Heat resistance of plastics

Heat deflection resistance describes the temperature up to which a plastic remains dimensionally stable under a defined mechanical load. It is therefore an important characteristic value for the temperature resistance of plastics, but must not be equated with the maximum service temperature or melting temperature.

For assessment, the heat deflection temperature HDT (Heat Deflection Temperature) and the Vicat softening temperature (VST) are used in particular. Both test values help designers, developers and buyers to compare plastics for components subjected to thermal and mechanical loads.

Especially in applications in mechanical engineering, vehicle construction, the electrical industry or medical technology, insufficient heat deflection resistance can cause a component to deform under heat and lose its function.

Heat deflection resistance and temperature resistance: what is the difference?

The terms heat deflection resistance and temperature resistance are often used interchangeably, but they describe different properties.

Heat deflection resistance indicates the temperature at which a plastic measurably deforms under a defined mechanical load. Standardised test values such as HDT or Vicat are used for this purpose.

Temperature resistance, by contrast, is a broader term. It describes the temperatures to which a plastic can be exposed briefly or permanently without losing the properties required for the application. In addition to dimensional stability, factors such as strength, ageing, chemical resistance and service life can play a role.

If you would like to compare plastics specifically for high continuous or peak temperatures, you will find further information under High-performance plastics for extreme temperatures.

Significance of heat deflection resistance

Heat deflection resistance provides information about how stable a plastic remains at high temperatures. It is particularly relevant in industries such as automotive engineering, electrical engineering and medical technology. In vehicles, this affects e.g. housings of control units in the engine compartment.

In medical technology, plastics must retain their shape even during sterilisation processes. In electrical engineering, in turn, it is decisive for insulators, switches or housings that must function reliably over many years. This characteristic value also plays a role in construction or in household appliances, for example in pipe systems, kitchen appliances or lighting components.

A component that deforms under heat can lose its function, lead to safety risks or cause high costs due to failures. Precise determination of this characteristic value is therefore essential in order to ensure safety and functionality over the entire service life. In addition, knowledge of the heat deflection resistance helps to design components more efficiently and avoid overdimensioning, which reduces costs, weight and material usage.

Test methods for determination

Standardised test procedures are available for determining heat deflection resistance. The HDT test and the Vicat softening temperature are particularly relevant. They measure different material reactions and must therefore not be directly equated with one another.

Vicat softening temperature (VST)

The Vicat softening temperature, often also referred to as the Vicat softening point, is determined in accordance with DIN EN ISO 306. A standardised test needle is placed on the test specimen with a defined force and the temperature is increased in a controlled manner.

The Vicat softening temperature is reached as soon as the needle has penetrated 1 mm deep into the plastic. The value thus describes when a thermoplastic plastic begins to soften significantly under the specified test conditions.

Vicat is particularly suitable for comparing different thermoplastic materials. However, the test value does not automatically correspond to the permissible continuous service temperature of a finished component.

Heat deflection temperature HDT

The heat deflection temperature HDT – in English Heat Deflection Temperature or Heat Distortion Temperature – describes the temperature at which a standardised plastic test specimen deforms under a specified bending load up to a defined extent. The test is carried out in accordance with DIN EN ISO 75, among others.

In contrast to the Vicat test, the HDT therefore focuses on dimensional stability under mechanical load. The characteristic value is therefore particularly relevant when plastic components are exposed to heat and mechanical forces at the same time.

The HDT is also not a universal limit value for the maximum service temperature. Load, component geometry, service life, moisture and other ambient conditions must also be taken into account when selecting the material.

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry examines the thermal behaviour of plastics at the molecular level. Small samples are placed in a heating chamber and compared with a reference sample. The device measures how much thermal energy is absorbed or released as the temperature rises.

This allows transition points such as glass transition temperature (Tg), melting point (Tm) or crystallisation temperature to be determined. This information is valuable for understanding the behaviour of plastics under repeated heating and cooling cycles. Particularly in the development of new materials or in the optimisation of formulations, DSC provides indispensable data that go beyond pure dimensional stability values.

Thermomechanical Analysis (TMA)

Thermomechanical Analysis measures the dimensional changes of a test specimen at increasing temperature under constant load. Typical measured variables are change in length, expansion or deformation. The method allows statements to be made about the linear coefficient of thermal expansion as well as about the point at which plastic deformation occurs.

TMA is particularly suitable for assessing the behaviour of thin-walled components, films or precision elements. By combining it with data from DSC, developers can gain a very differentiated picture of the structure and stability of the material.

Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analysis goes one step further by examining the viscoelastic properties of plastics under variable load. A test specimen is subjected to periodic loading (e.g. by bending or oscillation) while the temperature is systematically changed. Characteristic values such as storage modulus, loss modulus and damping factor are measured.

These values provide information about how a plastic stores or dissipates energy. With DMA, transition zones such as glass transition or relaxation processes can be identified very precisely. In practice, this means: developers can predict the service life of components exposed to alternating forces and temperatures much more accurately.

Typical values for common plastics

The values of heat deflection resistance are strongly material-dependent. Thermoplastics, thermosets and high-performance plastics differ considerably.

Plastic	Vicat softening temperature (°C)	HDT (°C)
PVC (rigid)	75–85	65–75
ABS	95–105	85–95
Polycarbonate (PC)	145–150	130–140
Polyamide (PA 6.6 GF)	200–220	190–210
PEEK	150–160	150–160
PTFE	> 200	> 200
Polyimide (PI)	> 250	> 250

These values are guide values. Composition, additives, glass fibre content and the type of processing can change the results considerably.

