How to mitigate slow crack growth in molded and extruded plastic parts

Plastics used in products with a long life expectancy may be prone to slow crack growth (SCG). SCG can be precipitated by product design, material selection, production methods and the environment in which the part or device is used. For example, the shift toward harsher chemicals to disinfect medical devices and lower the rate of hospital-acquired infections is causing stress cracking, or crazing, in plastic parts at an increasing rate. Christian Herrild, Director of Growth Strategy at Teel Plastics (Baraboo, WI) will do a deep dive into the causes of SCG in medical devices and suggest ways in which designers and manufacturers can mitigate this phenomenon during a conference session at the forthcoming co-located Medical Design & Manufacturing (MD&M) West and PLASTEC West event in Anaheim, CA. He recently shared a preview of his presentation with PlasticsToday.

Stress cracking in plastics
Microscopic image of stress cracking
in plastic tubing.

Cracking in plastics is slow in chemical terms and requires sustained stress, explained Herrild. While it is not an instantaneous event caused by impact or torque, for example, it can still happen on a relatively fast basis, even in less than a day in some cases. “This failure mode is inherent in any type of extruded or molded plastic that is in a stressed state,” said Herrild.

Stress in an injection molded part originates in the manner in which it is molded or assembled, said Herrild. In extruded products, “you see it in coil- and clip-type applications or those involving braiding or fastening, where something is clipped on or over fittings,” he explained. Some polymers, Herrild added, are more vulnerable to residual stress and will crack faster than others. It goes to follow that proper polymer selection is the primary tool for preventing SCG.

Chris Herrild, Teel Plastics
Christian Herrild, Director of Growth Strategy at Teel Plastics.

You rarely see SCG in many common polymers, especially for tubing, according to Herrild. “Flexible PVC and some of the thermoplastic urethanes and elastomers are very resistant to SCG. Unless you’re putting the part or device under some kind of extreme situation—exposure to certain chemicals or kinking the tubing—it’s very unlikely you would see any cracking under normal use conditions,” said Herrild.

SCG is a molecular process that does not entail chemical changes in the polymer. Herrild describes it as “molecular chain disentangulation,” by which the polymer branches come apart. “Some disinfectants penetrate polymers very well and lower the stress crack activation energy, making it much easier for the polymer branches to pull apart. Some polymers handle disinfectants well and some cannot tolerate them at all. If you know the type of disinfectant your device is likely to face, you can select a material that will withstand it,” Herrild told PlasticsToday.

Christian Herrild will present a paper on preventing slow crack growth to create safer medical devices at MD&M West/PLASTEC West on Feb. 5 at 4 PM. His presentation is part of the Technical Solutions conference track at the event, which runs from Feb. 5 to 7, 2019, at the Anaheim Convention Center in Anaheim, CA. For more information and to register to attend, go to the PLASTEC West website.

If you are locked into using a specific material, post processing and annealing can help to minimize the onset of SCG. “You should employ proper mold design principles so you’re not accidentally designing in a lot of stress. Over-torquing of screws is an issue, as is uneven cooling causing a slight warpage that is ironed out in assembly. If you like to do a pre-assembly wipe-down with isopropyl alcohol, which is standard practice for some OEMs, you should evaluate if that is really necessary. Exposure to that solvent can lower the activation energy to start crack formation, making the part more susceptible to long-term cracking,” said Herrild.

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