Heat deflection temperature, or HDT, measures how much heat a plastic can withstand before it starts to bend. It is a key check for thermal resistance. This guide explains the test, common grades, and how to use the data when choosing materials for structural parts.
- HDT is a standard test that shows the temperature where a plastic bar begins to bend under a specific load.
- Higher HDT grades resist heat better but often cost more and may be harder to process.
- Always check the test conditions, like load and support span, when comparing resin datasheets.
- Use HDT alongside other properties like tensile strength and melt flow index before placing an order.
What heat deflection temperature actually measures
Heat deflection temperature, commonly called HDT, is a test that finds the point where a plastic sample starts to bend under a known weight. It is not a melting point. A resin may melt at a high temperature, but it can soften and deform at a much lower level. HDT tells you where that softening begins under load.
The test usually involves a small bar of the plastic. A weight is placed in the middle of the bar, which rests on two supports. The sample is heated slowly in an oven or air stream. A probe or sensor watches the center of the bar. When the bar drops a set amount, the temperature at that moment is recorded as the HDT.
This method is widely used because it is simple and repeatable. It gives a single number that engineers can compare across different resin grades. It also helps answer a practical question: will this part hold its shape in the engine bay, near a heater, or in a hot environment?
Why the test conditions matter
The HDT number on a datasheet is only useful if you know how it was obtained. Two suppliers may list the same resin but show different values if they use different test setups. The load applied to the bar changes the result. A heavier load causes the plastic to bend at a lower temperature. A lighter load pushes the result higher.
The span between the supports also affects the outcome. A longer span makes the bar more flexible, leading to a lower HDT. The rate of heating matters too. Faster heating can cause the sample to deform before the temperature fully stabilizes.
When reviewing supplier data, check the test standard referenced. Even if the specific standard number is not listed, look for the load value and the deflection amount. For example, a datasheet might state that the test used a 0.5 kg load and measured bending at 0.25 mm. Without this context, comparing HDT values between different materials or grades is risky.
Common HDT ranges in industrial plastics
Different polymer families have different thermal behaviors. Engineering plastics generally perform better than commodity resins. The table below shows general patterns you will see across common resin types.
| Resin Type | General HDT Range (Deg C) | Typical Use Case |
|---|---|---|
| Polyethylene (HDPE) | Lower | Chemical tanks, flexible tubing |
| Polypropylene (Standard) | Moderate | Automotive under-hood, consumer goods |
| Polypropylene (Glass Filled) | Higher | Structural brackets, electrical housings |
| Polyamide (Nylon 6, 6) | Moderate to High | Gears, connectors, mechanical parts |
| Polycarbonate (Standard) | Moderate to High | Lenses, clear covers, impact shields |
| PEEK and High Performance | Very High | Aerospace, medical, chemical processing |
These ranges are broad. A specific grade of polypropylene with a higher filler content can outperform a standard grade. A nylon with moisture absorbed may show a lower HDT than a dry sample. Always treat these ranges as a starting point, not a final verdict.
How HDT influences sourcing decisions
When you are selecting a resin, HDT helps you avoid over-specifying or under-specifying the material. If a part sits in a hot environment, a low HDT grade will fail. The part may look fine at room temperature but warp after a few hours in heat. This leads to scrap, rework, or customer complaints.
Conversely, using a very high HDT grade when the environment is mild wastes money. High performance resins are expensive. They can be difficult to process, requiring higher injection molding temperatures and specialized equipment. If the application only needs moderate thermal resistance, a standard engineering grade is usually the better choice.
Consider the service temperature of the part. Service temperature is the temperature the part will experience in use. HDT should be higher than the service temperature. A margin is wise. If the part will sit in a 90 degree Celsius environment, a resin with an HDT of 95 degrees is too close. You want headroom.
Also consider the load. HDT is a loaded test. If the part is not under load, it may withstand higher temperatures without deforming. But if it is a structural bracket, the load is real. In that case, the HDT number is directly relevant.
A worked example: Choosing a resin for a housing
Imagine you are designing a plastic housing for a small electronic device. The device will be used in a car. The housing will sit near the dashboard, where temperatures can rise. The part will support a weight of a few hundred grams. It does not need to be clear, so aesthetics are less important than strength and heat resistance.
