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Resources3D Printing DesignHeat Deflection Temperature: Definitions, Importance, and Factors That Affect HDT
Worker using heat gun to bend electric PVC pipe. Image Credit: Shutterstock.com/BaLL LunLa

Heat Deflection Temperature: Definitions, Importance, and Factors That Affect HDT

Learn more about property and various factors that affect it.

Xomety X
By Team Xometry
March 23, 2023
 10 min read
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Heat deflection temperature (HDT) is a crucial property of polymers. It determines their ability to remain rigid and maintain their shape under high temperatures and constant loads. HDT is an essential indicator of a material's maximum temperature for structural applications. It can be a good measure by which to compare materials with one another. HDT is also crucial in the injection molding industry, where it helps determine the cooling rate and temperature of the material during molding. 

This article will discuss the definition, importance, and factors that affect HDT as well as the way to measure it in polymers.

What Is Heat Deflection Temperature (HDT)?

Heat deflection temperature (HDT) refers to a polymer's ability to remain rigid or “stiff” under a constant load and elevated temperatures. It indicates at what temperature a polymer starts to “soften” under a fixed load. According to ASTM D 648, the HDT is the temperature at which a sample bar of standard dimensions (127x13x12 mm) deflects by 0.25 mm when it undergoes a centered standard flexural stress of either 46 kPa (67 psi) or 1.8 MPa (264 psi).

This property is known by several other names as well: (DTUL) deflection temperature under load, (HDTUL) heat deflection temperature under load, or (HDT) heat distortion temperature. 

What Is the Importance of Heat Deflection Temperature (HDT)?

HDT is a helpful gauge for determining the temperature at which a material cannot be used for structural applications. This value can be used to compare materials with one another. It also helps indicate how a molded plastic part will perform when placed under mechanical load and exposed to heat. HDT is an important property in the injection molding industry. It can give insight into the polymer’s moldability. 

The ejection stage of any injection molding process puts a load on the freshly molded object. The material must be stiff enough that the ejector pins will properly force it out of the mold. The relative difference between two materials’ HDT values can help you decide on the proper cooling rate and temperature. The ejector pins will sink into soft, flexible plastic that is still above its HDT. However, below that temperature, it gets hard and is simpler to push out of the mold. A higher HDT enables a faster molding process.  

Heat Deflection Temperature (HDT) is a critical property for 3D printing materials, especially when using fused deposition modeling (FDM) and Polyjet devices. It becomes an important consideration because the material is heated and melted to create a 3D object. During the printing process, the material is subjected to high temperatures. It must maintain its shape and structural integrity so that the final product is durable and looks right. 

Different 3D printing materials have different HDT values. Your choice of material depends on the specific application and operating conditions. For example, if a 3D-printed part will be used in high-temperature environments, it must have a high HDT value to maintain its shape and properties. For more information, see our guide on 3D Printing.

How To Measure Heat Deflection Temperature (HDT)?

A polymer testing rod is set up on a device similar to the one in the diagram below for an ASTM D 648 test.

Bars for this test are molded to a particular width and thickness. The sample is loaded to the required bending stress in a three-point bend test beam configuration that is submerged in oil, as indicated in the schematic testing arrangement in Figure 1 below:

Slide 1 of 1
heat deflection temperature testing apparatus
heat deflection temperature testing apparatus
heat deflection temperature testing apparatus

HSD testing apparatus.

Image Credit: https://polymerdatabase.com/

Typically, mineral oil or silicone oil is used as the heat-transfer medium because it has no impact on the specimen's mechanical characteristics. 

After that, the oil bath is gradually heated at a rate of 20.2°C/min. The device presses down on the test bar's midpoint with a constant applied load of either 0.46 MPa or 1.8 MPa. The HDT is the temperature at which a bar of material deforms by 0.25 mm.

What Are the Common Loads Used in Heat Deflection Temperature (HDT)?

In tests for heat deflection, the two typical loads are:

  1. 0.46 MPa (67 psi): Typical load for softer plastic grades like polyethylene (PE) or LDPE.
  2. 1.8 MPa (264 psi): Utilized for stronger and more durable plastics like PEEK or polycarbonate (PC).

Other loads can also be used to test plastics, like 5.0 MPa (725 psi) or 8.0 MPa (1160 psi). However, these are not as common. 

What Device Is Used in Measuring Heat Deflection Temperature (HDT)?

The equipment used to test this value is called an HDT tester or an HDT apparatus. It typically 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 determine the deflection of the sample at various temperatures.

The HDT test setup consists of a rectangular-shaped sample of a specific size and thickness that is loaded in a three-point bending configuration. The sample is heated at a constant rate, and the temperature at which the sample begins to deform under the specified load is recorded as the HDT.

1. 0.46 MPa (67 psi)

The 0.46 MPa heat deflection method is the most common version of the test for softer plastic grades like PE and LDPE. The heated sample is subjected to a load of 0.46 MPa (67 psi), and the temperature is gradually increased at a specified 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. Note that 0.25 mm is an arbitrary amount and has no significant value in and of itself. 

2. 1.8 MPa (264 psi)

The 1.8 MPa HDT test is very similar to the 0.46 MPa test, with the only difference being the load that is applied to the specimen. The 1.8 MPa (264 psi) HDT test is typically used to evaluate the thermal properties of high-performance thermoplastics and thermosetting materials designed for high strength and rigidity at elevated temperatures. Materials that are tested with this method include PC, PEEK, and epoxy resins. 

