Information on the most widely used ASTM standards within the materials testing industry
ISO 306 Plastics -- Thermoplastic materials -- Determination of Vicat softening temperature (VST)
ISO 306 used to determine the Vicat Softening Temperature (VST) of thermoplastic materials. The VST is defined as the temperature at which a thermoplastic material starts to soften rapidly under a specified load and heating rate.
Vicat softening temperature or Vicat hardness is the determination of the softening point for materials that have no definite melting point, such as plastics. It is taken as the temperature at which the specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 mm2 circular or square cross-section. For the Vicat A test, a load of 10 N is used. For the Vicat B test, the load is 50 N.
Standards to determine Vicat softening point include ASTM D1525 and ISO 306, which are largely equivalent.
The vicat softening temperature can be used to compare the heat-characteristics of different materials.
Test Principle
The Vicat softening temperature (VST) is the temperature at which a flat‑ended cylindrical indenter penetrates 1 mm into a thermoplastic specimen under a constant specified load while heated at a uniform controlled rate. This temperature indicates the point at which the thermoplastic material begins to soften rapidly under low indentation loading.
Specific Test Methods
ISO 306, ASTM D1525 Standard specifies four methods for the determination of the Vicat softening temperature (VST) of thermoplastic materials:
- method A50 using a force of 10 N and a heating rate of 50 K/h;
- method B50 using a force of 50 N and a heating rate of 50 K/h;
- method A120 using a force of 10 N and a heating rate of 120 K/h;
- method B120 using a force of 50 N and a heating rate of 120 K/h.

The methods specified are applicable only to thermoplastics, for which they give a measure of the temperature at which the thermoplastics start to soften rapidly.
Test Equipment required for ISO 306 Plastic Vicat softening temperature (VST)
| VST Testing Machine | Recommend UnitedTest HVT series HDT/VICAT Tester. |
| Indenter | Hardened steel, circular cross‑sectional area (1.00 ± 0.015) mm² (diameter 1.128 ± 0.008 mm), flat tip free of burrs.
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| Heating equipment | Thermostatically controlled bath (liquid/fluidized bed/direct contact), with stirring/fluidizing system; heating rate tolerance: 50 ± 5 °C/h or 120 ± 10 °C/h. |
| Loading weights | Total load 10 ± 0.2 N (A methods) or 50 ± 1.0 N (B methods). |
| Temperature‑measuring device | Resolution 0.1 °C, accuracy ±1 °C; sensor ≤12.5 mm from the indenter contact point. |
| Penetration‑measuring device | Readable to 0.01 mm (e.g., dial gauge, LVDT). |
| Dimension gauge | Readable to 0.01 mm, complying with ISO 16012. |
Test Specimen Information:
Dimensions: Specimens must have a thickness between 3 mm and 6.5 mm and a surface area of at least 9.5 mm by 9.5 mm (or a diameter of 9.5 mm) .
Surface Quality: Surfaces must be flat, parallel, and free from flash or imperfections.
Preparation: Specimens can be prepared by molding (following relevant material standards or ISO 293/ISO 294 series), by machining (ISO 2818), or by cutting from finished parts or sheets .
Granules/powders: Moulded per ISO 293, ISO 294‑1/‑2/‑3, or ISO 20753.
Thick sheets (>6.5 mm): Machined to 3–6.5 mm (one surface machined, test on the intact surface).
Thin sheets (<3 mm): Stack up to 3 layers (total thickness 3–6.5 mm; top layer ≥1.5 mm).
Stacking Rule: If a sheet is thinner than 3 mm, up to three pieces can be stacked to reach the minimum thickness, provided the top measured piece is at least 1.5 mm thick .
Quantity: A minimum of two specimens must be tested for each sample.
Key Test Parameters
Applied Force: Precisely controlled at 10 N (±0.2 N) or 50 N (±1.0 N) depending on the selected method .
Heating Rate: Uniform elevation of temperature at 50°C/h (±5°C) or 120°C/h (±10°C) .
Starting Temperature: The heating apparatus must start at a maximum of 25°C, or at least 50°C below the expected VST .
Penetration Endpoint: The test automatically concludes when the indenter penetrates 1.00 mm (±0.01 mm) into the specimen from its initial position.
Test Notice:
Applicable only to thermoplastic materials.
The indenter must be ≥3 mm from the specimen edge.
Liquid‑bath heating at 50 °C/h is the reference method for disputes.
If the range of individual VST values exceeds 2 °C, repeat with another 2 specimens and report all data.
Results from different heating modes or methods are not directly comparable without correlation.
Specimen preparation history (moulding, annealing) may affect results and must be reported.
ISO 306 VST Test Procedure:
1, Mounting: Place the specimen horizontally under the indenter. If using a liquid bath/fluidized bed, assemble the rod/frame and place it in the heating equipment. If using a direct-contact heater, position the specimen between the heating blocks .
