How to alleviate the slow crack growth in molded and extruded plastic parts

Plastics used in long life products can be prone to slow crack growth (SCG). SCG can be triggered by product design, material selection, production methods, and the environment in which the part or device is used. For example, the shift towards more aggressive chemicals to disinfect medical devices and lowering the rate of hospital infections is increasingly leading to stress cracks or hairline cracks in plastic parts. Christian Herrild, Director of Growth Strategy at Teel Plastics (Baraboo, WI) will delve into the causes of SCG in medical devices and suggest ways in which designers and manufacturers co-located this phenomenon during a conference session on the upcoming. The Medical Design & Manufacturing (MD&M) West and PLASTEC West event in Anaheim, CA. He recently previewed his presentation with. divided Plastic today.

Microscopic image of stress cracks
in a plastic tube.

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

Stresses in an injection molded part arise in the way it is molded or assembled, Herrild said. With extruded products, “you can see this in coil and clip-type applications, or those with braids or fasteners where something is clipped onto or over fittings,” he explained. Some polymers, Herrild added, are more prone to residual stress and tear faster than others. It follows that the right polymer selection is the most important tool in preventing SCG.

Christian Herrild, director for growth strategy at Teel Plastics.

According to Herrild, SCG is rarely seen in many common polymers, especially in tubing. “Flexible PVC and some of the thermoplastic urethanes and elastomers are very resistant to SCG. Unless you expose the part or device to an extreme situation – exposed to certain chemicals or kinking the tubing – it is very unlikely that you will see any cracks under normal operating conditions, ”said Herrild.

SCG is a molecular process that does not result in any chemical changes in the polymer. Herrild describes it as “molecular chain disentanglement” in which the polymer branches become detached. “Some disinfectants penetrate polymers very well and lower the activation energy for stress cracks, which makes it much easier for the polymer branches to pull apart. Some polymers tolerate disinfectants well, while others do not tolerate them at all. When you know what type of disinfectant your device is likely to encounter, you can choose a material that will withstand it, ”said Herrild Plastic today.

Christian Herrild will give a lecture on the prevention of slow crack growth in order to create safer medical devices at the MD&M West / PLASTEC West on February 5th at 4:00 p.m. His presentation is part of the Technical Solutions Conference track at the event, which will take place February 5-7, 2019 at the Anaheim Convention Center in Anaheim, CA. Further information and registration for participation can be found on the PLASTEC West website.

If you want to use a specific material, post-processing and annealing can help minimize the occurrence of SCG. “You should use the correct design principles for shapes so that you don’t accidentally design under a lot of stress. Over-tightening the screws is just as much a problem as uneven cooling, which causes a slight distortion that is ironed out during assembly. If you want to wipe down with isopropyl alcohol before assembly, which is standard with some OEMs, you should check if it really is necessary. Exposure to this solvent can lower the activation energy to start cracking, making the part more prone to long term cracking, ”said Herrild.

When it comes to tubing, the conversation usually boils down to questions about stress concentration, Herrild added. “How big is the coil radius? Bending radius of the packaging? If you have clips, do they have sharp edges? “

Teel Plastics recently expanded its injection molding capabilities.

Herrild said he had observed a number of corrective actions related to medical devices with SCG. “There have been complaints about equipment failing prematurely and not meeting its expected lifespan. This is definitely an issue that plastics processors can help OEMs with, ”said Herrild.

Support with polymer selection and part design is part of his company’s service portfolio. In addition to extrusion, rapid prototyping, assembly and some design and development and contract manufacturing services, Teel Plastics expanded its range to include injection molding around six months ago.

“We brought four injection molding machines with us and hired additional engineers and specialists to support this initiative,” said Herrild. The company currently operates 110, 165 and 350-ton Arburg machines. The smaller injection molding machines are all electric, while the 350-ton machine is a hybrid to handle higher cavitation or larger volume products, Herrild said. “About half of the currently approved forms are medical forms and we are working to offer additional business for forms, mainly in the medical market,” he added. This will undoubtedly be an important topic of conversation at the company’s booth (# 2184) at MD&M West / PLASTEC West next month. With the addition of injection molding capacities, Teel Plastics is positioning itself more than ever as an outsourcing partner for medical OEMs, said Herrild.

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