Transmission laser welding of plastics

TWI Bulletin, July - August 1998

Senior Research Engineer, helped develop techniques for high power laser processing at TWI. Initially in welding of steel and alumimium, but more recently for plastics joining as part of Advanced Materials and Processes department.

Ian Jones reviews the latest developments in the use of lasers for plastics processing, and describes a new process for laser welding of plastic sheet and mouldings.

Lasers are very attractive tools for materials processing in many industrial areas. Their features allow for precise delivery of a controlled amount of energy exactly to the point where it is required. The beam from a laser may be focused to a small spot giving very high power densities, capable of vaporising any known material or expanded to carry out a broad surface treatment. Since the construction of the first laser in 1962, these special features have been put to use in an ever increasing variety of applications ranging from CD players to ship construction.

In application to plastics processing, lasers may be used to deliver energy for welding cutting, drilling or surface treatment. Process advantages include:

• high speed
• no contact with a heated tool
• highly automated and robotically manipulated
• controlled heating for low thermal damage or distortion

In addition, lasers are available with outputs covering a range of wavelengths, and this has a large bearing on the interaction of the light with the plastic material. An understanding of these absorption characteristics has led to the development of other novel applications in plastics processing.

In this report we review the laser types which are available for plastics processing and compare them in terms of their beam properties, cost of ownership and interaction with plastics materials. The application capabilities are discussed with reference to the alternative methods available for plastics processing culminating in a description of the recent developments in transmission laser welding.

Laser types and their interaction with plastics

The main commercially available laser types of interest in plastics processing are listed in Table 1. The different applications possible with each laser type are dependent mostly on the wavelength of light produced, which dictates the form of energy absorption in the plastic. However, for a given laser type, the processing behaviour also varies with the material type being used. The table also compares the cost of ownership of the lasers and the beam output available. Infrared lamps are included because the range of wavelengths available from them overlaps with the lasers and they therefore serve as a cost effective alternative in some applications where a large beam diameter can be used. In contrast to lasers the infrared lamp emits a broad band of wavelengths of light which cannot be focused to small spot sizes (<3mm) and high power densities.

Table 1: Comparison of commercially available laser and infrared lamp sources

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 CO₂ 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 CO₂ 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 CO₂ 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.

The Nd:YAG laser is also well established for material processing, and recent developments have led to increases in the power available to above 4kW and reduced the laser size. In general, the light from Nd:YAG lasers is absorbed far less readily in plastics than CO₂ laser light. The degree of energy absorption at the Nd:YAG laser wavelength (1.064µm, 1.2eV photon energy) depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastic, the laser will penetrate a few millimetres into the material. The absorption coefficient can be increased by means of additives such as pigments or fillers, which absorb and resonate directly at this photon energy or scatter the radiation for more effective bulk absorption. ^[1] The Nd:YAG laser may therefore be used for heating plastics to depths of a few millimetres or for heating a more highly absorbent medium (either metal or a plastic containing suitable additives) through or within a transmissive plastic part. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

High power diode lasers (>100W) have been available since early 1997. They are now available up to 2kW and are competitively priced compared to CO₂ and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than CO₂ (10%), Nd:YAG (3%) or excimer (<1%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers, and applications overlap. The beam from a diode laser is rectangular in shape, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing. ^[2]

Excimer lasers were first operated in 1975, some years after the CO₂ and Nd:YAG lasers. They are available with average powers up to about 1kW, but with pulse powers greater than 10 ⁸W, and focusable to very high power densities. There is a family of wavelengths available by exciting different gases within the laser. These are all in the ultraviolet (0.15-0.35µm, 3.5-7.9eV photon energy), and lie in a photon energy range capable of breaking chemical bonds and splitting molecules. The C-H bond, for example, has a bond energy of 3.5eV. This effect leads to many precision machining and surface treatment applications which are unique to excimer lasers. Excimer laser light is absorbed by molecules in the surface of plastics (<10µm depth), and rapidly breaks the molecular bonds within the polymer structure. This leads to a rapid increase in pressure and expulsion of material over a very precise region defined by the laser beam size. The excimer laser beam is transmitted by mirrors and often focused through masks to give the required features on the material surface. ^[3]

