Principle
A Shore hardness tester is an instrument that evaluates a material's hardness by measuring its ability to resist a specific presser needle. The principle of operation is based on a spring-loaded needle pressing into the surface of the material under standard pressure, which is converted into the corresponding hardness scale value by measuring the pressing depth or degree of resistance of the pressing needle. For polymers such as cast resins, their hardness after curing is a key index for evaluating their cross-linking density, mechanical properties, and end-use suitability. During measurement, the durometer needle is pressed into the resin surface under the action of a specified test force, and the elastic deformation and plastic deformation of the material together determine the depth of the press, resulting in the Shore hardness value. This value is usually correlated with the elastic modulus of the material, which can effectively reflect the curing state and mechanical properties of the resin.
Measurement method
Measuring the Shore hardness of cast resins follows a standardized testing process to ensure repeatability and comparability of results. Internationally commonly referenced standards include ASTM D2240 and ISO 868. Before testing, the sample should be conditioned for at least 24 hours in a standard temperature and humidity environment, usually under conditions of 23±2°C and 50±5% relative humidity. The surface of the sample should be flat, smooth, and bubble-free, with a thickness of not less than 6 mm, and the test points should be at least 12 mm away from the edge, and the distance between each test point should not be less than 6 mm. The hardness tester is placed vertically on the surface of the sample, and after the needle is in full contact with the surface, the maximum pressure is smoothly applied within 1 second, and the instantaneous hardness value is read within 1 second after pressing, or the reading after the specified time according to the standard requirements. At least five points are measured for each sample, taking the arithmetic average as the final result.
Applicability
Shore hardness testers are mainly divided into A-type, D-type and other scales, which are suitable for materials with different hardness ranges. For most cast resins, such as polyurethane, epoxy resin, etc., if the material is relatively soft, use the A-type scale; If the material hardness is high, close to hard plastic, it is advisable to use the D-type scale. Selection is based primarily on the material's expected hardness range and recommendations based on relevant standards. Improper scale selection can lead to measurements that exceed the valid range or insufficient resolution, affecting data accuracy. Therefore, the appropriate scale should be confirmed according to the estimated hardness of the resin or through pre-experiments before testing.
Influencing factors
The Shore hardness measurement of cast resins is influenced by a number of factors. Firstly, the degree of curing of the resin is crucial, and incomplete curing can lead to low hardness values. Secondly, ambient temperature and humidity affect the movement ability of polymer chains, and increasing temperature usually causes a slight decrease in hardness. When the sample thickness is insufficient, the underlying support can affect the needle pressing, resulting in high readings. Consistency in manipulation, such as pressure speed and timing of readings, can also introduce bias. In addition, the cleanliness of the needle, the calibration status of the spring, and the accuracy of the hardness tester itself are all fundamental to ensuring accurate measurements. Regular calibration of instruments using standard hardness blocks is necessary to maintain measurement reliability.
Analysis of results
Measurement data should be systematically recorded, and statistics such as mean, standard deviation, etc. should be calculated to evaluate the uniformity of the material and the repeatability of the test. The Shore hardness value itself is a dimensionless scale value used for relative comparison. During analysis, hardness data can be correlated with resin formulation (e.g., curing agent ratio, filler content) and curing process conditions (e.g., temperature, time) to optimize production parameters. The trend of hardness values can help determine the aging properties of resins or their stability in different environments. It is worth noting that Shore hardness mainly reflects the indentation resistance of the material's surface, which is an aspect of the comprehensive mechanical properties of the material, and usually needs to be fully evaluated in combination with other tests (such as tensile strength and wear resistance).
Notes:
To ensure measurement validity, pay attention to the following: Avoid testing on areas with curvature, texture, or unevenness on the resin surface; The hardness tester should remain stable and not shake during testing; For resins that may continue to cure slowly over time, test should be performed during a relatively stable period of time after curing is completed, and the time from completion of the test should be recorded; The same batch of samples should be tested with the same calibrated hardness tester to reduce system errors. The measurement report should clearly indicate the type of Shore hardness scale used, the test environmental conditions, the sample preparation method and the basis of the test standard.
Reference Standards
ASTM D2240-15, Standard Test Method for Rubber Property—Durometer Hardness.
ISO 868:2003, Plastics and ebonite — Determination of indentation hardness by means of a durometer (Shore hardness).
GB/T 531.1-2008, Vulcanized rubber or thermoplastic rubber - Press-in hardness test method - Part 1: Shore hardness test.