170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



Table 3. — Compressive strength/weight values of plastics and metals 



Material 



Canvas laminate 



Paper laminate 



Glass fabric laminate 



Impact phenolic, molded 



Wood-flour-phenolic, molded 



Asbestos laminate 



Magnesium alloy 



Chrome-moly steel 



Stainless steel 



Aluminum alloy 



Pregwood 



Ratio 



(canvas 



laminate = 



100) 



100 

 93 

 93 

 89 

 78 

 75 

 69 

 68 

 68 

 51 

 39 



Sheets of plastic cannot be drawn to new shapes as easily as metals, 

 but usually when odd shapes are wanted they can be created in plastics 

 before making the sheet itself; in other words, sheets of metals are raw 

 materials suitable for a great deal more working than sheets of plastics. 

 The plastics man starts one step back of the sheet, uses the same basic 

 material and pre-forms it to the desired shape instead of into the 

 sheet itself. 



There is one unfortunate characteristic of plastics that occurs also 

 in metals but not so badly, and that is creep. Creep (cold flow in this 

 case) is the constant change in dimension under stress. The thermo- 

 plastic plastics are strongly subject to it and therefore are avoided for 

 stressed service; in the thermosetting materials creep is present but 

 unless the material is used with practically no factor of safety it may be 

 discounted entirely. 



CLASSES OF PLASTICS 



In any comparison of plastics and metals it should be remembered 

 that there are many types and forms of "plastics.." All plastics are 

 divided into thermosetting and thermoplastic types. The first group, 

 the themosetting, are somewhat like concrete in their internal action. 

 As raw materials they will first soften under heat and then harden. 

 Practically no further softening will take place right up to the charring 

 point. A part once molded wrong might as well be scrapped, since the 

 material canot be used over again. (Like all technical statements, that 

 one must immediately be qualified. It has been reported that the H. J. 

 Heinz Co., of 57-variety fame, developed a liquid that would soften 

 a completely cured phenolic part, than which there is no more com- 

 pletely set item thinkable, so that the part could be manipulated by 

 hand, and would come back to its original hard state when dried. Also, 

 some of the airplane companies on the west coast have a process whereby 



