ISO 75, ASTM D648 Plastic Temperature of deflection under load

Information on the most widely used ASTM standards within the materials testing industry

Descriptions of the most prominent lSO standards within the materials testing industry

Standards including EN, DIN, JlS, and more

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Flexural & Bend tests evaluate material resistance to bending forces, measuring strength, stiffness and ductility for quality assurance. UnitedTest provides flexural testing machines (3-point/4-point bend) compliant with ASTM/ISO standards, offering tailored solutions for composites, plastics, metals and construction materials.

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ISO 75 Plastics -- Determination of temperature of deflection under load.

Introduction to ISO 75 Series:

ISO 75 is a three-part international standard for determining the Heat Deflection Temperature (HDT), Heat Distortion Temperature test or Temperature of Deflection Under Load (TDUL) of plastics. This property indicates the temperature at which a plastic specimen deflects a specified amount under a constant flexural load, providing critical data on short-term heat resistance. ISO 75-1 establishes general principles, ISO 75-2 specifies conditions for standard plastics and ebonite, while ISO 75-3 addresses high-strength thermosetting laminates and long-fiber composites.

Part 1	ISO 75-1: Specifies the general test method, apparatus requirements, and standard procedures applicable to all materials.
Part 2	ISO 75-2: Provides specific requirements for most plastics and ebonite, particularly those with short-fibre reinforcements (fibre length ≤ 7.5 mm prior to processing).
Part 3	ISO 75-3: Provides specific requirements for high-strength thermosetting laminates and long-fibre-reinforced plastics (fibre length > 7.5 mm prior to processing).

Test Principle of ISO 75:

The core principle of the ISO 75 test is three-point bending under a constant load.

A test specimen is placed on two supporting hinges (spanning a specific distance) and subjected to a defined constant force applied by a loading nose in the center. The assembly is then heated in a fluid (like silicone oil) or an air circulating oven at a uniform rate (typically 120 °C/h or 50°C/h). The temperature at which the specimen deflects (bends) by a specified amount (0~2mm) is recorded as the Temperature of Deflection under Load (HDT). This measures the material's ability to retain rigidity under load at elevated temperatures.

Test methods:

The test methods are categorized based on the constant bending stress applied to the specimen.

Method	Flexural Stress	Application
A	1.80 MPa	Used for rigorous evaluation of rigid plastics.
B	0.45 MPa	Used for materials that soften easily or for evaluating the softening trend.
C	8.00 MPa	Exclusively used for high-strength thermosetting laminates and certain long-fibre composites.

Test Specimen Information:

Specimen dimensions and preparation are critical for accurate results and vary slightly depending on the specific material being tested:

General Dimensions: Specimens are usually rectangular bars. Common dimensions are approximately 80 to 120 mm in length, 10 mm in width, and 3 to 13 mm in thickness.

Specific Examples: For hard rubber or plastics (ISO 75-2), a preferred size is often 80 mm × 10 mm × 4 mm. For larger spans, specimens might be 120 mm × 15 mm × 10 mm.

Preparation: Specimens must have smooth, flat surfaces free of flash, burrs, or scratches. They can be injection molded, compression molded, or cut from finished products. If machined, care must be taken to avoid overheating or inducing internal stresses.

Conditioning: Specimens must be conditioned in a standard atmosphere (e.g., 23 °C and 50% RH according to ISO 291) for a specified period before testing to remove residual moisture and stresses.

Sample hegiht (thickness h) in mm	Standard deflection
3.8	0.36
3.9	0.35
4.0	0.34
4.1	0.33
4.2	0.32
Standard deflections for different test-specimen heights for a 80 mm length X 10 mm width specimen tested

Test equipment for ISO 75 Plastic Heat Deflection Temperature (HDT) Test:

recommend UnitedTest HDT/VICAT testing machine.

Heating Device	A thermostatically controlled liquid bath (e.g., silicone oil) or a forced-air circulating oven. The heating rate must be strictly maintained at 120 ± 10 °C/h.
Loading Device	A mechanism to apply the specified constant force (weight) to the specimen via a loading nose, creating the required bending stress.
Deflection Measurement Instrument	A dial gauge or linear variable differential transformer (LVDT) with a precision of at least ±0.01 mm to measure the mid-span deflection of the specimen accurately.
Temperature Measurement Instrument	Thermocouples or resistance thermometers with an accuracy of at least ±1 °C, placed near the specimen to monitor the fluid/air temperature.
Support Fixtures	Two parallel supporting hinges and a central loading nose, all made of stainless steel with specified radii to ensure proper three-point bending contact without inducing stress concentrations

Test Procedures of ISO 75 Test:

Measurement: Measure the exact dimensions (length, width, thickness) of the conditioned specimen at the mid-span.

