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Transmission laser welding of plastics (May 2002)

 
P A Hilton*, I A Jones* and Y Kennish**

*TWI Ltd, Cambridge, UK

**Engineering Department, University of Cambridge, UK 

Paper presented at International Congress on Laser Advanced Materials Processing (LAMP 2002), May 2002

Abstract

The use of lasers for welding plastics was demonstrated in the early 1970's. However, it was not until late in the 1990's that production applications started to be considered widely. This followed the broad realisation that by selection of a suitable combination of radiation wavelength and plastics additives, to control light transmission and absorption, heat could be generated at the joint of a pre-assembled part without melting its outer surfaces. It is of added benefit that the window of transmission for an unpigmented and unfilled plastic typically covers the wavelengths delivered by small and cost effective diode lasers. Recent developments in the transmission laser welding process for plastics are discussed, including methods for the generation of welds between two clear plastics, application of similar techniques to the joining of thermoplastic textiles and new equipment, able to heat a complete joint and assist in the sealing of assemblies where the joint surfaces are not particularly smooth. An analytical heat flow model for the welding of clear plastics is shown in use for selecting process parameters.

Introduction

There are more than twenty separately identifiable techniques for welding thermoplastics, some of which have been commercially available for many years. These include manual processes, such as hot gas welding and extrusion welding, processes using vibration and frictional heating between the materials, such as ultrasonic and linear vibration welding and processes using an electromagnetic heat source, such as resistive implant welding and dielectric welding. [1] Laser welding may now be considered as another alternative welding method for plastics, with distinct processing and performance characteristics.

Early CO 2 lasers were first used to weld plastics (stake welds in a lap joint configuration) as long ago as 1970. [2] CO 2 laser light (10.6µm wavelength) tends to heat most plastics from the surface down with a very rapid heating action achievable. The CO 2 laser has found wide use in the cutting of plastic sheet material with high speed and accuracy. Thin polyolefin films (up to 0.1mm thick) have been welded with a CO 2 laser at speeds in excess of 500m/min. [3,4,5] However, the use of CO 2 lasers for welding of plastics has not entered wide use and only recently have production examples of package sealing in the medical sector been made public. Only after the development of an alternative method of applying the laser energy using diode or Nd:YAG laser sources, has the laser found extended application for welding plastic components.

Nd:YAG and diode laser light (800-1100nm wavelength) will transmit through several millimetres of unpigmented polymer. The polymer can be designed to absorb and heat in these laser beams with the addition of an absorber. Transmission laser welding of thin and thick materials is therefore possible where a transmitting plastic overlays an absorbing plastic. This results in a method of welding plastics that does not mark the outer surfaces of the component. The melting is carried out only where it is required at the interface between pre-assembled parts. The absorbing material used in the process is typically carbon black. This process was first described in 1985 for welding automotive components. [6] The first (published) part, mass produced using transmission laser welding was a keyless entry device for Mercedes in 1997. [7] A further development in laser welding in 1998 was the invention of the Clearweld ® process, [8,9] which now allows infrared transparent plastics to be welded, thus further extending the range of possible applications.

CO 2 laser welding - for films and thin plastics

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Fig.1. Lap weld in 0.1mm thick polyethylene made with a 900W CO 2 laser at 100m/min

The CO 2 laser is a well-established materials processing tool, available in power output up to 20kW, and most commonly used for metal cutting. 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 if the laser excites a resonant frequency in the molecule. In practice the absorption coefficients for the CO 2 laser light is high in most plastics. Very rapid processing of thin plastic film is therefore possible, even with fairly modest laser powers (<1000W) available from compact sealed CO 2 lasers. 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. Figure 1 shows a section through a lap weld in 0.1mm thick polyethylene made at TWI with a CO 2 laser power of 900W and a speed of 100m/min.

