Omar Khalid
The hardness of plastics is an important metric that determines material durability and suitability for applications. Testing the hardness of plastics helps to understand the sample's durability and ability to withstand pressure. The most common industry-standard tests for plastic hardness are the ASTM D785 and ISO 2039 tests, which describe methods for determining the hardness of plastics using a loaded ball indenter. The Rockwell hardness test is also a popular and non-destructive method for testing the hardness of plastics, which is performed in two stages to eliminate errors caused by surface irregularities and incorrect measurements. The Shore hardness scale is another reliable method for testing the hardness of rigid plastics, which is measured with a Durometer instrument.
| Characteristics | Values |
|---|---|
| Test Method | ASTM D785 and ISO 2039-1/-2 standards |
| Test Object | Hardness of plastics, hard rubber, and related electrical insulating materials |
| Test Instrument | Loaded ball indenter (weighted steel ball) or Rockwell hardness tester |
| Test Force | 49-980.7 N |
| Test Calculation | Surface area of indentation, hardness value |
| Test Result | Hardness value composed of two components |
| Test Application | Determining material durability, suitability for applications like food packaging or automobile interiors |
| Test Variations | Shore hardness scale (for softer plastics), Durometer instrument |
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What You'll Learn
Rockwell hardness test
The Rockwell hardness test is a popular and non-destructive method for determining the relative hardness of materials such as plastics. It is a quick and direct test that provides results in a matter of seconds without the need for secondary dimensional measurements. This test is used to assess how hard a sample is and to consider its durability and ability to withstand pressure.
The Rockwell hardness test is based on measuring the depth of an indentation made by an indenter, which can be a diamond cone or a hardened steel or tungsten carbide ball. The indenter is forced into the specimen using a combination of a preliminary minor load and a subsequent major load. The difference in depth between the zero datum position (before the application of the major load) and the maximum penetration position results in a Rockwell number that represents the hardness of the material.
The total test force in the Rockwell method is applied in two steps. The first step involves pressing the indenter into the specimen with a test preload up to indentation depth h0, which serves as the reference plane for the subsequent measurement. In the second step, an additional test force is applied for a defined dwell time, causing the indenter to penetrate the specimen to the maximum penetration depth h1. The sum of the test preload and additional test force is the total test force.
The Rockwell hardness (HR) can then be calculated using a formula defined in standards such as ISO 6508 and ASTM E18. The hardness value is expressed as a combination of a hardness number and a scale symbol representing the indenter and the loads used. The Rockwell scale consists of 30 different scales, each covering different hardness ranges and applications. The choice of indenter and test force depends on the characteristics of the material being tested, with softer materials typically tested using tungsten carbide balls and harder materials tested with diamond indenters.
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Ball indentation hardness
The ball indentation hardness test is specified in the ISO 2039-1 standard, which is often referred to as the ASTM D785/ISO 2039-1 standard. This standard describes the procedure for determining the hardness of plastics and related materials using the ball indentation method. The test is performed by applying a load to the indenter, typically in two stages, to eliminate the influence of surface roughness and measurement errors. The indenter is pressed into the specimen with a test preload, and then the full test force is applied for a specified dwell time. The test forces can be selected from 49 N, 132 N, 358 N, and 961 N, and the desired indentation depth after 30 seconds of application should be between 0.15 and 0.35 mm.
After the test, the indenter is removed, and the permanent indentation depth, also known as the depth difference, is measured. This depth is used to calculate the surface area of the indentation. Finally, the ball indentation hardness (HB) value is calculated using a formula defined in the standard. The HB value is represented by the two capital letters "HB," with "H" representing hardness and "B" representing ball indentation. For example, a hardness value of 45 HB would be read as "45 ball indentation hardness."
It is important to note that the test indentations must be positioned with sufficient distance from the specimen edge and between individual test indentations. A minimum distance of 10 mm is recommended to ensure accurate measurements. Additionally, the ball indentation hardness test is suitable for measuring both filled and unfilled plastics, making it a versatile method for evaluating the hardness of plastic materials.
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ISO 2039 standards
The ASTM D785 and ISO 2039 standards describe methods for determining the hardness of plastics, hard rubber, and related electrical insulating materials. The ISO 2039 standard specifically focuses on the ball indentation method for measuring hardness.
