Mechanical testing is used to assess the performance of plastic materials under different types of stress. The overreaching goal is to determine the sample material’s characteristics and to identify possible shortcomings before bringing products to market or selling plastic components to manufacturers down the production chain.
More specific reasons to perform a comprehensive set of mechanical tests include the following:
Ensuring compliance with national or international standards and clients’ safety and quality requirements
Testing new batches to verify that their quality is comparable to previous ones
Assessing the quality of plastic raw materials provided by a new supplier
Determining whether the performance of a novel biobased material is similar to that of traditional plastics
How is mechanical testing performed?
Mechanical testing can be either fundamental or imitative. Fundamental testing is performed on a standardized plastic piece rather than the finished product. This allows for a direct comparison of different materials or batches of the same material, making fundamental testing crucial for R&D and Q&A efforts.
In imitative testing, a finished plastic product is placed under a specified type of mechanical stress. The aim is to closely simulate and evaluate the material’s performance in the kind of use it is designed for. Suitable samples for imitative testing include, among others, plastic pipes, toys, and containers.
In most cases, mechanical testing is performed with static loading over a relatively short period. If the long-term performance of the material or product needs to be studied, however, cyclic testing is recommended. In cyclic testing, loads are applied, removed, and reapplied repeatedly to determine how well the material withstands stress over time.
Mechanical testing standards are published by various international and national standardization bodies, including ISO, ASTM, and DIN. The standards that apply to plastics are mostly industry-specific – and practically countless, as polymer-based materials are so widely used. In some cases, different standards may even apply to similar products within a single industry. A good example of this is the automotive industry, where each manufacturer has its own testing requirements for plastic components.
When plastic raw materials and intermediate-stage products are tested instead of finished articles, the applicable standards are determined based on the type of material and the property under inspection.
Which properties can be assessed through mechanical testing?
The choice of properties and testing methods depends on the plastic material’s composition and intended use. For example, different mechanical tests are performed on packaging films and injection-molded plastic caps. Some of the most commonly tested properties are outlined below.
Tensile testing is used to determine how plastics perform under controlled tension. During the test, a standardized specimen is placed in a testing machine (usually the universal tester) and pulled at a constant rate until it breaks or when the maximum extension has been reached. The displacement of the material is measured against the applied force to determine its tensile strength, yield point, tensile modulus, and other tensile properties.
Standardized tensile tests exist for various plastic materials, including films and injection molded plastics. Plastic films can be tested according to the ISO 527-3 standard, whereas injection and extrusion molded plastics are usually tested as per ISO 527-2.
Plastic’s flexural properties are determined by monitoring the force required to bend and break the material beyond repair. Flexural testing is similar to tensile testing but easier to perform, as it does not require a dog-bone-shaped test specimen.
Flexural tests are typically performed with a universal tester using the three-point bending method, but single cantilever and four-point bending methods are also available. Plastic’s flexural properties can be tested according to the ISO 178 standard.
During compression testing, a standardized specimen or a finished plastic product is subjected to squeezing by a compression load. The material’s displacement is calculated against the applied load to determine compressive properties like compression strength and modulus. ISO 604 standard applies to compression testing of rigid and semirigid plastics.
Impact testing measures the energy absorbed by a material during fracture and provides information on its toughness. In the case of solid plastics, testing is typically performed by hitting the specimen with a hammer (Charpy or Izod impact tests). For films, a typical impact test is done with the free-falling dart method.
Tearing properties of plastic films
The trouser tear test is most commonly used to measure the resistance of plastic films to tearing. During the test, a sample strip is cut from the middle and pulled in two directions while the energy needed to tear the sample is measured.
Assessing the thermal properties of plastics
Plastic’s mechanical properties are extremely sensitive to temperature. The tests described above are most often performed at room temperature, but they can be modified to test the material’s performance in elevated or lowered temperatures. There are also separate analyses specifically meant to test the heat characteristics of plastics.
HDT and Vicat softening
The heat deflection temperature (HDT) is the temperature at which a standard test specimen deflects 0,025 mm under a standardized load of either 0.455 MPa or 1.82 MPa. The higher this temperature is, the better the plastic is suited for use in elevated temperatures.
The principle of the Vicat softening point is similar to that of HDT, but the setup is slightly different. Vicat softening point is the temperature at which a 1 mm2 probe penetrates the sample material to a depth of 1 mm upon being loaded with either 10 N or 50 N. While HDT testing is only suited for rigid plastics, the Vicat test can be performed on plastic films by stacking them to a thickness of 3 mm.
Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) is another method used to assess the thermal properties of plastics. DMA also provides information about the material's viscoelastic properties, transition temperatures, and much more. Due to the method’s complexity and the amount of work that goes into interpreting the results, other techniques are usually favored when doing simple mechanical evaluation and characterization.
At Measurlabs, we offer mechanical testing services by various international and national standards, including ISO, EN, ASTM, and DIN. Non-standardized testing options are also available upon request. If you wish to discuss your company’s testing needs with one of our experts, contact us at firstname.lastname@example.org.