432 BELL SYSTEM TECHNICAL JOURNAL 



rather than total porosity then becomes the problem, since, in sintering, 

 only the smaller pores may become closed. At present, the manufacturer 

 of metal powders cannot guarantee his particle size distribution, nor can 

 the user determine and specify exactly what he needs. The grades can 

 be approximated, only, and the t\^es required must be determined in an 

 empirical manner-^. 



The apparent density (or loading weight) is the ratio of weight in grams 

 to volume in cubic centimeters of powder, measured according to some 

 specified method of filling a designated receptacle. It is of considerable 

 practical importance since it has effect on several of the operations of powder 

 metallurgy, especially that of pressing the compact. The lower the apparent 

 density of a powder as compared with the actual density of the solid melal, 

 the greater will be the volume of powder required to produce a briquette 

 of given size. This necessitates deeper dies and longer plungers than for 

 denser materials, and for very low apparent densities may become a serious 

 design problem. Powders can usually be supplied in a range of densities, 

 and the proper powder selected for use. For proper blending and mixing 

 of different metal powders for producing solid metal parts, it is advisable 

 to select grades having comparable apparent densities. An example of 

 the use of a low-density copper powder may be cited. For the manufacture 

 of starting brushes in the electrical industry, copper powder and carbon 

 powder are mixed together and compressed. By using copper powder of 

 a low apparent density, approaching that of the carbon (1.2), good blending 

 is assured and the danger of segregation eliminated-^. 



Low rate of flow of metal powders interferes with automatic pressing 

 operations and may make it necessary to install vibrating equipment on 

 the feeder hopper or even on the die itself. Rate of flow is influenced by 

 particle size distribution, particle shape, and amount of absorbed moisture. 



2. Mixing 



When only one metal is to be pressed and sintered, there is usually no 

 necessity for mixing since the powder as received from the manufacturer 

 is generally well blended. Where several batches of the same metal of 

 different particle size distribution are to be added, or where different metal 

 powders are to be used, it is necessary to mix them thoroughly prior to 

 pressing and sintering. This may be done in any of the standard type mixers 

 with the precaution, in some instances, of providing against oxidation of 

 the powders. 



3. Pressing 



For preparation of the compacts, the pressing operation may be done 

 at either ordinary or elevated temperatures. The majority of parts pro- 



