This image was created with artificial intelligence – marked under Art. 50 of the AI Act, Regulation (EU) 2024/1689. It is a placeholder until the company's photos arrive.Heat deflection resistance: What does it mean for material selection?
Find out what heat deflection temperature means for plastics, how it is measured and why it is crucial for material selection in engineering, automotive and medical technology.
Heat deflection resistance is a key parameter when selecting engineering plastics. It describes how well a material retains its shape under the influence of temperature – a criterion that determines functionality, safety and durability in many applications. This material parameter plays an essential role particularly in sectors such as mechanical engineering, the automotive industry or electrical engineering.
With increasing miniaturisation, a rising degree of automation and growing thermal loads, the demands placed on plastics are rising continuously. In this context, it is not enough merely to know the melting temperature of a material. What is decisive, rather, is how stable a plastic remains under mechanical load and high temperatures. Only in this way can permanently functioning components be designed that do not fail when the service temperature rises.
What does heat deflection resistance mean?
Heat deflection resistance describes the temperature at which a plastic begins to deform plastically under defined mechanical load. This property is not to be confused with the melting temperature or the glass transition temperature. Two common methods of determination are:
- Vicat softening temperature (VST): Determines the temperature at which a defined needle reaches a specified penetration depth in the plastic test specimen.
- Heat deflection temperature according to Martens (HDT): Measures the temperature at which a standardised test specimen deforms by a certain amount under bending load.
Together, these two parameters provide a reliable picture of how a material behaves under the influence of temperature in real applications. Particularly important here is the distinction between short-term and continuous temperature exposure, since plastics react very differently depending on molecular structure and filler content.
Relevance for material selection
Permanent dimensional stability
In applications where a plastic must withstand temperatures above 80°C permanently, a high heat deflection resistance is decisive. Classic standard plastics such as PE or PP are not sufficient here. Instead, high-performance plastics such as PPS, PEEK or PI are used.
Safety and function
In vehicle construction, for example, components in the engine compartment – such as sensor housings, plug connectors or pump components – must remain functionally reliable even at ambient temperatures of 120 to 150°C. Material failure due to deformation can have safety-relevant consequences here.
Miniaturisation and wall thicknesses
In electronic components, miniaturisation requires extremely high dimensional accuracy even at the smallest wall thicknesses. Materials such as LCP (Liquid Crystal Polymers) are distinguished by excellent heat deflection resistance combined with a low tendency to creep.
Factors influencing heat deflection resistance
- Fillers such as glass fibres or talc generally increase dimensional stability.
- Crystallinity: Semi-crystalline plastics (e.g. PA66, POM) have higher dimensional stability than amorphous ones (e.g. PC, PMMA).
- Moisture absorption can impair dimensional stability (e.g. in polyamides).
Typical areas of application
The fields of application for heat-resistant plastics are diverse and cover virtually all industries in which thermal loads occur. What is decisive here is not only the maximum temperature, but also the duration of exposure, the alternation between temperature cycles and the combination with mechanical or chemical loads.
Mechanical engineering
Sliding and guide elements exposed to thermal and mechanical load must maintain their function even at continuous operating temperatures above 100°C. Glass-fibre-reinforced or semi-crystalline plastics are often used here.
Electrical engineering
In high-current and high-voltage engineering, components such as insulating housings, plug connectors or coil formers require not only excellent insulating properties but also high heat deflection resistance – particularly in devices with poor heat dissipation.
Automotive
Engine compartment parts, transmission components, thermostat housings or air intake modules often have to withstand temperatures of up to 150°C – combined with vibrations, contact with chemicals and continuous load. Here OEMs are increasingly relying on high-performance plastics that enable metal-saving designs.
Medical technology
Sterilisable plastic parts (e.g. in autoclaves) in medical technology require heat deflection resistance > 120°C. At the same time, they must not lose their mechanical properties or be chemically attacked. This applies in particular to surgical instrument accessories or reusable aids.
Table: Comparison of typical plastics by heat deflection resistance
| Plastic | HDT (ISO 75, 1.8 MPa) | Vicat (ISO 306, B50) | Remark |
|---|---|---|---|
| PE-HD | approx. 75°C | approx. 125°C | For low-temperature applications |
| PA6 | approx. 200°C | approx. 215°C | Note water absorption |
| POM | approx. 165°C | approx. 170°C | Good dimensional stability |
| PC | approx. 135°C | approx. 150°C | Amorphous, transparent applications |
| PPS | approx. 230°C | approx. 260°C | Very high chemical resistance |
| PEEK | approx. 315°C | approx. 340°C | For the highest thermal requirements |
Conclusion
Heat deflection resistance is a decisive factor in the selection of engineering plastics, particularly in environments with high continuous temperatures or thermal load cycles. Knowledge of the respective limit temperatures and influencing factors is indispensable for designers and material experts in order to realise safety-critical or durable plastic components.
FAQ
What is the difference between melting temperature and heat deflection resistance?
The melting temperature marks the transition from the solid to the liquid state. Heat deflection resistance, by contrast, indicates when a plastic begins to deform plastically under load.
Which plastic has the highest heat deflection resistance?
PEEK is among the plastics with the highest heat deflection resistance and is suitable for applications up to over 300°C.
What role does component geometry play?
Thin-walled or delicate geometries can deform more quickly under temperature load. Good material selection therefore also takes the specific design into account.
