Paper published in Medical Device Technology Vol.14, No.3, pp.28-30, April 2003
Introduction
Polymeric materials may be processed in many ways using lasers, for melting, vaporisation or ablative action. Recent advances have been made utilising laser techniques for welding of plastics. Lasers are now being considered as an alternative to vibration, ultrasonic, dielectric, hot plate or hot bar welding and adhesive bonding, for medical devices, tubular systems, films and synthetic fabrics.
Transmission laser welding is used to carry out rapid welding of plastics by transmission of light through to an absorbing interface, where very localised, precise heating takes place, minimising any thermal damage. The process lends itself to automation, either by robotically manipulating the laser head to work over large components, or by scanning the laser beam via mirrors to weld small complex parts. The process is often considered for electronic components where ultrasonic vibration may be damaging, or in applications where the weld should not affect the appearance of the product. The main limitation of the laser welding process is that the material on at least the upper side of the joint must transmit at least 10% of the laser energy. This excludes plastics with high contents of some pigments such as TiO 2 or carbon black and metallic additions or coatings. The techniques required to put laser welding methods into practice are now described.
Laser selection for welding polymers
A laser source for transmission laser welding must deliver a radiation wavelength in a range where the polymer transmits. Absorbers can then be chosen to be applied where the beam needs to be absorbed to create a weld. [1] Typically a polymer will transmit visible and near-infrared radiation. Readily available and moderately priced sources that fit this requirement include the high power diode and Nd:YAG laser sources. They both emit in the near-infrared range, 808-980nm for diode sources and 1064nm for Nd:YAG sources. The diode or Nd:YAG laser may therefore be used for welding plastics when a suitable absorbing medium is positioned at the joint line, and the beam is transmitted through the upper layer of plastic. [2] The diode or Nd:YAG laser beam can be transmitted down a fibre optic enabling easy flexible operation with gantry or robot manipulation. Additionally the whole diode laser source is small and light enough to be manipulated by a robot arm or on a gantry. [3] The beams from both laser types may also be scanned using galvanometer controlled mirrors to allow following of complex weld line shapes.
Many welding applications can be carried out with a laser of less than 100W. Typically such a diode laser would cost approximately £15k and a Nd:YAG laser around £30k. The laser or workpiece manipulation costs need to be added to this and clearly depend on the complexity involved. If higher speed processing is required then laser sources delivering up to 6kW are available. Radiation generation from a diode source is more energy efficient than from aNd:YAG laser, so diode systems have lower running costs in general. The laser selected must also have the appropriate beam shape and size for the application. The diode laser focus spot is typically rectangular (this is modified to round however, if fibre delivery is used), with a minimum dimension of approximately 0.3mm for a 100W laser. If smaller weld dimensions are required then a Nd:YAG laser will be used. The rectangular format of the diode laser beam is,however, often well suited to delivering an even energy profile from one side of the weld to the other for larger weld widths.
Use of diode lasers for welding plastics
The basic requirements for welding plastics are heat, time and pressure to ensure that there is sufficient interdiffusion of polymer molecules at the component interface to generate the strength required. A certain energy/unit area can be defined experimentally for a weld, which will be dependant on the material thermal properties and the amount of melt needed to close any gaps at the joint. The heating required may range from 0.1-5J/mm 2.
For laser welding two plastic parts a specific laser beam absorber is arranged to be positioned at the joint. Typically this is either carbon black additive in the lower part or an infrared absorber (known as Clearweld ®)* either printed or sprayed onto one or both of the joint surfaces before welding. The parts are clamped and the joint is heated by passing the laser beam through the upper part of the component. The Clearweld process was developed so that components did not have to incorporate a black part. It allows two similar clear or coloured parts to be welded by using an interface absorber. The absorber is designed to absorb at the wavelength of the laser being used, which is, in this case, infrared and therefore beyond the range of human vision. The absorbent material has been taken through a series of cytotoxicity tests and found to be non-toxic in its original form, as applied on polymer and following laser processing.
*Clearweld ® is a registered trademark of TWI.
