Compared with the heat distortion test (HDT), the vicat test reveals the temperature at which the specimen loses its “stability-form” and softens, while HDT test measures the temperature at which the specimen loses its “load-bearing” capability.  VSP characteristic of plastic samples is an important tool for material selection process as well as for product design and manufacture.

During VSP test, a flat-ended needle loaded with a specified mass is placed in direct contact with a test specimen, which is immersed in a heat transfer oil bath.  The load applied to the specimen could be either 10 N or 50 N.  Subsequently the specimen and needle are heated at either 50 ^oC/h or 100 ^oC/h.  Finally, the temperature at which the needle penetrated to a depth of 1 mm is recorded and it is the Vicat softening point.