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.