Omar Khalid
Hardness is a material's resistance to penetration or permanent indentation. It is a critical factor when selecting a material for a specific application. Hardness can be measured in various ways, including scratch hardness, indentation hardness, and rebound hardness. Scratch hardness measures the resistance to fracture or permanent deformation due to friction from a sharp object. Indentation hardness measures the depth of indentation created by a given force. Rebound hardness is determined by measuring the frequency of an oscillating rod. In the context of plastics, hardness is particularly important for understanding their behaviour and suitability in different applications. Durometer or Shore durometer is a standard method for measuring the hardness of plastics and rubbers. The Rockwell method is another widely recognized approach for testing the hardness of plastics, specifically harder plastics like polycarbonate and nylon. These techniques provide valuable insights into the characteristics of plastics, aiding in their selection for specific applications.
| Characteristics | Values |
|---|---|
| Definition | Hardness is a measure of the resistance to localized plastic deformation, such as an indentation or a scratch induced mechanically by pressing or abrasion. |
| Hardness Tests | Durometer Shore Hardness Scale, Ultrasonic Contact Impedance (UCI) method, Rockwell hardness test, Mohs scale |
| Hardness Scales | Rockwell, Shore (durometer) |
| Hardness Numbers | Measurements made using specific testing equipment and testing methods |
| Hardness of Plastics | Determined using a loaded ball indenter or a Shore hardness gauge in accordance with standard ISO 2039-1 or ISO 2039-2 |
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What You'll Learn
Durometer Shore Hardness Scale
Hardness is a measure of a material's resistance to indentation or penetration by a harder object. It is also a measure of the resistance to localized plastic deformation. The toughness of a material is the maximum amount of energy it can absorb before fracturing, which is different from the amount of force that can be applied.
Durometer or Shore durometer is a standardized way to measure the hardness of non-metallic materials like plastics, rubbers, and elastomers. The scale was defined by Albert Ferdinand Shore, who developed a device to measure hardness in the 1920s. The Durometer term is often used to refer to both the measurement and the instrument. The device is called a 'Shore Hardness' gauge and looks like a round tire pressure gauge. The needle is placed against the material and pressure is applied. The depth of the indentation created by the needle is then measured and compared against a scale to determine the material’s hardness.
There are about 12 to 15 different Shore hardness scales, with the three most commonly used being Shore 00, Shore A, and Shore D. The Shore 00 scale is used for very soft materials, Shore A is for softer, more flexible materials, and Shore D is for semi-rigid plastics and harder materials with minimal flexibility. The scales overlap but are independent of each other. The Shore hardness value does not provide direct information about other properties like strength or resistance to scratches, abrasion, or wear.
The Shore durometer is generally preferred for testing the hardness of plastics and rubbers because it is fast and easy to measure. The hardness of a material is important to consider when choosing it for a specific application.
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Scratch hardness
Hardness is a measure of the resistance of a material to indentation or scratching. In materials science, hardness is defined as the resistance to localized plastic deformation, such as an indentation or scratch, induced mechanically by pressing or abrasion.
There are various methods to test scratch hardness, the most common being the Mohs scale, which classifies materials from 1 to 10, with talc at 1 and diamond at 10. Each material on the scale can scratch the materials that have a lower Mohs number. However, the Mohs scale is not very useful for classifying plastic materials, as most common plastics fall in the 2-3 Mohs range. Nevertheless, the basic technique of scratch hardness can be used to compare different plastic materials by testing their ability to scratch one another. Another method of testing scratch hardness is the pencil test, where a pencil is clamped at a 45-degree angle to the surface to be tested and then slid across it. The first pencil whose tip does not leave a noticeable scratch is considered the characteristic hardness rating. For example, PLEXIGLAS® has a 5H hardness rating, which is superior to many other plastics.
In addition to scratch hardness, there are other types of hardness tests and scales, such as the Rockwell and Shore (durometer) scales. While Rockwell hardness is typically used for metals, there are scales suitable for harder plastics. Shore durometers are generally preferred for rubber and plastic hardness because they are fast, easy to measure, and widely adopted. Durometer measurements range from 0 to 100 and are a relative comparison of hardness between different but similar materials tested using the same durometer scale and device.
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Ultrasonic Contact Impedance (UCI) method
The Ultrasonic Contact Impedance (UCI) method is a portable hardness testing technique that can be used to measure the hardness of a variety of materials, including metals, alloys, plastics, and rubber. It is a non-destructive testing method as it creates microscopic indentations that are only visible under a high-powered microscope. The UCI method was invented by Claus Kleesattel and has been used in the metal-processing industry for over 50 years.
