Once a process window has been defined for manufacturing medical device components, processors need a resin that can consistently yield parts. From the perspective of the processor, the ideal resin shows minimal fluctuations in viscosity and consistently yields good parts with minimal scrap. In this article we take a closer look at the processability of medical-grade resins with particular emphasis on spiral flow and how viscosity changes with temperature. Both of these attributes, which provide important insights on how easily a resin can deliver consistent part quality, are used to compare three medical-grade resins: Makrolon®, Apec® and a copolyester.
The resins examined were Makrolon® 2458, Apec® 1745 and a medical-grade copolyester. The drying and processing temperatures used in these studies followed the recommendations provided by the manufacturers. The spiral flow of each resin was determined with a standard Covestro LLC procedure with a Milacron FANUC S2000 Roboshot SiB-110 injection molding machine. A spiral length along a 1.5mm thick channel was measured for each resin at an injection speed of 10 cm/sec. Melt Volume Rate (MVR) testing was conducted at a temperature and force that was appropriate for each resin: 300°C/1.2 kg for Makrolon®, 330°C/2.16 kg for Apec® and 265°C/5 kg for the copolyester. Values at temperature variations of +/- 5°C were also recorded.
Results and Discussion
The table below shows the glass transition temperature and tensile modulus for the three materials we studied.
|Material||Tg [°C]||Tensile Modulus [MPa]|
Apec® 1745 is a high-heat copolycarbonate for medical applications that offers a higher glass-transition than Makrolon®, yet still combines a high modulus with toughness. Copolyesters are relatively new materials that are found in some foodware applications. Copolyester processing literature claims that the material’s lower glass transition temperature helps it achieve fast cycle times. Our own study published in 2013, however, suggests that copolyester’s significantly lower modulus plays a determining role in extending the cycle time to avoid damage during ejection. In that study, Makrolon® and Apec® were able to achieve much faster cycle times thanks to the higher modulus and higher glass-transition temperature.
The gallery shows consecutive Makrolon® and copolyester spiral flow samples stacked in the sequence in which they were molded.