2, Initial Load & Zeroing: Apply the specified weight (10 N or 50 N) to the specimen via the indenter. After 5 minutes, set the penetration-measuring device to zero (or record the initial reading) .
3, Heating: Begin raising the temperature of the heating medium at the specified uniform rate (50°C/h or 120°C/h). Ensure the medium is stirred well .
4, Determine VST: Continuously monitor the penetration depth. Record the exact temperature of the medium at the instant the indenter penetrates exactly 1.00 mm into the specimen . This temperature is the VST for that specific specimen.
5, Result Calculation: Calculate the arithmetic mean of the VSTs from all tested specimens. If the range between the highest and lowest individual results exceeds 2°C, the individual results must be reported, and the test repeated on a new set of specimens .
Test Applications (Industry Fields)
The VST test is a fundamental quality control and research metric in virtually all sectors utilizing thermoplastic polymers:
Automotive: Assessing dashboard components, trim, and under-hood parts for heat resistance.
Building & Construction: Evaluating PVC pipes, window frames, and insulation materials.
Consumer Electronics: Ensuring plastic casings and connectors can withstand soldering temperatures and operational heat.
Medical Devices: Testing the thermal stability of polymer housings and components during sterilization.
Packaging: Determining the suitability of plastics for hot-fill applications or microwave heating.
Related test stanadrd:
| ASTM D1525 | Standard Test Method for Vicat Softening Temperature of Plastics |
| DIN 53460 | Testing Plastics to Determine the Vicat Softening Temperature of Thermoplastics |
| NF T51-021 | Plastics – Thermoplastic materials – Determination of Vicat softening temperature (VST) |
| CSN 64 0521 | Plastics. Determination of the Vicat softening temperature of thermo- plastics |
| ISO 2507-1 | Thermoplastics pipes and fittings. Vicat softening temperature. General test method |
| ISO 2507-2 | Thermoplastics pipes and fittings — Vicat softening temperature — Part 2: Test conditions for unplasticized poly(vinyl chloride) (PVC-U) |
| ISO 2507-3 | Thermoplastics pipes and fittings - Vicat softening temperature - Part 3: Test conditions for acrylonitrile/butadiene/styrene (ABS) and acrylonitrile/styrene/acrylic ester (ASA) pipes and fittings |
Test Scope:
The Vicat softening temperature is the temperature at which a flat-ended needle penetrates the specimen to the depth of 1 mm under a specific load. The temperature reflects the point of softening to be expected when a material is used in an elevated temperature application.
Specimen Size:
The test specimen must be between 3 and 6.5 mm thick and at least 10 mm in width and length. No more than three layers may be stacked to achieve minimum thickness.
Data:
The Vicat softening test determines the temperature at which the needle penetrates 1 mm.
Related products and device
Related Standard
ASTM D1525: Standard Test Method for Vicat Softening Temperature of Plastics
ASTM D1525 test method covers determination of the temperature at which a specified needle penetration occurs when specimens are subjected to specified controlled test conditions. It's equivalent to standard ISO306.
Test Procedure:
A test specimen is placed in the testing apparatus so that the penetrating needle rests on its surface at least 1 mm from the edge. A load of 10N or 50N is applied to the specimen. The specimen is then lowered into an oil bath at 23 degrees C. The bath is raised at a rate of 50° or 120° C per hour until the needle penetrates 1 mm.
ISO 75 Plastics -- Determination of temperature of deflection under load.
ISO 75 specifies a general test method for the determination of the temperature of deflection under load (flexural stress under three-point loading) of plastics. Different types of test specimen and different constant loads are defined to suit different types of material. It is determined for materials such as thermoplastics, hard rubber and duroplastic laminates.
Q&A FAQ: ISO 306 Vicat Softening Temperature (VST) Test
Q1: What is the ISO 306 test and what does it measure?
A: ISO 306:2022 specifies a method for determining the Vicat Softening Temperature (VST) of thermoplastic materials. It measures the temperature at which a thermoplastic starts to soften rapidly under a specified load and heating rate, indicated by a 1 mm penetration of a standardized indenter into the specimen.
Q2: Why is the Vicat softening temperature important?
A: The VST is a critical indicator of a thermoplastic’s heat resistance. It helps manufacturers and designers select materials that can withstand processing temperatures (e.g., injection molding, extrusion) and service temperatures without deforming. It is also essential for quality control, comparing material grades, and ensuring compliance with industry specifications.
Q3: When must I repeat the test?
A: Repeat if the range of individual VST results exceeds 2 °C.
Q4: Is ISO 306 suitable for thermosets?
A: No, it applies only to thermoplastic materials.
Q5: Which method is used for dispute resolution?
A: Liquid-filled heating bath at 50 °C/h (the reference method).
Q6: What is the indenter specification?
A: Hardened steel, flat tip, cross-sectional area 1.00±0.015 mm² (diameter 1.128±0.008 mm).
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