Omar Khalid
Determining the hardness of plastics is essential for engineers and quality professionals to consider the material as a possible substitute for metal. Plastic hardness testing is crucial to ascertain the different types of plastic material quality. There are several methods to test the hardness of plastics, including the Rockwell hardness test, Shore hardness test, Vickers hardness test, ASTM D785, and ISO 2039-1/-2 standards. These tests involve applying an external force or load to the plastic and measuring the depth and shape of the resulting indentation to determine its hardness. The Shore hardness test uses a spring-loaded needle, while the Vickers hardness test uses a diamond-shaped indenter. The Rockwell hardness test is widely recognized and suitable for harder plastics, with variations in the testing procedure and preload or test force applied. The Vickers hardness test is similar to other testing methods but offers a sharp indenter made from hardened steel. The ASTM D785 and ISO 2039-1/-2 standards specify various Rockwell hardness scales and describe methods for determining hardness using a loaded ball indenter or Rockwell hardness tester.
| Characteristics | Values |
|---|---|
| Hardness testing methods | Rockwell hardness test, Shore hardness test, Vickers hardness test, Durometer hardness test |
| Rockwell hardness test | An external force is applied in two stages to eliminate the impact of specimen surface roughness and measuring errors caused by backlash in indentation depth measurement. The indenter is a steel ball, with different diameters depending on the method. |
| Shore hardness test | Uses a spring-loaded needle to apply force and penetrate the plastic. The Shore hardness scales used for measurement are usually Shore A and Shore D. |
| Vickers hardness test | Similar to other testing methods, but the indenter is diamond-shaped. Measurements can be done both during and after the load is applied. |
| Durometer hardness test | A standardized way to measure the hardness of plastics and other materials like rubber. Durometer measurement scales range from 0 to 100, with higher numbers indicating harder materials. |
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What You'll Learn
Rockwell hardness test
The Rockwell hardness test is one of the most widely recognized groups of hardness scales, the other being the Shore hardness test. The Rockwell hardness test is used to compare the hardness of metals, but there are scales suitable for harder plastics such as polycarbonate (PC), nylon (PA), and acetal (POM). There are thirty different Rockwell hardness scales, and the Rockwell hardness number is directly related to the indentation hardness of the plastic material—the higher the reading, the harder the material.
The Rockwell hardness test is carried out using the Rockwell hardness tester. The test specimens are placed on a steel anvil and are brought into contact with a minor load (10 kg), which for testing plastic materials is a steel ball. The dial is adjusted to zero under the minor load, and a major load (60 or 100 kg) is released, which causes the ball to indent into the plastic test specimen. After 15 seconds, the major load is removed, and a partial recovery from the indentation takes place. Then, after another 15 seconds, the hardness is read on the gauge (dial) of the instrument with the minor load still applied while the surface recovers. The distance between the initial and final indentations is used to calculate the Rockwell hardness.
The Rockwell hardness test is similar to the Vickers hardness test, which is the most common test for hard metals. The Vickers hardness test involves a diamond-shaped indenter that can be measured both when the load is applied and after it is removed. The Rockwell hardness test is also similar to the Knoop hardness test, which is used when the anisotropy of the material is of interest.
The Rockwell hardness test is a useful method for testing the hardness of plastic. Plastic hardness testing is crucial to ascertain the different types of plastic material quality, and the type of material determines the method to avoid measurement errors and reading errors.
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Shore hardness test
The Shore hardness scale typically uses either the Shore A or Shore D scale, which range from 0 to 100. Shore A is used for softer plastics, while Shore D is used for harder plastics. The Shore D indenter has a sharper point, which allows it to penetrate harder materials. The Shore hardness test can also be used to measure the hardness of other materials, such as rubber, elastomers, and silicones.
The durometer is the instrument used to measure the hardness of materials, specifically their resistance to indentation. It consists of a calibrated spring and an indenter that is pressed into the material to form an indentation. The hardness value is then displayed on a scale, with higher values indicating harder materials. The durometer measurement is dimensionless, meaning there is no unit of measurement. Instead, the durometer number represents a relative comparison of hardness between different but similar materials tested using the same scale, device, and standard.
To perform a Shore hardness test, the test sample should be placed on a hard, flat, or rigid surface. Pressure should then be applied to the durometer quickly and steadily, without any sudden impacts or shocks. The hardness should be measured at five different points on the sample, and the average of these measurements should be calculated. The needle on the durometer will indicate the material's hardness value on the scale.
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Vickers hardness test
The Vickers hardness test is a versatile hardness test method used for both macro and micro hardness testing. It is suitable for a wide range of applications and materials, including harder plastics. The test was developed in 1921 by Robert L. Smith and George E. Sandland during their work at Vickers Ltd. It was designed as an alternative to the Brinell hardness test, but with a simpler scale.
