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Application studies using through-transmission laser welding of polymers (April 2006)

   
Marcus Warwick and Marcus Gordon

TWI Ltd, Granta Park, Great Abington, Cambridge, CB1 6AL, UK

Paper presented at Joining Plastics 2006, London, National Physical Laboratory (NPL), 25-26 April 2006.

Dr Marcus Warwick completed his PhD, on the thermo-mechanical properties of metal-matrix composites, in the Department of Materials Science and Metallurgy, University of Cambridge. He has extensive practical experience, gained over 15 years in the materials industry, including development and introduction of new manufacturing processes in the fields of adhesives, composites and plastics joining. He currently works in the polymers section at TWI, where he has led a number of projects developing applications of plastics laser welding.

Abstract

Through-transmission laser welding offers significant benefits when compared with other polymer welding techniques such as vibration welding, hot-plate welding or ultrasonic welding. This paper reviews the benefits and limitations of the process and highlights recent technical developments in the field. Process fundamentals are reviewed, outlining critical process parameters, and the range of equipment available is discussed. Laser welding has the same limitations as other plastics welding techniques for joining dissimilar plastics, and a compatibility table is provided. The paper concludes with two case studies of successful through-transmission laser welding development projects carried out by TWI.

1. Introduction

The purchase and running costs of near infra-red (NIR) lasers, particularly diode lasers, have fallen over recent years. This, together with innovations in equipment, materials and processes, has opened up a wide range of applications to through-transmission laser welding. Amongst the benefits being achieved by users are:

  • Production of hermetic joints with good environmental stability via a true welding process.
  • Packaging of components that are sensitive to vibration, as there is no mechanical movement of the part.
  • Reduced distortion, and packaging of heat-sensitive components, as the process only generates a very limited heat-affected zone.
  • Production of welds with an excellent appearance without the use of a flash-trap, because only a thin film of polymer is melted, so it is possible to weld with very little flash.
  • Reduced contamination and polymer fumes, because molten material is contained within the joint and does not come into contact with the processing equipment.
  • Lower energy consumption, because the laser radiation is concentrated only where it is needed to form a joint, and only the minimum volume of polymer is melted, making the process highly efficient.
  • High processing speeds of at least 30m/min have been demonstrated, and some processes allow short cycle times of 1-3s.

This paper provides an overview of the process, and two application case studies.

2. Through-transmission laser welding

2.1. Process fundamentals

2.1.1. Process description

Through-transmission laser welding relies on the fact that many polymers, in their natural state, are not strong absorbers of NIR radiation. The window of NIR wavelengths that can be used, beyond the visible spectrum, from 800nm to approximately 2000nm, is illustrated in Fig.1.

Fig.1. Absorption spectrum of a typical polymer
Fig.1. Absorption spectrum of a typical polymer

The process differs from most other plastics joining processes, and from the use of lasers for joining metals, in that the energy from the laser is concentrated at the joint rather than melting material from the surface inwards. The process is illustrated in Fig.2. The energy of the laser must be concentrated at the joint in order to melt the polymer to form a weld. This has conventionally been achieved by adding a broad-spectrum absorber, such as carbon-black, to the lower workpiece, although a number of alternatives, giving greater design flexibility, are now available (see Section 2.3.1). The workpieces are clamped together, to provide intimate contact at the joint, and the part is irradiated. The laser passes through the upper workpiece to the joint, where the radiation is converted to heat by the absorbent additive. This melts a thin film of plastic on either side of the joint, resulting in a permanent weld when the joint has cooled.

