264 BELL SYSTEM TECHNICAL JOURNAL 



Further development work was then confined to methods of process- 

 ing this body to obtain satisfactory results on an automatic machine. 

 A survey of available commercial pressing equipment indicated that 

 machines of the type used in the manufacture of various pharma- 

 ceutical tablets or pellets offered the most promise for adaptation to 

 molding protector blocks. The development of suitable equipment 

 was complicated by the extremely thin walls of the parts and the 

 necessity for rapidity of operation. In the hand molding method 

 commonly used in the industry, a slow application of the molding 

 force was possible at the end of the stroke and likewise a gradual 

 withdrawal of the top die was possible after completion of the forming 

 operation. 



After some preliminary work with various types of tableting ma- 

 chines, we concentrated our efforts on single-plunger-type machines 

 with double dies. One of the major problems was a satisfactory 

 method of die lubrication since with the machine operating at twenty- 

 eight strokes a minute, the die surfaces were exposed for oiling only 

 an instant during each cycle. The use of an atomizer-type device with 

 a mixture of lard oil and kerosene was finally adopted with the amount 

 of lubricant closely controlled by oil sight cups. Exact timing of the 

 application of the spray to dies was obtained by automatically operat- 

 ing air check valves. This method proved more satisfactory than 

 wiping with saturated felt or incorporating a lubricant in the body 

 particles before molding. 



The various stages of the molding cycle are shown in Fig. 4. The 

 cycle of operation at twenty-eight strokes per minute was as follows: 

 as the bottom die reached the lowest position, a feed hopper was 

 vibrated over the cavity. The withdrawal of this hopper removed 

 excess material, after which the top punch descended forming the 

 part. The bottom and top punches then moved upward until the 

 bottom of the part was flush with the top of the die. A projection 

 on the hopper then pushed the part free. Before the hopper reached 

 a position over the cavity, any particles of clay adhering to the dies 

 were blown off and the lubricant was sprayed over each die. The 

 lower and upper dies were then returned to the original positions. 



A large amount of work was also necessary to adjust the size and 

 moisture characteristics of the pressing material not only to secure 

 well formed parts and prevent sticking but also to secure fired parts 

 meeting the desired requirements. A mixture of colored and un- 

 colored particles was used in the study of these characteristics in 

 order that the flow movements in the die during compression could 

 be studied. As a result of this work, it was found that most satis- 



