June 2017

Consistency with every shot: Makrolon® and Apec® for medical applications

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.

Experimental

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.[2] 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]
co-polyester 107 1550
Makrolon® 2458 145 2400
Apec® 1745 167 2400
Basic material properties of materials studied in this work.

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.[2] 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.[1]

The gallery shows consecutive Makrolon® and copolyester spiral flow samples stacked in the sequence in which they were molded.

Spiral Flow Parts

At recommended conditions, Makrolon® 2458 achieved a longer spiral flow length than the competitive copolyester.

The spiral flow parts reflect how far a resin can travel until it freezes during filling. At recommended conditions, Makrolon® 2458 achieved a longer flow length than the competitive copolyester. More importantly, Makrolon® and Apec® (not shown) had much more consistent shot-to-shot consistency. The table below summarizes the statistical data from the Spiral Flow specimens collected from our experiments.

Product Tmelt (°C) Tmold (°C) Spiral Flow (Cm) Standard Dev (%)
Makrolon® 2458 300 80 19.7 0.3
Apec® 1745 335 85 17.8 0.9
copolyester 282 65 17.8-19.1 2.6
Data recorded from Spiral Flow Experiments conducted in this work

Makrolon® and Apec® gave standard deviations on flow length that were less than 1%, which demonstrates the excellent shot-to-shot consistencies of both. The copolyester showed much greater flow-length variations. The image in Figure 1b suggests that the fluctuations are not the result of mold temperature fluctuations for which a trend to either increasing or decreasing length would have been expected. Thus, the higher shot-to-shot variation may be inherent to the copolyester. Our results suggest that it may be more difficult for copolyesters to achieve a consistent process.

To further investigate the shot-to-shot variations observed with the copolyester, we measured the Melt Volume Rate (MVR) at three temperatures within a 10°C temperature range to determine how flowability was influenced by temperature. Figure 2 compares how MVR deviated with temperature changes of ± 5°C.

Change in MVR when temperature is either decreased or increased by 5°C.

Both Apec® and Makrolon® showed smaller MVR changes than a medical copolyester within a 10°C window. This illustrates that Makrolon® and Apec® are less sensitive to temperature fluctuations and can maintain greater consistency during molding.

Conclusion

Our own studies show that Makrolon® and Apec® offer tangible advantages over a copolyester in terms of processability and shot-to-shot consistency. The better consistency of Makrolon® and Apec®, as well as lower viscosity fluctuations with temperature, means these resins can help deliver consistent parts within a validated process window. Moreover, shot-to-shot consistency can be very beneficial to further minimizing processing costs by minimizing scrap rates in a manufacturing process. Finally, the high flow and high rigidity for Makrolon® and Apec® can both be leveraged to produce thin-walled parts with less concern about filling.

References

  1. Appeared in part in “Material Properties And Their Influence On Molding Productivity And Efficiency Of Medical Resins”, Proc. SPE Annu. Tech. Conf. 71 (2013) 893-897.
  2. a) “Processing data for the injection molder”, available for download from www.plastics.covestro.com
    b) Eastman Tritan Processing Guide – SP-MBS-1465

For more information, email plastics@covestro.com.

« Back to Overview