Credit: Eastman and IPG Photonics.
Fibre laser welding of Eastman's Tritan copolyester enables clear seams to be produced without the need for additives.
Despite having been invented over 50 years ago in the USA, lasers in all of their many forms are still often perceived as an exotic energy source that is difficult for laymen to understand. But a new laser type and a new laser wavelength starting to be used in the medical device industry is changing that—the high power fibre laser. Dr Tony Hoult of laser welding equipment maker IPG Photonics in California introduces our first innovation.
The high power fibre laser has been dramatically altering the face of the laser industry since 2001. In older technologies, lasers are assembled from many separate optical components and may be delivered to the workpiece via a fibre optic cable, a technology that has been around for more than 25 years.
In the case of the fibre laser (pictured), however, the laser beam is actually generated within the fibre itself and all of the optical components are optically spliced together into one all-in-fibre beam path. The many advantages of this approach are easily understood by anyone familiar with optics.
Going back to basics, a laser beam is simply a beam of light energy which can be focused down to a very small spot, and this property alone is responsible for many of the high power industrial applications for which lasers are used—like cutting and welding thick steels.
In addition to this ability to focus on a very small spot is another property of a laser beam that is perhaps more responsible for their esoteric reputation—most laser beams produce light of a fairly well-defined wavelength.
Most readers will be familiar with the concept of the wavelength of light. But when the discussion moves to the relationship between wavelength and energy—which is at the heart of all branches of physics—many non-scientists and even some engineers may disengage.
However, this important relationship is central to understanding why clear polymers can now be laser welded without the complications of using additives, different colours or additional inks and dyes. This is illustrated in the news that IPG has worked with Eastman to invisibly weld pure Tritan copolyester without additives.
Until recently, significant average power for laser materials processing in industry was only available from a very limited number of laser types, either solid state lasers emitting in the near infra-red 1.07 µm wavelength regime or carbon dioxide gas lasers emitting at the longer 10.6 µm wavelength regime.
The advent of new versions of the standard industrial fibre laser now produce up to 120 watts of power at an intermediate wavelength regime known as the short wavelength infrared regime. The lower photon energy related to this longer wavelength, which is longer than the near infrared regime, has many important effects.
Because specific photon energies are absorbed by particular molecular bonds via a resonance mechanism, this longer wavelength is absorbed differently by many different molecules.
Of particular interest to us in the field of medical polymers is the improved absorption in the carbon-hydrogen (C-H) molecule, which is of course the background chain of all organic polymers. The end result of this is that absorption of this laser beam in clear polymers is greatly increased to the point at which highly controlled melting through the thickness of optically clear polymers is possible.
The availability of this new wavelength at high average power has, therefore, led to a greatly improved and simplified technique for laser welding clear polymers for the medical device industry.
IPG Photonics already has a dominant position in the market for conventional high average power fibre lasers for cutting metals. But in a recent development, IPG Photonics has shown that these lasers may now be used economically for the welding of polymers, of interest to the medical device industry.
At the recent medical technology manufacturing trade show MD&M West in California, held in February, IPG demonstrated how Eastman’s clear Tritan copolyester can be invisibly welded without additives.
Tritan, a relatively new material, has a balance of properties, including clarity, toughness and chemical resistance. It offers similar properties to polycarbonate but does not contain bisphenol A.
Eliminating the need for additives may result in two benefits for manufacturers. One is a potential cost saving, the other is not having to make sure the additive has been tested for biocompatibility.
Additionally, because the welding work is done via laser, no tools come into contact with the medical device material, making the process clean and controllable.
Dr Hoult reiterates the benefits: “For medical device manufacturers, the benefits of this welding technique include fully hermetic leak-proof precision joints which have a smooth weld that will not entrap bio-burden.”
Sterilisation of clear reusable durable devices made from Tritan copolyester is a critical consideration for manufacturers. Gary Hawkins from Eastman told Medical Plastics News that while they haven’t yet completed post ethylene oxide or radiation sterilisation laser weld quality or lap shear strength comparisons (they are due shortly), they expect post sterilisation performance of weld seals to be good based on experience with other welding methods. He added: “We have completed quality and lap shear evaluations after 120-hour exposure to isopropyl alcohol and sodium hyper chloride disinfectants and found strong performance.”
Sub-Micron Laser Welding of Plastics Goes to 500 nm
Recent developments by UK-headquartered TWI (The Welding Institute) and Vistec Lithography in the Netherlands have allowed welds with a width of 500 nm to be made in plastics. Medical Plastics News understands that these are the smallest welds ever reported in these materials. With the drive towards ever smaller scale complex plastic products such as biological analysis chips, chemical micro-reactors and electronics products in plastics, there is a need for welding processes that meet these challenges.
