GB/T 1633-2025Plastics - Thermoplastic materials - Determination of Vicat softerning temperature(VST) (English PDF)

5.1 Device for generating penetration

The device consists of a rigid metal frame containing a rod, that can move freely in the vertical direction. A load plate is mounted at one end of the rod; the indenter is mounted at the other end. A support plate or other suitable load application device is mounted at the bottom of the metal frame, as shown in Figure 1 or Figure 2.

Figure 1 -- Schematic diagram of test equipment with heating device as liquid bath or powder fluidization bed

Figure 2 -- Schematic diagram of test equipment with heating device as direct contact heating device

The load rod and rigid metal frame should be made of rigid material with a low coefficient of thermal expansion. Vertical components shall have the same coefficient of thermal expansion to avoid errors in the specimen penetration reading, which is caused by changes in the length of these components during testing.

After manufacturing, repairing, or replacing the specimen holder, a blank test shall be performed on each metal holder, which has a rigid material with a low coefficient of thermal expansion and a thickness comparable to the test specimen. This test shall cover the actual operating temperature range of the test material; a correction term shall be assigned for each temperature. If the correction term is greater than or equal to 0.02 mm, the value and algebraic sign shall be recorded; this shall be added algebraically to the penetration measurement reading. This correction term is used in each test.

Note. Quartz and borosilicate glass are suitable materials for blank specimens.

5.2 Indenter

The indenter shall be a hardened steel cylinder, which as a length of not less than 2 mm, a cross-sectional area of (1.000 ± 0.015) mm², a diameter of (1.128 ± 0.008) mm, fixed to the bottom of the load rod. The lower surface of the indenter in contact with the specimen shall be flat, perpendicular to the axis of the load bar, free of burrs.

5.3 Heating equipment

The heating equipment shall be a liquid bath, fluidization bed, or direct contact heating device, as shown in Figure 2.For heat transfer media other than gas (air), the specimen immersion depth shall be at least 35 mm.

A high-efficiency stirrer or device for flowing the solid heat transfer medium shall be provided. If a liquid is used for heat transfer, it shall be ensured that the selected liquid is stable at the operating temperature and has no effect on the specimen, such as expansion or cracking.

If different heating equipment selections lead to differences in test results, a liquid bath shall be used.

The heating equipment shall be equipped with a temperature controller capable of uniformly increasing the temperature at a rate of (50 ± 5) °C/h or (120 ± 10) °C/h. The heating rate shall be periodically verified, by automatically checking the temperature readings or manually checking the temperature.

The heating rate shall be verified periodically, by checking the automatic temperature readings or manually checking the temperature. If the temperature change is (5.0 ±

0.5) °C or (12.0 ± 1.0) °C every 6 minutes during the test, the heating rate requirement shall be considered met.

Since most instruments use a proportional-integral-derivative (PID) controller to control heating, temperature changes exceeding the specified tolerance are permissible in the first 10 minutes of heating or before reaching 40 °C. Under normal circumstances, the controller will adjust itself to the correct power and interval requirements to achieve the desired heating rate.

Note. Accelerated cooling is preferable for heating equipment.

Liquid paraffin, transformer oil, glycerin, silicone oil can be used as liquid heat transfer media, or other liquids. For fluidization beds, alumina powder may be used.

5.4 Counterweight

The load applied to the specimen shall be (10 ± 0.2) N (A50 and A120 methods) or (50

± 1) N (B50 and B120 methods).

5.5 Temperature measuring device

A temperature measuring device with an appropriate measuring range shall be used, with a minimum graduation of 0.1 °C and a maximum permissible error of ±1 °C. The temperature measuring device shall be calibrated, within the specific immersion depth used by the equipment and within the range including the Vicat softening point temperature to be measured. For multi-station heating equipment, a separate temperature measuring device shall be provided at each station. In this case, the temperature sensing part of the instrument shall not be more than 12.5 mm away from the point where the indenter contacts the specimen. The temperature sensing part of the instrument shall not contact the specimen or any part of the metal frame.

Note. Calibration methods for the temperature measuring device include static calibration (at one or more constant temperatures) and dynamic calibration (using a constant heating rate).

Dynamic calibration can measure the temperature hysteresis of built-in temperature sensors, but requires a reference temperature sensor with suitable dynamic characteristics.

Thermocouples shall conform to GB/T 16839.1 and GB/T 4990.Resistance thermometers shall conform to GB/T 30121.

5.6 Penetration measurement device

A calibrated dial indicator, linear variable displacement sensor (LVDT), or other suitable measuring device shall be used to measure the penetration of the indenter into the specimen. The reading shall be readable to 0.01 mm.

When using a dial indicator, the dial indicator's thrust shall be recorded and considered as part of the thrust exerted on the specimen. If the dial indicator's spring force is upward, it shall be subtracted from the load; if this force is downward, it shall be added to the load. Throughout the process, due to the significant variation in the force applied to the dial indicator spring, the measurement shall be taken at a depth of 1 mm into the specimen. The total downward thrust determined during equipment calibration, including the load bar, indenter, upward or downward force applied by the dial indicator spring within the measuring range used during testing, shall not exceed 1 N.

5.7 Micrometers and gauges

Used to measure the width and thickness of specimens, with a minimum graduation of

0.01 mm, conforming to GB/T 44535.