Your choice of impact-testing equipment depends on what you will use it for-R&D, quality control, material characterization, checking incoming material, or product design. In most cases, your customers dictate what is required. Automotive specs, for example, can cite ASTM, ISO, or SAE test methods or each automaker's own proprietary standards.

The ASTM, ISO, and other test standards are very clear on what plastics they pertain to. For example, thin-films may be tested according to ASTM D1709, while polycarbonate used in automotive parts would be covered by either ASTM D3763 or ISO 6603-2.

In addition, there is a whole realm of non-standard tests with no industry specifications. These might be tests on actual products or assemblies (e.g., plastic gas tanks). Such cases require understanding of the end-use conditions. For example, a manufacturer of food trays tests them at subambient temperatures to simulate freezer storage.

For QC of finished products that are not covered by a particular test requirement, a processor may opt for a basic pendulum or falling-weight impact tester. In general, falling-weight tests are easier to use because no notching of samples is required. Yohn from Tinius Olsen says, "A falling weight, such as Gardner impact, may be used for relatively flat objects like plastic sheet or vinyl siding. For testing say, a toothbrush, a pendulum-style unit may be used, but a method of holding the part securely has to be developed on a case-by-case basis."

Nova's Elston, who is ASTM section chair for static properties, sees no appetite in the industry for migrating from ASTM notched Izod to Charpy ISO standards for testing commodity resins. However, for engineering and specialty resins, there is indeed a move toward Charpy impact testing. "The automotive people are now on the ISO/CAMPUS bandwagon," he says, referring to the Consortium for Computer-Aided Preselection by Uniform Standards (CAMPUS), a global alliance of materials suppliers committed to ISO standards.