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Rockwell
Hardness
Rockwell Hardness
can differentiate relative hardness of different types of
a given plastic. But since elastic recovery is involved
as well as hardness, it is not valid to compare hardness
of various kinds of plastic entirely on the basis of this
test. Rockwell hardness is not an index of wear qualities
or abrasion resistance. For example, polystyrenes have high
Rockwell hardness values but poor scratch resistance.
A steel ball under
a minor load is applied to the surface of the specimen.
This indents slightly and assures good contact. The gauge
is then set at zero. The major load is applied for 15 seconds
and removed, leaving the minor load still applied. The indentation
remaining after 15 seconds is read directly off the dial.
The size of the balls
used and loadings vary, and values obtained with one set
cannot be correlated with values from another set. ASTM
D-785.
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