Laser welding for plastic components (May 2002)

Published in Assembly Automation vol. 22, no.2, May 2002

Abstract

The latest developments in the use of lasers for welding plastics are reviewed. Lasers were demonstrated as being suitable for welding plastics in 1970. However, it is only now that they are finding wide application following technical developments in transmission laser welding and Clearweld®, and the availability of small, economic diode laser systems.

Keywords

Laser, Welding, Plastics, Equipment, Joint design, Clearweld®

Introduction

There are more than fifteen 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, dielectric welding ( Wise, 1999). Laser welding may now be considered as another alternative welding method for plastics with distinct processing and performance characteristics.

Since early in the development of lasers for materials processing (the first multi-kilowatt CO ₂ laser was developed in 1970), it has been shown that lasers may be used for welding plastics ( Silvers et al, 1970). 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 therefore found wide use in the cutting of metal and 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 ( Jones et al, 1994). However, the use of CO ₂ lasers for welding of plastics has not entered wide use in production. Only after the development of an alternative method of applying the laser energy using diode or Nd:YAG laser sources, has the laser found 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 absorbent. 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 use mechanical vibration and 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. This process was first described in 1985 for welding automotive components ( Toyota, 1985). The first (published) part mass produced using transmission laser welding was a keyless entry device for Mercedes in 1997 ( Puetz et al, 1997). The absorbing material used in the process is typically carbon black. A further development in laser welding in 1998 was the invention of the Clearweld® process ( Jones et al, 1998 and 1999), which now allows two similar clear or coloured plastics to be welded, thus further extending the range of possible applications.

CO ₂ laser welding - for films and thin plastics

The CO ₂ laser is a well established materials processing tool, available in power output up to 45kW, 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 with most plastics is very high. Very rapid processing of thin plastic film is therefore possible, even with fairly modest laser powers (<1000W). 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. CO ₂ laser welding of thin film is possible at very high speeds as shown in Fig.1. 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. This technique may be applied as an alternative to ultrasonic, hot wire, dielectric or induction 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 to cut 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 ( Potente et al, 1995). This a variation on hot plate welding in which the joint surfaces are heated against a hot plate before butting together. 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.

Diode and Nd:YAG laser welding - for film, sheet and moulded components

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 CO ₂ (10%), Nd:YAG (3%) or excimer (<1%) lasers. High power diode lasers are available with wavelengths of 810nm, 980nm and 940nm. The degree of energy absorption at this wavelength depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastic, the laser will penetrate a few millimetres into semi-crystalline plastics, further through unpigmented amorphous plastics. So if two unpigmented plastics are clamped together and irradiated with a diode laser, typically no welding would occur. An absorbent must be added at the interface to allow welding to be carried out.

The absorption coefficient of plastics 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 (Seredenko, 1994). A diode or Nd:YAG laser may therefore be used for welding plastics if the upper material transmits energy and an absorptive medium is present at the joint interface or in the bulk of the lower material. Typically carbon black is used as an absorbent for the laser beam, in which case the lower part of the component has to be black (see Fig.2a). If the Clearweld® absorbent is used applied at the interface the lower part of the joint can be the same as the upper part. The Clearweld® absorbents used have very little visible colour and so do not affect the appearance of welds, even in clear plastics (see Figs.2b and 2c).

Equipment Variations

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 3. 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. 

1. 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 weldsmall 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 toweld completely circular pieces.
2. 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.
3. 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.
4. 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.

Joint Designs

The joint designs appropriate for laser welding have flat abutting faces where the weld will be generated. The weld area is defined by the structural performance requirements of the joint balanced with the time and thermal energy required to complete the weld. Preferably there also needs to be a minimal thickness of material between the laser source and the joint line, particularly if the polymers are semi-crystalline of if they are heavily filled. In many cases it is appropriate to limit the edges of the weld by a change in the profile of the joint. This can be used to assist in the generation a small weld bead and can reduce stress concentrating features that may otherwise develop between two flat pieces.

A number of potential joint designs are shown in figure 4.

The design shown at the right end of the middle row represents the use of two laser beams with different wavelengths, and two absorbing mediums placed at the two joint interfaces. The beams are matched to the laser wavelengths such that they will pass through one absorbent and be absorbed by the other. There is great potential for multi-layered structures and components with joints required within a cavity. Diode laser welding has also been shown to be very useful for films, textiles and joining films to rigid components.

The Clearweld® Laser Welding Process

The Clearweld® process was developed for laser welding 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 absorbent that has very little visible appearance. The absorbent is designed to absorb strongly only at the wavelength of the laser being used, which is, in this case, infrared and therefore beyond the range of human vision. The absorbent needs to be applied or incorporated at the joint interface at some stage before the welding process.

