PLASTICS AND METALS — SCRIBNBR 167 



tween these two sections is where molded plastics may have an even 

 chance. 



Other things being equal, the greatest advantage plastics have to 

 offer the prospective buyer is built-in color, and its twin brother, sur- 

 face permanence. In the case of plastics neither corrosion nor elec- 

 trolysis will alter what was bought and paid for. 



Die-cast or stamped metal is most certainly indicated when thin 

 sections must be rigid or bear any appreciable strain. Strong, thin 

 plastic parts can easily be made in limited shapes by using reinforce- 

 ments of laminates or fiber stocks, but they will not be rigid. On the 

 other hand, plastics can carry threads very nicely, as nicely indeed as 

 metals if there is enough "meat" around the thread to prevent its 

 cracking out. 



ENGINEERING FACTORS IN SELECTION 



Basically the first factor determining the choice of plastics or 

 metals is the available space or volume of the part. If it is relatively 

 small, plastics are almost automatically out unless their use is de- 

 manded by one of the six "exclusives" mentioned earlier. 



After the space factor comes cost. All plastics cost more than 

 metals, on a pound basis. The saving grace for plastics comes from 

 the advantage of their low specific gravities. (See table 1.) Even 

 magnesium at 1.80 is higher than the average range of 1.06 to 1.50 for 

 plastics. Perhaps, indeed, a correction of the usual strength values 

 for the specific-gravity effect may bring plastics up into the range of 

 the engineering metals. Unfortunately, to make any showing at all 

 for plastics we must choose the very best plastics material, and be 

 careful to select one of the weaker alloys for comparison. 



On this basis, for instance, let us make some tensile-strength com- 

 parisons. (See table 2.) Chrome-moly steel and stainless steel run 

 around 180,000 pounds per square inch, aluminum alloy 62,000, and 

 magnesium alloy 46,000. One of the strongest plastics we have is 

 Pregwood, impregnated plywood. Pregwood's tensile strength is 

 30,000 pounds per square inch. Other "strong" plastics are paper 

 laminate at 12,500 pounds per square inch, canvas laminate 9,500, and 

 wood-flour-phenolic molded parts 8,500. It is evident that plastics 

 offer no competition in direct comparisons of this type. 



Now suppose we set up our system of handicaps based on dividing 

 the strength figures by specific gravity so we can determine specific 

 tensile strength or tensile strength per pound. (Again see table 2, in 

 which the materials are listed in their order of merit in this respect.) 

 Immediately one of the plastics contestants, Pregwood, moves up to 

 third on the list at 23,000, behind magnesium alloy at 25,400, and 