Laser welding developments in recent years have shown that the laser beam size can be used to limit the weld size. Welds of the order of 10 um in width have been demonstrated using a focused infrared laser beam of similar dimension. Welds much smaller than this are not possible using this technique due to the resolution limits inherent in focusing a near infrared laser beam.
An alternative technique has been investigated using precise patterning of laser absorber dye on the plastics surface to define the weld position. This has enabled joints to be made an order of magnitude smaller than this. The new method used electron beam lithography to apply laser absorber in precise patterns, mimicking the methods used to build micro-electronic circuits. The absorber tracks were then used to generate welds between two plastic parts using laser heating.
Polymethylmethacrylate (PMMA) substrates were coated with lithography resist material modified to absorb infrared radiation, and to be weld compatible with the substrate. Developments were carried out to use electron beam lithography to pattern the resist coatings. This left a high resolution pattern of laser absorbent tracks on the surface of the PMMA, enabling selective heating by a laser beam that was much larger than the tracks of absorber.
Tests of the weld strength on larger scale samples with the same materials and techniques showed that successful high strength welds were produced.
Although tests with simple linear patterns have been carried out to date, the potential for welding of high resolution complex patterned devices is clear. These could include small channels, which may have sensitive surfaces such that only the edges of the channels must be heated or detailed areas with very complex welding requirements. The lithographic patterning techniques have been amply demonstrated within the scope of the electronics industry for the deposition of thin layers in high resolution patterns. When the laser is applied it will heat and weld only where the tracks of absorber are present, with the beam passing through the other areas with very little effect, hence protecting sensitive parts of the products.
There is potential for welds of even smaller dimensions as well as potential for use in other plastics using similar techniques.
Medical Plastics News would like to thank Ian Jones, principal project leader for polymers at TWI for this information.
Rotating Fibre Optics Welding
At Medtec Europe, held in February 2013 in Germany, manufacturer of laser production equipment Rofin, also based in Germany, showcased a rotating laser processing head which allows for circular laser processing of miniature workpieces sitting in a tray or bulky parts whose size makes them unsuitable for mounting on a rotary axis. A synchronously rotating gas nozzle precisely delivers assist gas for circular weld seams. According to Rofin, typical applications include cannula, medical instruments and valves for assembling endoscopes.
In non-welding applications Rofin demonstrated StarFemto, a finishing machine which cuts, drills and structures materials using lasers which pulsate as often as every 300 femtoseconds (one femtosecond is 1x10-15 seconds).
In addition to precision finishing, the StarFemto is said to allow cold temperature processing with brittle and temperature sensitive materials like ceramics, glass and polymers without burrs.
Desktop Galvo Fibre Laser Welding System
Fibre lasers were also on show at MD&M West at the stand of US manufacturer of welding equipment and laser processing systems, Miyachi Unitek.
The company demonstrated its newest welding system, its Mu desktop galvo fibre laser welding system, as well as its LF series of fibre lasers.
Miyachi describes the desktop welding system as follows. “It features the motion capabilities of a stage-based system in a highly compact footprint. Fibre and Nd:YAG compatible, the new system has an average power of up to 500 W, and high speed galvo motion. The Class 1 mini workstation is available in a number of configurations, one with a vertical door that provides quick access for loading and unloading. Options include through-the-lens vision, extraction, and custom tooling. The Mu is also available as a Class 4 system, and may also be configured as a welding sub-system for machine integration.”
For the LF fibre lasers, the company states: “The new LF series fibre lasers are ideal for micro spot and seam welding applications as well as fine cutting, with power levels from 100-500 W and a selection of beam qualities to suit each application need. A high definition large screen pendant enables quick and easy programming.”
Laser Welding of Plastic Microfluidic Devices
Frank Brunnecker, vice president of laser welding at German laser welding equipment supplier LPKF has written about how his company has developed laser welding equipment for two microfluidic point of care diagnostic devices.
The first is a device manufactured by the former medical plastic device manufacturer Wilden group, acquired by Germany’s Gerresheimer Regensburg in January 2007.
LPKF’s technology has been used to laser weld a measuring system for combined DNA and RNA analysis (figure 1). The system consists of an analyser and a one-way microfluidic cartridge. The latter is manufactured via injection moulding and joined by laser welding.
For many years, DNA and RNA analysis was a complex process that required fully equipped laboratories. Now, the microfluidic point-of-care “mini-lab” described above allows small research laboratories, surgery centres, clinics, and even food production facilities to dive in to rapid nucleic acid analysis. For example, if a patient is admitted to a hospital, the device allows DNA and RNA analysis to be carried out immediately by testing to identify the presence of any antibiotic-resistant pathogens.
