ISO 75 defines methods for measuring the heat deflection temperature (HDT) of plastics under specified loads. It helps compare material responses to heat and mechanical stress, supporting selection for thermal applications. The standard covers both thermoplastics and thermosets, but has limitations for partially crystalline plastics above their glass transition temperature.
ISO 75 explained
ISO 75 is an internationally recognized standard that specifies methods for determining the temperature at which a plastic material deforms under a specified load. Known as the “Heat Deflection Temperature” (HDT), this property helps manufacturers and engineers evaluate how a plastic will perform when exposed to elevated temperatures in practical applications. The results are essential for selecting materials used in environments where thermal resistance is needed, as they indicate the maximum temperature at which a material maintains its mechanical integrity under stress.
Scope and limitations
ISO 75 covers thermoplastic and thermosetting plastic materials, giving guidelines for preparing samples, determining appropriate loads, and measuring the heat deflection temperature. The test measures either 0.34 MPa or 1.8 MPa flexural stress, depending on the material and intended use. While highly informative for comparing similar materials, it is not suitable for partially crystalline plastics above their glass transition temperature, nor does it exactly simulate every real-world application because the loading and heating rates are standardized.
Applicability of ISO 75 data
The data obtained through ISO 75 assists in new product development, material comparison, and quality control. Manufacturers use it to confirm compliance with technical requirements or specifications and to ensure that end products meet expected thermal performance standards. The results help in communicating product properties within supply chains and can be critical when entering regulated markets or meeting customer requirements.
Takeaway
ISO 75 is a key standard for assessing the ability of plastic materials to retain mechanical strength when exposed to elevated temperatures, playing a central role in material selection and quality validation.
Preparing for the ISO 75 test
The first step in ISO 75 testing is preparing the test specimens according to the criteria set out in the standard. Samples must be of precise dimensions, typically rectangular bars, and may be conditioned to a controlled environment prior to testing, unless otherwise specified. Accurate sample preparation is essential to ensure valid and reproducible results across multiple tests and different laboratories.
Setting up the apparatus and test conditions
ISO 75 requires the use of a dedicated HDT apparatus. Each specimen is placed horizontally and subjected to a specified three-point bending force (either 0.34 MPa or 1.8 MPa, depending on the material and application). The temperature of the surrounding medium, usually a fluid bath, is gradually increased at a controlled rate, typically 2°C per minute.
Executing the test and recording data
During the test, the temperature is raised uniformly until the specimen bends to a set deflection, commonly 0.34 mm. The temperature at this point is recorded as the heat deflection temperature (HDT). This process is repeated for all test specimens to ensure consistent and reliable results. The average value from multiple samples is typically reported, in line with standard requirements.
Reporting and interpreting the results
The measured HDT results are compiled and reported according to ISO 75 guidelines. This includes recording the applied load, test deflection, rate of temperature increase, and any notable sample or method deviations. Results are then used to compare material grades or to check compliance with specific product or industry requirements.
Common uses of ISO 75 heat deflection temperature
The HDT obtained by ISO 75 plays a key role in R&D and production quality control. It helps material scientists and engineers judge whether a material is suitable for use in environments subjected to mechanical load and elevated temperatures, such as for pipes, automotive parts, or electrical housings. While highly informative, results should be considered alongside other material properties and specific design requirements.
Summary
Following the ISO 75 procedure ensures reliable, comparable results for evaluating the thermal performance of plastics under load. This supports accurate product development, compliance, and quality control in a wide range of plastic applications.
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