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Laser welding of polymers is a series of techniques to join thermoplastic components through laser radiation. The wavelength of laser for welding and processing applications is usually between 800 and 11,00 nm. This technology was firstly applied in the market to produce ignition key for Mercedes-Benz vehicles in the late 1990s. Due to the benefits including high joining speed, low residual stress and excellent weld appearance, laser welding of plastics is commonly used for industrial applications such as automotive taillight and instrument panels.

Laser equipment

Various types of lasers can be used in the welding of polymers. A list, and brief description, of each applicable laser type is given below.

Carbon dioxide lasers

Carbon dioxide lasers typically have a typical wavelength of 10.6μm which can be rapidly absorbed by the surface of most polymers. The penetration of CO₂ laser is less than 0.5 mm thickness and it is has been mostly applied for welding thin film and surface heating. Because this beam cannot be transmitted by silicon fiber, it is commonly delivered by mirrors.

Nd:YAG lasers

Nd:YAG lasers typically have a wavelength in the range of 0.8 - 1.1μm, with 1064nm being the most common. These lasers provide high beam quality allowing for small spot sizes and can be delivered via fiber optic cable.

Diode lasers

The wavelength of diode lasers is typically in the 780 - 980nm wavelength range. Compared with Nd:YAG laser and CO₂ laser, diode laser has supreme advantage in 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 2100nm. The expanded range of wavelengths has allowed for the development of through transmission welding without being absorbed additives.

Laser interaction with polymers

There are three types of interactions that can occur between laser radiation and plastics: reflection, absorption and transmission. The extent of individual interaction is dependent upon materials properties, laser wavelength, laser intensity and beam speed.

Reflection

Reflected and transmitted energy

Reflection of incident laser radiation by a homogenous plastic material is typically on the order of 5 to 10% in most polymers, which is low compared with absorption and transmission. The fraction of reflection (R) can be determined by the following equation,

$R={\frac {(n-m)^{2}}{(n+m)^{2}}}$

where $n$is the index of refraction of the plastics and $m$is the index of refraction of air (~1).

Transmission

Transmission of laser energy through certain polymers allows for processes such as through transmission welding. When the laser beam travel through the interfaces between different medium, the laser beam is refracted unless the path is perpendicular to the surface. This effect needs to be considered when laser travels through multi-layer to reach the joint region.

Internal scattering occur when laser pass through the thickness in semicrystalline plastics, where crystalline and amorphous phase have different index of refraction. Scattering can also occur in crystalline and amorphous plastics with reinforcement like glass fiber and certain colorants and additives. In transmission laser welding, such effect can reduce the effective energy of laser radiation towards joint area and limit the thickness of components.

Absorption

Laser absorption can occur at the surface of plastics or during transmission through thickness. The amount of laser energy absorbed by a polymer is a function of the laser wavelength, polymer absorptivity, polymer crystallinity, and additives (i.e. composite reinforcements, pigments, etc.).The absorption at surface has two possible ways, photolytic and pyrolytic. Photolytic occurs at short wavelength radiation (less than 350 nm or ultraviolet (UV)), when the photon energy is sufficient to break chemical bonds.

Heat is generated within the polymer when the photon energies of the laser beam cause the molecular bonds to vibrate. The heat distribution within a laser welded polymer is dictated by the Bouger-Lambert law of absorption,

$I_{t}=I_{0}\exp {(-\alpha t)}$

where $I_{0}$is the beam energy intensity at the surface, $\alpha$is the absorption constant and $t$is the thickness.

Effect of additives

Polymers often have secondary elements added to them for various reasons (i.e. strength, color, absorption, etc.). These elements can have a profound effect on the laser interaction with the polymer component. Some common additives and their effect on laser welding are described below.

Reinforcements

Various fibers are added to polymeric materials to create higher strength composites. Some typical fiber materials include: glass, carbon fiber, wood, etc. When the laser beam interacts with these materials it can get scattered or absorbed, changing the optical properties from that of the base polymer. In laser transmission welding, a transparent material with reinforcement may absorb or dilute the energy beam more, affecting the quality of the weld. High contents of glass fiber content increase the scattering within the plastics and raise the laser energy input for welding a certain thickness.

