Vicat Softening and Heat Deflection Temperature
Vicat softening point and heat deflection temperature both describe how a plastic behaves as it heats up, but they answer different design questions. One tells you when a surface starts to give. The other tells you when a part carrying a load starts to sag.
Two Numbers a Designer Reads Differently
When a plastic part has to work in heat, a designer needs to know two distinct things. The first is the temperature at which the material surface begins to soften under a concentrated point of pressure. The second is the temperature at which a part under a bending load deflects by a defined amount. Vicat answers the first and heat deflection temperature answers the second. A specification often calls for both because a part can be exposed to surface pressure, a sustained structural load, or both at once.
Vicat Softening Point
The Vicat softening point is the temperature at which a flat-ended needle 1 mm in diameter penetrates the specimen to a set depth under a specified load, while the temperature is raised at a controlled rate. ASTM D1525 and ISO 306 covers this testing. The result is a single temperature that marks where the material starts to yield to a concentrated point of pressure.
Vicat is most useful for a part where a surface has to hold its form against localized contact in heat: a clip, a boss, a fastening point, a contact face. It describes the onset of surface softening, which is the property that governs whether that feature holds.
Heat Deflection Temperature
Heat deflection temperature, or HDT, is the temperature at which a specimen under a constant flexural load deflects by a specified amount as the temperature is raised. ASTM D648 and ISO 75 cover this testing. The result describes the temperature at which the material can no longer hold its shape under a sustained bending load.
HDT is the number a designer reads for a part that carries a structural load while warm: a bracket, a housing, a support that has to stay rigid near a heat source. It speaks to load-bearing behavior in heat, not surface contact.
| Property | Vicat softening point | Heat deflection temperature |
|---|---|---|
| What it measures | Onset of surface softening under a point load | Deflection under a sustained bending load |
| Method | ASTM D1525, ISO 306 | ASTM D648 (Method B), ISO 75-1, 75-2 |
| Read it for | Clips, bosses, fastening and contact faces | Brackets, housings, load-bearing parts near heat |
Choosing the Right One for the Part
The two are not ranked, and a higher number on one does not substitute for the other. They describe different exposures. The way to choose is to look at how the part is actually loaded in heat. A feature that resists a concentrated point of contact is a Vicat question. A part that has to stay rigid under its own mounting or service load is an HDT question. When a part sees both, the specification calls for both, and each is read against its own limit.
Frequently Asked Questions
What is the Vicat softening point?
It is the temperature at which a standardized needle penetrates a plastic specimen to a set depth under a defined load as the temperature rises. ASTM D1525 and ISO 306 cover this, and it marks the onset of surface softening under a concentrated point of pressure.
What is heat deflection temperature?
HDT is the temperature at which a specimen under a constant flexural load deflects by a specified amount as it is heated. ASTM D648 and ISO 75 covers this, and they describes when a material can no longer hold its shape under a sustained bending load.
What is the difference between Vicat and HDT?
Vicat measures the onset of surface softening under a point load, while HDT measures deflection under a sustained bending load. One speaks to surface contact features, the other to load-bearing behavior in heat.
Which one does my part need?
It depends on how the part is loaded in heat. A surface-contact feature points to Vicat, a load-bearing part points to HDT, and a part that sees both is usually specified for both. The material specification defines the limits.
Ghesquiere Plastic Testing, Inc. is an A2LA accredited testing laboratory in Harper Woods, MI, specializing in weathering and flammability testing for automotive materials. Request a quote at quotes@gptesting.com.
