276 HANDBOOK OF MECHANICAL DESIGN 



POWDERED METAL PRESSINGS 

 Design Factors 



Formability. — Direct pressure must be applied to the entire cross section of the part when mold- 

 ing. The amount of pressure required to obtain a required density in the compressed compact 

 depends upon the malleability of the metal powder used. 



Powdered metal materials have almost no lateral flow in the mold in response to pressures 

 applied axially, therefore reentrant angles cannot be molded in the compact. If reentrant angles 

 are required at planes normal to the axis, thej^ must be machined to shape by conventional methods. 



Hot pressing may be resorted to as a means of obtaining solid, pore-free compacts. With this 

 method, however, the operation is slow, also die and maintenance costs are higher. 



Size and Shape Limitations. — Capacity of press available determines the maximum cross-sec- 

 tional area that can be compacted. Pressures for compacting vary from 30 to 60 tons per sq. in. 



The working stroke of the press, the compression ratio of the powder selected, and the density 

 required all determine the length of part that can be compacted. Compression ratios range between 

 2 to 1 and 20 to 1 for various metal powders. Length is limited bj^ minimum density desired because 

 frictional losses prevent the compacting pressure from being uniformly transmitted throughout the 

 depth of the mold. 



Shapes are confined to simple contours without undercuts in surface parallel to the axis. 



Dimensional Tolerances. — Possible to hold ver}^ close tolerances in cross-sectional dimensions. 



Tolerances in axial dimensions must be more liberal than those in cross sections, because all 

 the variables add up in the length of the briquette or the sintered piece. 



Tolerances for concentricity depend largely upon the clearance that must be provided between 

 the force and the mold, since this clearance is likely to be all on one side when the compacting pressure 

 is applied. Eccentricity can be corrected by operatioas subsequent to sintering, such as swaging or 

 rolling, but this means additional cost. 



Physical Properties. — Tensile strengths depend upon unit pressures employed to briquette the 

 powders, the length of heat-treatment, and the care exercised in control of powder. 



With heat-treating and quenching, it is possible to produce from alloy powders, gears that have 

 higher strength, wear, and impact resistance than case hardened low carbon steel. 



Strength and density may also be improved by re-pressing or cold-working if the sintered piece 

 is sufficiently malleable. 



Design Advantages. — Parts having selected properties can be made. Two or more metal pow- 

 ders can be used to produce alloys which retain proportionately the individual characteristics of each 

 constituent. Many special properties can be obtained by incorporating nonmetallic ingredients 

 with the metal powder, but this I'educes strength. 



Economical for the production of parts which if made by other methods would involve con- 

 siderable cost for machining operations in comparison with the cost of the material, or where scrap 

 losses would be high. The more complicated the machining required by a piece made by other 

 methods, the smaller the quantity that would have to be produced from metal powders in order to 

 carry the expense for tools and equipment. 