Clamping to keep the films in contact at the joint line is the most important feature of a system designed to carry out laser welding of plastic films. This technique may be applied mainly as an alternative to ultrasonic welding, where a fast, clean, fully automated joint is required. A simultaneous cut/seal may also be carried out for packaging or bag making purposes by controlling the laser beam power distribution using a diffractive focussing element, [10] cutting two films in contact whilst leaving a welded region at the edge of the cut. CO 2 laser welding of plastics greater than 0.5mm thick is not possible at high speeds unless the joint surfaces are melted directly with the laser and then butted together. [11] This is a variation of hot plate welding in which the joint surfaces are heated against a hot plate before joining. Despite these various techniques for using CO 2 laser for welding plastics, they have not been used extensively in production. Diode laser welding has been far more successful.

Transmission laser welding - for films, sheets, moulded components and polymer based fabrics

The nature of the interaction between the 10.6µm beam from the CO 2 laser and thermoplastic materials, meant an analogue to the deep penetration process used to weld metals could not be developed for plastics. The CO 2 laser beam is absorbed at the surface of the plastic, relying on conduction to heat through the thickness of the material, which results in decomposition, vaporisation and charring, before any significant depth of material is melted. The increasing use of Nd:YAG solid state lasers, and the advent of diode lasers (both producing beams with a near-infrared wavelength), meant laser beams became available that interacted with thermoplastics in a different manner to that of the far infrared CO 2 laser wavelength. The shorter wavelength beam available from both these near-infrared types of laser will transmit through several millimetres of unpigmented polymer, but can be absorbed by the polymer if an appropriate filler is added. [12,13] Based on this filled polymer technique, a transmission laser welding process was developed for thermoplastics where the transmitting plastic overlays an absorbing plastic. The laser beam passes through the top (transmitting) layer and is absorbed by the filler in the lower layer, producing sufficient heat to make a weld at the interface between the two parts. This process was first described for welding automotive components. [6] Typically, carbon black is used as the absorbing pigment or filler, resulting in one half of the assembly being coloured black. [14] A refinement of this process is currently being used by Mercedes Benz, to manufacture a sealed electronic key fob. A black dye that is an absorber in the visible region, but transmissive in the infrared, is used in the overlaying component, giving a welded assembly that appears completely black to the human eye. [7]

b3943f03.jpg

Fig.2. Laser transmission weld in 4mm thick polypropylene using a 100W Nd:YAG laser at a speed of 1.6m/min. The weld is at the interface between the light and dark materials. The melt zone is approximately 0.3mm thick

An example of the transmission welding technique, made with an Nd:YAG laser, utilising a visually transmissive plastic material for the upper section and a carbon black loaded plastic for the lower layer, can be seen in Figure 2.

High power diode lasers (>100W) have been available since early 1997, and are now available up to 4kW in power 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 at ~30%, than CO 2 (~10%), Nd:YAG (~3%) or excimer (<1%) lasers. High power diode lasers are commercially available with wavelengths of 808nm, 980nm and 940nm. The degree of energy absorption at these wavelengths in plastics, depends again, largely on the presence of additives. If no fillers or pigments are present in the plastic, the laser will penetrate a few millimetres into semi-crystalline plastics and further through unpigmented amorphous plastics.

When the suitability of the wavelength and the high efficiency of the diode laser is coupled with the fact that the natural focus of this type of laser system is very close to that required for transmission laser welding, it is easy to see why all of the commercial applications of this welding technique utilise diode laser light. [15]

Overlap and T-joint welding can be performed using the transmission technique. Thermoplastic welding processes require good contact between the two surfaces to be joined and application of pressure. It is interesting to note that a pressure applicator, if transparent to the diode laser light, can be placed directly over the position of the weld. The major benefits of the transmission laser welding technique can be summarised as:

  • a weld seam of high quality;
  • two part assembly;
  • no direct contact with the welding 'tool';
  • flexible joint configurations;
  • controlled and localised energy input;
  • high joint strengths.

