During the bandsawing and milling operations, the masking paper was left on 

 to protect the acrylic plastic against scratches. When the machining was com- 

 pleted, the masking paper was removed and the blank placed (chamfered edge 

 downward) into the mold with the aid of a hoist equipped for this purpose 

 with a vacuum-actuated suction disc. Four 0. 1 25-inch-diameter holes were 

 drilled 2 inches deep into the edge of the disc for placement of thermoelectric 

 couples. After placement of thermoelectric couples, the mold and blank 

 assembly were moved into a walk-in oven and the temperature control set at 

 1 65°F. After approximately 1 6 hours, the temperature control was set for 

 310°F. In approximately 8 hours, thermoelectric couples in the acrylic plastic 

 indicated that a minimum temperature of 300°F was reached. At that time, 

 a vacuum of 25 inches of mercury was applied to the mold. The blank sagged 

 immediately and completely. After 15 minutes, the oven was turned off, but 

 the vacuum was left on and the door closed to permit slow cooling of the 

 acrylic plastic to 1 10°F. It took 16 hours for the temperature in the acrylic 

 plastic to drop to 1 10°F. At that time, the doors to the oven were opened 

 and the formed sector and the mold removed (Figure 28). 



Since little was known about the formability of such thick acrylic 

 plastic blanks in a vacuum mold, a careful postforming inspection was per- 

 formed on the first molded acrylic plastic sector for the 66-inch capsule. 

 The inspection consisted not only of accurately measuring the thickness of 

 the formed sector, but also the sphericity of its concave surface. The spher- 

 icity of the concave mold was also measured, as without knowledge of 

 deviations in its sphericity, few conclusions could be drawn on the causes 

 of asphericity in the formed acrylic plastic sector. 



The results of measuring the sphericity of the mold (Figure 29), as 

 well as the sphericity and thickness of the formed acrylic plastic sector, are 

 quite interesting (Figure 30). First, the sphericity of the mold was found to 

 deviate at some points on the mold as much as 0.040 inch from the specified 

 33.000-inch radius of the formal sector, with the least deviation measured at 

 a distance of 12.000 inches from the center of the mold as would be expected 

 from a sphericity-measuring technique that used the surface of the mold at 

 that location as reference datum. Second, the deviations from the nominal 

 sphericity of the formed spherical sector followed closely the trend of the 

 deviations of the surface mold. But since the sphericity of the sector was 

 measured on its concave surface, and the thickness of the sector varied 

 somewhat from point to point (Figure 30), a quantitative comparison 

 between the sphericity of the convex surface on the sector and the spher- 

 icity of the concave surface on the mold could not be made accurately. 

 Third, the radius of curvature on the concave surface of the sector appeared 

 to be greater than the 30.500 inches specified. 



54 



