HDT and Vicat Testing Instruments

Vicat softening temperature (VST) provides a value for the temperature at which a thermoplastic material begins to soften rapidly. This temperature is determined using a needle-shaped indenter with a round cross-sectional area of 1 mm².

Test sequence

• The indenter is placed on the plastic specimen and loaded with a defined test weight.
• The specimen is then heated at a specified rate.
• The VST is obtained when the needle penetrates to a depth of 1 mm.
• ISO 306 and ASTM D1525 are identical with regard to the test sequence.

ISO 306 distinguishes between 2 procedures and 2 different heating rates, giving 4 different test methods in all:

• Method A = 10 N load, heating rate 50 K/h
• Method A = 10 N load, heating rate 120 K/h
• Method B = 50 N load, heating rate 50 K/h (preferred method for comparative tests ISO 13050-1)
• Method B = 50 N load, heating rate 120 K/h

The classical medium used for heat transfer is oil. An alternative is the "dry method", in which heat transfer is via heating blocks. A special version of the ZwickRoell instrument, the Vicat Dry, is available for this method.

Heat deflection temperature (HDT) (ISO 75-1/2/3) indicates the relative behavior of different types of material under load at elevated temperatures. It is determined for materials such as thermoplastics, hard rubber and duroplastic laminates.

Test sequence

• A specimen is placed in a 3-point flexure test kit and loaded with the test weight required to achieve the flexural stress specified in the standard.
• The temperature of the oil-bath is then increased at a uniform heating rate of 120K/h.
• The temperature at which the specimen reaches the deflection specified in the standard is determined.