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How ultrasonic and laser welding solves joining challenges

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Didier Perret, medical business development manager, Branson Welding and Assembly at Emerson discusses some of the material changes in the medical device sector, as some device manufacturers pivot away from PVC and PC.

Saengsuriya_5DIV Getty Images/iStockphoto

Medical device manufacturers are actively exploring alternatives to two of the polymer mainstays in the industry, namely polyvinyl chloride (PVC) and polycarbonate (PC), as concern and outright opposition mount due to the negative effects of some ingredients and/or components.

At the same time, the trend toward at-home testing and treatment is driving higher production volumes. Consequently, as device makers seek to minimise toxicology issues for the product itself and throughout the entire production process, they are also looking at ways to reduce costs and improve sustainability. One approach involves switching to other materials, particularly polypropylene (PP) and polyethylene (PE), collectively known as polyolefins. 

However, they are discovering traditional joining techniques — like conductive (heat) welding, radiofrequency (RF) welding (also known as high-frequency or dielectric welding), as well as solvent welding and adhesives — are not effective on these new resin choices. 

Pressure to discontinue use of PVC

The American Medical Association, among other healthcare and professional organisations, is encouraging hospitals and physicians to reduce the use of and eventually phase out PVC, which is used in 40% of all plastic-based medical devices, according to PVCMed Alliance (Brussels). PVC is used to make most of the tubing and IV bags available today, as well as many masks, breast-pump kits catheters and more. However, dioxin, a known human carcinogen, can be formed during the manufacture of PVC, and toxic chlorine may be released during processing and assembly. In addition, DEHP, a phthalate plasticiser commonly used to soften PVC, is a known endocrine-disrupting compound that, it is feared, can leach into the patient’s bloodstream, and potentially cause fertility and other reproductive-related problems. 

PVC can be welded very effectively using conductive (heat) welding, radiofrequency (RF) welding (also known as high-frequency or dielectric welding), as well as solvent welding and adhesives, and many manufacturers have used these processes for years. 

However, replacing PVC with PP or PE is not so simple from a joining perspective. That’s because polyolefins are nonpolar, so they are impervious to the electromagnetic waves that generate heat during RF welding. Likewise, PP and PE have excellent chemical resistance and are not easily bonded using solvents, and their low surface energy also means that adhesives are not very effective. 

By far the most effective technology for assembling polyolefins, as well as multilayer films incorporating these and other materials to form IV bags, is ultrasonic welding. Ultrasonic welding uses high-frequency vibrations to generate frictional heat between layers, softening the plastic so that it merges into a high-quality seal when the films are held together under pressure. This is an extremely fast joining process that can be applied to almost any thermoplastic, including PVC. While the equipment costs are higher than other technologies, there are numerous benefits that ensure a relatively quick ROI:

Replacing polycarbonate

Like PVC, polycarbonate has been used for years in a wide variety of medical applications ranging from drug delivery pens and pumps to dialysis filtration units, intravenous access components, and cardiac bypass systems including blood oxygenators, reservoirs, and filters. However, because polycarbonate contains bisphenol-A (BPA), a known endocrine-disrupting chemical, it has largely been eliminated from use in baby bottles and the water bottles used by athletes, and activist groups continue to call for an outright ban. 

While low levels of BPA have not been deemed especially problematic, there is still concern that direct and sustained contact with blood can lead to higher serum BPA levels, especially in patients with chronic renal disease. This, along with other material advantages, has manufacturers looking at PP and PE as potential alternatives to polycarbonate.

As in PVC replacement, ultrasonic welding is a good joining option, delivering the same benefits listed above, but in applications where polyolefins are being used in PC applications, laser welding is often considered as well. Components are pre-assembled before welding, and no vibration or movement is required to produce clean, particulate-free welds. Multiple laser beams apply energy along the full length of weld surface. One surface freely transmits the laser energy (without itself being affected) through to the second (laser-absorbing) surface where laser energy is converted to heat that is conducted across the interface, creating the weld. Benefits of this process include:

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