A comparison of oven sag temperature and heat deflection temperature for measuring heat resistance

The plastics industry places great emphasis on heat deflection temperature (HDT) as a measure of heat resistance when designing a plastic product. However, many manufacturers will evaluate a material's heat resistance by exposing a finished part to meximum use temperature under gravity load rather than the high loadings associated with the heat deflection test. The oven sag test is an alternate method which looks at the sag of a molded bar under gravity load. One end of the bar is clamped while the other end is free to sag. By employing a range of temperatures, the temperature at which gross sag takes place can be found. This study investigated how well the heat deflection temperature test and the oven sag test correlate to the performance of an injection molded box subjected to high temperatures using one and two phase PVC blends. The oven sag test was found to provide a more practical and more accurate measure of a material's performance under high temperatures and no external load. It is particularly well suited for evaluating two phase polymer blends.