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9/8/98 AC 43.13-1B Clean glass or polyethylene mixing containers are preferred. (b) Because of their short pot life (approximately 45 minutes) Cement PS-30 and Weld-On 40 must be used quickly once the components are mixed. Time consumed in preparation shortens the effective working time, making it necessary to have everything ready to be cemented before the cements are mixed. For better handling pour cement within 20 minutes of mixing. (c) For maximum joint strength, the final cement joint should be free of bubbles. It will usually be sufficient to allow the mixed cement to stand for 10minutes before ce menting to allow bubbles to rise to the surface. (See figure 3-20.) FIGURE 3-20. Applying pressure to acrylic plastics. (d) The gap joint technique can only be used with colorless PLEXIGLAS acrylic or in cases in which joints will be hidden. If in conspicuous joints in colored PLEXIGLAS acrylic are needed, the parts must be fitted closely, using closed V groove, butt, or arc joints. (3) Cement forms or dams may be made with masking tape as long as the adhesive sur face does not contact the cement. This is easily done with a strip of cellophane tape placed over the masking tape adhesive. The tape must be chosen carefully. The adhesive on ordinary cellophane tape prevents the cure of PS-30 and Weld-On 40. Before actual fab rication of parts, sample joints should be tried to ensure that the tape system used will not harm the cement. Since it is important for all of the cement to remain in the gap, only con tact pressure should be used. (4) Bubbles will tend to float to the top of the cement bead in a gap joint after the ce ment is poured. These cause no problem if the bead is machined off. A small wire (not cop per), or similar objects may be used to lift some bubbles out of the joint; however, the cement joint should be disturbed as little as possible. (5) Polymerizable cements shrink as the cement hardens. Therefore, the freshly poured cement bead should be left above the surfaces being cemented to compensate for the shrink age. If it is necessary for appearances, the bead may be machined off after the cement has set. 3-24. REPAIR OF PLASTICS. Replace, rather than repair extensively damaged trans parent plastic, whenever possible, since even a carefully patched part is not the equal of a new section, either optically or structurally. At the first sign of crack development, drill a small hole with a # 30 or a 1/8-inch drill at the ex treme ends of the cracks as shown in fig ure 3-21. This serves to localize the cracks and to prevent further splitting by distributing the strain over a large area. If the cracks are small, stopping them with drilled holes will usually suffice until replacement or more per manent repairs can be made. The following repairs are permissible; however, they are not to be located in the pilot’s line of vision during landing or normal flight. Par 3-23 Page 3-19PDF Image | AFS-640
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