Ethan Clarke
The hardness of a material refers to its ability to resist indentation or deformation. In the context of plastics, hardness is typically measured using the Shore hardness scale, which provides a numerical value that indicates the force required to create an indentation. This scale includes Shore A for softer plastics and Shore D for harder plastics. Other methods for determining the hardness of plastics include the ASTM D785 and ISO 2039-1/-2 standards, which specify the use of a loaded ball indenter or a Rockwell hardness tester. These methods involve calculating hardness based on the depth of an indentation made in the plastic.
| Characteristics | Values |
|---|---|
| Definition of hardness | A material's ability to resist indentation or localized deformation |
| Measurement technique | Shore hardness scales (Shore A, Shore B, Shore D, Shore OO) |
| Shore hardness scale range | 0 to 100 |
| Shore hardness measurement procedure | Place the test sample on a hard, flat, or rigid surface. Apply pressure to the durometer quickly and steadily. Take hardness measurements at specified intervals. Measure the hardness at five different points on the test sample. Calculate and record the average of these measurements. |
| Durometer | An instrument used to measure the hardness of materials, specifically their resistance to indentation |
| Durometer composition | Calibrated spring and an indenter that is pressed into the material to form an indentation |
| Ball indentation hardness | HB (H = hardness, B = ball indentation) |
| Rockwell hardness | HR (calculated using a formula defined in the standard, taking into account the Rockwell scale used) |
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What You'll Learn
Measuring Shore hardness
Shore hardness is a widely used technique for assessing the hardness of plastics and other flexible materials. It is a depth measurement method that quantifies a material's ability to resist indentation or localized deformation. The hardness value is determined by measuring the true indentation depth of a standardised presser foot, rather than the permanent impression made by the indenter. This is done using a device known as a durometer, which consists of a calibrated spring and an indenter that is pressed into the material to form an indentation. The resulting number is then rated on the Shore hardness scale. The scale ranges from 0 Shore (2.5 mm indentation depth) to 100 Shore (0 mm indentation depth), with 0 representing the softest possible material and 100 representing the hardest.
There are several scales used to measure the hardness of different materials, including Shore A, Shore D, Shore 00, Shore B, Shore C, Shore 0, Shore 000, and Shore D0. Shore A is used for softer materials such as rubbers, silicones, and thermoplastic elastomers (TPEs), while Shore D is used for harder, more rigid materials, including hard plastics and harder elastomers with little flexibility. The Shore D scale is often used to measure the hardness of more rigid plastics and elastomers such as vulcanized rubber, hard TPEs, and thermoplastics like ABS, PC, and PVC.
The Shore hardness measurement procedure involves the following steps:
- Place the test sample on a hard, flat, or rigid surface.
- Apply pressure to the durometer quickly and steadily, avoiding any sudden impacts or shocks.
- Take hardness measurements at specified intervals, typically at 3 seconds and then at 15 seconds.
- Measure the hardness at five different points on the test sample to account for any variations.
- Calculate and record the average of these measurements.
The Shore hardness test is standardised under ASTM D2240 and ISO 48-4, which provide guidelines for measuring the hardness of rubber, elastomers, and plastics. The test is also covered by ISO 868, which focuses on measuring the indentation depth of a standardised specimen. These standards ensure that Shore hardness measurements are consistent and comparable across different materials and applications.
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The Rockwell hardness scale
The hardness of plastics is a measure of a material's resistance to indentation, scratching, and abrasion. It is a key indicator of durability and resistance to surface damage. Various scales can measure the hardness of plastics, with the Shore hardness scale being the most commonly used. However, the Rockwell hardness scale is also used to test harder plastics.
The Shore hardness scale, on the other hand, is a more versatile technique that can measure the hardness of very soft materials like thermoplastic elastomers (TPEs) and soft rubbers, as well as harder materials like thermoplastics and thermosets. The two most commonly used Shore scales are Shore A and Shore D. Shore A is used to measure the hardness of flexible and soft materials, including rubbers, silicones, TPEs, and softer thermoplastics and thermosets. Shore D, on the other hand, measures the hardness of harder, more rigid materials, including hard plastics and harder elastomers with little flexibility.
The Shore hardness measurement procedure involves placing a test sample on a hard, flat, or rigid surface, and then applying pressure to the durometer quickly and steadily, avoiding any sudden impacts or shocks. Hardness measurements are taken at specified intervals, typically at 3 seconds and 15 seconds. The hardness is measured at five different points on the test sample to account for any variations, and the average of these measurements is calculated and recorded. The durometer used in Shore hardness testing consists of a calibrated spring and an indenter that is pressed into the material to form an indentation.
While the Shore hardness scale is widely used for plastics, it is important to note that the Rockwell hardness scale is specifically designed for metals and harder materials. Therefore, when testing harder plastics, the Rockwell hardness scale may be more suitable.
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Ball indentation hardness
The hardness of plastics can be determined using a loaded ball indenter in accordance with standard ISO 2039-1 or ISO 2039-2. ISO 2039-1 describes the determination of the hardness value by means of a ball indentation test, while ISO 2039-2 describes the determination of the hardness value with a Rockwell hardness testing machine.
The total test force is applied in two stages to eliminate the impact of surface roughness and measurement errors. First, the indenter is pressed into the specimen with a test preload of 9.8 N up to an indentation depth of h0, which defines the reference plane for the subsequent measurement of the indentation depth (h). The test force is then applied for a specified exposure time, and the penetration depth is measured. The test force can be selected from the values 49 N, 132 N, 358 N, and 961 N, with the desired indentation depth after 30 seconds being between 0.15 and 0.35 mm. If the final indentation depth falls outside this range, the test force must be adjusted.
