HDT and Vicat Instruments

Standard-compliant testing to ISO 75, ISO 306, ASTM D1215 and ASTM D648

With the Amsler HDT/Vicat Allround 6-300, ZwickRoell offers a motorized instrument with a fully automated test sequence for convenient and easy testing in the field of research and development, incoming goods inspection, production monitoring and for teaching and training purposes. All standards to ISO 75, ISO 306, ASTM D1215 and ASTM D648 are covered with the tests.

Overview of HDT and Vicat testing instruments

Determining the Heat Deflection Temperature

Many plastics applications are placing increasing demands on heat resistance. An important value with regard to the temperature resistance of plastics is the heat deflection temperature. This can be divided into Vicat softening temperature and HDT (heat deflection temperature). In both methods an oil-bath is heated at a defined rate and the temperature is recorded at a deflection or indentation depth specified by the standard.

Vicat softening temperature to ISO 306 and ASTM D1525 (VST: Vicat softening temperature)
Vicat softening temperature as per ISO 306 and ASTM D1525 as "dry method"
Heat deflection temperature ISO 75 ASTM D648 (HDT: heat deflection/distortion temperature)

Vicat Softening Temperature (VST) to ISO 306

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, totaling 4 different test methods in all:

Method A 50 = 10 N load, heating rate 50 K/h
Method A 120 = 10 N load, heating rate 120 K/h
Method B 50 = 50 N load, heating rate 50 K/h
Method B 120 = 50 N load, heating rate 120 K/h

Heat deflection temperature to ISO 75 (HDT)

The heat deflection temperature (HDT) to ISO 75 indicates the relative behavior of different types of material under load at elevated temperatures. It is determined on materials such as thermoplastics, hard rubber or duroplastic laminates.

Test procedure

For the test to ISO 75, 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 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.

Parameter for tests to ISO 75

Deflection:Deflection is specified by ASTM as an absolute deflection of 0.25 mm; ISO standards define an increase in flexural strain of 0.2%.
Specimen positioning:The standard also specifies how the specimen is to be positioned on the anvil: flatwise or edgewise.
Flexural stress:
The standards also specify different flexural stresses:

ISO 75

Method HDT A: flexural stress = 1.8 MPa
Method HDT B: flexural stress = 0.45 MPa
Method HDT C: flexural stress = 8.0 MPa

ASTM D 648

1.82 MPa and 0.455 MPa
Duropl. laminates:1/1000 of flexural Young's modulus

Test weight

Accurate measurement of specimens before the test, or the use of precisely identical specimens, is therefore extremely important with this method. Use of the standard HDT weight set is only worthwhile if specimen dimensions are adhered to exactly (±0.05 mm).In all other cases the universal set of weights is used.