RIGIDITY AND DENSITY
When making a part out of polyurethane,
the first two properties to consider are usually rigidity and
Rigidity, which correlates somewhat with durometer (hardness), is
mostly determined by chemical composition, and can be anywhere from
that of the hard plastic shell of your computer mouse (about 85 Shore
D) to the supple softness of a bass worm lure (about 40 Shore A).
Density, as in pounds per cubic foot (pcf), is varied by
foaming--adding a blowing agent to the mix that gasses out during the
chemical reaction, producing a cellular foam structure--much the same
way bread rises from the CO2 that the yeast produces--and can be
anywhere from solid (which, for most urethanes, is around
70 pcf) to
feather-light like a foam ice bucket (around 2 pcf).
The chart below visually illustrates
the combined variability of rigidity and density. If you want to see
how this relates to specific applications, click on any of the
examples in the list.
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