect the finish of the hole. However, large diameter drills require slower rotative speeds for best results. Also, the Plexiglas should be backed with wood and the feed slowed as the drill point breaks through the underside of the sheet.
Whenever holes are drilled completely through Plexiglas, the standard twist drills should be modified to a 60° tip angle, the cutting edge to a zero rake angle, and the back lip clearance angle increased to 12-15°.
Drills specially modified for drilling Plexiglas are available from authorized distributors and dealers of Plexiglas.
For accuracy and safety, Plexiglas parts should be clamped or held rigidly during drilling:
SHALLOW HOLES-Hole depth/ hole d'ameter ratio of less than 1½ to 1, use slow spiral twist drills with wide flutes modified as for through drilling.
Chip removal is no problem in drilling shallow holes and no coolant is needed.
MEDIUM DEEP HOLES-Hole depth/hole diameter ratio from 1½ to 1 up to 3 to l.
Use slow spiral twist drills with polished flutes which should be as wide as possible to aid in removing a continuous ribbon of material. The opti- 216