The 'Oval' logo is a registered trademark of Gentex Corporation.
In laser welding the main process parameters are:
- Laser power
- Laser beam size/beam uniformity
- Welding speed or time
- Absorption properties of material at interface
- Clamping pressure
The diode laser source can be used in a number of different ways due to its small size and weight. The different equipment variations being considered for application are shown diagrammatically in figure 1. The choice of equipment for a given application will depend on a number of factors, including the size and weight of the component being welded, the rigidity of the material, whether it is film based or moulded, and the number of components that are required in a given design. The major equipment variations are summarised below:
- Fixed laser moving workpiece - This equipment generally operates as a single pass process, with the joint heated as the workpiece passes beneath the laser source. It may be used with a two-axis flat bed table to weld small to medium sized flexible components with a 2-D joint line or in the form of a continuously moving substrate for welding thin films or textiles for packaging applications for example. A rotary axis can also be used in order to weld completely circular pieces.
- Moving laser, fixed workpiece - This equipment generally operates as a single pass process, with the joint heated as the laser beam passes over the workpiece. The laser may be manipulated by a robot for 3-D processing or attached to a moving gantry over a flat bed for 2-D processing. This type of equipment is most suited to large relatively flexible components.
- Fixed diode array and fixed workpiece - In this case the laser diodes are mounted in a frame designed to match the shape of the component being welded, rather than being put into a singular laser source. The process therefore operates with the whole of the joint irradiated for a given time. This procedure is suitable for small rigid moulded components that may not fit exactly at the joint line. The welding time would be set to heat and soften the weld line, which will flow under the clamping pressure and close any slight gaps.
- Scanning beam, fixed workpiece - The mirrors are programmed to move the laser beam around the joint line of the fixed component. The beam movement can be very fast (in excess of 2m/sec), and the joint can therefore be scanned many times per second. This effectively heats the whole joint line simultaneously and therefore the procedure acts in the same way as the laser diode array, and is suitable for small, rigid, moulded components. This equipment also has the added advantage that the weld line profile can be easily altered by loading a different program in to the scanning unit.
Fig.1. Equipment variations
a) Fixed laser, moving workpiece
b) Moving laser on a gantry or robot over a fixed workpiece
c) Fixed diode array and fixed workpiece
d) Galvanometer controlled mirrors scanning the laser beam round a fixed joint line
The samples shown in Figure 2 were welded with a 50W scanning diode laser system (supplied by Fisba Optik AG), using the fourth application method shown above. The joint faces for the process were prepared by applying Clearweld. The laser beam was moved round the circular joint profile at the rate of 20 revolutions per second. Welding times of 3-6seconds 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.
Fig.2. Test samples in which the top was welded using a scanning diode laser
a) black to clear PC
c) clear to clear PP
Fig.2a was using carbon black absorbent, b and c were using Clearweld ® absorbent.
Concluding remarks
The emergence of a new generation of low power lasers with unique properties has considerably increased the possibility of their use in many new exciting applications including welding of plastics and in medicine and non-invasive micro surgery (cornea corrections, kidney stones removal, etc).
Polymeric materials can now be laser welded using near infrared absorbing material placed at the joint as a mechanism to produce heat and localised melting. The success of the technique has been demonstrated on a wide range of applications including rigid and flexible polymers. The welding process is efficiently achieved using the very compact diode laser sources now commercially available, and lends itself easily to high levels of automation.
TWI has patented the Clearweld process of plastic laser welding and commercial welding consumable for the process is exclusively available from Gentex Corporation.
References
- Jones I A, Hilton P A, Sallavanti R, Griffiths J, 'Use of Infrared Dyes for Transmission Laser Welding of Plastics', Proc. ICALEO, Nov 1999.
- Potente H, Korte J and Stutz R, 'Laser-transmission welding of PE-HD', Kunstoffe 87 (1997) 3, pp348-350.
- Haug M and Rudloff T, 'Assessment of Different High Power Diode Lasers for Material Processing', SPIE Vol 3097 P5 and 3 Lasers in Materials Processing (Munich) June 1997.