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Fiber Lasers have matured into exceptionally reliable and stable industrial tools. These industrial Lasers have unique capabilities that enable a wide range of high quality micro-machining processes; of interest here is their ability to produce high quality welds between plastics.
Of special concern to the medical industry is the need to weld clear or light – coloured plastics. Information is presented here for the first time using Fiber Lasers to weld clear-to-clear plastics. A novel Laser source is therefore used in combination with a novel Laser welding technique to produce results of real interest to the Medical Device and other industries.
Although Laser welding of plastics was initially attempted using solid state Nd:YAG Lasers, these Lasers are essentially low duty cycle pulsed units. They also have low wall plug efficiency and high operating costs. Industrial Laser welding of plastics started to gain wider acceptance with the advent of direct diode Lasers in the 100W average power range, largely due to their continuous wave (CW) operating mode. Their major disadvantage is very low beam quality that necessitates the use of very short focal length lenses if small spot sizes are required.
These can be as short as 20 mm unless complex optics are used such that focal lengths and working distances become acceptable.
This increase in complexity comes at a price but nevertheless a specialised market area is now developing, particularly in the medical plastics industry.
High brightness Lasers are normally employed for applications where very high power density is required i.e. metal cutting. In this case, power densities in the order of 20 MW/cm2 (2 x 107 W/cm2) are required. In the case of plastics welding, typical power densities are several orders of magnitude less, ~ 100-300 W/cm2. Plastics are normally (although not exclusively) welded using a transmission welding technique where one component of the joint is transmissive and one is absorptive for the Laser wavelength used. In this way, controlled melting or wetting occurs at the joint interface.
Plastics welded by conventional thermal welding techniques can also be Laser welded. Specialised plastics are now becoming available that appear black but transmit enough of the Laser beam to allow them to be used as the upper component of a transmission welded joint. The absorbing component of the joint usually contains carbon which is of course a very efficient absorber of IR radiation.
The refinement of Laser welding of plastics known as the Clearweld™ technique is starting to be used in association with Laser welding and is gaining acceptance in the medical plastics field. The technique relies on the deposition of a highly specialised non carbon-containing coating in the area of the part that requires welding. The parts to be welded are clamped together at an appropriate pressure and the Laser beam irradiates the area of the joint.
Careful control of the Laser parameters leads to absorption of the Laser beam by the ink, resulting in a highly controlled melting of the faying materials in the joint area. Very high quality weld joints can be produced with a minimum of heat input to the surrounding material. Another important point of note is that the ink degrades and the joint area becomes clear on solidification.
A series of trials have been carried out using three generic material types that are appropriate to characterise for industrial Laser welding applications within the medical device industry.
All of these materials are known to be Laser weld-able but these trials were aimed at determining the suitability of the Fiber Laser for industrial Laser welding of plastics. Because of the slightly longer wavelength of the Fiber Laser, an appropriate Clearweld™ coating was employed on each material.
A series of 25 mm x 75 mm x 3mm thick samples were prepared from each of the materials. A 12.5 mm wide stripe of Clearweld™ coating was deposited at one end of 50% of the samples. Samples were then clamped together, the coated samples used as the lower component of the transmission weld joint, figure 1.
When welded, these samples conform to ASTM lap shear test requirements.
Figure 1: Welded polypropylene (top sample) and polycarbonate (lower sample) lap shear samples
In this case, a CW beam was used so the average power is the same as the peak power and hence peak power density (irradiance) is identical to the power density. Power density is therefore simply calculated by dividing power by spot area. Laser power was changed incrementally over the range of 30 – 60 watts, changing the power in increments of 10 watts for each material.
The Laser used in this work produces a 5.5 mm diameter; using a short focal length 50 mm collimated beam focal length lens it is possible to produce a <50μm diameter spot. This produces excessive power density and instantaneous material degradation at even moderate power levels. Average power would need to be reduced to the mW (milliwatt) regime to allow welding features of this size, and although readily achievable, this feature size was deemed inappropriate for an initial characterisation exercise. The 5.5 mm beam was therefore used throughout the trials.
There are several techniques used for manipulating either the Laser beam or the work piece to produce a Laser weld. In these trials a Scanlabs scanner was used to scan the beam in a slow raster over the stationary sample. In preliminary ranging trials, a wide scan speed variation was used. Similarly, when using a rastering technique, the raster increment is also important, a raster increment or overlap of 0.75 mm produced good bonds over the range of average power used here. Hence, to identify differences between the three materials, a single speed of 5 mm/s and a single raster overlap was used.
The well-defined wetted weld area is clearly shown in figure 1. The 12.5 mm x 25 mm area covered by the ink is almost completely wetted in all cases. The samples produced using higher Laser power showed some lack of wetting towards the edges of the samples. Mechanical testing of each of these well-wetted joints showed that the strength of the parent material was exceeded in every case. Material failure occurred as expected, adjacent to the weld zone, figure 2.
Other work has shown that high speed welding and smaller spots may also be used, this work will be reported later.
Figure 2: showing material failure (not at the point of weld) after testing
Eliminating the need for any Z-axis positioning by using a collimated (parallel) beam as used in these trials significantly reduces the complexity of welding equipment. Fiber Laser costs on the basis of $ per watt are now comparable with those of flash-lamp pumped solid state Lasers and are also comparable with direct diode systems.
A 100 watt CW/M Fiber Laser is a very useful tool for welding of plastics using the Clearweld™ technique. The slightly longer wavelength does not appear to reduce the efficiency of the process to a significant degree.
Plastics appropriate to the medical industry have been shown to be weld-able in a very controllable manner using an industrial Fiber Laser. High quality defect-free welds have been produced in different plastics materials using a Fiber Laser. Power levels required to achieve large area coverage were well within the range currently available.
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