Key Thermal Properties of Plastics: HDT, Service Temp, Melting/Glass Transition, and Expansion

When selecting a plastic for a high‑temperature application, understanding its thermal behavior is essential. Four core metrics—Heat Deflection Temperature (HDT), Continuous Service Temperature, Melting Point or Glass Transition Temperature, and Coefficient of Linear Thermal Expansion (CLTE)—provide the data needed to make informed material choices.

Heat Deflection Temperature (ASTM D648)

HDT, or heat‑distortion temperature, is the point at which a polymer bends under a prescribed load as it is heated or cooled. The ASTM D648 standard specifies the test method and ensures consistent, comparable results across manufacturers.

Continuous Service Temperature

Unlike HDT, the Continuous Service Temperature is the highest temperature at which a material can reliably perform over its expected service life. While no ASTM test exists for this metric, it is a critical design parameter for long‑term reliability.

Melting Point and Glass Transition Temperature (ASTM D3418)

Crystalline Polymers

Crystalline resins such as PEEK, PEK, PPS, and PFA melt at a defined temperature. Their regular molecular order grants high strength, stiffness, and chemical resistance, though they tend to be opaque.

Amorphous Plastics

Amorphous materials lack a true melting point. Instead, they soften over a broad temperature range, described by the glass transition temperature (T_g). These polymers are typically transparent and used in applications like plastic wrap and contact lenses.

Semi‑Crystalline Plastics

Semi‑crystalline polymers contain both crystalline and amorphous domains. They exhibit a melting point for the crystalline segments and a glass transition temperature for the amorphous portions.

Coefficient of Linear Thermal Expansion (CLTE)

CLTE quantifies how a material’s dimensions change with temperature: (CLTE) × 10⁻⁶ × length × ΔT (°C). Thermoplastics generally expand more than metals; for example, carbon steel expands at 10.8 × 10⁻⁶ m/m·K, whereas UHMW PE expands at 200 × 10⁻⁶ m/m·K—roughly 18 times more.

Table of Thermal Properties for Common Plastics

Material | HDT | Continuous Service Temp. | Melting Point / T_g | CLTE

Nylon (Crystalline) | 200 °F | 210 °F | 500 °F | 5.5 × 10⁻⁵

PPS (Semi‑Crystalline) | 250 °F | 425 °F | 426 °F | 2.8 × 10⁻⁵

PEI (Amorphous) | 410 °F | 340 °F | 410 °F (T_g) | 3.1 × 10⁻⁵

For high‑temperature applications, consult Craftech for material selection tailored to your specific needs.

Need assistance with 3‑D printing, troubleshooting, or component design? Contact Craftech Industrial Inc.