Heat Deflection Temperature
Encyclopedia
The heat deflection temperature or heat distortion temperature (HDT, HDTUL, or DTUL) 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.
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.
s 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 can take 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.
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
at which a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
or plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
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
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
components.
Determination
The Heat Distortion Temperature is determined by the following 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. This is similar to the test procedure defined in the ISOInternational Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
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 Component A of a system, acrylic or polycarbonatePolycarbonatePolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...
. Component 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 Component 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 °C or higher) due to the high viscosity of plastics in fluid form (this issue can be addressed to some extent by the addition of plasticizerPlasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
s 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 can take 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.