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But in larger parts it becomes a signifcant factor. Even after the dimensions stop changing at a measurable level, structural rearrangements continue to occur for weeks. Several years ago we studied the tensile strength and modulus, impact resistance, and dynamic mechanical properties of molded PP and acetal specimens over a period of fve weeks after molding. Properties were measured 48 hr after parts were produced, which is the ASTM guideline, and then additional tests were performed at intervals of seven days out to 35 days after the molding date. Tensile strength and modulus continued to rise over time, although the rate of increase declined as the time extended. Impact performance became worse. Some materials that were ductile two days after molding began to exhibit brittle behavior in 7 to 14 days. All of this was the direct result of continued crystallization. Most processors do not consider the effect of processing conditions such as mold temperature on part performance. They are much more concerned with making parts that are to print. So when a process produces parts that are undersized or on the low end of the tolerance range, processors think in terms of making adjustments to correct this condition. The adjustment of choice is almost always mold temperature. Reduce the mold temperature and the part will be larger. The reason is simple: A lower mold temperature limits the ability of the polymer to crystallize. Fewer crystals mean less shrinkage. Our Pump Systems Do What Other's Can't Maag gear pumps are engineered for some of the most demanding conditions to which a pump can be subjected: • pressures to 5,075 psi • temperatures to 600 °F • viscosities to 4,000,000 cP Clearances in four different pump areas are chosen for the particular application. Pumps can also be supplied stand-alone. Call to learn more. a brand of maag group Maag Automatik Inc. Charlotte, NC MaagAmericas@maag.com 704-716-9000 But in a material that has a Tg below room temperature, these gains may be temporary. In every material there is an ideal spacing between molecules. The material "knows" what this spacing is and given the opportunity it will do everything it can to achieve it. Suppressing crystallinity in a material that possesses structural mobility at room temperature is a short-term strategy. This is why the suppliers of acetal materials have been so insistent over the years on the importance of mold temperature and proper packing to prevent something they call post-mold shrinkage. It turns out that dimensional changes can also occur due to another mechanism that primarily affects amorphous materials, a class of polymers that is considered to have the upper hand when it comes to dimensional stability. However, these changes take much longer to unfold and they are smaller in magnitude. But if you are really focused on very close tolerances and the parts are large enough, these changes may still be of concern. We will deal with this mechanism in our next installment. about the author Michael Sepe is an independent materials and processing consultant based in sedona, ariz. with clients throughout north america, europe, and asia. he has more than 35 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 • mike@thematerialanalyst.com. The New MP1200 Makes It All So Simple. Whether you specify a manual or a motorized version, Tinius Olsen's next generation of melt indexer is equipped to maximize capability, efficiency and ease of use. • Three-zone heaters and controllers increase temperature stability. • Touch screen control is an intuitive interface. • Tapered weight design ensures safer loading and unloading. • Simple die release facilitates easy cleaning. • USB connectivity and Horizon software enables sophisticated SPC analyses. • Optional manual or motorized extrudate cutter. • Fully compliant with ASTM D1238 and ISO 1133 Melt Indexing Without the Stressing. (215) 675-7100 www.TiniusOlsen.com Plastics technology February 2013 19