SOME CERAMIC MANUFACTURING DEVELOPMENTS 263 



A study of the pressing behavior of the body under automatic 

 molding speeds and conditions indicated that development work would 

 be necessary to prevent the molded parts adhering to die surfaces. 

 An investigation of this factor indicated that the sticking to dies was 

 caused primarily not by adhesion between the metal and the molded 

 clay surface but rather by the vacuum effect of a dense air-tight layer 

 of material against the metal. This was shown by the facts that the 

 tendency for sticking decreased with (1) a decrease in the plastic 

 content of the body or a decrease in moisture content; (2) a decrease 

 in the viscosity of the die lubricant which thereby tended to clog the 

 pores of the molded surface to a less extent; and (3) an increase in the 

 volatility of the die lubricant. Two methods of overcoming the 

 sticking difficulties with molding compositions were therefore sug- 

 gested: (1) opening up the structure of the molded part by the use of 

 coarser material to provide capillaries for the escape of entrapped 

 air, and (2) the use of an improved lubricating compound. Since 

 it was not feasible to improve the lubricant sufficiently, an attempt 

 was made to obtain much coarser talc. The talc normally avail- 

 able at that time was such that on sieve tests approximately five 

 per cent to ten per cent remained on the 300-mesh screen. The 

 availability of coarser talc was investigated and it was found that 

 material coarser than eighteen per cent on 300-mesh was not available 

 at an economical price. In view of the fact that the talc was very fine 

 grained and non-plastic, it gave a very dense molded structure without 

 contributing materially to the strength required to hold the molded 

 part together. It therefore seemed advisable to use a clay, feldspar, 

 and silica body and to minimize abrasion by the selection of suitable 

 tool steels and the proper design of equipment. 



In arriving at a suitable body composition of the feldspar type it 

 was decided to use a composition which would mature at about cone 

 12 or 2390° F. Sufficient feldspar was used to obtain a low porosity 

 when fired over a reasonably wide temperature range. The amount 

 of clay used was governed by the raw strength required. Enough 

 silica was used to obtain sharp definite outlines and to avoid warpage. 

 The following composition was arrived at: 



Flint 22.5 



Feldspar 37.5 



Ball Clay 20.0 



Kaolin 15.0 



China Clay 5.0 



100.0 