Setup: Place the specimen on the supports of the testing apparatus, ensuring it is centered. Attach the deflection measuring device.

Loading: Apply the calculated weight (corresponding to the desired stress: 0.45 MPa, 1.80 MPa, or 8.00 MPa) to the loading rod. Allow the system to stabilize.

Heating: Start the heating device and the deflection recorder simultaneously. Ensure the temperature rises uniformly at 120 °C/h.

Recording: Continuously monitor the temperature and deflection. The test ends automatically or manually when the specimen's mid-span deflection reaches the predetermined standard deflection value (which corresponds to a flexural strain increment of 0.2% or 0.3%, typically around 0.25 mm to 0.34 mm depending on the span and thickness).

Result: The temperature at which the standard deflection is reached is recorded as the Heat Deflection Temperature (HDT) in degrees Celsius (°C).

Test Applications (Industry Fields)

The ISO 75 HDT test is fundamental across numerous industries for material selection and quality control:

Automotive: Evaluating under-hood components, connectors, and interior parts that may be exposed to high engine or environmental temperatures.

Electronics & Electrical: Assessing the thermal endurance of plastic housings, insulating parts, and circuit board components during soldering or operation.

Aerospace: Selecting lightweight polymer composites and thermoplastics for cabin interiors or secondary structures.

Medical Devices: Ensuring plastic syringes, casings, and instruments can withstand high-temperature sterilization (autoclaving) or hot water disinfection.

Consumer Goods & Packaging: Testing food containers for hot-fill applications or dishwasher safety.

Construction & Building: Verifying the thermal stability of PVC pipes, fittings, and insulation materials.

Test standard related:

ASTM D648	Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position
ISO 306	Vicat softening temperature; Measures temperature at which needle penetrates 1 mm under load
ISO 20753	Multipurpose test specimens; Specifies standard specimen dimensions
ISO 2818	Plastics — Preparation of test specimens by machining (used for specimen preparation).
GB/T 1634	Chinese standard Equivalent to ISO 75
DIN 53461	Testing of plastics; determination of heat deflection temperature under load.

Heat Distortion Temperature Tester for accurate measurement of plastic materials’ heat deflection under load, ensuring reliable thermal performance evaluation, quality control, and compliance with international standards.

Q1: What is the ISO 75 test?

A: The ISO 75 test is an international standard method for determining the Heat Deflection Temperature (HDT) of plastics. It measures the temperature at which a plastic specimen deflects (bends) by a specified amount under a defined constant load. It is a critical indicator of a material's short-term heat resistance.

Q2: Why is the HDT test so important for plastics?

A: HDT is crucial because it tells engineers and designers the upper temperature limit at which a plastic part can still support a load without excessive deformation. This prevents failures in real-world applications, such as:

A plastic car part warping under the hood.

A medical device deforming during steam sterilization.

An electronic housing softening near a heat-generating component.

Q3: What is the difference between the three parts of ISO 75?

A: The series divides responsibilities:

ISO 75-1: Covers the general principles and apparatus requirements applicable to all materials.

ISO 75-2: Provides specific details for most unreinforced and short-fibre-reinforced plastics (e.g., ABS, polycarbonate, glass-filled nylon).

ISO 75-3: Addresses high-strength thermosetting laminates and long-fibre-reinforced plastics (e.g., epoxy/G10 laminates, advanced composites).

Q4: What are the typical specimen dimensions?

A: Specimens are usually rectangular bars. Common sizes are:

Length: 80 mm to 120 mm

Width: 10 mm

Thickness: 3 mm to 13 mm

The exact size depends on the material type and the specific part of the standard being followed.

Q5: Does HDT indicate the maximum continuous service temperature of a plastic?

A: No. This is a common misconception. HDT is a short-term, accelerated test under a specific load. It does not predict long-term performance. For maximum service temperature, you need data from long-term thermal aging tests (like ISO 2578). However, HDT is an excellent screening tool for comparing materials quickly.

Q6: How does HDT relate to other thermal properties like Vicat Softening Temperature?

A: Both are short-term heat resistance tests, but they differ:

HDT (ISO 75): Uses a bending load and measures deflection. More relevant for structural parts under load.

Vicat (ISO 306): Uses a needle pressed into the material with a smaller force and measures penetration. More relevant for materials used in non-structural or lightly loaded applications.

Q7: Can I compare ISO 75 results with ASTM D648 results?

A: Yes, but with caution. ASTM D648 is the equivalent American standard. While the principles are similar, minor differences in specimen dimensions, heating rates, or deflection endpoints can lead to slightly different values. Always compare data generated under the same standard.

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