Specific gravity of plastic (the density {mass per unit volume} of any material divided by that of water at a standard temperature, usually 4 degrees C. Since water's density is nearly 1.00 g./cc., density in g./cc. and specific gravity are numerically nearly equal.) is one of the few properties where a single number provides an adequate description of a property. This is not to suggest that specific gravity does not change with temperature, but the effect is relatively minor until the material reaches the softening or melting point, and it is seldom a determining factor in part performance. There are a couple of benefits to knowing the specific gravity of a material. One of these has to do with the weight of the molded part. For a given volume of material, a compound with a lower specific gravity will produce a part with a lower weight. To put this another way, a given weight of raw material will make more parts in a resin with a lower specific gravity. This is important because while processors buy material by the pound or the kilogram, end users buy by the cubic inch or cubic centimeter.
Pocket Performance Specs for Thermoplastics provides twelve key properties for thousands of resin grades. Each property reported in this book (including specific gravity of plastic) can play a very important role in selecting plastic materials – especially if one understands a few basics and use scenarios for comparing. Buy it now for $59.95!
Specific gravity considerations also come into play in industries where weight savings is a key value-added feature. In the automotive industry, one of the key benefits of metal-to-plastic conversion is the fuel economy benefit of changing from steel, with a specific gravity of approximately 7, or aluminum, with a specific gravity of 2.7, to a polymer where the specific gravity usually ranges from 1-2. In aerospace this is an even more important consideration. Often engineers work in terms of strength-to-weight ratios.
Obviously, specific gravity is the key to the weight portion of this calculation. In high-performance markets, this factor is one of the reasons that the much more expensive carbon fiber is chosen over glass fiber. For example, this book shows that a 40% carbon fiber-reinforced polycarbonate is almost 7% lighter than the same polymer compounded with 40% glass but at the same time is 50% stiffer (modulus) and over 25% stronger (at room temperature, of course). Higher specific gravity materials can also wipe out an apparent cost saving. For example, replacing a $2.00/lb material with one that cost $1.60/lb may appear to an excellent decision until we realize that the "higher-priced" material has a specific gravity of 1.24 while the new material has a specific gravity of 1.57.
About
Michael Sepe Michael Sepe's involvement in the plastics industry
since 1977 lends credibility to his expertise in research, development and manufacturing
of plastics applications. Learn
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