Laser welding of plastic parts, though known from the seventies, has not been much used due to its high cost. However, the nineties saw a drastic drop in cost, largely increasing its usage for plastic part welding. Laser welding systems are most useful when the parts being joined are delicate (electronic components), or require sterile conditions (medical devices and food packaging). The relatively high speed of laser welding makes it valuable on assembly lines for plastic automotive parts. Laser welding can also join parts with complex geometries that would be hard to link with other welding methods.

Laser welding has many advantages. Some of the key benefits are:

•  No contact of equipment with part to be welded
•  High speed
•  No flash is produced
•  Welds are strong
•  High-precision joints can be produced
•  Technique is vibration-free
•  Gas-tight or hermetic seals are possible
•  Thermal damage and distortion are minimal

The most common form of laser welding is transmission laser welding. In this process, two plastic parts are clamped together, and a laser beam in the short-wavelength infrared (IR) region is directed at the section to be joined. The beam passes through the top layer which is transparent; it is absorbed by the bottom layer which is laser absorbing. Absorption of the laser energy causes the bottom layer to heat up, melting both upper and lower layers of plastic and causing them to fuse. The upper layer can be clear or colored, but must be sufficiently light-transmitting to allow the laser beam to pass through it.

Diode lasers, which possess wavelengths between 800-1,000 nm are the most energy-efficient lasers used in welding. They are highly compact, so they are easy to mount on a robot. Diode absorption characteristics are similar to those of Nd:YAG.
Carbon dioxide (CO2 ) lasers are also used in plastic welding. They emit light at a wavelength of 10,600 nm, which is more easily absorbed by plastics than emissions from Nd:YAG and diode devices. However, light from CO 2 lasers is not as penetrating as light from the other two lasers, so CO 2 units are typically used in film applications.
Transmission welding with Nd:YAG or diode lasers can join plastics of more than 1mm thickness at linear speeds exceeding 20 m/min. CO2 welding of films can be done even faster - at rates of up to 750 m/min.

Nearly all thermoplastics and thermoplastic elastomers can be welded with lasers. Common materials often joined with the technique include Polypropylene, Polystyrene, Polycarbonate, ABS Polyamide, Acrylic, Acetal, PET and PBT. Some engineering plastics, such as PPS and liquid crystal polymers are not well suited for laser welding because of their low levels of transmission of laser light. Carbon black is often added to the lower plastic layer to make it absorptive enough for transmission laser welding.
Both unfilled and glass-reinforced polymers can be laser welded. But increased concentrations of glass fillers scatter the IR radiation of lasers, reducing the overall light transmission through polymers. Colored plastics can be laser welded, but penetration of laser beams through plastics declines as pigment or dye concentrations increase.

In the automotive industry, laser welding of plastics has been used in the assembly of fuel injectors, gearshift housings, engine compartment sensors, cockpit housings, hydraulic oil tanks, filter housings, headlights and taillights. Other auto applications include production of air intake manifolds, and of auxiliary water pumps. In the medical area, laser welding is useful in the assembly of fluid reservoirs and filters, tube-to-tube connectors, ostomy bags, hearing aids, implants, and microfluidic devices used in analyses. As laser welding is a vibration-free technology, it is particularly valuable for assembling delicate electronic components. Devices fabricated by laser techniques include keyboards, mobile phones and connectors. Automotive electronic components made with laser welding include automatic door locks, keyless entry devices and sensors. Lasers can also weld thin plastic films together at their edges to form packaging enclosures. The operation can be done extremely rapidly. According to one source (TWI Ltd.), a 100W CO2 laser can weld 100 micrometer polyethylene films at 100 m/min.