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Laser welding of thermoplastic materials (June 2001)

 
Felicity A Chipperfield and Ian A Jones

Paper published in Medical Device Technology vol.12, no.5, June 2001, pp. 40-45

1. Introduction

Laser welding of thermoplastic materials is gaining significant interest in a number of industrial sectors: automotive, consumer goods, fashion, medical and packaging. There are three main laser types which are commercially available for producing high quality joints in thermoplastic components (see Table 1).

Table 1. Comparison of commercially available laser sources for plastics processing.

Laser TypeCO 2Nd:YAGDiode
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 0.5 - 5.0, often rectangular
Capital Cost *, £k 100W: £20k
1000W: £50k
100W: £40k
1000W: £80k
100W: £10k
1000W: £50k
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
Specific to this application Clear to clear, see section 3.1.2.1 Clear to pigmented or coloured surface, or Clearweld®, see section 3.1.2.2 Clear to pigmented or coloured surface, or Clearweld®, see section 3.1.2.3
* approximate figures for general case. Other equipment variants exist with different properties.

2. Types of laser

2.1 CO 2 Laser

Fig. 1. CO 2 laser welded polyethylene, joined at 100m/min
Fig. 1. CO 2 laser welded polyethylene, joined at 100m/min

The CO 2 laser is a well established materials processing tool, now available in power output up to 60kW, and most commonly used for cutting metals, plastics and ceramics. The CO 2 laser radiation (10.6µm wavelength) 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 due to the laser exciting a resonant frequency in the molecules. Very rapid processing of thin plastic film (<0.2mm) is therefore possible (high speed welding up to 500m/min has been demonstrated), even with fairly modest laser powers (<1000W). Figure 1 shows a weld made between two polyethylene films at 100m/min. The CO 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. It is not suitable for transmission laser welding.

2.2 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 6kW and reductions in the laser size. In general, the light from Nd:YAG lasers is absorbed far less readily in 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, 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. [1] Welds can be made by positioning such infrared absorbers to generate heat at a joint interface. This process is generally termed 'transmission laser welding' because the laser energy is transmitted through a proportion of the plastic to the joint. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

2.3 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 CO 2 and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than either CO 2 (10%) or Nd:YAG (3%) lasers. Typically diode lasers emit radiation at a wavelength of 0.8-0.95µm. Therefore, the interaction with plastics is very similar to that of the Nd:YAG laser, and applications overlap. The beam from a diode laser is generally 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. [2]

Nd:YAG and diode lasers can both be used to carry out transmission laser welding, given that one of the plastics is transmissive to the laser and the other absorbs. The process provides a means by which plastic parts with different absorption characteristics may be welded with no melt flash, no marking of the outer surfaces and only a small heat affected zone.

3. Transmission laser welding using Clearweld®

Fig. 2. Transmission laser welded samples made using a carbon black IR absorber (top) and the Clearweld® process (top)
Fig. 2. Transmission laser welded samples made using a carbon black IR absorber (top) and the Clearweld® process (top)
TWI and Gentex Corporation have developed a technique for laser welding plastics with infrared dye, creating a joint almost invisible to the human eye. Typically carbon black would be used as the absorbing medium for the laser light, however, this new approach enables two similar clear (or coloured) plastics to be joined with a minimal mark weld line, see Fig.2. The selection of IR absorbers with minimal visible light absorption is made to match the 1064nm wavelength (Nd:YAG laser), as well as at 808nm and 940nm (wavelengths available from the relatively new diode lasers), see section 3.1.

The Clearweld® technology works by using an almost colourless infrared absorber, which absorbs the infrared laser light very efficiently, converting the absorbed light energy to heat without significantly absorbing visible light. The infrared absorber is applied to one of the components to be welded, either to the surface by painting or printing, or into the bulk of the plastic.

3.1 Light absorption and energy conversion in organic dyes

Conventional dyes, by definition, absorb visible electromagnetic radiation (380-750nm), the process involving excitation of an electron from the highest occupied molecular orbital of the chromophore into the first vacant antibonding orbital. The process occurs without electron spin change and the dye is promoted from the singlet ground state to the first excited singlet state. This phenomenon has been used extensively in heat mode optical data recording, where a finely focussed laser beam is absorbed by a dye and the heating effect produces a change in the polymer substrate, by melting or ablation. It has been shown, for example, that using energies as low as 0.1 nanojoule per square micron can produce local temperatures up to 300°C, sufficient to melt most plastics and dyes, if a dye layer absorbing some 99% of the incident energy is employed. [3]

For maximum conversion of light energy to thermal energy by a dye, the dye should absorb as much of the light as possible, and, if broad band radiation is employed, then the naturally broad bands of most dyes are an advantage. However, if laser sources are employed, it is important to match the λ max (the wavelength at which maximum absorption occurs) of the dye as closely as possible to the laser wavelength. In addition, a narrow band dye is likely to have a much higher molar absorption coefficient at its λ max than a structurally similar dye with a broad absorption band, and consequently less dye will be needed to achieve maximum absorption of the incident light.

3.2 Methods of introducing absorbers for laser welding of plastics

When the incident laser light is absorbed, the dye molecules dissipate the absorbed energy principally as heat to the dye molecules and their local environment.

Dye concentrations of approximately 0.02% on a film weight basis are typically adequate but are a function of the particular dye used as well as the plastic being welded. If the dye is introduced into the bulk of the plastic, clearly this has the advantage that any area subject to the incident laser light will be heated, however the cost of applying dye to the whole material could be economically non-viable in some applications.

Therefore the methods of locally introducing the IR absorber to the joint include the following:

  • As a thin film
  • Sprayed or printed from a solvent

3.3 Welding of fabrics

The Clearweld® technology is not limited to sheets and plaques of material, it can also be used for fabrics. Figure 3 shows continuous and hermetic overlap welds made in the waterproof fabric Goretex TM using a Nd:YAG laser beam of approximately 100W in power and welding speed of 500mm/min.
Fig. 3. Continuous overlap welds made using infrared absorbing dye in the fabric Goretex TM
Fig. 3. Continuous overlap welds made using infrared absorbing dye in the fabric Goretex TM

The advantage of using laser welding over traditional stitching is that the surface of the material is unbroken, and so the fabrics remain waterproof around the seams.

Figure 4 shows a cross-section through a similar overlap joint between two pieces of Tactel TM , a nylon based fabric employing a polyurethane top layer, welded at 1500mm/min. Blue fabric has been welded to yellow fabric to show the degree of mixing in the melt region.

Fig. 4. Cross section of a joint made in the fabric Tactel TM utilising infrared absorbing dye
Fig. 4. Cross section of a joint made in the fabric Tactel TM utilising infrared absorbing dye

Individual unmelted fibres can be clearly seen, resulting in little change to the external appearance of the joint. This indicates the level of control available over the process.

4. Summary

Clearweld® is an exciting development, for welding transparent polymers or various fabrics with great versatility. It is a technology on the verge of exploitation and awaits industrial needs before specific applications are defined. The Clearweld® process is being commercialised by Gentex Corporation and TWI.

For more information, please contact us.

References

  1. SeredenkoM M: 'Determining spectral characteristics of pigment absorption and scattering in the middle IR spectral range'. Optics and Spectroscopy. 1994 76 (3) 418-420
  2. HaugM and Rudloff T: 'Assessment of different high power Diode lasers for material processing'. Proc conf 'Lasers in Materials Processing', Munich, Germany. June 1997, SPIE 3097, P5 and 3.
  3. HurditchR: Colour Science '98 ed. J. Griffiths, University of Leeds Print Services, Leeds, 1999.

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