Laser welding of plastics

Laser welding was first demonstrated on thermoplastics in the 1970's, but has only recently found a place in industrial scale situations. The technique, suitable for joining both sheet film and moulded thermoplastics, uses a laser beam to melt the plastic in the joint region. The laser generates an intense beam of radiation (usually in the infra red area of the electromagnetic spectrum) which is focussed onto the material to be joined. This excites a resonant frequency in the molecule, resulting in heating of the surrounding material. Two forms of laser welding exist; CO2 laser welding and transmission laser welding. CO2 laser radiation is readily absorbed by plastics, allowing quick joints to be made, but limiting the depth of penetration of the beam, restricting the technique to film applications. The radiation produced by Nd:YAG and diode lasers is less readily absorbed by plastics, but these lasers are suitable for performing transmission laser welding. In this operation, it is necessary for one of the plastics to be transmissive to laser light and the other to absorb the laser energy, to ensure that the beam focuses on the joint region. Alternatively, an opaque surface coating may be applied at the joint, to weld two transmissive plastics. Transmission laser welding is capable of welding thicker parts than CO2 welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.

The technique
CO2 laser welding

The CO2 laser is a well established materials processing tool, available in power outputs of up to 60kW, and most commonly used for metal cutting. The CO2 laser radiation (10.6µm) is rapidly absorbed in the surface layers of plastics. Absorption at these photon energies (0.12eV) is based on the vibration of molecular bonds. The plastics will heat up if the laser excites a resonant frequency in the molecule. In practice the absorption coefficients for the CO2 laser with most plastics is very high. Very rapid processing of thin plastic film is therefore possible, even with fairly modest laser powers (<1000w).> 2 laser beam cannot be transmitted down a silica fibre optic, but can be manipulated around a complex process path using mirrors and either gantry or robotic movement.





A CO2 laser weld in 100µm polyethylene film at 100m/min with 100W laser power




Transmission laser welding - Nd:YAG laser

The Nd:YAG laser is also well established for material processing, and recent developments have led to increases in the power available to above 6kW and reduced the physical size of the laser. In general, the light from Nd:YAG lasers is absorbed far less readily in unpigmented plastics than CO 2 laser light. The degree of energy absorption at the Nd:YAG laser wavelength (1.064µm, 1.2eV photon energy) depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastics, the laser will penetrate a few millimetres into the material. The absorption coefficient can be increased by means of additives such as pigments or fillers, which absorb and resonate directly at this photon energy or scatter the radiation for more effective bulk absorption. The Nd:YAG laser may therefore be used for heating plastics to depths of a few millimetres or for heating a more highly absorbent medium (either metal or a plastic containing suitable additives) through or within the transmissive plastic part. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

Transmission laser welding - Diode laser

High power diode lasers (>100W) have been available since early 1997. They are now available up to 6kW and are competitively priced compared to CO2 and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than CO2 (10%) or Nd:YAG (3%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers, and applications overlap. The beam from a diode laser is typically rectangular in shape, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing.








Diagram of transmission laser welding







Comparison of commercially available laser sources for plastics processing

Laser Type	CO2	Nd:YAG	Diode
Wavelength (µm)	10.6	1.06	0.8-1.0
Max. power (W)	60,000	6,000	6,000
Efficiency	10%	3%	30%
Beam Transmission	Reflection off mirrors	Fibre optic and mirrors	Fibre optic and mirrors
Minimum spot size * (mm)	0.2-0.7 diam.	0.1-0.5 diam.	0.5x0.5
Capital Cost * (£k)	100W: £20k 1000W: £50k	100W: £40k 1000W: £80k	100W: £15-20k 1000W: £80-100k
Running Cost *(£/hr)	100W: £0.2-0.5 1000W: £2-4	100W: £0.1 1000W: £3-5	100W: £0.1-0.2 1000W: £1-2
Interaction with Plastics	Complete absorption at surface in <0.5mm	Transmission and bulk heating for 0.1-10mm	Transmission and bulk heating for 0.1-10mm

* Approximate figures for general case. Other equipment variants exist with different properties.

Scope

Laser welding is a high volume production process with the advantage of creating no vibrations and generating minimum weld flash. The technique relies on the initial outlay for a laser system, however, the benefits of a laser system include; a controllable beam power, reducing the risk of distortion or damage to components; precise focussing of the laser beam allowing accurate joints to be formed; and a non contact process which is both clean and hygienic. Laser welding may be performed in a single-shot or continuous manner, but the materials to be joined require clamping. Weld speeds depend on polymer absorption. It is possible to create joints in plastics over 1mm thick (with transmission laser welding) at up to at least 20m/min whilst rates of up to 750 m/min are achievable in the CO2 laser welding of films.

Adaptations of laser welding
Clearweld®

The Clearweld® process was invented, and has been patented, by TWI. It is being commercialised by Gentex Corporation. The process uses commercially available lasers in conjunction with infrared absorbing welding consumables.

The carbon black absorber traditionally used is replaced by a colourless, infrared absorbing medium thus expanding the applicability of the technique to clear plastics. The infrared absorbing medium is either printed/painted onto one surface of the joint, encompassed into the bulk plastic, or produced in the form of a film that can be inserted into the joint. It absorbs infra-red laser light allowing an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour. The process is especially suitable where the appearance of a product is important. In the case of fabrics joining, positioning of the infrared absorbing medium at the joint restricts melting to the interface rather than through the full thickness of the joint as occurs in other welding methods for fabrics. Consequently, flexible seams are produced making the process suitable for the joining of fabrics for clothing applications.

Additional information can be found on the Clearweld® website - www.clearweld.com.