As with all plastics welding processes there are three critical process parameters:

  • Temperature
  • Time
  • Pressure
Fig.2. Through-transmission laser welding - cross-section through the welding process
Fig.2. Through-transmission laser welding - cross-section through the welding process

2.1.2. Energy density

The energy density used during welding combines the process parameters of temperature and time. It is determined by the laser power, the spot size at the joint, and the irradiation time (for fixed processes) or welding speed (for processes in which the part moves with respect to the laser).

spcmwapr2006e1.gif

or

spcmwapr2006e2.gif

If the energy density is too low then insufficient heating takes place and the material at the joint is not held at a high enough temperature for a sufficiently long time to form a strong weld. If the energy density is too high, then excess heating can degrade the polymer at the joint, resulting in porosity, or, in extreme cases, burning or charring of the polymer. Either case results in a weld of lower strength than the optimum. In practice, a relatively wide processing window can usually be found within which satisfactory welds can be produced. Typically laser welding applications use an energy density within the range 0.1-2 J/mm 2 , although this will vary depending on the depth of melt required to ensure a satisfactory joint.

Although the energy density can be used to characterise the welding process, it should be treated with caution. The conduction of heat away from the joint during welding means that using the same energy density will not necessarily result in the same quality of weld. For example, with a constant spot width, doubling the power will usually allow the speed to be more than doubled, whilst retaining the same performance from the weld.

2.1.3. Clamping pressure

If the workpieces are not clamped together during welding, or if the pressure at the joint is insufficient, then the joint faces will not be in intimate contact. This results in:

  • Poor conduction of heat to the upper workpiece
  • Limited interdiffusion of polymer chains on either side of the joint.

Both effects result in a weld of lower strength than the optimum. Care is needed to ensure that a clamping load actually provides pressure at the joint. Clamping pressure in the range 0.1-1N/mm 2 is typically used. If the workpieces bend under the clamping load in such a way that the joint is distorted then a poor weld can result. For this reason, it is often useful to have some compliance, for example an elastomeric element, in the clamping system.

A wide variety of clamping systems have been used for through-transmission laser welding. They are mostly variants of the two systems illustrated in Fig.3.

a) Fixed clamp
a) Fixed clamp
b) Moving clamp
b) Moving clamp

Fig.3. Clamping systems for through-transmission laser welding: a) Fixed clamp; b) Moving clamp

Variants of the fixed clamp include systems using mechanical fastenings, rather than an actuator, to apply a load. In the simplest variant, if the part design allows it, a bolt can be passed through the workpiece to apply the load. The transparent cover must be rigid enough to provide the clamping pressure. Thick acrylic or plain plate glass can be used. Borosilicate glass is less vulnerable to thermal shocks during welding, but more expensive. For welding of high-temperature polymers, quartz glass may be used. In all cases it is important to ensure that suitable safety precautions are taken to avoid the risk of injury if the transparent cover breaks while it is under load.

The moving clamp can use bearings, rollers, or a simple sliding shoe to apply a clamping load. Because the load is applied only at the point where the joint is irradiated, clamping loads may be much lower when a moving clamp is used. There is therefore less risk of distorting the workpiece, and equipment can be less bulky. This is particularly advantageous for large components, where application of a suitable clamping pressure to a large area can require large loads.

2.1.4. Motion systems

A variety of methods are available for manipulating the welded component with respect to the laser. These are illustrated in Fig.4 and described in more detail below.

a) Moving workpiece
a) Moving workpiece
b) Moving laser
b) Moving laser
c) Curtain laser
c) Curtain laser
d) Simultaneous welding
d) Simultaneous welding
e) Scanning laser
e) Scanning laser
Fig.4. Welding methods: a) Moving workpiece; b) Moving laser; c) Curtain laser; d) Simultaneous welding; e) Scanning laser  

2.1.4.1. Moving part

With the laser fixed, the part can be manipulated to form a continuous weld. This can be achieved, for example, with rollers, or a single or two-axis moving table. This type of system is relatively simple to set up and program. It would not normally be used if three-dimensional welds are required.

2.1.4.2. Moving laser

The optical system for a fibre delivered laser, or the laser head for a direct diode laser can be mounted on a variety of robotic systems. These range from simple two-axis gantry systems to multiple-axis robotic arms. The laser is then manipulated around the part to be welded, potentially allowing complex, three-dimensional welds to be produced. To facilitate automatic production, it is feasible to combine a moving laser with a moving part, for example by using a rotating table to present different faces of a component to a laser mounted on a robot arm.

