Omar Khalid
Testing for wear and tear on plastics is an important part of product development and production. There are several methods to test the quality of plastic components, including radiographic testing, which involves exposing the plastic to radiation, usually x-rays, to detect flaws or contaminants. Another method is the melt flow test, which involves melting plastic granules and pouring them through an orifice to determine the flow behaviour of the plastic. The burn test can also be used to identify the type of plastic by heating it with a lighter and observing its behaviour, though this should be done with proper equipment due to the potential release of carcinogens. Other tests include the Izod impact test, which measures the energy required to break notched specimens, and the moisture analysis test, which determines water content in the plastic.
| Characteristics | Values |
|---|---|
| Tensile properties | The force required to pull the specimen apart and the amount it stretches before breaking |
| Elastic modulus | The ratio of stress to strain below the material's proportional limit |
| Flexural properties | The amount of load a specimen can withstand in the center before failing |
| Impact | The energy required to break notched specimens under standard conditions |
| Stiffness | |
| Hardness | |
| Flammability | |
| Shear strength | |
| Density | Whether the plastic is denser than water and will sink or float |
| Moisture analysis | The water content in the raw plastic |
| Radiographic testing | The plastic is exposed to radiation and any flaws are detected |
| Ultrasonic inspection | The plastic is placed in water and an electric transducer is used to release sound waves to detect defects |
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What You'll Learn
Moisture analysis
Moisture content is a critical parameter in plastics processing and manufacturing, and excessive moisture can adversely affect the material’s properties and processing characteristics. The moisture content in plastic pellets has to be controlled as it's an important aspect of producing high-quality plastic parts. The right moisture content of polymer resins ensures trouble-free compounding and injection moulding, smooth surfaces, and ideal mechanical properties of the injection-moulded parts.
The ASTM D7191 test method is used for the quantitative detection of water in polymers down to 20 ppm. An airtight, sealed flow system is used for this test to protect the analyzer from contamination by atmospheric water, as sample temperatures can approach or exceed 250°C.
The Karl Fischer titration method is the ASTM standard test method to determine the residual moisture content in plastics. The HX204 moisture analyser is ideal for applications with low moisture content such as plastic pellets. It offers a high resolution of 0.1 mg, which is necessary for determining small amounts of water content in a robust and accurate manner.
Other methods for determining moisture content in plastics include ASTM D6980 (Standard Test Method for Determination of Moisture in Plastics by Loss in Weight) and ASTM D6869 (Coulometric and Volumetric Determination of Moisture in Plastics Using the Karl Fischer Reaction). The latter uses the reaction of iodine (I2) with water (Karl Fischer Reaction) to determine the amount of moisture in a polymer sample.
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Radiographic testing
The basic principle of radiographic testing involves exposing the material to a beam of radiation, typically X-rays, or gamma rays for thicker materials. The radiation penetrates the object and its intensity as it passes through is measured and captured on photographic film. Any variations in the thickness or flaws in the material, such as contaminants, appear as dark spots on the film. This process is similar to how X-rays are used in medical settings to visualize internal body structures.
One critical factor influencing the accuracy of radiographic testing is the size of the radiation source. A larger source provides more uniform exposure, resulting in clearer and more accurate images. Conversely, a smaller source may lead to uneven exposure, making it challenging to interpret the radiographs correctly. Additionally, the quality of the radiographic equipment, including X-ray machines and gamma-ray sources, plays a significant role in the reliability of test results, with higher-quality equipment providing more consistent and accurate radiation output.
It's important to note that while radiographic testing is a valuable tool, it has certain limitations when detecting low-density materials like plastic. Standard X-ray systems are typically designed to detect items with a higher density level than water (1000 kg/m3), and plastics, with their relatively low density, can sometimes evade detection. However, newer technologies like Material Discrimination X-ray (MDX) have been developed to address this issue by identifying materials by their atomic number rather than density, making it possible to detect plastic contaminants that were previously undetectable.
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Melt flow testing
The Melt Flow Index (MFI) is a critical parameter in Melt Flow Testing. It is a quantitative measure of the rate at which a material flows when heated to a specific temperature. MFI is expressed in grams of material flowing per 10 minutes and is often used to determine the ease of processing and the quality of the final product. A low MFI indicates poor flowability, which can lead to issues with part quality, such as an uneven finish and inaccurate dimensions. Conversely, a very high MFI may cause the material to flow too quickly, resulting in improper part formation.
There are several methods to test the MFI of plastics. One common approach is to use a melting point apparatus to determine the temperature at which the material melts. Other equipment includes a melt index tester and a Brookfield Viscometer. When conducting these tests, it is important to consider the material's density, melt temperature, and viscosity. Additionally, safety precautions, such as wearing safety glasses and an apron, are essential.
