Plastic Strength Explained: Heat Deflection Temperature (HDT)
2025/11/6
Heat Deflection Temperature (HDT), also known as heat distortion temperature, is the temperature at which a plastic begins to deform under a specific load. It reflects a material’s heat resistance and high-temperature stiffness (elastic modulus), allowing engineers to compare plastics for performance under thermal stress.
Why Heat Resistance Matters For Plastics
Plastics are valued for being lightweight, transparent, and easy to process. Their main limitation has been heat resistance, as strength can drop under thermal stress. While standard plastics often struggle in high-temperature applications, some components are now being replaced with plastics instead of metals to reduce weight and cost.
High-performance “super-engineering plastics” can withstand temperatures over 100°C, and in some cases up to 400-500°C. This advancement allows plastics to replace parts that previously required metals. Additionally, plastics can be reinforced with additives, enhancing their versatility for demanding applications.
How Heat Deflection Temperature is Tested
HDT is measured using a three-point bending test. A sample is loaded, either low load (0.45 MPa) or high load (1.80 MPa), while the temperature rises gradually (typically 120°C/hour). The HDT is the temperature at which bending strain reaches 0.2%.
It’s important to note that the HDT value itself is not an absolute performance indicator. It is primarily a comparative measure. Materials with higher HDT values resist thermal stress better.
Short-Term Heat Resistance and Vicat Softening Temperature (VST)
Alongside HDT, the Vicat Softening Temperature (VST) measures a plastic’s short-term heat resistance. In this test, a penetrator presses 1 mm into the sample. Testing methods (A50, B50, A120, B120) specify load and heating rate. The B50 method (50 N load, 50°C/hour heating) is the most widely used in industry.
Step Up Your Project with Yumoto Electric
Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of engineering plastics, including PEEK, PPS, MC Nylon, and POM.
If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.