2.1.4.3. Curtain laser

The laser energy is spread into a line and then passed over the component, either by moving the laser or by moving the part. A mask is typically used to ensure that only the relevant areas of the component are exposed to the radiation. This is particularly suited to small components with a complex weld geometry. The process would usually be used only to produce two-dimensional welds.

2.1.4.4. Simultaneous welding

If a large number of identical welds is required then an array of diode lasers can be assembled in the shape of the weld. This is then used to irradiate the whole joint simultaneously, with a typical cycle time of 1-3s. This approach is well suited to automated assembly. The equipment used is frequently based on ultrasonic welding equipment, and this process is typically used in place of ultrasonic welding where a good cosmetic appearance is required, or for components that are sensitive to vibration. Two and three-dimensional welds can be produced. The entire joint is welded at the same time, allowing more collapse of the polymer at the joint, and therefore allowing wider part tolerances.

2.1.4.5. Scanning laser

The laser radiation is manipulated by a pair of orthogonal rotating mirrors over an area that may range from 50mm x 50mm up to approximately 1000mm x 1000mm. In general, a larger working area implies a longer working distance and a larger spot size. It is possible to co-ordinate an assembly of a number of scanning systems to give a larger working area. In general, only two-dimensional welds can be produced.

Repeatedly scanning the laser at high speed over the same path can be used to give quasi-simultaneous welding. As for simultaneous welding, this welds the entire joint area the same time, allowing more collapse of the material in the joint and potentially allowing wider tolerances.

2.2. Laser equipment

The main types of NIR laser used for through-transmission laser welding, and selected properties, are listed in Table 1. Additional details of each type of laser are given below.

Table 1 NIR Laser types

Laser typeNd:YAGDiodeFibre
Wavelength (nm) 1064 808 or 940 1000 - 2100
Maximum power (kW) ~5 ~7 ~10
Efficiency (%) 3 30 20
Approximate cost for 100W system (£k) 40 10 30
Beam quality High Low High
Fibre or direct beam delivery Fibre Fibre or direct Fibre

2.2.1. Nd:YAG lasers

Nd:YAG lasers are widely use in industry for materials processing. High-power systems are bulky, but lower power systems are relatively compact. Water-cooling is usually required. The beam is transferred from the laser to the work-piece via an optical fibre. It is feasible to combine the beam from more than one laser to produce higher powers if required. The high beam quality allows a relatively small spot size to be produced, giving a high power density suitable for welding of metals and for marking applications. However, in the field of plastics laser welding, a small spot size is needed for only a few precision applications, and the relatively high purchase and running costs usually mean an alternative laser source is selected.

2.2.2. Diode lasers

Diode lasers produce radiation at a wavelength of 808nm (InGaAlAs) or 940nm (InGaAs). Water-cooling is usually required. Their relatively low beam quality means that they cannot be used to produce a spot size as small as Nd:YAG or fibre lasers. However, this is rarely a problem for plastics laser welding, where the relatively low purchase and running costs have attracted a great deal of interest. The beam may be delivered by an optical fibre, but the diode is sufficiently small and light that it is often feasible to use a direct system, in which the diode is included with a lens system in a single unit, typically ~150 x 150 x 300mm. This unit can readily be mounted on a gantry system or robot arm to manipulate the beam.

2.2.3. Fibre lasers

Rare-earth doped fibre lasers typically supply a single wavelength in the range 1000nm to 2100nm. In the field of materials processing, much interest has focussed on wavelengths around 1100nm to provide a direct replacement for Nd:YAG lasers, with equivalent beam quality, but greater efficiency. Systems are relatively compact and can be air-cooled. In the field of plastics welding the use of fibre lasers has been demonstrated for a range of applications, including precision welding, films, textiles and larger moulded parts.

