Laser welding is an increasingly popular joining technique in medical device manufacturing for a number of reasons. It is often described as a “clean” process, and it’s a “very good alternative to solvent welding, and, as devices get more complicated, design restrictions may rule out ultrasonic welding,” explained Steve Duckworth, Global Head of Segment Medical and Pharmaceutical at materials supplier Clariant. Furthermore, laser welding is fast and reliable.
“The big thing is reliability,” Duckworth told PlasticsToday. “You need certainty that the weld is consistent and reliable, or you may end up having to carry out 100% inspections.” He cites the example of one customer who had been using ultrasonic welding for a canister. “He had a 15% reject rate,” said Duckworth. “The problem was to detect that reject rate, he had to do 100% inspection of the products via pressure testing to ensure the weld was good. He is switching to laser welding for the second generation of that product.”
|For laser welding applications, Clariant offers different color choices in a range of polymers, including polypropylene, ABS, polycarbonate, and PC/ABS blends. Image courtesy Clariant.|
Laser welding requires a transmissive plastic part that allows the laser energy to pass through and an absorbent part that receives the energy, which is converted to heat and melts the polymers at the bond line. Typically, carbon black enables absorption of the radiation in the polymer, but that gets “complicated if both colors need to be black,” noted Duckworth. “You can’t use carbon black on the bit that is doing the transmission or you block the laser. You need to come up with different formulations to arrive at the black without using carbon black,” explained Duckworth. It just so happens that is an area of expertise at Clariant.
A thorough knowledge of pigment and additive options for the absorbing and transmitting parts of the polymer part enables Clariant to offer different color choices in a range of polymers, including polypropylene, ABS, polycarbonate, PC/ABS blends, and more, said Duckworth. “In collaboration with laser equipment suppliers, we’ve developed analytical techniques that can be used to screen different solutions to find the best one,” he added. Clariant also applies what it calls a Quality by Design approach to ensure that the end result matches expectations.
Laser welding offers numerous advantages compared with other joining methods — surfaces do not require pre-treatment, the potential leaching of solvent residue is eliminated, and complex geometries can be joined with minimal stress on the joints — but those benefits can be compromised if the design inputs are not carefully accounted for in the early stages of product development. “Quality by Design requires thinking about all the various parameters of the finished part — the polymers, colorants, design aspects, and processing techniques — at the beginning of the process,” said Duckworth. For example, color decisions are often made later on in the development cycle by the marketing team or others not involved in the functional design process. That could have severe consequences if you are laser welding a part for a medical device.
“You have to think about this as a system, in terms of laser-transparent and laser-absorbent parts,” Duckworth explained to PlasticsToday. “These two parts need to be designed together, because you don’t want to go through all of the validation programs and then have marketing decide that the product should be pink. The laser won’t be able to penetrate and you’re in trouble. This has to be thought out at the beginning, hence the value of a Quality by Design approach, said Duckworth. “The end product’s quality is heavily dependent on having all design inputs in the beginning.”
Clariant offers materials optimized for laser welding in both masterbatch concentrates and ready-to-use compounds. Masterbatch added to the host polymer during processing may be convenient for introducing color, explained Clariant, but the processing equipment and flow path in the tool may lead to inconsistent pigment distribution at the weld line. A ready-to-use compound may solve this problem.
Available as part of the Mevopur medical-grade product line, the laser welding materials are manufactured in Clariant facilities certified to EN ISO 13485-2016 and have undergone extensive testing and include documentation attesting to their compliance to medical device standards.