You look at two options. Option A is a standard polyamide. Its HDT is around 90 degrees. Option B is a glass filled nylon. Its HDT is around 150 degrees.
The service environment for the housing is expected to reach 110 degrees during hot summer days. Option A is not safe. Even without a heavy load, the plastic may soften. The part could warp, and the device might shift.
Option B is the stronger candidate. Its HDT is well above the service temperature. The glass filler also adds stiffness, which helps the part resist bending under the weight of the electronics.
You would not choose a PEEK resin for this application. Its HDT is very high, but the cost is significantly higher. The processing requirements are also more demanding. For a car housing, the glass filled nylon offers the right balance of thermal resistance, strength, and cost.
HDT versus other thermal properties
HDT is useful, but it is not the only thermal property to check. Melting temperature is the point where the solid becomes a liquid. For crystalline polymers like polypropylene, the melting point is distinct. For amorphous polymers like polycarbonate, the material softens gradually.
Thermal stability is also relevant. Some resins degrade chemically over time when exposed to heat. HDT does not measure chemical breakdown. It only measures mechanical deformation. A resin might hold its shape at 150 degrees for a short test, but it could yellow or become brittle after years in that environment.
Thermal expansion is another factor. If the part is made of two different plastics, or if it is attached to a metal component, the plastics must expand and contract at similar rates. A high HDT resin that expands a lot could cause stress at the joint.
When you see a resin datasheet, look at the full thermal section. Check the heat deflection temperature, the melting point if applicable, the glass transition temperature for amorphous materials, and the long term thermal stability data if available.
Common mistakes when interpreting HDT
One frequent error is comparing HDT values without checking the test conditions. As mentioned earlier, a 0.5 kg load test and a 1.0 kg load test will give different numbers. If you compare a resin tested under a light load to one tested under a heavy load, the lighter load result will look better, but the material may be worse in reality.
Another mistake is assuming HDT equals service temperature. HDT is a point of deformation under load. In a real part, the load may be lower than the test load. However, the environment is rarely static. Heat, vibration, and moisture all work together. A part that passes HDT in a lab may fail in the field if the conditions are different.
Finally, do not ignore moisture. Hygroscopic resins like nylons absorb water from the air. Moisture plasticizes the material, lowering its HDT. A dry sample in the lab may show a high HDT. The same sample, after sitting in a humid environment, may show a much lower value. Always test the material in the state it will be in during use.
How to use HDT data in your specification
When writing your material specification, do not just list a resin name. Specify the minimum HDT required. State the test conditions if they are non-standard. For example, “HDT of at least 130 degrees Celsius, measured under a 0.5 kg load with a 12 mm span.”
This removes ambiguity for the supplier. It also helps you evaluate samples. If you receive a sample and run a quick HDT test, you can verify that the resin matches the datasheet. If the sample shows a lower value than expected, you need to investigate before releasing it into production.
Keep a record of the HDT data for each batch. Thermal properties can vary slightly between production runs. A log of HDT values over time can help you spot issues early. If a supplier changes a process or a raw material source, the HDT may drift. Catching this early saves you from a bad batch.
In short, heat deflection temperature is a practical tool. It helps you match the material to the environment. Use it to narrow your choices, verify your samples, and ensure your parts will perform as expected over time.
Frequently asked questions
Is heat deflection temperature the same as melting point?
No. HDT measures the temperature where a loaded sample begins to bend. Melting point is where the solid turns into a liquid. They are different properties and should not be confused.
Can I use HDT to compare two different polymers directly?
Only if they were tested under the same conditions, such as the same load and support span. If the test methods differ, the numbers are not directly comparable.
Does moisture affect the HDT of a resin?
Yes. Many resins, like nylons, absorb moisture. This lowers the HDT. Always check if the material was dried or conditioned before testing.
What is the best HDT for a plastic part?
There is no single best value. The HDT should be higher than the maximum service temperature with a safety margin. It must also fit your budget and processing capabilities.
How does filler change the HDT of a plastic?
Fillers like glass or mineral fibers usually increase the HDT. They add stiffness and help the plastic resist bending under heat. This makes filled grades suitable for structural applications.
