HDT — ISO 75 / ASTM D648 · Plastics Testing

What Is HDT?

The heat deflection temperature test is the test in which the temperature at which the sample, to which a defined stress is applied in a three-point bending arrangement is applied, is heated at a constant speed and reaches the deflection defined in the standard is determined.

Purpose of HDT

The aim is to obtain a comparative benchmark on the temperature limit at which plastic can be used under load.

Which Products Is HDT Applied To?

Engineering thermoplastics
Reinforced plastic parts
Electrical product housings
Automotive interior and engine compartment parts

In Which Sectors Is HDT Used?

PlasticsAutomotivePackagingConsumer Products

Standards Related to HDT

ISO 75-1
ISO 75-2
ASTM D648

HDT Conditions

Strain: Method defined in the standard: A (1.80 MPa), B (0.45 MPa) or C (8.00 MPa)
Heating rate: Constant speed defined in the standard (typically 120 °C/hour)
deflection criterion: Temperature at which the deflection value defined in the standard is reached
Specimen: In sizes defined in the standard; on flat or side surface

Equipment for HDT

HDT/Vicat tester and heating bath
Calibrated load set and deflection measurement setup
Temperature measurement and recording system
Specimen-preparation bench

Sample Requirements for HDT

Sample dimensions and surface smoothness must comply with the standard
The sample must be conditioned
The applied method (A/B/C) must be defined in the specification
The number of specimens is defined in the standard

Duration of HDT

Typically 2–3 days.

HDT Procedure

1Samples are prepared and conditioned.
2The method and stress to be applied are selected.
3Samples are placed in the fixture and load is applied.
4The bath is heated at a defined rate.
5The temperature at which the deflection criterion is reached is recorded.
6The result is reported by specifying the method.

HDT Acceptance Criteria

The acceptance criterion is the minimum HDT value given in the material standard or specification.

Evaluation of Results

HDT is not a service temperature limit; It is a comparative measure taken at a constant heating rate and a single stress. A given HDT value cannot be compared without specifying the method (A/B/C).

HDT Report

Standard and method applied
Strain and heating rate
Sample dimensions and orientation
Measured HDT values
Conditioning conditions

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Material product standards

Plastics Testing — What Does This Testing Field Cover?

The section below describes the field that the HDT test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Plastics testing determines the mechanical, thermal and burning behaviour of a polymer material. Unlike metals, plastics are VISCOELASTIC: their behaviour depends on loading rate, temperature and moisture. The same material therefore gives different results under different conditions, and the test conditions are an inseparable part of the result.

The practical consequence of viscoelasticity is this: a plastic part continues to deform over time under a constant load (creep) and the stress it carries at constant deformation falls over time (stress relaxation). A screwed joint loosens, a clamp loses its clamping force, a shelf sags over time. A short-term tensile test does not show this behaviour; a long-term test or an accelerated model is needed.

Thermal behaviour is the second main axis. Melt flow rate indicates the processability of the material and lot-to-lot consistency; heat deflection temperature and the Vicat softening point give a rough limit for service temperature. Thermal analysis (DSC, TGA, DMA) reveals the identity of the material: glass transition temperature, melting behaviour, crystallinity, filler content and decomposition temperature.

Burning behaviour is the third axis and ties directly into product standards. The UL 94 class and the limiting oxygen index are written into the material data sheet; a particular class is required for the housing of an electrical appliance, while a different classification system applies to a construction product.

Ageing is the most frequently skipped part of the field. Plastics degrade under UV, heat, moisture and oxygen: colour changes, the surface cracks, impact strength falls. For a part intended for outdoor use, what matters is not its values on day one but its values after ageing. Xenon arc and fluorescent UV equipment accelerate this degradation and allow comparison.

The output of this family is usually written into the material data sheet, and product standards refer to that sheet. But there can be a difference between the data sheet value and the behaviour of the actual part: orientation formed during moulding, weld lines and internal stresses make the part behave differently from the specimen.

The use of recycled content has added a new question to the field. Recycled resin varies from lot to lot; mechanical properties, colour and burning behaviour can shift between batches. For a manufacturer using recycled material, incoming material control is therefore not a choice but a necessity.

Method groups in this field

Mechanical
Tension, compression, bending, Charpy and Izod impact, hardness (Shore), creep and environmental stress cracking.
Thermal
Melt flow index (MFI), heat deflection temperature (HDT), Vicat softening temperature, linear thermal expansion.
Thermal analysis
Transition temperatures and crystallinity by DSC, composition and decomposition by TGA, dynamic mechanical behaviour by DMA.
Burning
UL 94 classification, limiting oxygen index (LOI), glow wire and needle flame tests.
Ageing
Accelerated ageing by xenon arc and fluorescent UV, hot air ageing, humidity and condensation.
Physical
Density, water absorption, ash content, gas and water vapour permeability of films and sheets.

Commonly confused in this field

A plastics result without a stated test rate and temperature cannot be interpreted; viscoelastic behaviour depends on these parameters.
A specimen result does not transfer directly to the actual part; moulding orientation and weld lines are decisive.
HDT and Vicat values are NOT service temperatures; they are a rough indication of an upper limit and do not apply under load.
The UL 94 class depends on thickness; a different wall thickness requires re-verification.
For moisture-absorbing polymers (such as polyamide), results are misleading unless conditioning is carried out.
ASTM D638 and ISO 527 results are not directly equivalent; the specimen geometries and rate definitions differ.

Which decisions these results support

Material selection and supplier approval
Meeting the flammability and thermal requirements of the product standard
Verifying the use of recycled content and lot control
Moulding process control and lot consistency
Demonstrating ageing resistance for outdoor use

Related Tests

Frequently Asked Questions About HDT

Is the HDT value the maximum temperature at which the product can operate?

No. HDT is a comparative value measured at a defined load and heating rate. Long-term thermal aging and RTI/relative thermal index type evaluations are required for sustained service temperature.