The UCI method involves using a probe with a rod that has a Vickers diamond attached to the contact end. This rod is vibrated at an ultrasonic frequency of about 70-78 kHz using piezoelectric ceramics. When this rod is pressed against the surface of the material with a predetermined force, it produces a small indentation. The depth and shape of this indentation depend on the hardness of the material and the force applied. As the diamond penetrates the material, there is a shift in the resonant frequency of the rod, which is used to calculate the hardness value. Harder materials result in shallower indentations and an increase in the resonant frequency.
The UCI method offers several advantages over traditional hardness testing methods. It can be used in situations where traditional methods are not possible or useful, such as on challenging surfaces or when testing heavy and/or immovable parts. It is also suitable for finished products or precision components as the indentations produced are often microscopic and less invasive. The UCI method is easy to align and adjust, and it provides highly reproducible results, even in the case of unsymmetrical test indentations.
The UCI hardness testing devices consist of a measurement probe (manual or motorized) and a testing device to assess, save, and compare measured values. Tests can be carried out in all directions, in tight spaces, and on challenging material geometries or difficult-to-reach locations. The results are displayed digitally and can be converted into common hardness scales such as Vickers, Brinell, or Rockwell through calibration processes.
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Rockwell hardness scales
Hardness is a material's resistance to being penetrated or permanently indented. It is measured in different ways, including scratch hardness, indentation hardness, and rebound hardness. Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object. Indentation hardness, on the other hand, measures the depth of indentation in the material created by a given force.
Durometer or Shore durometer is a standardized way to measure the hardness of materials like rubber (elastomers) and plastics. Shore durometers are generally preferred for referencing rubber and plastic hardness because they are fast and easy to measure. The Shore hardness scale measures the resistance of a material to indentation. A Shore hardness gauge has a needle on a spring protruding from one end. The needle is placed against the plastic and pressure is applied. Once the gauge is pressed firmly against the material, the measurement needle will indicate the corresponding hardness measurement.
The Rockwell hardness test is another commonly used test for plastics, especially polyolefins. Rockwell hardness is used to compare the hardness of metals but there are Rockwell hardness scales suitable for harder plastics such as polycarbonate (PC), nylon (PA), and acetal (POM). The Rockwell hardness test measures hardness in progressive numbers on different scales corresponding to the size of the ball indentor used. The Rockwell hardness number is directly related to the indentation hardness of the plastic material (i.e., the higher the reading, the harder the material).
The Vickers hardness (HV) test is the most common for hard metals, followed by the Rockwell hardness scale A test. Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads. The superficial Rockwell test is a widely used variant of the Rockwell hardness test, wherein the minor load is 3 N and the major loads are 15, 30, or 45 N.
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Brittleness
Plastic hardness is a measure of how resistant a material is to indentation or scratching. It is important to understand the hardness of a material before choosing it for a particular application. Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity.
There are several causes of brittleness in plastics. One cause is weak molecular bonding, which can result from improper mixing and melting of the plastic resin during the moulding process. This can be due to a low compression ratio, low barrel temperature, or insufficient cycle time. Another cause of brittleness is thermal degradation, which can occur due to excessive back pressure, screw RPM, or injection speed during processing. This can lead to the formation of weld line areas in the moulded part, resulting in weak spots that are prone to cracking. Condensation on the surface of a cold mould in humid conditions can also cause brittleness by forming a barrier to molecular bonding.
To reduce brittleness in plastics, several solutions can be implemented. One approach is to adjust processing parameters, such as increasing barrel temperature, cycle time, or injection pressure, to ensure proper mixing and melting of the plastic resin. It is important to be cautious when making these adjustments to avoid degradation of the material. Another solution is to use plasticizers, which are added to the plastic when it is hot and malleable to increase its flexibility. However, some plasticizers can evaporate over time, requiring the plastic to be remelted and new plasticizer added.
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Frequently asked questions
Hardness is a material's resistance to being penetrated or permanently indented. It is often measured by applying pressure and measuring the depth of the indentation.
There are a few different methods to measure the hardness of plastic. The Shore durometer is a widely used method that involves pressing a needle against the plastic and measuring the depth of the indentation. The Rockwell method is another common test that uses a steel ball to apply force in two stages, which helps to eliminate errors.
The Shore durometer scale ranges from 0 to 100. A higher number indicates a harder material. It is important to note that hardness numbers are only useful for comparing materials tested using the same scale, device, and measurement standard.
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