Laser welding

...

Welding with diode or Nd:YAG lasers has become a well-established processing method for joining thermoplastic components in industrial applications. The recognised procedure is transmission laser welding, where the upper plastic part is transparent to the laser and the lower part is absorbing (Fig. 1).

The absorbed radiation is converted into heat at the interface and a joint with the upper part created by heat conduction. To manufacture plastic materials that are absorbing, generally carbon black is used as the additive. However, with carbon black, plastics are restricted to dark colours and transparent plastic components are not possible.

Transmission laser welding - Fibre lasers

A recent advancement in laser welding is the development of fibre lasers. These operate at a wavelength of between 1 and 2 µm with laser power up to several kW and offer improved beam quality compared with diode and Nd:YAG lasers. Their use in plastics welding is under evaluation. A comparison of commercially available laser sources in shown in Table 1.

Laser absorbers

As noted, the process requires NIR absorbing materials to convert laser energy to heat. Carbon black dispersed throughout the lower substrate material has

---------------------------------------------------------------

served this purpose. However, it imparts color to the substrate, a distinct disadvantage in applications where appearance is important. In addition, it requires high loading in the absorptive material, up to 1.00%, to be effective.

Recently, however, laser-absorptive coatings and resins containing complex organic additives have been developed to provide greater colour and design flexibility in laser welding clear and opaque plastics.

Applied by liquid-dispensing systems, low-viscosity, solvent-based coatings form a thin, uniform layer of an NIR absorbing material at the joint interface. Since most plastics do not absorb NIR energy, only the areas where the coating has been applied and exposed to laser energy will melt and form a weld. Because they produce colourless welds, coatings are ideal for laser-welding colourless and transparent materials for medical components such as filters, containers, fittings, tube sets, and microfluidic devices.

Laser-absorptive coatings also can be used to weld coloured or opaque materials. They are compatible with the widest range of thermoplastics of all laser-welding techniques, and have been tested for biocompatibility and cytotoxicity. In addition, they permit selective welding without the need for masking and provide the capability to weld multiple layers simultaneously. If necessary, parts can be pre-coated off-line and inventoried for subsequent welding.

Alternatively, these absorbers can be added to resins, which, in turn, can be moulded or extruded into a part. Weldable resins also can be used to extrude films that can be die-cut and insert-moulded or used as intermediate layer for laser welding. Formulating them requires taking into account a range of variables, and the type of additive used is determined by polymer compatibility, colour requirements, and laser welding wavelength. As a result, custom formulations are designed specifically for each application. Weldable resins yield parts with a high level of visible transmission, but impart a slight tint to the substrate, which can then be colour-corrected. They are well suited to moulding coloured transparent and light- coloured opaque substrates for electronic housings, medical devices, fluid containers, and other relatively small components or components with large weld areas.

Substrate Material

The material requirements for laser welding are a top substrate that transmits the laser wavelength and a bottom substrate that absorbs the laser energy either throughout the bulk of the material or at the surface only. In addition, the two substrates should have similar melting temperature ranges and be miscible with each other.

---------------------------------------------------------------

Part design

When welding two surfaces, the joint design of the parts affects the welding. The design must allow the application of enough pressure to create a strong weld. The laser energy must reach the weld interface and the joint must be perpendicular or at an angle to the laser beam. If the joint is parallel to the beam, the laser energy is not sufficient for generating a weld.

The cleanliness of the surfaces that are to be welded may also affect the quality of the weld. If the surface is contaminated with a foreign material, the contamination may interfere with the transmission of the laser energy through the top substrate to the weld.

The texture of the surface may also affect the welding process. As the contact area between the two surfaces increases, a stronger weld will form. Smooth, moulded surfaces are therefore preferred. Rough surfaces can be welded, but the weld strengths may be reduced.

Finally, the materials that are being welded must be considered when developing a joint design. Plastics that do not transmit NIR energy very well will be limited by thickness when used as the top substrate. When welding through thick top substrates a higher energy density may be required.

In welding process such as ultrasonic welding, the process relies on the melting of a large amount of plastic. When the melting occurs under applied pressure, the parts will collapse to form the weld joint. However, there is little material collapse associated with laser welding in comparison to the collapse seen with other welding processes.

Clamping pressure

To obtain good weld quality, the substrates must be in intimate contact at the weld interface. The most common way of achieving this is to apply external clamping pressure. However, an interference fit may provide enough internal pressure to avoid the need for external clamping pressure. The pressure depends upon the materials being joined and the surface conditions at the weld interface.

This allows sufficient melt flow to weld compatible substrates.

Parameter Selection

Optimised welding parameters achieve the application requirements and include energy density (which is calculated by dividing laser power by the beam size and weld speed or time), clamping pressure and absorbing additive concentration. These weld parameters act in combination