The heart of the whole system is its one-way cartridge. It consists of several injection-moulded components which are joined together by transmission laser welding. This cartridge contains all the necessary reagents, and carries out the actual test. The liquid sample passes along the long microfluidic channels, going through numerous processing steps along the way. At the end, the processed sample lands in the actual detection field.
The technology for sealing the microchannels on the base of the cartridge is said to present a special challenge. The need for the nearly 2 m-long channel to be airtight is obvious. Moreover, the cross-sections of the channels must also guarantee the unimpeded flow of the sample through the maze-like network.
Welding challenges: To join the one-way cartridge, a number of different technologies could have been used. Methods where energy is input in the form of friction, such as ultrasound welding, were deemed unsuitable right from the start because of the specification for absolute particle-free production. These methods produce debris which could block the width of the channels. Thermocouple welding was also deemed technically unsuitable because it cannot satisfy the need for very fine contours.
After analysing all of the alternative technologies in detail, the choice was clear: transmission laser welding. Laser welding was the winner for several very compelling reasons: short cycle times, good options for process monitoring, reliability, high welding seam strength, and the fact that no additional materials were required. The technology is also perfectly suited for cleanroom applications.
The second application is the Medios series from German diagnostics manufacturer Pes DiagnoseSysteme. Here, the complete diagnostics unit is located in one analytical device which takes up about as much space as a desktop printer. The diagnosis cartridges in this appliance are joined by transmission laser welding.
For this application, the choice of material for the main component and the lower cover plate went to a polycarbonate (PC) to satisfy the medical and mechanical specifications. The upper cover plate is a two-component injection moulded part made of PC and a TPE. This is also joined to the main element by laser welding. The housing is made with either a black or transparent colour to absorb the laser energy. The transparent colour contains a special absorber tailored to the wavelength of the laser beam. The cover plate and the TPE in the front module are uncoloured and therefore transmissive to the laser beam.
The functional and production-specific parameters were already taken into account during the design of the cartridge. A key aspect here is ensuring that the welding zone is completely accessible to the laser beam. The stringent requirements of the application itself mean that the microfluidic components which are to be joined together must be made with very high geometrical precision.
After recently expanding production facilities, LPKF has enlarged its distribution capacities with the employment of two new experts—Thomas Eckert and Mario Gügel.
The appointments are to provide consulting services to support the company’s growth in laser welding.
Mario Gügel has experience in the assembly of new industrial installations as well as in their relocation—including industrial laser systems. He will strengthen the international customer acquisition and customer service activities, as well as assisting the LPKF branch office in the USA.
Thomas Eckert’s previous customer service experience was in the technical field service team for the customers and distributors in Germany and Europe of a manufacturer of plastic butt-welding equipment. He will assist the German distribution team from his base in Hamburg, Germany, from where he will also provide customer support to clients in northern Europe.
Transmission Laser Welding Module Optimised for Cleanroom
At Medtec 2012 German plastics joining equipment specialist Bielomatik highlighted the fact that its K3642 laser integration module, optimised for cleanroom class 7 and GMP grade C, allows laser transmission welding to be integrated into existing facilities. The company said that connecting the K3642 by Bielomatik to automated production lines and facilities, such as conveyor belts, rotary discs and so on, is also comparatively simple.
The laser unit is 300 mm wide and is said to offer both high installation flexibility and very good accessibility. The supply unit with laser source, control cabinet and pneumatic equipment can be installed remotely from the laser integration module. An IT connection via a range of interfaces including ethernet also ensures flexibility. Remote maintenance options are possible. The module can be equipped with either a diode or fibre laser in the performance range 25 W to 300 W (continuous wave, not taking into account the power reserve of approximately 20%) at the fibre optic termination. Intuitively operated via touch panel and visualisation software, all process variables can be monitored and adjusted by setting their target levels and upper and lower limits. Retaining force, laser power and scanning speed can be controlled dynamically. Process data can be archived on a network drive, an SD card or a USB stick.
The clamping and fixturing system consists of a pneumatic pressure cylinder and a two-column precision guide. Two points of rotation are used to position the clamping bracket and support for the mask exactly parallel to the capture tool. The clamping system uses reference-based position measurement with an accuracy of either 0.001 mm or 0.01 mm to facilitate the component/cover request and thus prevents unnecessary waste. During welding, the sinking depth is monitored exactly. Bielomatik says the tool can be changed manually in approximately five minutes.
It is claimed by the manufacturer that the laser welding method “meets very high requirements for tight, particle-free and clean welded connections”. It goes on to say: “A flow-optimised, easy-clean stainless steel hood encloses the laser welding integration module and covers encapsulate lubricated parts of the machine, as well as those parts which emit particles”.