The temperatures given serve exclusively for orientation. HDT and Vicat values depend, among other things, on the material type, degree of reinforcement and test conditions. For a specific material selection, the values of the respective technical data sheet and the actual operating conditions should therefore always be taken into account.

The thermal conductivity of a plastic is also to be distinguished from its heat deflection resistance. It does not describe the permissible temperature load, but rather how well heat is transported within the material. Further information can be found under Thermal conductivity of plastics.

Influencing factors

Heat deflection resistance is influenced by several parameters:

• Fillers and reinforcements: Glass fibres, carbon fibres or minerals can significantly raise the temperature resistance.
• Additives: Flame retardants, stabilisers or plasticisers change the resistance and must be taken into account when selecting the material.
• Processing conditions: Cooling rates, post-conditioning and the orientation of the molecular chains during injection moulding have a direct influence on dimensional stability.
• Moisture: Hygroscopic plastics such as polyamides lose thermal and mechanical stability at high humidity.
• Long-term load: Permanent heat cycles or creep behaviour can weaken dimensional stability in the long term.

Practical relevance

Heat deflection resistance is a key factor for the reliability of plastic components in high-temperature environments. Examples are:

• Automotive engineering: Engine compartment components, connectors, covers
• Electrical engineering: Switches, fuse holders, insulating bodies
• Medical technology: Instruments that must be sterilised repeatedly
• Mechanical engineering: Components in production plants with continuous load
• Aerospace: Lightweight components that must withstand extreme temperature fluctuations

A sound understanding of these values enables safe material selection and considerably reduces the risk of complaints and failures.

Challenges and limits

Although the test methods are precise and standardised, they reach their limits in practice. The results primarily reflect laboratory conditions, which can only be transferred to real application environments to a limited extent. In reality, several factors often act simultaneously – high temperatures in combination with moisture, continuous mechanical load or chemical media.

This can influence the behaviour of plastics considerably more than in standardised tests. Ageing processes, UV radiation or repeated heat cycles are also difficult to fully reproduce. Test data must therefore always be interpreted with safety margins. Companies that use plastics in critical applications therefore often supplement laboratory measurements with long-term tests, simulations and field trials.

Future prospects

The requirements for plastics are rising continuously. Developments in the field of high-performance plastics and nano-reinforcements enable materials that remain dimensionally stable even at over 300 °C. At the same time, sustainable biopolymers are gaining in importance, offering not only thermal resistance but also ecological advantages.

In addition, there are hybrid materials and multi-component systems that combine various properties such as thermal resistance, chemical resistance and mechanical strength. With advancing digitalisation, companies are increasingly relying on computer-aided simulations that can precisely predict the behaviour of materials. This allows development times to be shortened, material tests to be made more efficient and innovative products to be brought to market maturity more quickly.

In conclusion

The heat deflection resistance of plastics is an indispensable characteristic value in material selection. Test methods such as Vicat, Martens, DSC, TMA and DMA provide reliable data that must be considered in combination depending on the application. For engineers, this means: in addition to the characteristic values, influencing factors such as additives, processing and ambient conditions must also be taken into account.

At the same time, practice requires laboratory values to be questioned critically and supplemented by application-specific tests. Only in this way can components be developed that are reliable and durable under extreme conditions. In times of growing requirements for efficiency, safety and sustainability, heat deflection resistance thus gains a key role that can decide the success of products and applications.

FAQ on the heat deflection resistance of plastics

What is the difference between Vicat and HDT? Vicat measures the transition from the solid to a softer state, while HDT determines the deflection under load – i.e. assesses dimensional stability in a way that is closer to practice.

Why is heat deflection resistance not the same as melting temperature? Heat deflection resistance describes the temperature at which a component loses its shape, not the point at which the plastic melts completely.

Can additives improve heat deflection resistance? Yes, glass fibre reinforcements or special additives significantly increase stability.

Which plastics are particularly suitable for high temperatures? High-performance plastics such as PEEK, PI or PTFE are characterised by particularly high heat deflection resistance.

How do thermoplastics and thermosets differ in heat deflection resistance? Thermoplastics soften as the temperature rises, while thermosets remain considerably more dimensionally stable due to their close-meshed crosslinking and only degrade at very high temperatures.

What role do simulations play in material development? Modern simulation methods allow engineers to predict the behaviour of plastics under thermal load virtually and thus shorten development times and avoid wrong decisions.

What does heat deflection resistance mean for plastics?

Heat deflection resistance describes the temperature up to which a plastic sufficiently retains its shape under a defined mechanical load. Typical characteristic values for this are HDT and Vicat.

What is the heat deflection temperature HDT?

The HDT or Heat Deflection Temperature indicates the temperature at which a standardised plastic test specimen deforms under a defined bending load up to a specified extent.

What is the Vicat softening temperature?

The Vicat softening temperature indicates the temperature at which a standardised test needle penetrates 1 mm deep into a thermoplastic plastic under defined load.

Is heat deflection resistance the same as temperature resistance?

No. Heat deflection resistance describes deformation under defined test conditions. Temperature resistance takes a broader view of the temperatures to which a material can withstand briefly or permanently in a specific application.

What does thermal stability mean for plastics?

Thermal stability generally describes the ability of a plastic to retain its relevant properties at elevated temperature over a certain period. This can include dimensional stability, mechanical strength and ageing resistance.