Bill R. VanHoose, P.E., Crista K. McNish, Nicholas Piazza
Papers # 2014 Chicago
Material properties used in design of various plastic products should be representative of those present in the finished product. Impacts of processing, cure time, environmental conditions, etc. can considerably influence material properties in finished goods. While taking test specimens directly from finished goods, for example a pipe wall or pipe liner, may be desired, it is seldom practical. Consistency in the extraction of these specimens, manufacturing variations in wall thickness, and location of blade cuts all may contribute to wide distributions in test results. In comparison, the use of standard injection or compression molds provides more consistent test results but may not represent some of the manufacturing induced stresses present in direct samples.
Corrugated polyethylene (PE) and polypropylene (PP) pipe specifications and subsequently material specifications like ASTM D3350  or ASTM D4101  generally require compression molding PE specimens and injection molding PP specimens, respectively. This may prove to be a reliable means of specimen preparation for determining representative short term mechanical properties for each resin type. However, this may not be the most accurate means of specimen preparation for determining short term or long-term structural design properties, particularly considering cycle times, process temperatures and process conditions vary greatly from plaque preparation to finished goods production.
Considering time dependent creep behavior is a major design consideration for viscoelastic pipe materials, it is critical the test specimen used in testing generates a result that is accurately representative of the finished product. As such, this paper compares creep results for samples taken directly from corrugated PE and corrugated PP pipe walls with those prepared by injection and compression molding. Effort is made to make recommendations of a plaque preparation method to more closely represent properties found in finished goods.