Laser welding of polymers is a set of methods used to join polymeric components through the use of a laser. It can be performed using CO2 lasers, Nd:YAG lasers, Diode lasers and Fiber lasers.
When a laser encounters the surface of plastics, it can be reflected, absorbed or penetrate through the thickness of a component. Laser welding of plastics is based on the energy absorption of laser radiation, which can be reinforced by additives and fillers.
Laser welding techniques include:
Direct laser welding
Laser surface heating, Through transmission laser welding Intermediate film welding.
Because of high joining speeds, low residual stresses and excellent weld appearances, laser welding processes have been widely used for automotive and medical applications.
Laser sources
The types of lasers used in the welding of polymers include CO2 lasers, Nd:YAG lasers, Diode lasers and fiber lasers. CO2 lasers are mostly applied to weld thin films and thin plastics due to the high energy absorption coefficients of most plastics. Nd:YAG lasers and Diode lasers produce short wavelength radiation, which transmit through several millimeters of unpigmented polymer. They are used in the transmission laser welding techniques.
Carbon dioxide lasers
Carbon dioxide lasers have a wavelength of 10.6 μm which is rapidly absorbed by most polymers. Because of the high-energy absorption coefficients, processing of plastics using CO2 can be done rapidly with low laser powers. This type of laser can be used in direct welding of polymers or cutting. However, the penetration of CO2 lasers is less than 0.5 mm and is mostly applicable for the welding of thin film and surface heating. Because the beam cannot be transmitted by silicon fiber, the beam is commonly delivered by mirrors.
Nd:YAG lasers
Nd:YAG lasers have a wavelength in the range of 0.8 - 1.1 μm, with 1064 nm being the most common. These lasers provide a high beam quality allowing for small spot sizes. This type of beam can be delivered via fiber optic cable.
Diode lasers
The wavelength of diode lasers is typically in the 780 - 980 nm wavelength range. Compared with Nd:YAG laser and CO2 laser, diode laser has supreme advantage in energy efficiency. The high-energy light wave can penetrate a thickness of a few millimeters in semicrystalline plastics and further in unpigmented amorphous plastics. Diode lasers can be either fiber delivered or local to the weld location. The relatively small size makes assembling arrays for larger foot prints possible.
Fiber laser
Fiber lasers typically exhibit wavelengths ranging from 1000 to 3500 nm. The expanded range of wavelengths has allowed for the development of through transmission welding without additional absorbing additives.
Equipment
The equipment settings may vary greatly in design and complexity. However, there are five components included in most of the machines:
generator/power supply
control interface
actuator
lower fixture
upper fixture.
Generator/Power supply
This component transforms the received voltage and frequency to the corresponding voltage, current and frequency to the laser source. Diode laser and fiber laser are the two most commonly used system for laser welding.
Control interface
The control interface is an interface between operator and machine to monitor operations of the system. It is constructed by logic circuits to send operators the information of machine status and welding parameters. Depending on different laser modes, the control interface will vary the parameters allowable to change.
Actuator
This component is a press activated by pneumatical and electrical power. It compresses the part in the upper fixture to touch the components in the lower fixture and apply pre-determined loads during welding processes. Displacement controls are added to actuators to monitor precisely the movements.
Lower fixture
Lower fixture is a jig structure that locates the lower part of a joint. It provides locations and alignments that ensure the welding of components with tight tolerances.
Upper fixture
The upper fixture is the most complicated and important component in the whole system. Laser beam is generated in this component to heat up the welding parts. The design of upper fixture often varies from laser sources and heating modes. For example, when a YAG laser or a diode laser is used as the heat source, optical fibers are often employed to provide mobility. However, the welding part cannot move.