hydraulic press. After 3 minutes, the male die was raised while the application 

 of vacuum continued. When the temperature of the formed acrylic plastic 

 decreased to 160°F after about 12 minutes, the acrylic plastic was removed 

 from the female mold and cooled on a workbench. 



Although the supplier of acrylic plastic plate recommended heating 

 the acrylic plastic to 340°F and the dies to 1 70°F prior to forming, some 

 exploratory experiments were conducted to verify those settings. Experi- 

 mentation indicated that although the magnitude of residual stresses in 

 acrylic plastic after forming is inversely proportional to the temperature of 

 the molds and of the acrylic plastic, the number and sizes of surface blisters 

 and pits is directly proportional to the magnitude of those two temperatures. 

 By varying the temperature of the acrylic plastic and of the dies, it was found 

 that if the acrylic plastic was preheated to 310 to 320°F and subsequently 

 placed into 160°F dies, the number and severity of surface blisters was not 

 noticeable, while at the same time the residual stresses were not too excessive. 

 Lowering the temperature of acrylic plastic to 260°F eliminated all surface 

 bubbles and blisters, but introduced stresses of unacceptable magnitude. 



Figure 24. Hydraulic press with matched male and female die assembly for 

 pressing spherical sectors for model capsules. 



50 