Although several infrared absorbing pigments are available which can be incorporated into the laser assembly of the part being welded, there are several drawbacks to the standard transmission welding technique. Although a candidate absorbing plastic may appear to be non transparent as a result of a laser beam transition test, it may in fact not be a candidate for transmission laser welding due to it scattering laser light rather than absorbing the light. A classic example of an absorber for transmission laser welding is carbon black, but this limits the choice of colour for one part of the component.

An extension of the transmission laser welding process which allows completely clear or similarly coloured components to be welded by using an infrared absorbing medium, clear in the visible range of the spectrum, but tailored to absorb heavily the specific wavelength of the laser beam being used, has also been described. [9]

The nature of the infrared absorbers, means that it is possible to tailor the absorber to the wavelength of light being used to absorb energy efficiently. Thus only relatively small amounts of absorber at the interface between the two components to be welded are required. The absorber can be introduced as a discrete layer at the interface (in a film, as a coating, or in an absorber rich surface layer of the component), or incorporated into the bulk of the polymer of the underlying component. Development work on this process, which has the trademark Clearweld ®, at TWI, was mainly carried out using polymethylmethacrylate (PMMA) test specimens. An example of an overlap weld made by applying a painted layer of absorber to the joint region between two transparent sheets of 3mm thick PMMA can be seen in Figure 3. One of the obvious advantages of this technique is that almost any colour of plastic material can be used on either side of the joint.

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Fig.3. Transmission laser overlap weld in clear PMMA made with an infrared impregnated film interlayer

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Fig.4. Test samples in which the top was welded using a scanning diode laser

a) black to clear PC using carbon black absorbent

spiajmay2002f2b.jpg

b) clear to clear ABS using Clearweld ® absorbent

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c) clear to clear PP using Clearweld ® absorbent. The height of each component is about 30mm.

Figure 4 shows three variations of the same component, with the lid welded to the body using (a) basic transmission welding utilising a carbon black loaded lower body, (b) the Clearweld ® process with a clear ABS body and lid and (c) the Clearweld® process with a translucent to translucent polypropylene body and lid.

The samples shown in Figure 4 were welded with a 50W scanning diode laser system. The joint faces for the Clearweld ® process were prepared by applying absorbent very simply with a paintbrush. The laser beam was moved round the circular joint profile at the rate of 20 revolutions per second. Welding times of 3-6 seconds were used and the energy applied was 0.6J/mm 2 , for the clear to black samples and 0.75-1.5J/mm 2 , for the clear to clear samples, depending on the materials used. Leak tight joints were produced in all cases.

spiajmay2002f5.jpg

Fig.5. Clearweld ® laser weld in PMMA made with infrared absorbent impregnated film at the interface, shown in transmitted light microscopy between crossed polars

The form of the joint from the Clearweld ® process can be seen in Figure 5. The heat-affected zone is indicated by the change in birefringent colour viewed between crossed polarising optics. This is indicating a change in the orientation of polymer molecules, probably due to residual stress generation as a result of a heating cycle in constrained material. The lenticular HAZ section is characteristic of the process, and for this sample is a maximum of 0.5mm thick. The thin dark line at the centre of this lenticular region is the actual weld, which is very narrow. By suitable choice of the pigment in the top part of a totally black component, i.e. a pigment which absorbs visible light but transmits infrared light, it is also possible, using the Clearweld ® process, to produce a welded joint in a component which is black to the eye.

For the Clearweld ® process a number of application methods have been developed for the absorbent, some of which are ready for commercial use, and others that may be of use for future applications:

Commercially available:

  • Surface application in a solvent carrier by ink jet printing
  • Surface application in a solvent carrier by spraying or needle dispensing

Potential future use:

  • A thin film incorporating the infrared absorber placed at the joint.
  • In the bulk of the polymer (typically this is the method used with carbon black as the absorber).
  • Use of an absorber laden film used as a mould insert.
  • Surface application by dip coating, infusion, painting, or pad printing.
  • Co-extrusion.
  • Overmoulding.