Colorants

Colorants (pigments) are added to polymers for various reasons including aesthetics and functional requirements (such as optics). Certain color additives, such as titanium dioxide, can have a negative impact on the laser weldability of a polymer. The titanium dioxide provides a white coloring to polymers but also scatters laser energy making it difficult to weld. Another color additive, carbon black, is a very effective energy absorber and is often added to create welds. By controlling the concentration of carbon black with the absorbing polymer it is possible to control the effective area of the laser weld.

Laser heating mode

The laser beam energy can be delivered to the required areas through a variety of techniques. The four most common approaches include: contour heating, simultaneous heating, quasi-simultaneous heating, and masked heating.

Contour heating

In the contour heating (laser scanning or laser moving) technique, a laser beam of fixed dimension passed through the desired area to create a continuous weld seam. The laser source is manipulated by a galvanic mirror or a robotic system to perform scanning in a fast rate. The benefit of contour heating is the weld can be performed with a single laser source, which can be reprogramed for different applications. However, due to the localized heating area, uneven contact between welding components can occur to form weld voids. Sufficient pressing force is necessary to close the gap between two parts. Some of the important parameters for this technique include: laser wavelength, laser power, traverse speed, and polymer properties.

Simultaneous heating

In the simultaneous heating approach, a beam spot of appropriate size is used to irradiate the entire weld area without the need for relative movement between the work piece and the laser source. For creating a weld with large area, multiple laser sources can be assembled to melt the selected region simultaneously. This approach can be adopted to substitute ultrasonic welding in the case of welding components sensitive to vibration. Key processing parameters for this approach include: laser wavelength, laser power, heating time, clamp pressure, cooling time, and polymer properties.

Quasi-simultaneous heating (QSLW)

In the quasi-simultaneous heating approach a work area is irradiated by the use of scanning mirrors. The mirrors actuate the laser beam over the entire work area rapidly, creating a simultaneously melted region. Some of the important parameters for this technique include: laser wavelength, laser power, heating time, cooling time, polymer properties.

Masked heating

Masked heating is performed by a laser curtain translating through a mask blocking to melt selected regions. Masks are made of laser cut steel, or other materials that effectively block the laser radiation. This approach is capable of creating micro-scale weld on components with complex geometry. Key processing parameters for this approach include: laser wavelength, laser power, heating time, clamp pressure, cooling time, travel speed and polymer properties.

Laser welding techniques

Polymeric materials can be joined via laser by several techniques listed below

Direct laser welding

Direct laser welding of polymers

Similar to laser welding of metals, in direct laser welding the surface of the polymer is heated to create a melt zone that joins two components together. This approach can be used to create butt joints and lap joints with complete penetration. Laser wavelengths between 2 and 10.6μm are used for this process due to their high absorptivity in polymers.

Laser surface heating

Laser surface heating is similar to non-contact hot plate welding in that laser reflecting mirrors are placed between components to create a surface molten layer. The exposure duration is usually between 2-10s. Then the mirror is retracted and the components are pressed together to form a joint. Process parameters for laser surface heating include the laser output, wavelength, heating time, change-over time, and forging pressure and time.

Through transmission laser welding (TTLW)

Diagram of transmission laser welding of polymers

Through transmission laser welding of polymers is a method to create a joint at the interface between two polymer components with different transparency to laser wavelength. The upper component is transparent to the laser wavelength between 80 µm to 1050 µm, and the underneath component is either opaque in nature, or modified by the addition of colorants which promote the absorption of laser radiation. A typical colorant is carbon black that absorb most of the electromagnetic wavelength.

During laser welding, two components are held by the lower fixture to control alignment and a small clamping force is added to the upper part to form intimate contact. A melting layer is created at the interface between two components, composed of a mixture of two plastic materials.

There are four different modes of transmission laser welding: scanning mode, simultaneous, quasi-simultaneous, and mask heating.

Many benefits can be obtained by transmission laser welding such as fast welding velocity and flexibility, good cosmetic property and low residual stress. From processing perspectives, laser welding can be performed in pre-assembled condition, reducing the necessity for complex fixture. However, this method is not suitable for plastics with high crystallinity due to refraction and limited by sample geometry as well.

Intermediate film welding

Intermediate film welding is a method to join incompatible plastic components by placing an intermediate film. Similar to transmission welding, laser radiation pass through the transparent components and melt the intermediate layers to create a joint.This film can be made of an opaque thermoplastic, solvent, viscous fluid, or other substance that heats upon exposure to laser energy. The thin layer then generates the heat required to fuse the system together.