2.3. Recent developments

2.3.1. Materials

The Clearweld ® process was invented by TWI and has been successfully commercialised by the Gentex Corporation. A material with strong absorption at the laser wavelength, but little or no absorption at visible wavelengths is used to concentrate the laser energy at the joint. This allows a weld to be produced with little or no impact on the visual appearance of the part, giving designers flexibility in the choice of materials and colours. The absorbing material maybe applied as a coating, or may be used in the lower part via a resin additive.

Colour-matched systems also allow flexibility for the designer. These consist of pairs of additives, having the same appearance at visible wavelengths, one of which has low absorption at the laser wavelength (to be used for the upper part), the other having high absorption at the laser wavelength (to be used for the lower part). The first application was black-to-black, using carbon-black in the lower part and an infra-red-transmissive black dye in the upper part. A full range of colours is now available, including white.

2.3.2. Equipment

The Globo system from Leister is a novel laser welding technology, in which a spherical glass bearing acts both as the final focussing element of the laser optical system, and as the means of applying a clamping load to the joint. The system can be mounted on a robotic arm to give considerable flexibility in part and joint design.

Fig.5. Handlaser equipment for laser welding (Courtesy of Prolas)
Fig.5. Handlaser equipment for laser welding (Courtesy of Prolas)

The Handlaser system from Prolas, illustrated in Fig.5, offers a manual laser welding system, with an integrated pyrometer to control the weld temperature. A number of safety features ensure that the laser only operates when the unit is in contact with the polymer substrate. This gives great flexibility for the manufacture of prototypes and short production runs, avoiding investment in equipment to automate the laser welding process.

2.4. Process limitations

Some of the benefits that have been achieved by users of through-transmission laser welding were listed in the introduction. The drawbacks of the process include the following:

  • The need to provide access for the laser while applying a clamping pressure restricts the joint geometry. In practice, a wide range of joints can be used, but it is important that any limitations are considered early in the design process.
  • The lack of weld flash and associated collapse at the joint means that good part fit is required. In practice, there is a compromise to be made - wider tolerances may be acceptable, at the expense of using a higher energy density and causing more flash.
  • One of the parts must transmit NIR. This restricts the use of some additives and fillers, although recent materials developments have allowed much greater flexibility in design. It also limits the thickness of the upper workpiece, particularly for semi-crystalline polymers, which scatter the incident radiation rather than transmitting it to the joint. The joint can often be designed to avoid this limitation.

3. Dissimilar materials joining

Through-transmission laser welding is no different from other plastics welding processes in its ability to join dissimilar materials combinations. As a general rule, dissimilar plastics cannot be welded successfully, but there are a few exceptions to this rule. A typical example is a car rear light cluster. A PMMA (polymethylmethacrylate) lens can be hot plate welded to an ABS (acrylonitrile butadiene styrene) housing. Dissimilar combinations that can be welded must be chemically compatible, and must have a similar glass-transition temperature (for amorphous polymers) or melting temperature (for semi-crystalline polymers).

Table 2 outlines the main possibilities for dissimilar plastics welding. Any material not shown can, in general, only be welded to itself.

Table 2 Dissimilar plastics welding. [1-5]

 Not weldableABSABS/PCAcrylicModified PPOPA 12PBTPCHDPEPEIPETPolysulphone
xWeldable
-Some grades weldable
?Some reports of successful welding
Acrylonitrile butadiene styrene, ABS x x x ?   ? x   ?    
ABS/PC x x -       x        
Acrylic x - x       -        
Modified PPO ?     x     ?        
Nylon 12, PA12         x   ?        
Polybutylene terphthalate, PBT ?         x ?   ?    
Polycarbonate, PC x x - ? ? ? x   - ? -
High-density polyethylene, HDPE               x      
Polyetherimide,PEI ?         ? -   x    
Polyester, PET             ?     x  
Polysulphone             -       x
Polypropylene, PP               ?      
Polyphenylene oxide, PPO       x              
Polystyrene, PS -   - x     -        
Polyvinyl chloride, PVC -   ?     ?          
SAN-NAS-ASA - - - -              
Styrene butadiene copolymer, SBC               ?      
Styrene-butadiene-styrene, SBS -   x                