A study of the degree of absorption required to give high speed and high strength welding has been carried out using ink jet printing as the application method. The results are displayed in the graph shown in Figure 6. The study was based on lap welded polycarbonate sheet. There are indications that the amount of energy required to make a high strength weld varies with the energy absorption at the interface. A minimum value, for this particular case was found at an absorption of 21%. It is evident that effective welding can be achieved with the presence of only small amounts of absorbent material, and that there are no benefits obtained, in terms of productivity, by using large excesses of the absorber. In this experiment the degree of energy absorption was altered by changing the density of drop deposition from the ink jet printer.

sppahmay2002f6.gif

Fig.6. Applied laser energy required to achieve high weld strength in polycarbonate sheet when using different amounts of absorbent applied using ink jet printing

However, more recent work with higher power diode lasers, when welding polymer fabrics, has shown that for a similar spot size and amount of absorber, a seven fold increase in speed could be obtained by increasing the applied power for 150W to 900W. Welds made at both laser powers, exhibited very similar peel strengths when tested.

Triple layer joining

Figure 7, shows the absorption coefficient of two particular infrared absorbers (A and B) of the type used in the Clearweld ® process, plotted against laser wavelength. The 808nm and 940nm wavelengths were obtained from a diode laser and the 1064nm wavelength was obtained using an Nd:YAG laser. The figure shows clearly how wavelength sensitive the absorbers can be and this sensitivity can be put to some interesting uses. The nature in which a diode laser is constructed makes it relatively easy to assemble a single laser, capable of operating at two separate or combined wavelengths, simply by switching the power to different stacks of diodes. TWI has such a diode laser capable of producing 150W of power at either 808nm or 940nm. 300W of power is available if both wavelengths are used simultaneously. Conveniently, both wavelengths are focussed with the same set of optics. Using this laser and two selected absorbers, it is possible to simultaneously weld three sections of plastic material together, as depicted schematically in Figure 8.
sppahmay2002f7.gif

Fig.7. Absorption coefficient of two absorbers as a function of laser wavelength

Absorber B, highly absorbent at the shorter (808nm) wavelength, but able to transmit a significant fraction of laser power at the longer (940nm) wavelength is applied to the top surface of the intermediate layer, whereas absorber A, strongly absorbent at the longer (940nm) wavelength, is applied to the lower side of the intermediate piece. This assembly is then subjected to a single pass of a laser beam containing both wavelengths, resulting in a triple layer weld. The technique has been applied to both solid plastics and fabrics, as can be seen in the weld section shown in Figure 9, made using 300W of laser power, at a travel speed of about 500mm/min.

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Fig.8. Schematic representation of a triple element weld, made using two infrared absorbers and two laser wavelengths

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Fig.9. Section through a triple layer joint made using two infrared absorbers in 2mm thick sheets of clear PMMA

Welding of thermoplastic fabrics

Although the Clearweld ® example in Figure 3 is shown with two visibly clear sheets of PMMA, an absorber applied in this way can be used to join several other materials, coloured or otherwise. In this context the technique has also been used to perform transmission laser welding of polymer based fabrics, in an overlap joint configuration. [16]

When the incident laser light is absorbed, the absorbed energy is dissipated principally as heat to the absorber molecules and their local environment. In the case where the local environment is a thermoplastic polymer, melting occurs at the surface between the dye and the polymer. If a polymer that does not absorb near-infrared radiation but which may be clear or coloured, is adjacent to this surface, the melting will cause a weld to occur.

Thus for overlap joining of polymer fabric material, the absorber can either be incorporated between the two layers of fabric as a thin film, or incorporated into the bulk of the lower (with respect to the incident laser radiation) piece of material. In the latter case, only the absorber at the upper surface of the material will be used in the joining process, as most absorbers completely absorb the incident laser light in a depth of less than about 40µm.

