Tom Hoover/Senior Medical Segment Manager/Branson Ultrasonics Corporation, a business of Emerson
Because the structures of many plastic products are far too complex to mold in a single piece, it might be necessary to assemble their components into a finished product using one of three joining methods: Mechanical fasteners, adhesives or plastic welds.
Ultrasonic welding is a popular industrial assembly technique that uses heat and pressure to create solid-state welds between plastic components. Components are held in tooling, then subjected to high-frequency (10-70 kHz), low-amplitude (1-250 µm) mechanical vibration that generates intramolecular friction, melting the mating surfaces and creating a strong molecular bond. Ultrasonic welds are widely used to join thermoformed plastic assemblies because they eliminate the need for chemical solvents, adhesives, screws or additives.
The ultrasonic welding process integrates into high-volume part production and automation because weld cycles are fast – typically less than one second – and do not require consumables. Typical plastic component designs require only minor modification to ensure repeatable, high-strength ultrasonically welded assembly. The most common (and typically “mold-safe”) modification adds a small “energy director” to the mating parts interface. The energy director melts and flows to join the two surfaces. Under ideal conditions, polymer chains from each side of the mating parts migrate across the interface and become indistinguishable from the parent material.
Many thermoplastics, both amorphous (such as polystyrene) and semi-crystalline (such as nylon), can be ultrasonically welded. Ideally, both parts in a weld should be the same material. However, many combinations of dissimilar plastics can be ultrasonically welded if their melting temperatures (Glass Transition Temperature, Tg), are fairly close.
Ultrasonic welding can offer significant advantages to the assembly of medical devices that must otherwise be joined by screws and solvents. Adhesives and solvents have much longer processing times, can introduce contamination and be challenging to accurately dispense. Minimally invasive surgical instruments such as catheters, cannulas, luers and trocars often utilize ultrasonic joining with great success.
For applications in which the vibration used in ultrasonic welding could negatively impact microelectronic components or delicate part structures such as membranes or filters, other plastic joining technologies are readily available. One alternative is laser welding, a vibration-free process that produces clean and hermetically sealed welds between a wide range of dissimilar polymers. Benefits like these, together with exceptional aesthetics, have made laser welding the technology of choice for joining plastic parts used in many advanced medical applications, including everything from in vitro diagnostic test products to wearable technology for remote monitoring or microfluidic drug delivery.
There are many choices, questions and challenges inherent in the design, development and production of reliable, repeatable joining process solutions. A material joining supplier provides expertise that can have a positive influence on everything from proof of concept to prototype development, scalability, data collection, regulatory compliance and more. Involving an experienced engineering expert in early stages enables a manufacturer to determine the best joining technology for their product. For medical device marketers who design and manufacture on multiple continents, it is also valuable to partner with a technology supplier with global capabilities that can provide local welding design and production expertise and support where the device assembly is performed.