Omar Khalid
Plastic hardness testing is crucial to determine the quality of different types of plastic materials. The hardness of a plastic material refers to its ability to resist irreversible indentation or perforation by a harder force or body. This ensures the durability and strength of the plastic, which is essential for its performance in various applications, such as industrial parts and consumer products. There are several standard methods for testing the hardness of plastics, including the Rockwell hardness test, Shore hardness test, and Vickers hardness test. These tests involve applying an external force or indentation to the plastic sample and measuring the depth and shape of the resulting concavity to determine its hardness.
| Characteristics | Values |
|---|---|
| Definition of hardness | A material's ability to resist irreversible indentation. |
| Importance of testing hardness | To determine the quality of plastic before industrial use. |
| Testing methods | Rockwell hardness test, Shore hardness test, Vickers hardness test, Ball indentation hardness test |
| Rockwell hardness test | An initial preload is applied, followed by a larger load, and then removed. The indentation is measured on a scale. |
| Shore hardness test | Uses a durometer with a sharp indenter made from hardened steel. |
| Vickers hardness test | N/A |
| Ball indentation hardness test | The hardness value is determined by measuring the surface of the indentation and then calculating the hardness from the measured indentation depth. |
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What You'll Learn
Rockwell hardness test
The Rockwell hardness test is a common method for determining the hardness of plastic materials. Hardness, in this context, refers to a material's ability to resist localized plastic deformation, or more specifically, its resistance to indentation.
The Rockwell hardness test involves applying a minor load, followed by a major load, to a test specimen. The minor load establishes the zero position, and the major load is applied and then removed while maintaining the minor load. The depth of penetration from this process is then measured, with a harder material giving a lower measure. The Rockwell hardness number is directly related to the indentation hardness of the plastic material—the higher the reading, the harder the material.
The test specimen must meet certain requirements. It must have a minimum thickness of 0.25 inches (6.35 mm) and be free from surface irregularities such as sink marks or protrusions. The specimen can be molded or cut from a plastic sheet, but it must have flat and parallel surfaces.
The Rockwell hardness test is performed in two stages to eliminate potential errors caused by surface irregularities and incorrect measurements due to indentation depth. A steel ball (the indenter) is pressed into the plastic material at a specified force, normally 9.8N, referred to as the preload. The preload force and the resulting indentation depth are critical to obtaining accurate results. The indentation depth should be between 0.15 and 0.35 mm, and the thickness of the test specimen should be at least ten times the depth of the indentation. Convex surfaces should be avoided as they yield lower readings.
There are several Rockwell hardness test scales, each employing different-sized steel balls and loads. The scales are denoted by letters, with E, M, R, and L being the most common for soft materials like plastics and rubbers. The result of the test is a dimensionless number, such as HRA, HRB, or HRC, where the last letter corresponds to the Rockwell scale used.
The Rockwell hardness test is widely used in industries such as automotive component manufacturing, where understanding the hardness and scratch resistance of materials is essential. However, it is important to note that this test does not provide a comprehensive assessment of a material's wear qualities or abrasion resistance.
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Shore hardness test
The two most commonly used durometer scales in Shore hardness testing are Shore A and Shore D. The Shore A scale is used for softer plastics, while the Shore D scale is used for harder plastics and elastomers. The Shore D indenter has a sharper point measured from the horizontal, allowing it to penetrate harder materials due to the smaller area over which the force is applied. The hardness values in both scales range from 0 to 100. The Shore OO scale is another durometer scale used to measure the hardness of materials, although it is not as commonly employed as the Shore A and Shore D scales.
The ASTM D2240 standard, also known as "Rubber Property—Durometer Hardness," is a specific type of Shore hardness test used for plastics and polymers. This test involves pressing the indentation tip against the material and then reading the hardness directly from the device's scale. The hardness value is determined by the amount of pressure needed to form an indentation at a particular depth. The ASTM D2240 test is frequently used to test composites and polymers and can be employed in accelerated aging tests to monitor the evolution of mechanical properties in relation to exposure.
The Shore hardness measurement procedure involves placing the test sample on a hard, flat, or rigid surface. Pressure is then applied to the durometer quickly and steadily, avoiding any sudden impacts or shocks. 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. If the needle on the durometer has penetrated the plastic to its maximum depth, the needle will indicate the material's hardness value on the scale.
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Vickers hardness test
The Vickers hardness test is one of the best options for testing the hardness of plastics. It is also referred to as the Vickers procedure or the microhardness test method. It was developed in 1921 by Robert L. Smith and George E. Sandland as an alternative to the Brinell hardness test. The Vickers hardness test is ideal for testing a wide range of materials, from hard materials like steel to softer materials like plastic.