In its simplest form, the absorber can be dissolved in a suitable host and painted onto the joint line. More sophisticated versions of this technique would employ the use of stick-on films containing the absorber, or by sprayed application from an ink jet printer. Absorber concentrations of approximately 0.02% on a film weight basis are typically adequate but are a function of the particular absorber used as well as the plastics being welded. A film thickness of 10-40µm is also typical. One advantage of using a film/tape containing the absorber or using a spray application is that the film is only needed where a joint is required. If the absorber is introduced into the bulk of the polymer fabric, clearly this has the advantage that any area subject to the incident laser light will be affected, however the cost of applying absorber to the whole fabric could be uneconomic in some applications. The method employed in any production process would depend on the particular application and the restriction of added manufacturing steps and cost.

As with the Clearweld ® process for solid plastics the absorber at the interface between the materials acts as the site where the light from the laser is absorbed and converted into heat in a well-defined area. The materials to be joined are placed one on top of the other, and clamped together. The area of heating and hence joining may be defined by either the size of the laser beam or the extent of the absorber containing region. In the experiments reported here, both Nd:YAG and diode laser light have been used. Both these laser wavelengths are easily transmitted via optical fibres, which enhances the flexibility of the process in industrial terms.

Nd:YAG lasers for this type of work are usually employed in a de-focus position to produce a spot of laser energy some few mm in diameter. The current methods of beam forming used with diode lasers, tend to produce a natural focus of the diode light which is rectangular in shape, a few mm by a few mm in size. This energy profile is almost ideal for the thermoplastic fabric welding process. The welding occurs as the heat generated in the absorber is sufficient to melt of the order ≤ 0.1mm of the polymer fabric. The heat generation at the interface is controlled by the absorption coefficient of the layer and the processing parameters. The main parameters, for a given width of weld, are laser power, energy distribution in the focus, and the welding speed.

Figure 10 shows continuous overlap welds, providing hermetic sealing, made in a waterproof fabric using this technique and an Nd:YAG laser beam of approximately 100W in power. The welding speed was 500mm/min.

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Fig.10. Continuous overlap welds made using the Clearweld ® technique in a waterproof fabric

Figure 11 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. In this case, both yellow and blue fabric have been used and the joint has been made between the uncoated surfaces. In these particular sections the upper polyurethane layer on the blue fabric is visible, as are some unmelted blue fibres. An homogenised melt region can be seen in the centre of the picture and lower down, unmelted fibres from the yellow material are visible. The presence of unmelted fibres is advantageous in that a degree of the flexibility of the original fabric can be maintained after welding.

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Fig.11. Cross section of a joint made utilising the Clearweld ® technique in the fabric Tactel TM

Table 1 lists the results of mechanical testing of a range of lap joints made on a selection of woven fabrics manufactured from nylon 66.

Table 1. Results of mechanical testing on a range of woven fabrics

Material ColourThickness
(mm)
Peel
Strength
(N/mm)
Lap/Shear
Strength
(N/mm)
Parent
Strength
(N/mm)
Brown
Orange
Bronze
Yellow
0.19
0.23
0.16
0.41
0.70
2.16
2.07
4.40
2.08
5.22
2.76
6.79
8.47
13.95
9.38
16.12

For these experiments an Nd:YAG laser, with a 7mm diameter focal spot was used at powers between 50 and 100 watts. The welding speeds were in the range 500-1000mm/min. The peel and lap/shear tests were performed on 25mm wide samples at a test rate of 5mm/min. The test results are quoted as the maximum applied force per mm of seam. Each result is the average of three tests. It can be seen that, as a percentage of the strength of the parent materials, values between 25 and 40 percent were obtained for the welded joints in a simple lap configuration.

The textile industry has the problem of producing hermetic seams in man-made fabrics. There are a wide range of applications which require such seams including outdoor clothing, sports goods, industrial and safety clothing, sails, tents, inflatable goods and even balloon envelopes, where laser welding using the Clearweld ® technique could be used to advantage. The laser process also offers the possibility of automation in what is largely a manual labour-based industry sector.

The Clearweld ® technique is so flexible in the area of polymer based fabric joining that several complete garments, containing no stitching, have already been assembled.