 Not weldablePPPPOPSPVCSAN-NAS-ASASBCSBS
xWeldable
-Some grades weldable
?Some reports of successful welding
Acrylonitrile butadiene styrene, ABS     - - -   -
ABS/PC         -    
Acrylic     - ? -   x
Modified PPO   x x   -    
Nylon 12, PA12              
Polybutylene terphthalate, PBT       ?      
Polycarbonate, PC     -        
High-density polyethylene, HDPE ?         ?  
Polyetherimide,PEI              
Polyester, PET              
Polysulphone              
Polypropylene, PP x            
Polyphenylene oxide, PPO   x          
Polystyrene, PS     x   -   -
Polyvinyl chloride, PVC       x      
SAN-NAS-ASA     -   x   -
Styrene butadiene copolymer, SBC           x  
Styrene-butadiene-styrene, SBS     -   -   x

4. Case studies

4.1. Inflatable neck brace

4.1.1. Background

The Royal National Hospital for Rheumatic Disease (RNHRD) in Bath, UK, has developed the concept of an inflatable brace for the immobilisation of neck injuries. This can be inflated to a high-pressure when a high-level of support is required, for example when the patient is travelling. The pressure can be reduced when greater mobility is required, for example while eating. TWI was approached for assistance in developing a manufacturing technique to assemble the brace. The project was funded by a Department of Health New and Emerging Applications of Technology (NEAT) award.

4.1.2. Process requirements

The material selected for the brace was Hytrel ®, a thermoplastic polyester elastomer from DuPontTM. For simplicity, two sheets were welded together, forming a number of interconnected inflatable chambers. A key objective of the work was that the brace should be customised for each patient. This steered the choice of welding process towards laser welding, using equipment that could be rapidly programmed to produce different sizes tailored to individual patient measurements. It was desirable to use a single grade of translucent material for the brace, which led to the use of Clearweld ® to weld the sheets together. The design went through a number of iterations during process development, and ended with the patterns shown in Fig.6. The two parts are joined together (via a temporary fastening) at the uninflated cells, which sit at either side of the patient's neck. A moulded threaded lug was used with an elastomer gasket to attach the inflation tubes to the prototype part. For higher volume production a moulded lug can be welded directly to the Hytrel ® sheet.

a) Back
a) Back
b) Front
b) Front

Fig.6. Design of demonstrator neck brace (courtesy of RNHRD)

4.1.3. Development work

For convenience during the trials, Clearweld ® ink, supplied by Gentex Corporation, was sprayed over the entire surface of the sheet. This allowed the location of the weld to be controlled simply by exposure to the laser. A two-axis moving table was used to manipulate the Hytrel ® sheets beneath the laser. The sheets were held beneath a sheet of 3mm thick acrylic, and a clamping load of ~400N was applied to the acrylic sheet via a pneumatic actuator attached to a 20mm diameter, ring-shaped, sliding, clamp. The equipment is shown in Fig.7.

Fig.7. Equipment used for ink spraying and welding trials
Fig.7. Equipment used for ink spraying and welding trials

Due to the complex loading situation, notably high stress concentrations at the ends of the welds at the air gap between chambers, a strength requirement could not be identified for the welds. However, process conditions were identified to give a weld that generated failure in the film before failure of the weld when tested in peel.

4.1.4. Outcome

Demonstrator parts were fabricated successfully, and detail of the back is shown in Fig.8. The welds are between the dark tramlines. The prototype part has been successfully tested, and is shown in Fig.9. The inflatable neck brace is now being developed further by RNHRD.

