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Analysis of Causes of Hardness Deviation in TPE / TPR Testing of Elastomers

Hits:Updated:2020-05-21【Print】

Hardness is an important performance parameter of thermoplastic elastomers TPE and TPR. The hardness of TPE and TPR materials for general products is required to be within the standard hardness range of 3 to 5 degrees (A). Some products with high requirements for soft hardness, such as headphone cables and soft TPE products (such as ultra-soft sex toys with a hardness of 0 ~ 15A), even require that the hardness of the TPE material be within the standard hardness ± 2 degrees (A). The determination of hardness is very important for the confirmation of new product TPE and TPR material samples. Sometimes, the inaccurate grasp of the hardness of the material sample may cause the inaccuracy of the hardness of the sample material, which may cause the hardness of the sample to fail to meet customer requirements.

The TPE / TPR sample hardness test is inaccurate.

1. Factors of Shore hardness tester

1.1 Influence of the length of the pressure needle on the test results

The standard stipulates that the height of the pressure pin of the Shore A hardness tester to be exposed to the pressure surface is 2.5 mm. In the free state, the pointer should point to zero. When the pressure pin is on a smooth metal plate or glass, the instrument pointer should point to 100 degrees. If the indication is greater than or less than 100 degrees, the exposed height of the pressure pin is greater than or less than 2.5mm or less than 2.5mm. In this case, it should stop. Use for correction. When the exposed height of the pressure pin is greater than 2.5mm, the hardness value measured is high.

1.2 Influence of the shape of the end of the pressure needle on the test results

Under the long-term action, the end of the pressure pin of the Shore A hardness tester causes wear, causing its geometric size to change, which affects the test results. The diameter of the end after wear is larger. The measured result is also large because of its unit area. Due to different pressures. The larger the diameter is, the smaller the pressure is. The measured hardness value is too large, otherwise it is too small.

2. Ambient temperature factor

Rubber is a polymer material, and its hardness value changes with the change of the environment. The higher the temperature, the lower the hardness value. Different rubbers have different degrees of influence. For example, natural rubber with a slow crystallization speed has a smaller effect on temperature, while neoprene and styrene-butadiene rubber have a significant effect. The same formula of TPE and TPR materials is about 5 degrees (A) harder in winter than in summer.

3. The effect of reading time

The reading time of the Shore A hardness tester has a great influence on the test results. After the pressure pin and the sample are pressed, the reading is immediately taken and the pointer is stabilized, and then the reading is different. The results obtained are very different, the former is high, and the latter is low. More significantly, this is mainly caused by the creep of the rubber compound after compression. General specimens should be read immediately after compression (Note).

4. The influence of the thickness of the sample (test piece)

The hardness of the measured TPE material is related to the measurement method and the thickness of the part. The same TPE model material, the larger the thickness of the product, the harder it feels, although the thickness is actually independent of the hardness of the material. The ASTM method stipulates that the thickness of the test sample is 0.250 inches (approximately 6.35mm thickness of the test piece), which is the range where the hardness is not affected by the thickness.

Note: Most materials can withstand the initial pressure, but over time, they will yield due to creep and relaxation. The hardness tester's reading can be taken immediately or after a certain delay time, usually after 5 to 10 seconds. Instant readings usually show higher (or harder) readings than delayed readings. Delayed readings are more representative of both the hardness of the material and its resilience. Weak, less elastic materials are more prone to creep than stronger, more elastic materials.

related information:

1. The penetration depth of the pressure needle.


The most commonly used instrument for measuring hardness is called a Shore hardness tester. The instrument measures penetration depths from zero to 0.100 inches (approximately 2.5mm). A zero reading on the scale means that the indenter penetrates the extreme depth. Similarly, a reading of 100 indicates no penetration of the indenter.
 
2. Selection of hardness tester for materials with different hardness ranges.
 
There are three types of Shore hardness testers, namely Shore A hardness tester, Shore D hardness tester, and Shore OO hardness tester. Shore A hardness tester A, generally the hardness range of 30 ~ 90A is relatively accurate. If the hardness is lower than 30A, use the Shore OO test. If the hardness is higher than 90A, use the Shore D standard test. The test value is more accurate (see below). Figure).
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