In addition to the benefits exhibited by the transmission laser welding process described earlier, the Clearweld ® technique:

  • Allows virtually any coloured plastic to be joined.
  • Produces a joint which is almost invisible to the eye.
  • Allows the joining of flexible polymer based fabrics as well as rigid plastics.

Light absorption and energy conversion in organic substances

Conventional absorbing media, by definition, absorb visible electromagnetic radiation. The process involves 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 absorber is promoted from the singlet ground state to the first excited singlet state. The energy increase is quite considerable (ca. 150-300kJ.mol -1 ), and the absorber molecule must lose the energy rapidly by various processes, returning to the ground state, where it is again available to absorb a photon of light. The most common deactivation process is internal conversion, followed by vibrational relaxation. In this case the molecule passes from the zero vibrational level of the excited state into a high vibrational level of the ground state, and then undergoes rapid vibrational relaxation (ca.10 -13 seconds) to the lowest vibrational level of the ground state, with the excess thermal energy being transferred to the surrounding host molecules. Other deactivating processes, such as fluorescence, or vibrational relaxation via the triplet state (intersystem crossing) are generally less important for most absorbers. The overall effect is for the host matrix to become heated, and the local temperature rise will be determined by the wavelength and intensity of the laser light, and the thermal conductivity of the host. This phenomenon has been used extensively in heat mode optical data recording, where a finely focussed laser beam is absorbed and the heating effect produces a change in the polymer substrate, by melting or ablation. It has been shown, for example, that writing energies as low as 0.1 nanojoule per square micron can increase local temperatures up to ca, 300°C, sufficient to melt most plastics and dyes, if a dye layer absorbing 99% of the incident energy is employed. [17]

If broad band radiation is employed, then the naturally broad bands of most absorbers are an advantage. However, if laser sources are employed, it is important to match the λ max of the absorber (the wavelength at which maximum absorption occurs) as closely as possible to the laser wavelength. In addition, a narrow band absorber is likely to have a much higher molar absorption coefficient at its λ max than a structurally similar absorber with a broad absorption band. This results in less absorber being needed to achieve maximum absorption of the incident light.

The ideal near-infrared absorber for laser welding, whilst producing minimal marking in the visible region, should have the following attributes:

  • A narrow absorption band near the laser wavelength with a high molar absorption coefficient.
  • Little if any absorption in the region 400-700nm.
  • Good solubility in the host material.
  • Good stability towards the incorporation method used.
  • Should not degrade to coloured by-products.

If all the known near-infrared absorber systems are considered, the vast majority can be discounted on the grounds that they have pronounced visual colour. Others can be disregarded because of their instability or their low absorption intensity. Selection of the most appropriate candidates from the remainder then becomes a matter of trial and error. Examples of three absorber types which can satisfy all of the above requirements are the cyanine absorbers, e.g. ( Fig.12), the squarylium absorbers ( Fig.13) and the croconium absorbers ( Fig.14).

spiajmay2000f2.gif

Fig.12. Example of cyanine absorber [ λ max = 785nm; ε max ≈ 360,000 1.mol -1 .cm -1 in dichloromethane]

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Fig.13. Example of squarylium absorber [ λ max ca.800nm; ε max ≈ 150,000 1.mol -1 .cm -1 ]

spiajmay2000f4.gif

Fig.14. Example of croconium absorber [ λ max ca.820nm; ε max ≈ 200,000 1.mol -1 .cm -1 ]

Equipment developments

As described earlier, the diode laser source, with its compact size, high efficiency and almost ideal spot size, is an excellent choice for transmission laser welding of plastic materials. Because of its small size and weight, it can be used in several ways. The first two variations are fairly conventional with the laser either fixed and the component manipulated by CNC control, or with the laser manipulated, sometimes in the arm of a robot, over a stationary workpiece. More recent developments involve the potential to keep both laser and workpiece stationary during welding. The first involves a diode laser equipped with a set of computer controlled scanning optics, similar in concept to those developed for laser marking systems. For small components, of the order 100 x 100mm, this allows the laser beam to be traversed very rapidly over any pre-programmed shape. The second concept in this area relies on an array of fixed laser diodes configured to radiate on the area to be welded as a single pulse of light. Both these techniques have the capability to effectively heat the whole of the joint line simultaneously (for small components). This means that if pressure is applied during welding, at the point when the joint becomes molten, some flow of the melt becomes possible. Thus surfaces slightly rougher than those required for the 'moving source' type applications can be accommodated. It is likely that both these latter systems will soon be seen in production applications of the transmission laser welding technique for plastics.

