A thorough evaluation indicated that free-blowing flat plates into 

 spherical modules would result in the largest variation in spherical radius 

 from one module to another, and pressing the flat plates between male 

 and female molds of appropriate radii would result in the least variation. In 

 addition, the free-blowing process produces the largest variation in thickness 

 between different points on the module, while the pressing process produced 

 not only a uniform thickness- from point to point on a module, but also from 

 one module to another. Thus, it was obvious that the forming process by 

 pressing was the most applicable to fabrication of a spherical acrylic plastic 

 capsule, in which undue deviations from sphericity and uniformity in thick- 

 ness would exert a deleterious effect on its implosion pressure. 



On the basis of this selection process, pressing dies were designed for 

 the fabrication of 15-inch models and the 66-inch operational prototype (see 

 Appendix A for detailed dimensions of the 66-inch capsule). However, only 

 the dies for the model were built initially as it was felt that some experience 

 in using them was required before the decision could be made to invest a 

 considerable sum of money for fabrication of the dies for the prototype 

 66-inch acrylic plastic sphere. As soon as the press die assembly was built, 

 it was placed in operation for molding structural modules for the 15-inch 

 capsules. 



Model structural modules were formed in a female die with a 7.5-inch 

 concave radius and a male die with a 7-inch convex radius (Figure 24). The 

 female die had holes in its bottom through which vacuum could be applied 

 to the space between male and female dies. Both dies were placed in a verti- 

 cal hydraulic press that applied sufficient force to bring the male die to its 

 bottom position 0.5 inch above the female even if the acrylic plastic was 

 several thousands of an inch thicker than 0.500. The alignment between 

 the male and female dies was assured by rigid mounting in the press platens 

 and by steel guide pins on the male die that fitted holes in the female die. 

 Bottom stops were provided on the guide pins to assure that the male die 

 would not move any closer to the female die than 0.5 inch even if the acrylic 

 plastic blank was under 0.5 inch in thickness. Finish on the die surfaces con- 

 tacting the acrylic plastic was 32 rms. 



The first step in the forming process was bandsawing and subsequently 

 turning in the lathe a 9.875-inch-diameter disc from nominal 0.5-inch grade 

 G Plexiglas acrylic plastic stock. Subsequently, it was stripped of protective 

 paper covering and was placed into an oven whose operational temperature 

 was adjustable. After heating the acrylic plastic to the forming temperature 

 (discussed below) at a rate of approximately 15°F/minute, it was removed 

 from the oven and placed into the open die assembly which was preheated 

 by an electric heater in the press platens to 160°F. Vacuum was immediately 

 applied to the female die followed by closing of the die assembly with the 



49 



