Heat Deflection Temperature (HDT): Method, Importance, Application

Heat Deflection Temperature (HDT) is an important characteristic of polymers, particularly those utilized in resin 3D printing and polymer additive manufacturing. It describes a polymer’s capacity to remain rigid or “stiff” under constant load and high temperatures. It denotes the temperature at which a polymer begins to “soften” under a constant load.

Heat Deflection Temperature (HDT) Method, Values, Importance, Application
Heat Deflection Temperature (HDT) Method, Values, Importance, Application

The HDT is an important indicator of a material’s maximum temperature in structural applications. It can be a useful tool for comparing different materials. HDT is especially important in the injection molding sector since it helps determine the cooling rate and temperature of the material throughout the molding process.

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What is Heat Deflection Temperature (HDT)?

The heat deflection temperature (HDT) is the temperature at which a material deflects under a certain load. It’s also called the “heat distortion temperature” or “heat deformation temperature.” The HDT is a significant metric for determining the mechanical properties of materials, particularly plastics, and it is frequently used to compare similar materials. It indicates a material’s ability to withstand deformation when subjected to heat.

To measure a material’s HDT, place a sample on a heated plate and apply a load. The temperature at which the sample deflects a predetermined distance is then measured. The load used is commonly a 1 or 0.45 mm stainless steel ball. The HDT is often represented in degrees Celsius or Fahrenheit.

Importance of Heat Deflection Temperature (HDT)

  • HDT is the highest temperature at which a material can be employed in structural applications without considerable deformation.
  • HDT is useful for comparing thermal properties of various materials.
  • It aids in predicting how a molded plastic part will perform when subjected to mechanical pressure and heat.
  • The injection molding industry relies heavily on HDT to understand the moldability of polymers.
  • During the ejection stage, the material must be firm enough to allow the ejector pins to force it out of the mold.
  • HDT readings help determine the correct cooling rate and temperature for the molding process.
  • Materials below their HDT grow harder and easier to eject, whilst those above it remain soft and flexible, producing problems with ejector pins.
  • A greater HDT enables a faster molding process, increasing total manufacturing efficiency.

Applications of Heat Deflection Temperature

The Deflection Temperature (HDT) is the temperature at which a plastic or polymer material deforms under a certain stress. It is an important feature for determining the performance of plastic materials in high-temperature environments.

Some common applications of HDT are:

  1. Injection molding: HDT is critical in establishing whether a plastic material can tolerate high temperatures during injection molding. It ensures that the plastic substance retains its shape and integrity during the molding process.
  2. Aerospace industry: The aerospace industry employs HDT to assess the performance of plastic materials in high-temperature conditions. The HDT test examines whether the materials can endure the extreme temperatures and stress that will occur during flight.
  3. Automotive Industry: HDT plays an important role in influencing the performance of plastic materials in the automobile industry. It assesses whether the materials can endure the extreme temperatures and strains that they will face in engine components, beneath the hood, and elsewhere.
  4. Consumer Goods Industry: HDT is used to assess the performance of plastic materials in a variety of consumer product applications. For example, it examines if plastic materials can survive high temperatures during cooking or food preservation.
  5. Electrical Industry: The electrical sector employs HDT to evaluate the performance of plastic materials in high-temperature electrical applications. It assesses whether the materials can endure the high heat and strains that they will face in electrical components.
  6. Building and Construction Industry: HDT is used to assess the performance of plastic materials in high-temperature conditions in the building and construction industries. It examines if the materials can endure the high temperatures and pressures that will occur in roofing, piping, and other applications.

Device Used in Measuring Heat Deflection Temperature

This value is tested using equipment known as an HDT tester or HDT instrument. It usually consists of a heating system to raise the temperature of the sample, a loading system to apply force to the sample, and a measuring system to detect the sample’s deflection at different temperatures.

The HDT test setup includes a rectangular-shaped sample with a defined size and thickness that is loaded in a three-point bending arrangement. The HDT is calculated by heating the sample at a continuous rate until it begins to distort under the prescribed load.

0.46 MPa (67 PSI)

  • The 0.46 MPa heat deflection method is the most widely used form of the test for softer plastic grades such as PE and LDPE.
  • The heated sample is subjected to a 0.46 MPa (67 psi) load, and the temperature is steadily increased at a controlled rate.
  • The deflection of the sample is measured at various temperatures, and the temperature at which the sample deflects 0.25 mm under the load of 0.46 MPa (67 psi) is recorded as the HDT.
  • It is important to note that 0.25 mm is an arbitrary number with no inherent significance.

1.8 MPa (264 PSI)

  • The 1.8 MPa HDT test is nearly identical to the 0.46 MPa test, with the sole change being the load applied to the specimen.
  • The 1.8 MPa (264 psi) HDT test is commonly used to assess the thermal properties of high-performance thermoplastics and thermosetting materials designed to maintain high strength and rigidity at high temperatures.
  • Materials tested with this approach include PC, PEEK, and epoxy resins.

Heat Deflection Temperature (HDT) Measurement Procedure

Heat Deflection Temperature (HDT) is a crucial characteristic that indicates a polymer’s ability to withstand structural changes under a specified load at higher temperatures. It is also known as Heat Distortion Temperature, Deflection Temperature Under Load (DTUL), or Heat Deflection Temperature Under Load (HDTUL).

