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Plastics & Elastomers
The material selection platform
Plastics & Elastomers

Heat Deflection Temperature of Plastics

Heat Deflection Temperature (HDT) of Plastics HDT stands for Heat Deflection Temperature or Heat Distortion Temperature. The heat deflection temperature is a measure of polymer’s resistance to distortion under a given load at elevated temperature.

In other words, it is the temperature at which a given polymer test bar will be bended of 0.25 mm under a given load. It is one of the two basic methods for assigning a value to the performance of plastics at high temperature. The value of 0.25 mm is an arbitrary value and has no specific significance.

The deflection temperature is also known as the 'deflection temperature under load' (DTUL), 'heat deflection temperature under load (HDTUL)', or 'heat distortion temperature' (HDT).

HDT is a measure of the “stiffness” of the material
as the temperature increases

The two common loads used in heat deflection testing are:

  »  0.46 MPa (67 psi) 
  »  1.8 MPa (264 psi)

Although tests performed at higher loads such as 5.0 MPa (725 psi) or 8.0 MPa (1160 psi) are occasionally encountered but they are not discussed here.

Limitations that are associated with the determination of the HDT is that the sample is not thermally isotropic and, in thick samples in particular, will contain a temperature gradient.

Applications include:

  • Screen materials for injection molding developments
  • Manufacture of products using thermoplastic components
  • Defines use of materials in heating elements

Check out more on HDT:

  »  Significance of HDT
  »  How to Calculate the HDT for plastics
  »  Heat Deflection Temperature Vs Vicat Softening Temperature
  »  Heat Deflection Temperature Vs Glass Transition Temperature (Tg)
  »  Factors Affecting Heat Deflection Temperature
  »  HDT Values of Several Plastics

Significance of HDT

  »  Represents a value which can be used to compare different materials with each other
  »  Applied in many aspects of product design, engineering and manufacture of products using thermoplastic components
  »  Higher heat deflection temperature means a faster molding process in injection molding processes

How to Measure HDT Value for plastics?

The ASTM test is ASTM D 648 (which is equivalent to the ISO 75).

  »  In the case of the HDT @ 1.8 MPa the load value is exactly 1820 kPa, which is commonly referenced as 1.8MPa (264 psi)
  »  In the case of the HDT @ 0.46MPa the load value is 0.46 MPa (66 psi)

The test is performed using an apparatus as shown below.

How to measure HDT value for plastics?
Source: The Effect of Long Term Thermal Exposure
on Plastics and Elastomers by Laurence W. McKeen

A test bar is molded of a specific thickness and width. The test sample is submerged in oil for which the temperature is raised at a uniform rate (usually 2°C per minute). The load is applied to the midpoint of the test bar that is supported near both ends. The temperature at which a bar of material is deformed 0.25mm is recorded as the HDT.

Factors Influencing Thermal Performance

This test measures short term performance under load at elevated temperatures for a by measuring the effect of temperature on stiffness. However, this is only an estimate, and cannot predict how the final part will perform. Other factors such as:

  »  The time of exposure to elevated temperature
  »  The rate of temperature increase
  »  The part geometry

… will significantly influence the final thermal performance of an application.

It is also important to note that HDT for a specific polymer grade will depend on the base resin and on the presence of reinforcing agents, fillers and plasticizers.

  »  Reinforced and filled grades have high HDT (harder and stiffer under the heat), while
  »  Plasticizers decrease HDT by making polymer softer and more flexibility

Heat Deflection Temperature Vs Vicat Softening Temperature

Vicat test is used to identify a temperature at which a needle of specified dimensions penetrates into plastic specimen a specified distance under a given load.

Compared with the Heat Deflection Temperature (HDT) test, the vicat test reveals the temperature at which the specimen loses its “stability-form” and softens, while HDT test measures the temperature at which the specimen loses its “load-bearing” capability. The Vicat point is closer to the actual melting or softening point of the polymer, the Vicat number will typically be higher.

Heat Deflection Temperature Vs Glass Transition Temperature (Tg)

As discussed above, HDT gives an indication of how the material behaves when stressed at elevated temperatures. A material can have only “one” HDT. While Glass transition temperature of a material is entirely different.

The Glass transition temperature is the temperature at which a crystalline or semi-crystalline portion of the polymer melts and changes from an ordered structure to an amorphous structure. And, since it depends primarily on the polymer structure or morphology, Tg for a material is very little affected by addition od reinforcement, fillers, plasticizers or any other type of additive. Though it is also important to note- It is possible that a material can have more than one Tg, one for each structurally distinct crystalline phase.

Factors Affecting Heat Deflection Temperature

  »  Extrusion Conditions - Higher Orientation levels generally lead to higher HDT values
  »  Morphology - Higher crystallinity levels mean higher HDT values
  »  Filler Loadings - An increase in filler loadings leads to an increase in the HDT

HDT Values of Several Plastics

  »  HDT @0.46 Mpa (67 psi)
  »  HDT @1.8 Mpa (264 psi)

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