Fig.8. Detail of inflatable part (Courtesy of RNHRD)
Fig.8. Detail of inflatable part (Courtesy of RNHRD)
Fig.9. Prototype neck brace undergoing testing (Courtesy of RNHRD)
Fig.9. Prototype neck brace undergoing testing (Courtesy of RNHRD)

4.2. Lithographic processing tank

4.2.1. Background

Barkston Plastics Engineering, based in Leeds, UK, produces a range of structural components from plastics. The company recently identified a need to improve the reproducibility of the production of a range of polypropylene tanks. These are used to contain liquids in lithographic (computer to plate) processing equipment, and have traditionally been assembled by attaching end plates to a formed cross section using manual hot-gas welding. Barkston approached TWI for assistance in developing a more automated manufacturing operation, and after considering a number of alternatives, transmission laser welding was selected as a candidate process.

4.2.2. Process requirements

The two critical requirements for the welding process are:

  • Weld strength must be at least as high as that produced by hot-gas welding.
  • The weld must be leak tight.

4.2.3. Development work

Previously the tanks were manufactured entirely from natural polypropylene. To facilitate laser welding, the end plates were machined from natural polypropylene, which transmits NIR wavelengths, while the cross sections were formed from a grey coloured polypropylene sheet, which absorbs the NIR radiation.

A preliminary programme of work at TWI's laboratories demonstrated the feasibility of welding through relatively thick polypropylene sections, and identified suitable process conditions. The welds were found to be as strong as those achieved using hot gas welding - in both cases failure occurred in the parent material and not at the weld. A further set of trials produced small tanks with a simple cross-section. These demonstrated that leak-tight welds could be prepared. Favourable feedback from Barkston's customers then led to manufacture by TWI of several full-size tanks using a six-axis robot supplied by Motoman manipulating a high-power diode laser from Laserlines. Clamping pressure was applied via a pneumatic actuator attached to a ring-shaped sliding clamp. This equipment is shown in Fig.10. The finished tanks, shown in Fig.11, were put through in-service testing with satisfactory results.

a) Overview of welding process
a) Overview of welding process
b) Diode laser head and clamping unit
b) Diode laser head and clamping unit
Fig.10. Equipment used to weld demonstrator full-size tanks at TWI: a) Overview of welding process; b) Diode laser head and clamping unit
Fig.11. Finished tanks (Courtesy Barkston Plastics)
Fig.11. Finished tanks (Courtesy Barkston Plastics)

4.2.4. Outcome

Barkston have now installed and commissioned laser welding equipment, with the following benefits:

  • Increased automation allows improved quality and better reproducibility.
  • A cleaner and more efficient process because molten plastic is contained within the joint, and the only plastic melted is that needed to create the weld.
  • A significant reduction in the time required to weld a component.
  • Continued design flexibility for the polypropylene tanks. Additionally, a laser welding system may be used for other product lines.

The last point was particularly important for the investment. A number of other products are now welded using the new equipment, including some fully acrylic parts using the Clearweld ® process.

5. Acknowledgements

The authors wish to thank James Doel, Nigel Harris and Professor David Blake at RNHRD, and Tony Blackmore at Barkston Plastics Engineering, for their help with the work described in this paper. The assistance of Ian Jones and Mike Troughton at TWI is also gratefully acknowledged.

References

  1. Reinhold Martin, 'The Use of Lasers with Technical Polymers', AILU Conference, 'Laser Processing of Polymer-Based Materials', February 2004
  2. Grewell, Benatar & Park, 'Ultrasonic Welding' in Plastics & Composites Welding Handbook, Hanser 2003, ISBN 3-466-19534-3
  3. Benatar, 'Implant Induction (Electromagnetic) Welding' in Plastics & Composites Welding Handbook, Hanser 2003, ISBN 3-466-19534-3
  4. Froment, 'Linear & Orbital Vibration Welding' in Plastics & Composites Welding Handbook, Hanser 2003, ISBN 3-466-19534-3
  5. Watson, Rivett & Johnson 'Plastics - an Industrial & Literature Survey of Joining Techniques', The Welding Institute Research Report 301/1986

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