Standard Testing Method ASTM D648

The ASTM D648 (equal to ISO 75) test is an international standard that specifies how to quantify the HDT of polymers. In this procedure, a test bar constructed of the polymer to be examined is inserted into an oil-filled 3-point bending device. The temperature of the oil is gradually increased while a known constant load is supplied to the center of the polymer bar. The temperature of the oil is raised until the polymer bar deforms by 0.25mm, indicating the Heat Deflection Temperature (HDT).

  • A test bar with a specified thickness and breadth is manufactured.
  • The test sample is immersed in oil and heated at a constant rate (typically 2°C per minute).
  • The load is applied to the middle of the test bar, which is supported at both ends.
  • The HDT is the temperature at which a 0.25mm-thick bar of material deforms.

Factors Affecting Heat Deflection Temperature (HDT)

The Heat Deflection Temperature (HDT) analyzes how temperature affects a polymer’s stiffness. While it is a useful estimate, it cannot accurately forecast the performance of the final component. Several variables affect HDT, including:

  • Extrusion Conditions: During the extrusion process, polymer chains’ orientation can be adjusted. Higher levels of orientation typically result in higher HDT values, as aligned chains are more resistant to deformation under heat.
  • Morphology: Polymers with higher crystallinity have higher HDT values. This is because crystalline portions are more stiff and thermally stable than amorphous regions, which improves the material’s resistance to heat-induced deformation.
  • Filler Loading: Adding fillers, such as glass fibers or mineral fillers, to a polymer improves its stiffness and thermal resistance. As a result, materials with higher filler loads have higher HDT values. Heat causes these reinforced and filled grades to harden and stiffen.
  • Presence of plasticizers: Plasticizers are additions that make polymers more flexible and less brittle. However, their presence reduces the HDT by making the polymer softer and less resistant to deformation at high temperatures.
  • The time of exposure to elevated temperature: The duration of a material’s exposure to elevated temperatures influences the HDT. The longer the material is heated, the greater the likelihood of deformation under load.
  • Part geometry: The HDT is heavily influenced by the test specimen’s geometry. The specimen’s thickness and shape can influence the rate of heat transfer and stress distribution during the test.
  • Rate of temperature increase: The HDT is affected by the rate at which the temperature increases during the test. If temperatures rise slowly, the HDT may be higher, but a rapid pace of heating can result in a lower HDT. The pace of temperature change can also influence the material’s mechanical properties, such as modulus of elasticity, tensile strength, and elongation at break.

Heat Deflection Temperature Values for Common Polymers

Polymer NameMin Value (°C)Max Value (°C)
ABS – Acrylonitrile butadiene styrene88100
PA – Nylon Polyamide, 66 30% Glass Fiber230255
PAI – Polyamide-Imides (TORLON)275280
PC – Polycarbonate, high heat140180
PE – Polyethylene, 30% glass fiber121121
PEEK – Polyetheretherketone150160
PEI – Polyetherimide (ULTEM)190200
PP – Polypropylene (30-40% Glass fiber-reinforced)125140
PP – Polypropylene Homopolymer/Copolymer5060
PS – Polystyrene, high heat85100
PSU – Polysulfone160174
PTFE – Polytetrafluorethylene4550
PVC – Polyvinyl chloride, rigid5475
PVDF – Polyvinylidene fluoride (KYNAR)50125

Difference Between Heat Deflection Temperature (HDT) and Glass Transition Temperature (Tg)

Heat Deflection Temperature (HDT)Glass Transition Temperature (Tg)
A macroscopic measurement of a material’s stiffness as temperature increasesThe temperature at which vitrification or glassification occurs
The temperature above which the material cannot be used for structural purposesThe temperature at which molecular chain segment movements begin to “freeze”
Indicates stiffness and load-bearing capabilityIndicates the transition from a rubbery state to a glassy state
Significantly alters HDT by stiffening the materialMay not have as pronounced an effect on Tg
Plasticizers soften the polymer, reducing HDTPlasticizers also reduce Tg by increasing chain mobility
Better indicator of the temperature application limit for structural applicationsUseful for understanding the material’s behavior at a molecular level
Often related to Tg or Tm, but not interchangeableRelated to HDT but describes different material behavior
Used to determine maximum operational temperature for load-bearing applicationsUsed to understand the thermal behavior and processing conditions


  • https://infinitalab.com/blogs/common-uses-of-heat-deflection-temperature-hdt/
  • https://omnexus.specialchem.com/polymer-property/heat-deflection-temperature-of-plastics
  • https://aipprecision.com/understanding-heat-deflection-temperature-plastics/
  • https://www.makeitfrom.com/glossary/Heat-Deflection-Temperature
  • https://www.xometry.com/resources/3d-printing/heat-deflection-temperature/

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Kabita Sharma

Kabita Sharma, a Central Department of Chemistry graduate, is a young enthusiast interested in exploring nature's intricate chemistry. Her focus areas include organic chemistry, drug design, chemical biology, computational chemistry, and natural products. Her goal is to improve the comprehension of chemistry among a diverse audience through writing.

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