Wondering If Prototypes Will Withstand the Heat?

Although the question "Is the prototype capable of withstanding high temperatures?" may seem simple to answer, there are a variety of factors to consider when evaluating product information based on reported heat data such as heat deflection temperatures (HDTs) and glass transition temperatures (Tg).

HDT is a relative measure of a material's ability to perform for a short time at elevated temperatures while supporting a load. The test measures the effect of temperature on stiffness. A standard test specimen is given a defined surface stress and the temperature is raised at a uniform rate. Although widely used to indicate high-temperature performance, the HDT test only simulates a narrow range of conditions. Many high-temperature applications involve higher temperatures, more loading and unsupported conditions. Therefore, the results obtained by this test method do not represent maximum use temperatures, because in real life, essential factors such as time, part geometry, the rate of temperature increase, loading and nominal surface stress may differ from the testing conditions.

There are two primary standards for measuring HDT, one based on ASTM standards and the other on ISO values. Some changes may be found in the reported ASTM standards when compared with ISO values because of the different dimensions of test specimens. ASTM D648 is defined as the temperature at which a test bar, loaded to the specified bending stress, deflects by 0.010 inch (0.25 millimeter). The two common loads used are 0.46 megaPascal (66 psi) and 1.8 megaPascal (264 psi), although tests performed at higher loads such as 5.0 megaPascal (725 psi) or 8.0 megaPascal (1,160 psi) occasionally are encountered.

Amorphous polymers, such as SL materials, have no defined melting temperature. Molders of traditional plastics use the Tg of amorphous polymers to define the point at which they are processed, which is in their rubbery state. Because SL materials are processed via light-imaging techniques, the Tg is not as important from a mold processing perspective. However, what is important to note is that the mechanical properties of the resin change once the Tg is reached. The Tg is the point at which parts start to soften, and is the more scientifically accepted way of evaluating how parts will perform when exposed to higher temperatures because it is not as dependent on all the different variables involved in calculating HDT. However, comparing Tg values often can lead to some confusion because of the three different methods of testing Tg that all lead to slightly different results and can vary an estimated five to ten degrees. The different methods are the differential scanning calorimeter (DSC), thermal mechanical analysis (TMA) and dynamic mechanical analysis (DMA). The correct method for measuring Tg differs depending on what type of material is being evaluated and often is left to the judgment of the laboratory performing the test. However, regardless of the Tg value, SL prototypes are used successfully at temperatures much higher than the Tg would suggest appropriate, once again highlighting the geometry-specific applications of the parts.