Heat deflection temperature

The heat deflection temperature or heat distortion temperature (HDT or HDTUL) is the temperature at which a polymer or plastic sample deforms under a specified load. This property of a given plastic material is applied in many aspects of product design, engineering, and manufacture of products using thermoplastic components.

Determination
The Heat Distortion Temperature is determined by following the test procedure outlined in ASTM D648. The test specimen is loaded in three-point bending in the edgewise direction. The outer fiber stress used for testing is either 0.455 MPa or 1.82 MPa, and the temperature is increased at 2 °C/min until the specimen deflects 0.25 mm.

Limitations that are associated with the determination of the HDT is that the sample is not thermally isotropic and, thick samples in particular, will contain a temperature gradient. The HDT of a particular material can also be very sensitive to stress experienced by the component which is dependent on the component’s dimensions. The selected deflection of 0.25 mm (which is 0.2% additional strain) is selected arbitrarily and has no physical meaning.

Design
Thermal simulations of a system will show temperatures that will be encountered by a specific component of that system. Knowing what temperature that a specific component will have to endure during use will allow the determination of the best material for that application.
 * Example: One of two materials may be used for Widget A of a system, acrylic or polycarbonate.  Widget A will have to endure temperatures of 120 °C during use.  Polycarbonate (HDT=140 °C) will not deform at 120 °C but acrylic (HDT=90 °C) would deform.  Polycarbonate would be used for Widget A in this case.

Injection molding
An injection molded plastic part is considered "safe" to remove from its mold once it is near or below the HDT. This means that part deformation will be held within acceptable limits after removal. The molding of plastics by necessity occurs at high temperatures (routinely 200 degrees Celsius or higher) due to the high viscosity of plastics in fluid form (this issue can be addressed to some extent by the addition of plasticizers to the melt). Once plastic is in the mold, it must be cooled to a temperature to which little or no dimensional change will occur after removal.

A major drawback of this to industrial applications is that, in general, plastics do not conduct heat well and so will take quite a while to cool to room temperature. One way to mitigate this is to use a cold mold (thereby increasing heat loss from the part). Even so, the cooling of the part to room temperature takes too long for the mass production of parts.

As such, the heat deflection temperature plays an important role, as it allows for manufacturers to achieve a much faster molding process than they would otherwise. The HDT does not signify the part to be non-susceptible to dimension changes, but, as mentioned earlier, these dimension changes will be within certain acceptable limits.