The test involves dropping a load onto a diamond indenter, which creates an indentation on the surface of the material. The size of the indentation is then measured and converted into a hardness value. The Vickers hardness number (VHN) is determined by the ratio F/A, where F is the force applied to the diamond in kilograms-force and A is the surface area of the resulting indentation in square millimetres. The Vickers hardness number can also be converted into SI units by multiplying the hardness number in kilograms-force per square millimetre by the standard gravity and then dividing by 1000.
The Vickers hardness test is a useful method for testing plastic hardness because it can be used on a wide range of materials, including thin and small sections of test material. The test is also relatively simple, with a wide hardness range, and the indenters have a universal shape, which means the procedure takes less time in calibration. The diamond indenters are also quite resistant to deformation and do not require frequent replacement.
To ensure accurate results, the sample must have a flat and smooth surface and be small enough to fit the scale. The specimen's surface should be polished to help evaluate the size of the impressions. The Vickers hardness test blocks can also be used to properly calibrate the tester.
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Ball indentation hardness test
The ball indentation hardness test is one of the standard methods for determining the hardness of plastics, hard rubber, and related electrical insulating materials. This test is carried out in accordance with ISO 2039-1, which outlines the procedure for determining the hardness value through a ball indentation test.
The test force is applied in two stages, with a preload or pre-force applied first, followed by the main test force. The preload is applied with a force of 9.8 N to establish a reference level for the subsequent measurement of the indentation depth. The test force is then applied for a specified exposure time, with the indentation depth being measured at the end of this period. The force values for the main test force are 49 N, 132 N, 358 N, and 961 N, and the desired force value depends on the required indentation depth range. The acceptable range for indentation depth is between 0.15 and 0.35 mm, and if the depth falls outside this range, the test force must be adjusted accordingly.
The measured indentation depth is then used to calculate the surface area of the indent, and subsequently, the ball indentation hardness (HB) is calculated using a formula defined in the standard. The test indentations must be positioned with sufficient distance from the specimen edge and between individual test indentations (no less than 10 mm).
The ball indentation hardness test is suitable for measuring both filled and unfilled plastics and provides a strong average of the measured value due to the high test loads and deep indentations.
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Using a durometer
Durometer, or Shore durometer, is a standardised way to measure the hardness of materials like rubber and plastics. It is a specialised tool that evaluates a material's resistance to indentation, providing a numerical value on a standardised durometer scale. The durometer itself looks like a round tyre pressure gauge with a needle on a calibrated spring extending from one end.
To test for durometer hardness, an engineer places the needle against the plastic and applies pressure. The needle penetrates the material, and the measurement needle indicates the corresponding durometer hardness on the appropriate scale. The durometer scale ranges from 0 to 100, with lower numbers indicating softer plastics and higher numbers indicating harder plastics. For example, a 90A durometer polyurethane tubing is harder than a 70A polyurethane tubing.
There are different Shore hardness scales, with the two most common being Shore A and Shore D. Shore A is for softer materials, ranging from flexible rubbers to semi-rigid plastics with almost no flexibility. Shore D is for harder materials, including hard rubbers, semi-rigid plastics, and hard plastics. It is important to remember that durometer hardness does not directly correlate to the flexibility of the material.
When performing a Shore durometer hardness test, a flat sample of sufficient thickness is prepared to avoid the "anvil effect". The durometer is applied perpendicular to the surface, and the hardness value is immediately displayed. Analog models are simple and durable, while digital versions offer features like precise readings and data storage.
Durometer is an important factor to consider when manufacturing with plastic, as it helps engineers and product teams understand the mechanical specifications of each prospective material. It also helps determine whether a material is suitable for specific applications.
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Frequently asked questions
Plastic hardness is the material's ability to resist irreversible indentation, perforation, and wear.
Testing the hardness of plastics is important to determine the quality of the plastic material before it is used in industrial components or consumer products. The hardness of a plastic determines its durability and strength.
Some standard methods to test the hardness of plastics include the Rockwell hardness test, the Shore hardness test, and the Vickers hardness test. The ASTM D785 and ISO 2039-1/-2 standards also describe methods for determining the hardness of plastics using a loaded ball indenter.
The Rockwell hardness test uses a steel ball to create an indentation in the plastic. The depth and shape of the indentation, as well as the time taken to complete the test, are used to determine the hardness of the material.
