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The Shore test has been used since 1907 to determine the hardness of a wide
variety
of rubber and soft plastics. Originally there were only 4 different scales for
rubbers.
However, now there are 12 scales to allow testing an even wider range of
materials from small
rubber O rings to very soft foam products. The testers that perform Shore tests
have been
commonly referred to as Durometers and the results frequently called Durometer
hardness.
With the exception of the M scale testers, all Durometers can be used either as
a portable
unit or in an operating stand. This flexibility adds greatly to the usefulness
of the Shore
scale.
Standards
Shore test methods are defined in the following standards:
- ASTM D-2240
- DIN 53 505
- ISO 7619 Part 1
- JIS K 6301*
- ASKER C-SRIS-0101
*Note: The JIS standard is very similar to the ASTM 2240
standard. However,
there are small but important differences.
Shore Test Method
The Shore test uses a hardened indenter, an accurately calibrated spring, a
depth indicator,
and a flat presser foot. The indenter is mounted in the middle of the presser
foot and extends
2.5mm from the surface of the foot. In the fully extended position the
indicator displays zero.
When the indenter is depressed flat even with the presser foot's surface,
the
indicator displays
100. Therefore, every Shore point is equal to 0.0025mm penetration (M scale is
0.00125mm).
In use the unit is placed on the sample so that the presser foot is held firmly
against the
test surface. The spring pushes the indenter into the sample and the indicator
indicates the
depth of penetration. The deeper the indentation the softer the material and
the lower the
indicator reading.
The different Shore scales, A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R are
created by using
7 different indenter shapes, 5 different springs, 2 different indenter
extensions an 2 different
presser foot specifications. The A and D scales are by far the most commonly
used. The M scale
uses a very low force spring and was developed to allow testing very small
parts like O rings
that can not be tested in the normal A scale. Because different materials
respond to the test
scales in different ways, there is no correlation between the different scales.
Applications
All Durometers except for the M scale units can be used as a portable device.
Test stands are
recommended for best accuracy and are required for M scale testing because of
its increased
sensitivity. Some stands have extra weights to make sure that the force on the
presser foot is
constant from test to test. Normally multiple tests are done on each sample and
the average
result is used.
Strengths
- Fast, easy to use
- Inexpensive
- Wide range of materials can be tested
- Non destructive, part can normally be used after testing
Weaknesses
- Dwell time variables can cause poor readings.
- Inconsistent force on the presser foot will cause errors.
- Difficulties keeping the indenter perpendicular to the test surface will cause
errors.
- The test surface must be large enough to support the presser foot.
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