HDT is the initial temperature of deformation of plastics under load and has been used for decades to characterise the thermal resistance of plastics. It is a so-called one-point value, valid only under the test conditions. For many years now, researchers who have been studying the properties of plastics in greater depth have been trying to convince industrialists that dynamic mechanical analysis (DMA) should be used instead, but most plant engineers consider this procedure too complicated. In a series of articles, M. Sepe, head of a US consultancy, shows that DMA curves can provide very important information on the properties of plastics without the need for complicated mathematical calculations. Part 1 of our publication deals with how much more DMA reveals about the elastic modulus of the plastic under investigation than HDT (which is essentially the temperature at which the elastic modulus of a polymer suddenly decreases); Part 2 in the next issue will reveal further useful information that can be read from DMA curves.