165 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



Outside this area the two types of materials are mutually exclusive — 

 each supreme in its own right. Plastics cannot be used where con- 

 tinuous heat above 400° F. will be applied. Plastics lack the surface 

 hardness of most metals, although many plastics seem to be better 

 under abrasion than many metals. Again, if only a small space is 

 available and the cross section therefore limited, metals are invariably 

 chosen because they provide the requisite strength. 



SOME BROAD DISTINCTIONS 



Plastics are beyond the competitive reach of metals where (1) 

 satisfactory electrical insulating quality is required; (2) a material is 

 needed that will waterproof textiles so as to leave them flexible enough 

 to use as raincoats, shower curtains, and the like; (3) transparency 

 is required; (4) a material is sought that is suitable for protection 

 against ordinary heat conduction in such simple applications as coffee- 

 pot handles, gas-stove handles, electric-iron handles and thousands 

 of like parts; (5) the material must feel warm and comfortable to the 

 touch, so that one's skin won't freeze to it in cold weather, and must 

 not get too hot to handle in very hot weather; and (6) a choice of 

 built-in colors is desired. 



If those were the sole bases of comparison between plastics and 

 metals, metals would already be at the start of a long decline. How- 

 ever the problem cannot be settled so simply in favor of either side. 

 Let us go back to the two most important fields where the choice may 

 be difficult — formed metals versus molded plastics and sheet metals 

 versus sheet plastics, and see if it is not necessary to draw some finer 

 distinctions. 



To begin with, not all metals can be formed with equal ease. As 

 with plastics, compromise is usually invoked. A certain alloy steel 

 may be ideal as to service quality but to get it in the shape needed might 

 involve so much cost as to make its specification unwise. If some 

 bright engineer found an airplane propellor design that needed a 

 metal so hard to work that it would cut the entire propeller production 

 of the country to one-tenth its present rate, it is unlikely that the 

 advantages accruing from its use would be considered enough to 

 justify the loss of production. 



Molded plastics need rather expensive steel molds, but so do metal 

 die-castings. Sand-cast metals offer a cheaper approach, but may 

 leave too much machining to be done to get the needed accuracy. 

 Plastics molded parts seem to fit into a middle band of accuracy. If 

 the part is really rough and a sixteenth of an inch means nothing, a 

 sand-casting is certainly indicated; if the tolerances, however, are 

 less than 0.001 inch, carefully machined metal is a better choice; be- 



