TWI Knowledge Summary

Laser welding of plastics

Introduction

Laser welding was first demonstrated on thermoplastics in the 1970s, but has only recently found a place in industrial-scale situations. The technique, suitable for joining both sheet film and moulded thermoplastics, uses a laser beam to melt the plastic in the joint region. The laser generates an intense beam of radiation (usually in the infrared area of the electromagnetic spectrum) which is focused onto the material to be joined, providing very good control over the region heated and the amount of heat applied. Two general forms of laser welding exist: direct laser welding and transmission laser welding. Direct laser welding usually uses CO₂ laser radiation, which is readily absorbed by plastics, allowing quick joints to be made, but limiting the depth of penetration of the beam and restricting the technique to film applications. The shorter wavelength radiation produced by Nd:YAG, fibre and diode lasers is less readily absorbed by plastics, but these lasers are suitable for performing transmission laser welding. In this operation, it is necessary for one of the plastics to be transmissive to laser light and the other to absorb the laser energy, to ensure that the heating is concentrated at the joint region. Alternatively, an opaque surface coating may be applied at the joint, to weld two transmissive plastics. Transmission laser welding is capable of welding thicker parts than direct welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.

The technique

Direct Laser Welding

In direct laser welding the materials are heated from the outer surface possibly to a depth of a few millimetres. Normally, no specific radiation absorber is added to the plastics. Laser sources from 2.0-10.6µm wavelength are typically used. At 10.6µm (CO₂ laser), radiation is strongly absorbed by plastic surfaces, allowing high speed joints to be made in thin films. Developments have also been made using a CO₂ laser transmissive cover sheet as a clamp and heat sink to make welds in thicker plastics without material loss at the surface. At 2.0µm, where the absorption is less strong, a fiber or Holmium:YAG laser can be used to make welds in sheet a few millimetres thick. Direct laser welding is not widely applied for joining plastics but has potential for wider use.

Fig.1. CO₂ laser weld in 100µm polyethylene film at 100m/min with 100W laser power (direct laser welding)

Transmission laser welding

Transmission laser welding is now widely used for joining thermoplastics in industry, using laser sources with wavelengths from 0.8-1.1µm, such as diode, Nd:YAG and fiber lasers. The radiation at these wavelengths is less readily absorbed by natural plastics. Laser absorbing additives are therefore put into the lower part or applied as a thin surface coating at the joint. The parts are positioned together before welding and the laser beam passes through the upper part to heat the joint at the absorbing surface of the lower part (Figure 2). The absorber in or on the lower plastic is typically carbon or an infrared absorber with minimal visible color (Clearweld^®), which allows a wide range of part colours and appearances to be welded. Transmission laser welding is capable of welding thicker parts than direct welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.

Fig.2. Diagram of transmission laser welding

The maximum thickness of the upper part is determined by the transmission properties of the material; transmission laser welding is only possible if over 10% of the energy is transmitted to the joint interface.

Transmission laser welding can also be used to weld film and sheet materials. The laser source is scanned over the two parts just in advance of clamping using a roll processing method.

Comparison of commercially available laser sources for plastics processing

	Nd:YAG	Diode	Fiber	CO₂	Ho:YAG or Tm:YAG
Wavelength (nm)	1064	780 - 980	1000 - 2100	10,600	~ 2000
Efficiency¹⁾	3	30	20	10	3
Approximate cost for 100W system (k)	60	15	40	15	150+
Beam quality²⁾	High	Low	High	High	High
Interaction with plastics	Transmission and bulk heating 0.1-10mm	Transmission and bulk heating 0.1-10mm	Transmission and bulk heating 0.1-10mm	Surface heating to < 0.5mm	Bulk heating 0.1-3mm

¹⁾ Efficiency is the percentage of the electrical power consumed by the laser that is emitted in the beam.
²⁾ Beam quality is the ability to focus the beam to a small spot size with a high energy density.

Scope

Laser welding is a high volume production process with the advantage of creating no vibrations and generating minimal weld flash. The technique relies on the initial outlay for a laser system, however, the benefits of a laser system include; a controllable beam power, reducing the risk of distortion or damage to components; precise focussing of the laser beam allowing accurate joints to be formed; and a non contact process which is both clean and hygienic. Laser welding may be performed in a single-shot or continuous manner, but the materials to be joined require clamping. Weld speeds depend on polymer absorption. It is possible to create joints in plastics over 1mm thick (with transmission laser welding) at up to at least 20m/min whilst rates of up to 750 m/min are achievable in the CO₂ laser welding of films.

Adaptations of laser welding

Clearweld^®

The Clearweld^® process was invented, and has been patented, by TWI. It is being commercialised by Gentex Corporation. The process uses commercially available lasers in conjunction with infrared absorbing welding consumables. The carbon black absorber traditionally used is replaced by a colourless, infrared absorbing medium thus expanding the applicability of the technique to clear plastics. The infrared absorbing medium is either printed/painted onto one surface of the joint, encompassed into the bulk plastic, or produced in the form of a film that can be inserted into the joint. It absorbs infra-red laser light allowing an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour. The process is especially suitable where the appearance of a product is important. In the case of fabrics joining, positioning of the infrared absorbing medium at the joint restricts melting to the interface rather than through the full thickness of the joint as occurs in other welding methods for fabrics. Consequently, flexible seams are produced making the process suitable for the joining of fabrics for clothing applications.

Additional information can be found on the Clearweld^® website - www.clearweld.com

Fig.3. Clearweld^®: Upper left sample shows conventional transmission laser welding with carbon black as the laser absorbing material. Lower right sample shows the use of a novel infrared absorbing medium to create a low visibility joint

Cut/seal

In the cut/seal process, careful control of the laser beam profile allows film to be both welded and cut in a simultaneous operation. This development is of particular use in the production of packaging items and plastic bags.

Applications

Laser welding has proved to be especially effective in the welding of thermoplastic films in a lap joint configuration. The speeds attainable with laser welding make it especially suitable for use in the packaging and automotive industries, including use for welding pressure vessels, whilst biomedical applications exploit the cleanliness of the process. The Clearweld^® process extends its applicability to circumstances where final appearance is important. Applications in the areas of food packaging, medical devices and packaging, electronic displays and fabrics are being developed.