Heat Deflection Temperature (HDT): Definition, Significance, and Influencing Factors

When choosing a manufacturing material, particularly plastic, the technical data sheet will give you a bunch of useful information to help in your selection. The heat deflection temperature (HDT) is one such tidbit, and it’s an important one to know if your product needs to keep its shape and size under pressure.

What is Heat Deflection Temperature?

A material’s HDT is the temperature at which it starts to deform when it’s put under a specific load at high heat. It measures the plastic’s ability to stay stiff or rigid in these circumstances. High HDT plastics will stay more rigid in higher heats, and low HDT ones, not so much. And yes, this is only relevant to polymers and plastics, so metals will have to sit this one out (they have their own versions, like hot hardness and creep tests).

HDT is valuable (a must, in fact) for the engineering, design, and manufacturing of plastic products (or those with any parts made of plastic) in many industries, including:

• Packaging
• Aerospace
• Automotive
• Electronic
• Medical
• Consumer goods
• Construction industry
• Injection-molding

In packaging, containers at the grocery store, post office, and even at home, are stacked and squashed—and sometimes in heat—but they can’t warp under pressure. The same goes for vehicle parts, like dashboards. A car often sits in direct sunlight for hours on end, but it would be a total disaster if the dash sunk in when you put your bag or phone on it. In injection molding, a material’s HDT is one of the factors that will determine the process cycle time. It’s also a big decider in whether or not a plastic is suitable for use as a structural component, as these understandably have a pretty tough job.

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.

To learn more, see our guide on How 3D Printing Works.

How To Measure Heat Deflection Temperature (HDT)?

To work out a material’s HDT, we need heating, loading, and measuring systems—all of which are provided by the HDT test equipment/apparatus (pictured below). The standard measuring system for HDT is the American Society for Testing and Materials (ASTM) D 648—which is comparable to the ISO 75 international standard.

The HDT measuring process consists of the following 4 steps:

1. The sample is prepped and loaded on the machine in a three-point bending configuration, meaning it’s supported at its two ends so that pressure can be applied in the middle.
2. A constant bending stress (either 0.46 or 1.8 MPa, depending on what you’re testing) is applied to said middle.
3. A mineral or silicone oil that won’t affect the plastic is used as the heat transfer, and its temperature is gradually increased at a controlled rate of 2°C/min (±0.2°C/min).
4. The sample is monitored throughout all stages and temps, and when it deforms by 0.25 mm, we have the HDT.