Infrared lamps operate by the electrical resistance heating of a wire filament. In contrast to lasers this leads to the emission of photons with a range of wavelengths, with the peak emission being at a wavelength dependent on the temperature of the filament. ^[4] Tungsten filament lamps emit most of their power at 0.7-2.5µm, which gives them absorption characteristics in plastics similar to the Nd:YAG and diode lasers. Absorption coefficients tend to be higher than for lasers, however, because there are more wavelengths of light available to couple into the molecular vibrations of the materials. Infrared lamps are relatively cheap and lightweight heat sources which are generally applied using an elliptical focusing mirror but without transmission of the light over long distances.

Review of processing capabilities and developments

A summary of the processing capabilities of the laser types is shown in Table 2.

Table 2: Summary of application capabilities for the laser and infrared lamp types

CO₂ lasers are widely used for high speed cutting and drilling of plastics. The rapid heating and thermal decomposition of the plastic enables a 500W CO₂ laser to carry out cutting of sheet up to at least 10mm thick. The cut may be carried out using a compressed gas (usually air) jet flowing coaxially with the laser. The gas jet removes the molten or vaporised material to leave a cut of 0.1-0.6mm width. ^[8] The process is non-contact, so soft materials such as foams are readily cut without deformation of the edge. Laser cutting or drilling may be used as an alternative to ultrasonic, die, saw or other mechanical cutting methods.

CO₂ lasers can drill holes of 0.15-1mm diameter in 1-50msec per hole. ^[9] Even higher precision drilling and micromachining is carried out using excimer lasers. Features with dimensional accuracy of as good as 1µm are possible in most plastics. Such control over removal of material allows for a range of applications, such as wire stripping, sensor nozzle drilling, and surface marking or coding, to be carried out. ^[3] Some of these tasks are not possible using other methods.

Excimer lasers have also been used to modify polymer surfaces prior to adhesive bonding. The photochemical reaction during the laser treatment can improve the shear strength of the lap joints compared to similar control joints made with abraded/degreased/abraded and primed surfaces. ^[10]

Opportunities for transmission laser welding

Nd:YAG and diode lasers (and infrared lamps) can be used to heat surfaces which are in contact by transmitting energy through one side of the joint, given that one of the plastics is transmissive to the laser and the other absorbs. This process of transmission laser welding is shown in Fig.2. An example of polypropylene welded using this technique is shown in Fig.3. The process provides a means by which plastic parts with different absorption characteristics may be welded with no melt flash, no marking of the outer surfaces and only a small heat affected zone. The welding is carried out with either a Nd:YAG laser or a diode laser, and narrow welds can be made at up to 10 m/min even with lasers of 100W or less. Infrared lamps can also be used to carry out similar joints but the broader wavelength band gives less selectivity between absorption in the two materials to be welded. In many cases the infrared lamp may offer a cheaper alternative to lasers for large joint ar eas.

The process requires two materials which transmit the laser to different degrees. This can be arranged either by using filler or pigment in one side of the joint. Successful welds can be made with as little as 0.05% carbon black in the absorbing layer, ^[11] and with dyes in the transmissive layer which absorb visible light, but do not absorb near infrared light. ^[12] So it is possible to weld two similar coloured materials with different dye types. Other requirements for successful application of the process include intimate contact at the weld interface and a smooth interface surface. Positive pressure is required over the joint area, though this is assisted by the thermal expansion at the interface.

The process has already reached production for the manufacture of keyless entry cases by a German company - Marquardt. ^[12] Ten 30W lasers are used to make a 76µm wide weld at 5 m/min. It may also be used to heat and seal metal parts within a plastic, or to weld elastomers to plastics, where ultrasonic or vibration welding would not be possible. Other advantages include the ease of automation, process monitoring and control of quality, easier change of tooling than for vibration welding and reduced weld surface contamination compared to hot plate welding.

Developments relating to this process are still underway. A greater understanding of the effect of fillers and pigments and other additives in materials, and the effect of different laser types on the welding process is required. There is also a need for short and long term testing to be carried out before the process can be applied to structural components.

References