Heat Deflection Temperature

Heat Deflection Temperature

1. HDT is a relative measure of a material’s ability to perform for a short time at elevated elevated temperatures while a load.

2. The test measures the effect of temperature on stiffness: a standard test specimen is given a defined surface stress and the temperature is raised at a uniform rate.

3. Defined as the temperature at which a standard test bar (5x½x¼ in ) deflects 0.010

inch under a stated load of either 66 or 264 psi.

4. HDT values are used to compare the elevated temperature performance of the

materials under load at the stated conditions.

Test Method:

• ASTMD 648, ISO 75 -1 and 75-2

Test Specimen:

• 127mm (5 in.) in length, 13mm (½ in.) in depth by any width from 3mm ( ⅛ in.) to 13mm ((½ in.)

Conditioning:

• 23 2℃ and 50 5% RH for not less than 40 hrs prior to test.

Test Method

• Specimen Supports: Metal supports for the specimen of 100 ± 2mm

• Immersion Bath

• Deflection Measurement Device

• Weights: 0.455 MPa (66 psi) ± 2.5% or 1.82 MPa (264 psi) ± 2.5%.

• Temperature Measurement System

Two replicate specimens are used for each test

The weight of the rod used to transfer the force on the test specimen is included as part of the total load. The The load (P) is calculated as:

                         P= 2Sbd2/ 3L

Where,

S= Max. Fibre stress in the specimen of 66 Psi / 264 Psi 

b= Width of specimen

d= Depth of specimen

L= Width of span between suppor

HDT and Amorphous & Semi-Crystalline plastics

1. In amorphous polymers, HDT is nearly the same as the glass transition temperature Tg of  the material.

2. Because amorphous polymers have no defined melting temperature, they are processed in their rubbery state above Tg.

3. Crystalline polymers may show low HDT values and still have structural utility at higher

4. Temperatures.

5. HDT test method is more reprodcible with amorphous plastics than with crstalline.

6. With some polymers it may be necessary to anneal the test specimens to obtain reliable 

results.

7. Addition of glass fibres to the polymer will increase its modulus. Since the HDT represents a temperature where the material exhibits a defined modulus, increasing the modulus will also increase the HDT.

8. The results obtained by this test method do NOT represent maximum use temperatures, because in real life essential factors such as time, loading and nominal surface stress may differ from the testing conditions.

9. Glass fibres have a more significant effect on the HDT of crystalline polymers than on amorphous polymers.

10.The data are not intended for use in design or predicting endurance at elevated temperatures.

11.Used for screening and ranking materials for short-term heat resistance.

Factors Influencing HDT

• HDT of unannealed (heat treatment) specimen is usually lower than that of annealed specimen.

•Specimen thickness is directly proportional to HDT because of the inherently low thermal conductivity of plastic materials.

• Higher the fibre stress or loading lower the HDT.

• Injection moulded specimen tend to have a lower HDT than compression – moulded specimen.

• Compression moulded specimen are relatively stress free.