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The PRW monthly Design Guides provide practical guidance for de-signers, toolmakers and moulders. Every month a different aspect of de-sign technology is tackled and to-gether 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, vibra-tion and induction techniques. This month we conclude the top-ic 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.

An exploration of laser welding● 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 fl ash traps● Laser welding is good for very large and very small parts, for clean applica-tions, and for assemblies with delicate components

Designer’s notes

Transmission joint types for laser welding, source: After Leister

The principle of laser welding

offset the relatively high cost of the equipment. It is a non-con-tact process so it can be used with fragile components such as elec-tronics modules.

Heat is generated directly at the joint so temperature increas-es can be confined and mini-mised. No flash or debris is generated. Computer-guided la-sers can deal with complex three-dimensional joints.

Laser welding is therefore par-ticularly suitable for very large and very small parts, for medical and other ‘clean’ applications, and for assemblies incorporating delicate components.Clive Maier, Econology

How laser welding can be used when joining moulded plastic parts and its benefi ts in small applications

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 mate-rial 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 energy-absorbent material.

Transparency to laser radia-tion 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-

dymium/yttrium aluminium garnet) laser with a power of 10-30W or more. This is suffi-cient for rapid welding at speeds up to 10metres per minute.

There are two ways of apply-ing laser radiation to the joint. In the first method the laser beam is adapted to irradiate the entire joint profile simultane-ously. 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 al-most all thermoplastics, includ-ing dissimilar plastics provided their respective melt tempera-ture ranges overlap.

The essential point is that one of the joint materials must be transparent to laser radiation, while the other absorbs the radia-tion. This difference is brought about by colouring the materials

with pigments that are either la-ser transparent or absorbent.

By using different types of pig-ment it is possible to laser weld materials that are the same col-our. Alternatively, a laser-opaque coating may be applied at the joint surface.

Laser welding is not much af-fected 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 sim-ple. 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

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