Tech Tip #2

 

 

Question
I understand that durometer is a measure of rubber “hardness”. But what do the numbers mean, and what are some rules for picking a durometer for my applications?

 

Answer:
Durometer is a means of expressing the hardness of cured rubber components through either initial indentation, indentation after a specified period of time, or required readings from both methods.

The test method is based on the penetration of a specified type of indentor when forced into the material under specified conditions. Durometer readings are expressed in terms of a Shore “?” hardness; for example, Shore “A”.  The “?” can be A, B, C, D, DO, O, OO, and M. The geometry of the indentor and the applied force influences the measurements such that no simple relationship exists between the measurement obtained with one type of durometer and those obtained with another type of durometer or other instruments used for measuring hardness such as a Wallace.

 

Below is a picture taken from ASTM showing the difference in the indentors shape:

 

 

The more sharper an indentor is, will determine deeper penetration into the rubber sample.

A rounded point is used for the measurement of hardness on sponge, blown and/or very soft material.

The spring force is also modified to allow measurement of harder materials.

The scale of a durometer goes from 0 to 100 but is most accurate within the range of 20 to 80.  As one approaches the extremes of the range, the next level of durometer is selected to maintain within the accuracy range of the durometer.

 

The Shore “A” durometer is the standard for most rubber products.  Shore “D” is for rubber of high hardness, generally exceeding 80 Shore “A”.  Shore O, OO, and DO are used for blown and/or low durometer materials.  Shore “M”, also known as microhardness, is used for thin profile products.

A key point in durometer hardness testing is no simple relationships between indentation hardness and any fundamental property of the material being tested.

 

In ASTM D2000 and SAE J200, durometer is listed in grouping based on hardness ranges such as 30, 40, 50, 60, 70, 80, 90, and will always be listed as a denomination of 10.  The second part of ASTM and SAE will be the Tensile strength requirements in Mega Pascal (Mpa).  From that the elongation requirements will be listed but the actual formulation and performance will be based on actual product performance.

Durometer is a good quality control test once the proper material has been selected and verified.  Durometer is also good for estimating what range of properties an application may be looking for such as; 40 Shore D would indicate a material that will be very hard and will have short elongation but high modulus.  Such a material may be used as a vibration mount or a coupler.

 

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