Heat models for plastics welding with lasers

An analytical heat conduction model [18] has been developed to investigate lap welding of plastics using the Clearweld ® technique. Experiments using single lap joints in amorphorus polyethylene terephthalate (PETG) have been used to validate the model. The heat flow model provides an approximate solution for the peak temperature at the joint interface as a function of depth through the weld. The welding parameters include applied power, traverse speed, and beam radius. The model requires the physical properties of the plastic and knowledge of the absorption coefficients for the laser wavelength being used. For the purpose of validation it was postulated that a good weld was formed when the temperature of the polymer exceeded its melting point at the interface. 'Good' and 'Bad' welds, were made at a range of applied power densities and travel speeds welding PETG, with the process parameters selected to produce melting or not. In tensile tests all the 'good' welds, performed better than the parent material, while imperfections and lack of fusion could be seen in all the bad welds. The model also predicted that the temperature at the joint interface can rise to 850°C. The low thermal conductivity of the polymer and a short dwell time for the heat source, leads to a steep thermal gradient. The model also predicts that the weld depth (thickness) is only of the order 15 microns, but that the heat-affected zone can be of the order 0.25mm in thickness. These predictions were borne out by measurements of weld sections made using PETG.

Summary and conclusions

Pre-assembled thermoplastic materials can be welded by transmitting a laser beam through the top part of the joint and by generating heat at the interface in a pre-positioned absorbing medium. Carbon black or alternatively, the virtually colourless Clearweld ® infrared absorbent system can be used as the mechanism to produce heat and localised melting. The welds produced are cosmetically appealing and the upper and lower surfaces of the material are unaffected by the process.

The laser welding process is efficiently achieved using the very compact diode laser sources now commercially available, and lends itself easily to high levels of automation and rapid production. Use of diode arrays and scanning laser beams also produce components with minimal distortion. Potential applications of this technology exist in a wide variety of industry sectors with plastic joining requirements, including welding films for packaging, moulded components, and hermetic containers.

Polymer fabric materials can be laser welded using infrared absorbers as a mechanism to produce heat and localised melting. The success of the technique has been demonstrated on a wide range of flexible materials. The welds produced are cosmetically appealing and the upper and lower surfaces of the material are unaffected by the process. In mechanical testing, joint strengths of between 20 and 40 percent of the parent material strength have been achieved, in a simple lap joint.

Using the Clearweld ® technique and two different absorbing medium, a triple layer joint has been produced by using a single pass of a diode laser generating two discrete laser wavelengths.

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  16. Hilton P A, Jones I A, Sallavanti R: 'Laser welding of fabrics using infrared absorbing dyes'. Proc. ASM 2000, 9-12 Oct 2000, St. Louis, Missouri, USA.
  17. Hurditch R: 'Coulour Science '98. ed Griffiths J, Univsersity of Leeds Print Services, Leeds 1999, pp 202.
  18. Y C Kennish, Shercliff H R, McGrath G C: 'Heat flow model for laser welding of polymers', Proc. ANTEC 2002, May 5-9 2002, San Fransisco, CA.

The process of laser welding using an infrared absorbing interlayer has been given the trademark Clearweld ®. In addition, patent protection has been initiated by TWI on this process. Gentex Corporation, who manufactures suitable absorbers for the technique, is licensed by TWI to exploit the Clearweld ® process.

For more information please email:


contactus@twi.co.uk