PLASTICS AND METALS — SCRIBNER 



169 



THE CASE FOR PLASTICS 



Faced by this story of comparative performance, why don't plastics 

 fold up and leave the field of engineering materials entirely to the 

 metals ? In ninety-nine cases out of a hundred, when a plastic material 

 is faced with a really severe service application of an engineering na- 

 ture it does fade right out of the picture — unless the stronger metal 

 has its own handicap (such as poor resistance to corrosion or even 

 electrical characteristics) that may be even more serious than the 

 mechanical deficiency of the plastic. 



Fortunately for plastics there are enough applications for which any 

 material will have enough strength if it can be dropped six times from 

 the height of a table to a concrete floor without breaking. Certainly 

 if you can design around compressive strength, almost any kind of 

 thermosetting plastic material, except plywood, will give the best re- 

 sults on a pound basis. Under tension, plywood is right up in the 

 front fighting it out with the leaders in the metal world on a pound- 

 for-pound basis, asking no quarter and giving none. That is the 

 reason the famous Mosquito bombers are made of plywood (actually a 

 different kind of wood for each part of the structure, birch in one spot, 

 boxwood in another, etc.). 



Plastics have a few outstanding service qualities where these pre- 

 viously mentioned measures of strength are not all-important. One 

 of these is abrasion resistance. Ammunition chutes of laminated 

 phenolic outwear steel. Plastic gears are not only quieter but wear as 

 well as steel. Under impact plastic sheets won't dent and gradually 

 deform before failure, but will just break all of a sudden, while metal, 

 although it will not fail completely, will get battered out of any usable 

 shape. Plastics will dampen vibration better than metals, too, which 

 means less noise, or the avoidance of synchronous beats set up in other 

 parts of a machine. 



Table 2. — Comparison of strength/ weight properties of plastics and metals 



Material 



Ratio 

 (magnesium 

 alloy =100) 



Magnesium alloy 



Stainless steel 



Pregwood 



Chrome-moly steel. 



Aluminum alloy 



Paper laminate... 



Glass fabric laminate 



Canvas fabric laminate 



Wood-flour-phenolic, molded 



Asbestos paper laminate 



Impact phenolic, molded 



100 

 93 

 90 

 90 

 87 

 36 

 36 

 28 

 24 

 21 

 21 