ISO 2039-1/-2 Standards
The ISO 2039-1 standard specifies a method for determining the hardness of plastics using a loaded ball indenter. The hardness value is calculated from the measured indentation depth, which is the depth of the indentation left by the indenter under load. The test indentations must be positioned with sufficient distance from the specimen edge and between individual test indentations (no less than 10 mm). This method is suitable for measuring both filled and unfilled plastics and can provide valuable data for research, development, quality control, and acceptance or rejection under specifications.
ISO 2039-2 Standard
The ISO 2039-2 standard specifies various Rockwell hardness scales for plastic hardness tests. The Rockwell method aims to eliminate the influence of specimen surface roughness and measurement errors caused by play in the indentation depth measurement. It involves applying the total test force in two steps and measuring the indentation depth at a defined preload. The hardness value according to ISO 2039-2 consists of two components and can be calculated using a formula defined in the standard.
ASTM D785 Standard
The ASTM D785 standard is often mentioned alongside the ISO 2039 standard and provides additional methods for determining the hardness of plastics. It specifies the use of a Rockwell hardness tester, which is suitable for testing hardness in the macro range with test forces ranging from 49 to 980.7 N. The ASTM D785 standard also differs from ISO 2039-2 in terms of the diameter of the indentation ball and the level of the test force used.
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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 involves measuring the depth of an indentation made by a loaded ball indenter, also known as a Rockwell hardness tester, on the surface of the plastic or material being tested. The hardness value is then calculated based on the measured indentation depth.
The ASTM D785 standard specifies the use of a Rockwell hardness tester, which applies a test force in the range of 49-980.7 N. This force is applied in two steps: a minor load and a major load. The minor load is applied first, and the gauge is set to zero. Then, the major load is applied by tripping a lever. After a set amount of time, typically 15 seconds, the major load is removed, and the specimen is allowed to recover for another period, usually also 15 seconds. Finally, the hardness is read from the dial with the minor load still applied. The hardness number is directly related to the indentation hardness, with higher numbers indicating harder materials.
The ASTM D785 standard is often used in conjunction with ISO 2039-2, which specifies various Rockwell hardness scales for plastic hardness testing. The primary difference between ASTM D785 and ISO 2039-2 is the diameter of the indentation ball and the level of the test force used. Additionally, ASTM D785 is equivalent to Procedure A of ISO 2039-2, while Procedure B of ASTM D785 is equivalent to the test method in the integral annex part of ISO 2039-2.
The ASTM D785 standard is suitable for testing the hardness of both filled and unfilled plastics, as well as hard rubber and related electrical insulating materials. It is important to note that the results obtained by this test method are not generally considered a measure of the abrasion or wear resistance of the plastic materials being tested. Additionally, the time factors involved in applying major and minor loads can significantly affect the results, especially for materials with creep and recovery properties.
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Durometer testing
There are several different durometer scales used for different types of materials. The three most common scales are shore 00, shore A, and shore D. Shore 00 is used to measure the hardness of extremely soft materials, such as gels and rubbers, shore A measures flexible rubbers that can range from very soft to hard, and shore D is used for hard rubbers and plastics. There are also other less common scales, such as shore OO for soft gels and foams, and shore O for textile windings.
The durometer test is performed by applying the durometer tool perpendicular to the surface of a flat sample of the material being tested. The durometer tool has a needle on a calibrated spring extending from one end, which is pressed into the surface of the material. The depth of the resulting indentation is then used to calculate the hardness value, which is displayed on the durometer scale.
The ASTM D785 and ISO 2039 standards describe methods for determining the hardness of plastics using a loaded ball indenter or a Rockwell hardness tester. The Rockwell method involves applying the total test force in two steps to eliminate the influence of the specimen surface roughness and measurement errors. The hardness value is then calculated using a formula defined in the standard.
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Frequently asked questions
Plastic hardness testing is a method of assessing how hard a plastic sample is. This data is then used to determine the sample's durability and ability to withstand pressure.
The most common tests to check the hardness of plastics are the Rockwell Hardness Test and the Shore Hardness Test.
The Rockwell Hardness Test uses a diamond cone or a weighted steel ball, also known as a loaded ball indenter, which is pressed into the plastic material at a specified force. The indentation depth is then measured and used to calculate the surface area of the indent and the hardness of the plastic.
The Shore Hardness Test is performed with a Durometer, an instrument with a spring-loaded foot that is propelled into the sample being tested. The penetration of the indenter foot and the time taken for penetration are used to determine the hardness of the plastic.
