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9/8/98 AC 43.13-1B 3-18. GENERAL. Plastics cover a broad field of organic synthetic resin and may be di vided into two main classifications – thermo plastics and thermosetting plastics. a. Thermoplastics. Thermoplastics may be softened by heat and can be dissolved in various organic solvents. Two kinds of trans parent 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 speci fications of MIL-P-5425 for regular acrylic, MIL-P-8184 for craze-resistant acrylic. b. Thermosetting Plastics. These plas tics do not soften appreciably under heat but may char and blister at temperatures of 240 to 260 °C (400 to 500 °F). Most of the molded products of synthetic resin composi tion, such as phenolic, urea-formaldehyde, and melamine formaldehyde resins, belong to the thermosetting group. Once the plastic be comes hard, additional heat will not change it back into a liquid as it would with a thermo plastic. 3-19. STORAGE AND HANDLING. Be cause transparent thermoplastic sheets soften and deform when they are heated, they must be stored where the temperature will never be come 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 ar eas. 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 plas tic 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 °F (49 °C), 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 re tain the shape to which they were formed. Acrylic plastic may be cold-bent into a single curvature if the material is thin and the bend ing radius is at least 180 times the thickness of the sheet. Cold bending beyond these limits Par 3-18 Page 3-15 SECTION 3. TRANSPARENT PLASTICSPDF Image | AFS-640
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