What Is the Standard Measuring for Heat Deflection Temperature (HDT) in ASTM?

ASTM D 648, which is comparable to ISO 75, is the ASTM (American Society for Testing and Materials) standard for measuring HDT.

What Are the Examples of Heat Deflection Temperatures (HDT) of Different Materials?

The typical deflection temperatures under a 0.46 MPa (67 psi) load and a 1.8 MPa (264 psi) load, as well as the melting points of certain polymers, are listed in Table 1 below:

Table 1: Thermal Properties of Polymers
Polymer TypeHDT at 0.46 MPa [°C]HDT at 1.8 MPa [°C]Melting Point [°C]
Polymer Type
ABS
HDT at 0.46 MPa [°C]
98
HDT at 1.8 MPa [°C]
88
Melting Point [°C]
-
Polymer Type
Acrylic
HDT at 0.46 MPa [°C]
95
HDT at 1.8 MPa [°C]
85
Melting Point [°C]
130
Polymer Type
Nylon 6
HDT at 0.46 MPa [°C]
160
HDT at 1.8 MPa [°C]
60
Melting Point [°C]
220
Polymer Type
Polycarbonate
HDT at 0.46 MPa [°C]
140
HDT at 1.8 MPa [°C]
130
Melting Point [°C]
-
Polymer Type
Polyethylene
HDT at 0.46 MPa [°C]
85
HDT at 1.8 MPa [°C]
60
Melting Point [°C]
130
Polymer Type
PET
HDT at 0.46 MPa [°C]
70
HDT at 1.8 MPa [°C]
65
Melting Point [°C]
250
Polymer Type
Polypropylene
HDT at 0.46 MPa [°C]
100
HDT at 1.8 MPa [°C]
70
Melting Point [°C]
160
Polymer Type
Polystyrene
HDT at 0.46 MPa [°C]
95
HDT at 1.8 MPa [°C]
85
Melting Point [°C]
-

Table Credit: https://www.matweb.com/

These averages may not apply to all grades of these given materials. 

Where Does Heat Deflection Temperature (HDT) Commonly Apply?

HDT is a well-known property and gets used in the engineering, design, and production of items with thermoplastic components. A lot of industries use this value to select the correct polymer for the job. Some of the top industries for which HDT is relevant include: packaging, aerospace, automotive, electronic, medical, and injection-molding industries. 

What Are the Factors That Impact the Heat Deflection Temperature (HDT)?

Factors that can influence the HDT include: the presence of additives or fillers in the material, the molecular weight and molecular weight distribution of the polymer, and the degree of crystallinity in the material’s molecular structure. The three biggest factors, however, are listed below:

1. The Time of Exposure to Elevated Temperature

The HDT is influenced by the length of time a material is exposed to elevated temperatures. The longer the material is heated, the more likely it is to deform under load. 

2. The Part Geometry

The geometry of the test specimen plays a crucial role in determining the HDT. The thickness and shape of the specimen can affect the rate of heat transfer and the distribution of stress within the sample during the test.

3. The Rate of Temperature Increase

The rate of temperature increase during the test affects the HDT. If temperatures rise slowly, the HDT may be higher while a speedy rate of heating can result in a lower HDT. The rate of temperature change can also affect the mechanical properties of the material, including the modulus of elasticity, tensile strength, and elongation at break.

What Is the Heat Deflection Temperature (HDT) of Nylon?

The heat deflection temperature (HDT) of Nylon will vary for different loads. The HDT is typically determined at a specific load, and increasing the load will result in a lower HDT.

For example, the HDT of unfilled and unmodified Nylon 6 and Nylon 6/6 at a load of 1.8 MPa (264 psi) can be in the range of 70-120°C. If the load is reduced to 0.46 MPa (67 psi), the HDT jumps into the range of 115-155°C.

It's important to note that HDT is just one of several factors to consider when selecting a material. The actual performance of Nylon in a given situation will depend on various factors such as the specific grade of Nylon, the geometry of the part, and the operating conditions beyond just the load and temperature.

Is Heat Deflection Temperature (HDT) Can Only Be Applied in Plastics?

Yes. Heat deflection temperature is a thermal property that’s unique to polymers and plastics. 

Can Heat Deflection Temperature (HDT) Be Also Applied to Steel?

No, HDT cannot be applied to steel. The heat deflection temperature (HDT) is a property that relates specifically to polymers and plastics. There are, however, other thermal properties that can provide valuable insights into how metals behave. 

What Is the Difference Between Heat Deflection Temperature and Glass Transition Temperature?

Amorphous materials' glass transition temperature (Tg) or melting temperature (Tm) are related to their heat deflection temperature. The HDT is a helpful indicator of the temperature over which the material cannot be used for structural purposes. HDT and Tg (or Tm) are, however, not interchangeable terms. The HDT is a macroscopic measurement of a material’s "stiffness" as the temperature increases, whereas the Tg specifies the temperature at which vitrification or glassification occurs (i.e. when the movements of molecular chain segments (kinetic units) begin to "freeze"). 

The two characteristics are usually closely related. Fillers and reinforcements, however, significantly alter the HDT since the former softens the polymeric component while the latter stiffens it. Depending on the type and amount of filler/fiber used, filled and unfilled polymers can have quite different HDTs. Plasticizers have similar impacts on both attributes. The HDT is frequently a better indicator of the temperature application limit for structural applications than is the Tg or Tm.

Summary

This article presented heat deflection temperature, explained what it is, and discussed how to measure it. To learn more about heat deflection temperature, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including 3D printing and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.