Eastman Explores Processing of Tritan Copolyester For Small, Multicavity Medical Parts
A sophisticated 32-cavity mold and all-electric press are being used to define the process window and critical mold and part design criteria.
Inherent purity, clarity, toughness, and resistance to harsh disinfectant chemicals, aggressive drug formulations, and sterilization procedures are qualities that recommend Tritan copolyester for medical applications. Eastman Chemical Co., Kingsport, Tenn., has commercialized this unique resin in a number of medical products since 2009. So far, those have been medium-to-large components of devices and electronic instruments. Now, Eastman is seeking to introduce Tritan into small, precision, multi-cavity injection molded devices and components as a bisphenol-free alternative to PC and acrylic.
Consequently, Eastman has launched an exhaustive research program to learn everything it can about optimal tooling, processing, and Design for Manufacturabilty for such applications at its Kingsport molding laboratory. Eastman invested several hundred thousand dollars in a Roboshot 165-ton injection machine from Milacron LLC, Batavia, Ohio, and a 32-cavity tool from Prestige Mold Inc., Rancho Cucamonga, Calif., with a valve-gate hot-runner system from Mold-
Masters, Georgetown, Ont., a Milacron company. The tool produces a universal female luer; it ran in Milacron’s booth at NPE2015 in Orlando, Fla., this past March.
According to Dr. Steven Givens, Eastman senior polymer applications scientist, one of the challenges of optimizing the process was cavity balancing; he notes that the material in the hot-runner system is 22 times the weight of all 32 parts together (0.9 g each). That was solved with a Mold-Masters Synchro-Plate system that mechanically actuates all valve gates simultaneously.
Other important molding considerations, notes Givens, are adequate venting and cooling. With regard to the latter, he says it’s important to freeze the outer skin of the part thoroughly, or it can stick. The test mold uses conformal cooling in circular paths around the parts. It was designed for a 14-sec cycle but ran at 10.96 sec at NPE. One of the most important things learned in trials with the mold was Tritan’s lack of sensitivity to mold finish. As an experiment, half the cavities were polished and half were left with a rough EDM finish. Givens says all parts eject with no problems, potentially saving $30,000 worth of mold-polishing costs. According to him, this would not be the case with PC, acrylic, or HIPS.
Givens also notes that barrel residence time must be controlled carefully, because copolyester can degrade from excessive heating without showing any visible signs, though its properties can be reduced. Givens recommends designing the system for a melt residence time of 5 min or less at 540 F (282 C); but at NPE, the tool ran on a larger press with a 7-min residence time and no degradation.
Givens says Eastman plans further trials to define the maximum process window and to test different screws, nozzle geometries, release packages, robotic demolding, inspection systems, and even post-mold laser engraving. The goal is to provide customers with complete part and tool design and processing information, supplementing what is already available at tritanmoldIt.com.
Plastics weigh in with added design freedom and environmental friendliness—especially when the alternative is lead.
Acetal materials have been a commercial option for more than 50 years.
After molding, acetal parts can continue to shrink at room temperature and even in the cold.