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Hardness

Shore (Durometer) Hardness Testing of Plastics


The hardness of plastics is most commonly measured by the Shore® (Durometer) test or Rockwell hardness test. Both methods measure the resistance of plastics toward indentation and provide an empirical hardness value that doesn't necessarily correlate well to other properties or fundamental characteristics. Shore Hardness, using either the Shore A or Shore D scale, is the preferred method for rubbers/elastomers and is also commonly used for 'softer' plastics such as polyolefins, fluoropolymers, and vinyls. The Shore A scale is used for 'softer' rubbers while the Shore D scale is used for 'harder' ones. Many other Shore hardness scales, such as Shore O and Shore H hardness, exist but are only rarely encountered by most people in the plastics industry.

The Shore hardness is measured with an apparatus known as a Durometer and consequently is also known as 'Durometer hardness'. The hardness value is determined by the penetration of the Durometer indenter foot into the sample. Because of the resilience of rubbers and plastics, the indentation reading my change over time - so the indentation time is sometimes reported along with the hardness number. The ASTM test method designation is ASTM D2240 00 and is generally used in North America. Related methods include ISO 7619 and ISO 868; DIN 53505; and JIS K 6301, which was discontinued and superceeded by JIS K 6253.

The results obtained from this test are a useful measure of relative resistance to indentation of various grades of polymers. However, the Shore Durometer hardness test does not serve well as a predictor of other properties such as strength or resistance to scratches, abrasion, or wear, and should not be used alone for product design specifications. Shore hardness is often used as a proxy for flexibility (flexural modulus) for the specification of elastomers. The correlation between Shore hardness and flexibility holds for similar materials, especially within a series of grades from the same product line, but this is an empirical and not a fundamental relationship.



Durometer Hardness- Shore A & D (ASTM D2240)


SOP

Varify that the Type A transducer is attached and level.
   2.   Turn the Durometer Hardness System Model 716 on and allow to warm up about 15 minutes.
   3.   Select the proper elasped time setting for the test. One second is the standard setting.
   4.   Raise or lower the the transducer arm to accomodate  the specimen, placing the indentor about 1/8" above the specimen.
   5.   For Type A transducer ( softer material) use only the small weight.
   6.   Place the weight on the shaft  the transducer is attached to.
   7.   Check all specimens carefully for proper contact with the indentor.
   8.   To test a specimen, place the specimen under the indentor foot and press down swiftly on the platform lever and hold until the reading is complete.  
   9.    Record 5 reading from various specimens and average results.

* 1.   Switch from Type A to Type D
  2.  Remove Type A by loosening the black bolt holding the transducer. Replace Type A with Type D.  Leave bolt slightly loose.
   3.   Use the large weight to help in leveling the transducer by placing the weight on the platform and lowering the  indentor foot into the hole in the weight.
   4.   This should level the transducer.
   5.   Type D has its own power supply, so the Model 716 does not have to be on.
   6.   Connect the power supply to the transducer and allow to warm up about 15 minutes.
   7.   The time elasped setting is on the left side of the transducer. Press the button in to change the settings.
   8.   Lower the transducer to about 1/8" above the specimen.
   9.   The Type D uses both the large and small weights. Put the small weight on first.
  10.  Check all specimens carefully before testing, ti insure proper contact with indentor.
  11.  To test a specimen, place specimen under indentor and press down swiftly on the platform lever until elasped time is complete.
  12.   Record 5 reading from various specimens and average results.



Rockwell Hardness (ASTM D785)

 The Rockwell method is generally used to measure hardness of metals and plastics. Measurements on plastics are based on the amount of permanent deformation as the result of penetration of a steel ball under high load for 15 seconds. Different diameter balls are used for materials with different hardness ranges. Each unique set-up is designated by a letter. Most polymers are covered by the R, M and E scales. Rockwell can differentiate between the hardness of various polymers, but it does not take the difference in elastic component of polymers into account. Determinations of hardness based solely on Rockwell are risky. Rockwell hardness is also does NOT provide a measure of relative wear, abrasion, or scrPDCh resistance.