The Vickers hardness test uses a diamond-shaped indenter, which is dropped onto a sample with a flat surface, leaving a mark. The depth and shape of the indentation determine the hardness of the material. The Vickers hardness number (VHN) is calculated by measuring the diagonal lengths of the impression left by the indenter and using a table or formula to convert these measurements to a Vickers hardness number. The corresponding unit of VHN is the kilogram-force per square millimeter (kgf/mm2) or HV number. To convert the Vickers hardness number to SI units, the hardness number in kgf/mm2 must be multiplied by standard gravity and divided by 1000 to get the hardness in GPa.
The Vickers hardness test is ideal for testing a wide range of materials, from hard materials like steel to softer materials like plastic. It is also relatively easy to use, as the indenter is the same for all materials, and the calculations are independent of the size of the indenter. The test is performed on a universal or micro hardness tester, and the sample must meet the test specifications to yield accurate results.
When performing the Vickers hardness test, it is important to consider the spacing between indentations to avoid misinterpretations of perceived hardness. The standards prescribe a minimum diagonal width of 20 µm for accurate reading of the indents. The Vickers hardness test is a useful procedure for determining the suitability of a material for a specific purpose, as the hardness of a material is a critical factor in its structural integrity.
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ASTM D785 standard
ASTM D785 is a standard test method for determining the Rockwell hardness of plastics and electrical insulating materials. It is a widely recognised hardness scale that is used to compare the hardness of metals and harder plastics such as polycarbonate (PC), nylon (PA), and acetal (POM). The test method involves applying an external force to a test sample using an indenter, typically a steel ball, and measuring the depth of the resulting indentation.
The ASTM D785 standard covers two procedures for testing the indentation hardness of plastics: Procedure A and Procedure B. Both procedures use a Rockwell hardness tester, but they differ in the way the indentation is measured. Procedure A measures the indentation remaining 15 seconds after a given major load is released to a standard 10-kg minor load. Procedure B measures the indentation of the indenter into the specimen after a 15-second application of the major load while the load is still applied.
The Rockwell hardness number is derived from the net increase in depth impression as the load on the indenter is increased from a fixed minor load to a major load and then returned to a minor load. The hardness number is always quoted with a scale symbol representing the indenter size, load, and dial scale used. Each Rockwell scale division represents 0.002 mm (0.00008 in.) vertical movement of the indenter. A higher Rockwell hardness number indicates a harder material.
The ASTM D785 standard is similar to the ISO 2039-2 standard, which also describes a method for determining the indentation hardness of plastics using a Rockwell hardness tester. However, the ASTM D785 standard differs in the diameter of the indentation ball and the level of the test force used. It is important to note that the ASTM D785 standard does not address all safety concerns associated with its use, and it is the responsibility of the user to establish appropriate safety and health practices.
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ISO 2039-1/-2 standard
The ASTM D785 and ISO 2039-1/-2 standards outline methods for determining the hardness of plastics, hard rubber, and related electrical insulating materials. The ISO 2039-1 standard specifies a ball indentation method, where a loaded ball indenter is used to create an indentation on the surface of the material, and the hardness value is calculated from the measured indentation depth. The ball indentation hardness (HB) is calculated using a formula defined in the standard. This method is suitable for measuring both filled and unfilled plastics.
The ISO 2039-2 standard, on the other hand, specifies the use of a Rockwell hardness tester to determine the indentation hardness of plastics. This method involves using a Rockwell hardness tester with different scales, such as M, L, and R, to measure the net increase in the depth of impression as the load on the indentor is varied between a fixed minor load and a major load. The hardness value according to ISO 2039-2 is made up of two components. This method is used to test hardness in the macro range with a test force between 49-980.7 N.
The test indentations must be positioned with sufficient distance from the specimen edge and between individual test indentations (no less than 10 mm). The high test loads and deep indentations allow for measurement across a large test surface and provide a strong average of the measured value. The Rockwell hardness tester is particularly useful for testing harder plastics such as polycarbonate (PC), nylon (PA), and acetal (POM).
In addition to the ISO 2039-1/-2 standards, other methods for measuring the hardness of plastics include the Shore hardness test and the Vickers hardness test. The Shore hardness test uses a spring-loaded needle to penetrate the plastic, and the hardness is estimated by measuring the indentation depth. The Vickers hardness method is similar, but the indenter is diamond-shaped, and measurements can be taken during and after the load is applied.
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Frequently asked questions
Plastic hardness testing is a process that determines the quality of different types of plastic materials.
The Rockwell hardness test, Shore hardness test, and Vickers hardness test are the most common methods for testing the hardness of plastics.
The Rockwell hardness test uses a steel ball to apply force to the centre of a test sample. The depth of the indenter's penetration determines the level of hardness.
The Shore hardness test uses a spring-loaded needle to apply force to the plastic. The depth of the needle's penetration determines the hardness of the plastic.
A durometer is a standardised way to measure the hardness of materials like rubber and plastics. The Shore durometer is a measuring device that uses specific, standardised techniques for measuring hardness.
