Vicat Softening Point vs HDT: Thermal Testing of Plastics Explained

How hot can a plastic part get before it stops behaving like a solid? Two laboratory answers exist: the Vicat Softening Point (VSP, ASTM D 1525 / ISO 306) and the Heat Deflection Temperature (HDT — also called heat distortion temperature, ASTM D 648 / ISO 75). They measure different things and appear on datasheets for different reasons.

Both are run on a combined VSP / HDT Apparatus — an oil bath with calibrated loading stations, dial or digital indicators, and a controlled heating rate.

What Each Test Measures

• Vicat (VSP) — temperature at which a flat-ended needle of 1 mm² penetrates 1 mm into the specimen under 10 N or 50 N load. It reflects surface softening — relevant to hot-fill containers, fittings and electrical parts.
• HDT — temperature at which a bar deflects a set amount under a three-point bending stress of 0.455 MPa or 1.82 MPa. It reflects load-bearing stiffness at temperature — the number designers use for structural parts.

Rule of thumb: Vicat runs higher than HDT for the same material, because localized needle penetration requires more softening than bulk flexural deflection. Quoting one when a buyer asked for the other is a common and costly specification error.

Running the Test Correctly

Specimens are immersed in a silicone-oil bath heated at exactly 120 °C/hour (or 50 °C/hour where specified). Heating-rate accuracy is the heart of the apparatus — a fast bath reads high, a slow one reads low. Multi-station units test 2–6 specimens simultaneously, and automatic units record the trigger temperature the instant deflection or penetration reaches the limit.

Related Thermal Tests

Thermal characterisation rarely stops at VSP/HDT. PE pressure-pipe grades additionally require Oxidation Induction Time (OIT) testing for stabiliser content, while ovens and furnaces cover heat reversion and ageing and ash content determinations.

Frequently Asked Questions