The measured indentation depth is used to calculate the surface area of the indent, and subsequently, the ball indentation hardness (HB) value. This value is represented as two capital letters, with H representing hardness and B representing ball indentation. For example, a hardness value of 45 HB means a hardness of 45.
Rockwell Hardness
The Rockwell hardness test is another method for determining the hardness of plastics and is described in ISO 2039-2. This test also uses a steel ball indenter, but the diameter of the ball and the test force differ from those used in the ball indentation hardness test. The Rockwell method involves applying the total test force in two stages, similar to the ball indentation hardness test, to eliminate the impact of surface roughness and measurement errors. The indenter is first pressed into the specimen with a test preload of 9.8 N to a penetration depth of h0, which serves as the reference level for measuring the indentation depth (h). The test force is then applied for a specified dwell time, and the Rockwell hardness (HR) is calculated using the residual indentation depth and a formula defined in the standard.
Shore Hardness
The Shore hardness scale is a widely used technique for assessing the hardness of plastics and rubbers. It measures the resistance of more flexible materials to indentation and provides a numerical value that indicates how much force is required to create an indentation. The two most commonly used Shore hardness scales for plastics are Shore A, which is used for softer plastics, and Shore D, which is used for harder, more rigid plastics.
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Durometer scales
The hardness of a material refers to its ability to resist indentation or localized deformation. This is often quantified using a durometer, a measuring device that uses specific, standardised techniques. Durometer measurement scales range from 0 to 100, with higher values indicating harder materials.
Durometer or Shore durometer is a standardised way to measure the hardness of materials like rubber (elastomers) and plastics. Shore durometers are measuring devices that use specific, standardised techniques for measuring hardness. The Shore hardness scale measures the resistance of a material to indentation, providing a numerical value that indicates how much force is required to create an indentation in the material. The hardness value is based on the indentation depth of the test.
There are two commonly used durometer scales: Shore A and Shore D. Shore A is used to measure softer materials, including rubbers, silicones, thermoplastic elastomers (TPEs), and softer thermoplastics and thermosets. Shore D is used for harder, more rigid materials, including hard plastics and harder elastomers with little flexibility. The indenter for Shore D has a sharper point, allowing it to penetrate harder materials.
The Shore hardness measurement procedure involves placing the test sample on a hard, flat, or rigid surface. Pressure is applied to the durometer quickly and steadily, and hardness measurements are taken at specified intervals. Measurements are taken at 3 seconds and 15 seconds, and the hardness is measured at five different points on the test sample to account for any variations. The average of these measurements is then calculated and recorded.
It is important to note that Shore hardness is specifically designed for plastics and rubbers, while other hardness testing methods like Rockwell or Brinell are more suitable for metals and harder materials. Comparing hardness numbers between different scales can be challenging due to the use of different instruments and testing methods.
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Testing procedures
Testing the hardness of plastic is a simple process, but it requires proper training to perform the test. The hardness of a material refers to its level of resistance to permanent indentation. There are many different techniques to measure hardness, and each test can give different results for the same material. Therefore, it is important to use the right test according to the nature of the material.
The plastic materials that are commonly used in hardness testing are molded parts and plastic sheets. Some common forms of plastic for hardness testing include thermo-polystyrene, cellulose acetate, Formica, thermoplastics, and Plexiglas. The thickness of the plastic can also affect the results.
There are three different types of hardness tests used for testing the hardness of plastic: D2240, ASTM D785, and E384/E92. In general terms, these three tests are known as shore-durometer-hardness, Rockwell hardness, and Knoop & Vickers test.
The Rockwell hardness test is a standardised procedure (ISO 2039) that is used to test hardness in the macro range with a test force between 49 and 980.7 N. This method is a depth-measurement method, which means that the permanent indentation depth caused by the indenter is measured. The Rockwell method applies the total test force in two stages, which eliminates the impact of the specimen's surface roughness and measuring errors caused by backlash in indentation depth measurement. The Rockwell hardness (HR) can be calculated using the residual indentation depth and a formula defined in the standard, taking account of the applied Rockwell scale.
The Shore hardness scale measures the hardness of various materials, providing a numerical value that indicates how much force is required to create an indentation in the material. Shore hardness is specifically designed to measure the resistance of more flexible materials to indentation. Shore hardness scales include Shore A, which measures the hardness of flexible and soft materials, and Shore D, which measures the hardness of harder, more rigid materials.
The ASTM D785 and ISO 2039-1/-2 standards describe methods for determining the hardness of plastics using a loaded ball indenter. The ISO 2039-1 standard uses a ball indentation test to determine the hardness value, while the ASTM D785 and ISO 2039-2 standards use a Rockwell hardness tester.
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Frequently asked questions
The hardness of plastic is measured using the Shore hardness scale, which indicates how much force is required to create an indentation in the material. The scale ranges from 0 to 100, with higher values indicating a harder material.
There are two commonly used Shore hardness scales for measuring the hardness of plastics: Shore A and Shore D. Shore A is used for softer plastics and rubbers, while Shore D is used for harder, more rigid plastics.
The Shore hardness of a plastic is measured using a durometer, which consists of a calibrated spring and an indenter that is pressed into the material to form an indentation. The test sample is placed on a hard, flat surface, and pressure is applied to the durometer steadily. Measurements are taken at specified intervals, and the average is calculated to determine the hardness value.