The MFI plays a significant role in the engineering of plastics. It helps determine the appropriate polymer grade for different processes. For example, in injection moulding, a high MFI is desirable to ensure the material can easily fill the mould. Conversely, for applications requiring high mechanical strength, a lower MFI may be chosen to achieve sufficient final strength.
In summary, Melt Flow Testing, particularly the measurement of MFI, is a valuable tool for assessing the suitability of plastics for specific applications. It provides insights into the flow behaviour and processability of molten plastics, helping engineers and manufacturers make informed decisions about material selection and process optimization.
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Tensile and flexural testing
Mechanical testing is used to assess the performance of plastic materials under specified types of stress. The goal is to determine the characteristics of the sample material and identify possible shortcomings before bringing products to market. ASTM D790 is a testing method that can be used to determine the flexural (bending) properties of plastics. This test is conducted on a universal testing system using a three-point bend fixture at a rate proportional to the depth of the specimen. ASTM D790 testing can be used to determine the flexural modulus, flexural strength, and flexural yield strength of a material. The flexural modulus is a measure of the material's stiffness, while the flexural strength is the maximum flexural stress obtained during a bend test. For some materials, the specimen may break before the yield point, in which case the flexural strength equals the flexural stress at break.
Instron offers a range of testing systems for tensile, compression, and bend tests, among others. Their Instron 3400 Series Universal Testing Systems can be used for a variety of flexural tests, including the determination of flexural modulus, flexural strength, and flexural yield strength. The system uses Bluehill Universal Software, which is designed for an intuitive user experience and offers features such as pre-loaded test methods and enhanced data exporting.
Tensile testing is another important method for evaluating the mechanical properties of plastics. While specific details on tensile testing of plastics are scarce, it likely involves applying tensile stress to a sample and measuring its response. This can include measuring the elastic deformation and plastic deformation regions, the yield strength, the ultimate tensile strength, and the failure point.
In addition to tensile and flexural testing, there are other mechanical tests used to evaluate the performance of plastics. The trouser tear test, for example, is used to measure the resistance of plastic films to tearing. During this test, a sample strip is cut from the middle and pulled in two directions while the energy needed to tear the sample is measured. Mechanical testing can also involve placing a finished plastic product under a specified type of mechanical stress to simulate and evaluate its performance in its intended use. This type of testing is known as imitative testing and is commonly used for products such as plastic pipes and containers.
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Impact testing
The Charpy impact test is one of the most widely used testing methods for plastics. It involves dropping a weighted pendulum from a certain height to make contact with the chosen specimen. The specimen then absorbs energy, which can be inferred by measuring the difference in the pendulum's height before and after it was fractured. The Charpy test can be performed at reduced and ambient temperatures, even as low as -45°F. It is usually performed on unnotched specimens with edgewise impact (1eU). If the specimen does not break, the test is performed using notched specimens. The test results are not directly comparable. The Charpy test is similar to the Izod impact test, which can also be used on plastics, but there are differences in specimen orientation and the point of impact. The Izod test uses a vertically clamped specimen with the notch positioned toward the pendulum, and the pendulum impacts the specimen at a point above the notch.
The instrumented Charpy impact test to ISO 179-2 offers a large measurement range. Unlike conventional pendulum impact testers, forces are measured instead of energy. The measurement electronics allow for precise measurements as low as 1/100 of the nominal force. The lower end of the measurable impact energy is determined by the test duration and the natural frequency of the measuring elements. The Charpy test according to ISO 179-1 and ISO 179-2 provides characteristic values for impact strength at high strain rates in the form of an area-related energy value. The tests are normally performed in a normal climate or at low temperatures.
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Frequently asked questions
Here are some common methods to test the wear and tear on plastics:
- The Izod impact test calculates the energy required to break notched specimens under standard conditions.
- The specimen is mounted between a pendulum head and crosshead clamp on the pendulum of an impact tester. The energy loss is recorded, along with the type of failure.
- Flexural properties of plastics are obtained by placing a specimen on two supports spaced 4 inches apart and applying a load in the center.
- A moisture analysis determines the water content in the raw plastic by drying it under a heat source and weighing the sample.
Radiographic testing is performed to determine the quality of plastic components in the injection molding process. This involves exposing the plastic to a beam of radiation, typically x-rays, and measuring the radiation's intensity as it passes through. Acoustic inspection uses sound waves to find flaws and defective areas, while ultrasonic inspection is a more intensive test that requires a source of high-frequency sound waves.
The melt flow test is used to understand how a thermoplastic will behave during the molding process. The plastic granules are melted and poured through an orifice for ten minutes. The amount of plastic that comes out is weighed and compared to the original amount to determine the melt flow index.