Clearweld® Consumable Application Methods

Given that the weld is generated only where the laser and absorbent positions coincide and that the generation of heat is dependent on the amount of absorbent applied, the placement of the infrared absorbent is of great importance to the performance of the welding process. An application method should allow for deposition of controlled amounts of absorbent to controlled positions at the joint interface. There are a number of application methods available ( Jones et al, 1999) some of which are ready for commercial use, and others that may be of use for future applications:

Commercially available:

• Surface application by ink jet printing
• Surface application by spraying or needle dispensing

Potential future use:

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

The method which is employed would depend on the particular application and the balance of performance, cost, colour and extra manufacturing steps.

Process Definition and Control

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 .

In laser welding the main welding parameters are:

• Laser power
• Laser beam size/beam uniformity
• Welding speed or time
• Absorption properties of material at interface
• Clamping pressure

Process control may be used to maintain these machine parameters within the required specification. Alternatively methods are currently being developed to monitor the weld as it forms. These may either sense the weld temperature, or monitor the changes that take place in the appearance of the joint. In addition it must be ensured that the joint has developed sufficient contact area following completion of the weld process.

Experimental Demonstration

The samples shown in Figure 2 were welded with a 50W scanning diode laser system (supplied by Fisba Optik AG). The joint faces for the Clearweld® process were prepared by applying absorbent with a paint brush. 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.

The form of the heated zone can be seen in Figure 5. This sample was made using a circular beam profile, which results in a lenticular shaped weld, and with absorbent applied in the form of a thin film, which can just be picked out at the joint area. 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.

Benefits and Limitations

Transmission laser welding and Clearweld® have the following process characteristics when compared to other plastics welding processes:

• High processing rates
• Low heat input hence low risk of thermal distortion or thermal damage
• Good melt zone temperature and therefore depth control
• Accurate control of weld dimensions and position
• Three dimensional joint lines can be welded
• Low tooling costs
• In-process quality control possible through use of a vision or a temperature measurement system
• Rapid change-over to different product designs
• Pre-assembly possible
• Little or no weld flash
• Equipment not component or process specific
• The outer surfaces of the component or textile remain unmelted
• The process is carried out without vibration of the component as a whole
• The welding process can be continuous for long welds or applied as a pulse of energy from an array of diodes to give a single-shot procedure. There is also a method of rapid scanning the laser beam with mirrors around a component to give a quasi-single-shot process
• In the Clearweld® process the absorbent imparts very little visible colour to the components, so welds can be made without changing the appearance of the polymer.
• Components can be in the same material and optically clear joints are possible if the Clearweld® process is used
• The absorbent materials for Clearweld® applied in the work to date have passed cytotoxicity tests - the first stage requirement for medical application

Disadvantages:

• The main limitation of the transmission laser welding or Clearweld® process is that at least one side of the joint must transmit a proportion of the laser radiation to the joint interface. This proportion may be as low as 10%,but the danger of overheating the top surface of the joint before welding occurs increases as the proportion of transmitted energy decreases. This puts a limit of about 10mm on the thickness of semi-crystalline materials that may be welded
• Good fit-up required
• Clean surfaces needed on transmitting component
• The process requires that some form of absorbent for the laser beam must be incorporated at the joint interface. This will always add an extra complication to the process, whether Clearweld® methods or methods using absorbing materials such as carbon black are applied.

Summary

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 an absorbing medium deliberately positioned there. 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. 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 and even synthetic textiles.

The process of laser welding using a specific infrared absorbing material has been given the trademark Clearweld®. TWI has initiated patent protection for this process. Gentex Corporation has been given exclusive license to commercialise Clearweld® and now has consumable products available providing a basis for the development of welding applications.

References

Jones I A and Taylor N S: 'High speed welding of plastics using lasers', ANTEC '94 conference proceedings, 1-5 May 1994, San Francisco, USA.

Jones I A and Wise R W: 'Welding Method'. Patent Application WO 00/20157, 1 Oct 1998.

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, Heil M and Korte J: 'Welding of plastics using CO ₂ lasers', IIW commission XVI document, XVI-681-95, 1995.

Puetz H et al: 'Laser welding offers array of assembly advantages', Modern Plastics International, Sept 1997.

Seredenko MM: 'Determining Spectral Characteristics of Pigment Absorption and Scattering in the Middle IR Spectral Range', Optics and Spectroscopy vol.76 no.3 1994 pp418-420.

Silvers H J Jr and Wachtell S: 'Perforating, welding and cutting plastic films with a continuous CO ₂ laser'. PA State University, Eng. Proc, pp.88-97, August 1970.

Toyota Jidosha K K: 'Laser beam welding of plastic plates', Patent Application JP85213304, 26 September 1985.

Wise R J: 'Thermal Welding of Polymers', Woodhead Publishing Ltd, Cambridge, Oct 99.