What Is a Rubber Hardness Tester?
A rubber hardness tester, as the name implies, is a device that measures the hardness of rubber.
Mainly, a measuring instrument called a durometer is used. This device measures the rubber hardness by the depth to which the needle is pushed into the sample when the sample is deformed by spring force.
Durometers can be classified into three types according to the hardness of the rubber to be measured: Type A, Type D, and Type E, which have different needle tip shapes. The hardness is determined by the force with which the needle is pressed against the sample and the area of the indentation.
Uses of Rubber Hardness Testers
Rubber hardness testers are used to evaluate the hardness of rubber. Specifically, they are used for car tires and erasers. In addition to rubber, it is also used to evaluate the hardness of elastomers and plastic products.
Principle of Rubber Hardness Testers
The most commonly used durometer for measuring hardness deforms a sample by pressing a needle against it under the force of a spring. At this time, the sample generates a repulsive force in response to the pressing force.
When both forces reach equilibrium, the amount by which the needle is pushed into the sample can be used to measure the hardness value. If the sample’s repulsive force is weak, the sample will be soft; if the repulsive force is strong, the sample will be hard.
Rubber Hardness Tester’s Information
1. How to Describe Hardness
The result of hardness measurement, not only for the durometer, is not a physical property value like weight and has no unit. It is the result of a certain method of testing, so it is necessary to describe the test method along with the numerical value. The measurement method and the notation of the results are defined in the standard.
The durometer type, indicated value, and time to reading are indicated in this way. (Omitted for measurements of 1 second or less.) Other standards use similar notation. It is important to check the notation method according to the intended test method.
The main standards that use the durometer are as follows:
- ASTM D2240-2005
Physical Properties of Rubber – Standard Test Method for Durometer Hardness - ISO 48-4
Rubber-Hardness Test Method with Pocket Mold - ISO 868-2003
Plastics – Durometer Test Method
2. Precautions for Measurement
When using the durometer, the following points should be noted.
- If the sample rubber is affected by temperature or humidity, it cannot be measured accurately.
- If the rubber hardness testers are pressed against the sample improperly, or if the surface of the sample is uneven or warped, accurate measurement will not be possible.
- Repeatedly measuring the same spot on the sample will result in lower hardness, so when measuring multiple locations, please measure at different points. Generally, it is recommended to separate the measurement points by at least 6 mm.
- Sample thickness also affects the measurement. Type A durometers require a thickness of 6 mm or more.