Design guide
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An exploration of laser welding How laser welding can be used when joining moulded plastic parts and its benefits in small applications The PRW monthly Design Guides provide practical guidance for designers, toolmakers and moulders. Every month a different aspect of design technology is tackled and together these guides are becoming an indispensable reference point for those designing successful products. Having started with relatively basic design guidance, more sophisticated and detailed issues are now being addressed as the series progresses. There are many ways of joining moulded and other plastics parts by welding. The last few Design Guides have covered welding by ultrasonic, hot plate, spin, vibration and induction techniques. This month we conclude the topic with a look at laser welding. In laser welding, the energy of a laser beam is used to raise the temperature above the melting point at the interface of two parts to be joined.
The principle of laser welding
For this to be effective, the plastics material between the joint interface and the laser source must be transparent to the laser energy, while the material below the joint interface must absorb the energy. The ideal is that almost all the energy should be absorbed at or near the surface of the energyabsorbent material. Transparency to laser radiation is not necessarily the same as transparency to light. It is quite possible for a material that appears opaque to the eye to be transparent to the laser. This is because of the difference in wavelength. Humanly visible light has a wavelength of 400-700nm, whereas plastics welding lasers operate in the range 800-1,100nm. Plastics can be welded with a laser diode or a Nd:YAG (neo-
Designer’s notes
Transmission joint types for laser welding, source: After Leister
dymium/yttrium aluminium garnet) laser with a power of 10-30W or more. This is sufficient for rapid welding at speeds up to 10metres per minute. There are two ways of applying laser radiation to the joint. In the first method the laser beam is adapted to irradiate the entire joint profile simultaneously. The alternative and more adaptable method is to guide a focused laser beam along the joint profile under computer control. Lasers can be used to weld almost all thermoplastics, including dissimilar plastics provided their respective melt temperature ranges overlap. The essential point is that one of the joint materials must be transparent to laser radiation, while the other absorbs the radiation. This difference is brought about by colouring the materials
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with pigments that are either laser transparent or absorbent. By using different types of pigment it is possible to laser weld materials that are the same colour. Alternatively, a laser-opaque coating may be applied at the joint surface. Laser welding is not much affected by other additives in the material; 50% glass reinforced plastics have been successfully welded. Joint strength is 80-100% of the base material. The key parameters affecting joint quality are laser power, beam focus spot size, welding pressure and speed/time. Laser joints can be very simple. The joint width is defined by the laser beam so there is no need for special joint geometries or flash traps, and this reduces mould costs. Laser welding has a number of advantages that can be used to
● One joint material must be transparent to laser radiation while the other absorbs the radiation ● Almost all thermoplastics can be laser welded, including reinforced and dissimilar materials ● Joints can be very simple and need no flash traps ● Laser welding is good for very large and very small parts, for clean applications, and for assemblies with delicate components
offset the relatively high cost of the equipment. It is a non-contact process so it can be used with fragile components such as electronics modules. Heat is generated directly at the joint so temperature increases can be confined and minimised. No flash or debris is generated. Computer-guided lasers can deal with complex threedimensional joints. Laser welding is therefore particularly suitable for very large and very small parts, for medical and other ‘clean’ applications, and for assemblies incorporating delicate components. Clive Maier, Econology
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