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News Item - Lasers Target Medical Plastics
The medical device industry has often been a driving force behind the development and evolution of many of today’s manufacturing technologies. One example of this is the increase in the use of lasers for welding applications on medical device components. The laser is one of the most flexible tools available to production engineers today. Indeed, new applications emerge on a regular basis as laser technology itself continues to evolve to meet the ever growing demands of manufacturers. With its stringent requirements for absolute precision, consistency and cleanliness at each stage in the manufacturing process, the medical device industry poses many challenges to equipment suppliers and manufacturers alike. Adhering to these demands is a must irrespective of the process which may be selected to join plastic components. The laser, however, is fast becoming the process of choice for many manufacturers as it offer a completely hygienic and particle free solution to plastics joining applications on medical device components. In this article, Dave MacLellan – Sales Manager for Rofin-Baasel UK’s Micro Division - explores both the pre-requisites and benefits of the process.
Lasers – Clean, Controllable and Consistent Of the many different types of laser sources available, Diode lasers are most commonly used to weld plastic components. Using wavelengths in the range of 808nm to 980nm and with up to 400W of power, these laser sources provide a highly flexible and powerful solution to plastic welding applications. The actual power required for a particular component is determined by the specific materials being joined, wall thickness and the welding speed or cycle time required. Laser welding of plastic components involves the joining of two layers of polymer, one which is transparent to the laser wavelength and another which absorbs the laser beam. The majority of polymers however, are naturally transparent to the typical laser wavelengths used and therefore additives or coatings are required to change the material properties and make it absorb the laser energy. Using the correct combination of materials allows the laser energy to pass through the upper, transparent polymer and heat up the lower, absorbent polymer. The heat generated from the laser beam then melts the adjacent surface of the transparent material, creating a molten pool which when solidified, becomes the weld joint. Laser welds on plastic are strong, precise and also aesthetic in their appearance, an important attribute for many medical device components. As for any joining process, the individual components must be presented repeatably and also held securely during welding. For laser welding applications, it is especially important that the two components to be joined are firmly in contact to enable the heat to be conducted from the absorbent polymer back to the transparent polymer. Uniform clamping around the weld joint will contribute to the high process quality demanded by the industry. Laser welding of plastics offers a number of distinct advantages over other joining technologies - as the thermal loads and mechanical stress generated during the welding process are extremely low, there is no surface damage to the components and the process is particle free. The capability of the laser to produce spot welds and seams in the sub-millimetre range means that the process can be used for many different components, including miniature parts.
Closed Loop Temperature Control Another characteristic of the laser, which makes it so appropriate for welding applications on medical device components, is the ability to accurately monitor, and in certain cases dynamically control, the process. The latest DILAS galvo scanner technology, available from Rofin-Baasel UK Limited, provides the advantages of closed-loop temperature control using an on-axis pyrometer which is fully integrated within the high speed beam deflection system. Galvo-scanners are commonly used when high speed positioning or movement of the laser spot is required. This type of system is ideally suited to applications such as quasi-simultaneous polymer welding where perhaps several components have to be processed within the working area without manipulating or re-positioning the parts. Compared to other laser types, diode lasers convert the input energy (current) directly into laser radiation. Diode lasers allow quick modulation of the laser power, which is an essential element in high speed processes such as closed-loop temperature control applications. When applied to contour welding of polymers the pyrometer sensor can be integrated into the processing optics and detect on-axis thermal radiation from the process area. To avoid interference between the pyrometer and the laser source, the sensor on the pyrometer uses wavelengths which are different from those of the laser. Pyrometers used in materials processing applications generally have sensors which operate in the 1800 nm–2100 nm range whereas diode lasers operate at the 808 nm or 980 nm wavelengths. The benefits of closed loop temperature control for plastic welding is an even more robust and stable process. The pyrometer controller also enables the storage of process data including weld temperature and laser output, which can subsequently be used for quality control, documentation and analysis purposes.
Polymer Welding Challenges Typical examples where lasers have been used to join and weld medical device components include inhalers and other drug delivery systems where the fine components of the assembly can be joined invisibly and hermetically without generating any debris. Other components include blood filters and diagnostic items where micro-fluidic channels are created to make a complete “lab on a chip”. The challenges in joining polymers using a laser are greatest on those polymers which are pure white or clear and transparent. The nature of the medical device industry is that often these colours are chosen over black or darker colours owing to the association of non-coloured items with “purity” and cleanliness. However has not been detrimental to the adoption of laser polymer welding, which continues to expand within the medical market. Today, new laser sources, with a wider range of wavelengths and a lower cost base are making polymer welding even more attractive and the uptake of this technology within the medical device industry looks certain to increase rapidly.
