What is Heat deflection temperature (HDT)?

Heat deflection temperature (HDT), also known as heat distortion temperature, is a technical parameter that indicates a thermoplastic material’s resistance to deformation under simultaneous mechanical load and increasing temperature. It is a key measure for evaluating how a plastic behaves under thermal stress in real-use conditions, especially in applications where parts are exposed to elevated temperatures, either continuously or intermittently.

HDT is expressed in degrees Celsius (°C) and is determined through a standardized test, typically according to ISO 75 or ASTM D648. In this test, a sample is subjected to a constant load (commonly 0.45 MPa or 1.8 MPa) while being heated at a controlled rate. The temperature at which the sample reaches a specific deflection (typically 0.25 mm) is recorded as the heat deflection temperature.

This value is essential when selecting materials for components that must retain shape, rigidity, or functionality in hot environments—such as automotive engine parts, appliances, electrical devices, or industrial systems operating at high temperatures. The higher the HDT, the better the material’s ability to resist heat-induced deformation under load.

Different plastics exhibit a wide range of heat deflection temperature values, for example:

• High-density polyethylene (HDPE): approx. 75 °C

• Polypropylene (PP): 60 °C to 120 °C (depending on grade and filler content)

• Polycarbonate (PC): 130 °C to 140 °C

• Polyamide (PA 66): 180 °C to 200 °C

• PEEK (polyetheretherketone): over 300 °C

It’s important to note that HDT does not represent the material’s maximum continuous service temperature but serves as a useful comparative indicator of thermal structural behavior. The presence of fillers such as mineral charges or glass fibers can significantly increase HDT, enhancing the material’s thermal stability.

At Plásticos Lezo, we consider heat deflection temperature a key criterion when selecting materials for technical parts. We assess the real thermal conditions where the part will be used and verify whether the base polymer—or its reinforced version—meets the required deflection resistance. We also work closely with clients to optimize part geometry, thickness, and reinforcement points to ensure functionality even near the material’s thermal limits.

HDT is also used as a benchmark when validating new compounds or recycled materials, particularly in sectors where thermal performance is critical. In specific projects, we complement this parameter with additional tests such as Vicat softening temperature, thermal creep, or accelerated aging.

In summary, heat deflection temperature (HDT) is an essential value to ensure that a plastic part maintains dimensional stability under heat and mechanical load. At Plásticos Lezo, we treat it as a fundamental design and quality control tool when developing technical solutions that demand reliable thermal and structural performance.