454 BELL SYSTEM TECHNICAL JOURNAL 



operation. Large presses are usually not of the automatic type, 

 which means hand operation with lower production rates and increased 

 cost. Low apparent density of most metal powders affects die 

 design and limits the thickness of parts produced. A compression 

 ratio of about 3 to 1 is generally assumed, which means mold depth 

 must be at least 3 times the thickness of the finished compact. 

 Other factors of die design are noted under item 7 of this section. 



5. The powder process is essentially one of mass production, and a 

 reasonable number of parts must, in general, be fabricated or the 

 costs per unit will be excessive. 



6. On a production basis, powder metal structural parts generally have 

 relatively low elongation, tensile strength and impact strength^^-^*-®^. 

 The mechanical properties of a sintered part depend to some 

 extent on its density, which itself is a function of the type of powder 

 used, the compacting pressure, and the sintering treatment. Because 

 of the voids normally present in powder metal parts, the ultimate 

 properties cannot be expected to be as good as those obtained on 

 cast and wrought materials^-. 



7. There are a number of design limitations for powder metallurgy 

 parts" ■« .61 .62. 



a. Sharp corners should be avoided and internal angles should have 

 fillets. 



b. Large and abrupt changes in thickness of parts should be avoided, 

 as should uneven cross sections. 



c. Re-entrant angles, grooves, and undercuts cannot be molded, 

 and if required, must be machined in an extra operation. Internal 

 and external threads, and holes at right angles to the central 

 hole or perpendicular to the axis of pressing, likewise cannot be 

 pressed, and must be machined. 



d. Length of pressed parts must be comparable to the cross-section 

 area because of pressing limitations. A long section may have 

 a soft central portion of low density. 



e. There is almost no flow of metal powders during compacting 

 because of friction between particles, and between particles and 

 the die walls'^'' •'^^^ 



8. Although powder parts can be produced to close dimensions by 

 careful control of the compacting and sintering operations and by 

 coining or re-pressing the sintered pieces, tolerances should, in general, 

 be fairly liberal if costs are to be kept down^^-^^. Close dimensional 

 tolerances may necessitate machining operations to meet specifications. 

 Eccentricity of cylindrical parts may be controlled fairly closely, 

 but concentricity may be troublesome because there must be clear- 



