278 BELL SYSTEM TECHNICAL JOURNAL 



Another major factor in the development of a suitable molding 

 compound was the abrasive effect of the molding body on the die 

 parts. Because of the close tolerances required, this factor was 

 important in order to avoid excessive tool expense. Talc was therefore 

 chosen as the chief body constitutent to obtain a long die life. The 

 balance of the body was made up of twenty-five per cent clay which 

 gave the desired density in both molded and fired states. With the 

 talc-wax compound, a long die life was obtained even with the close 

 tolerances required. As a result of the use of a combination of waxes 

 as a binder this composition had a low uniform shrinkage of approxi- 

 mately four per cent as compared to about ten per cent with most 

 dry pressed porcelains. In addition, variable shrinkage and warpage 

 resulting from drying strains were eliminated. 



In molding this body, the lubrication of die surfaces was found to 

 be critical because of the extreme thinness of the part. It was im- 

 practicable to apply a sufficiently exact amount of a liquid lubricant 

 to prevent the parts from either adhering to the dies or being weakened 

 from the absorption of the liquid. This problem was overcome by 

 tumbling the granulated molding material with a fraction of a per cent 

 of zinc stearate.^^ The stearate coated grains of material were then 

 molded without any additional die lubricant. 



Using the above composition, the process developed was as follows: 

 The talc and clay were thoroughly milled in a carbon tetrachloride 

 solution of the waxes. After drying, this mixture was disintegrated 

 and sized, after which the particles of compound were coated with 

 zinc stearate. The parts were then molded four at a time in a com- 

 mercial self-contained hydraulic press within an accuracy of ± 3 

 per cent of the total thickness and ± 1 per cent of the inside diameter. 

 After molding, any fins were removed and the parts trimmed within 

 ±1.5 per cent of the total thickness in a finning machine which was 

 an adaptation of a commercial automatic indexing head drill press. 

 In this machine, the parts were fed to a rotating end cutter by a 

 revolving indexing head and were held under this cutter by a vacuum 

 applied to the underside of the parts. Tungsten carbide cutters 

 were used to obtain long tool life. After finning, the parts were fired 

 in small trays in a continuous kiln. The parts were then individually 

 gauged for thickness, roundness and inside diameter and individually 

 inspected for cracks, flaws, and burrs. They were then examined 

 under a 10 to 1 glass for smoothness and regularity of inner edge 

 before being used in the assembly of the transmitter. 



"JW. J. Scott Patent 1,847,196, "Ceramic Article and Method of Making the 

 Same," March 1, 1932. 



