Laser plastic welding is a joining method that utilizes high-energy-density laser beams as the heat source. Its basic principle involves clamping two plastic components together under pressure, then directing a short-wave infrared laser beam onto the desired bonding area. The laser passes through the upper transparent material and is absorbed by the lower layer. This absorbed energy is converted into heat, melting and fusing the two layers together. At the same time, thermal expansion generates internal pressure within the materials, working in conjunction with external clamping force to ensure a strong, durable weld.

Below, Wanzhong Laser will reveal the specific requirements that plastic materials must meet for successful laser welding applications.

1. Material Absorption Characteristics in the Heat-Affected Zone

In the heat-affected zone (HAZ), the material should have good absorption characteristics for the laser wavelength used. Typically, absorbers are added to enhance this performance. A commonly used and ideal absorber is carbon black , which can absorb almost all the energy from infrared lasers. This significantly improves the thermal absorption efficiency, allowing the HAZ material to melt faster and more effectively. However, using carbon black as an absorber tends to darken the weld seam, making it visibly different from the base material color.

Other colored dyes can also serve as absorbers. For example, a transparent dye developed by The Welding Institute (TWI) of the UK allows visible light transmission while absorbing laser energy, resulting in nearly invisible or transparent weld seams.

2. Light Transmission Properties of the Non-Heat-Affected Zone

For the non-absorbing (transmissive) part of the joint, the material should exhibit good optical transmittance, especially when performing lap welding on two thin plastic parts. In transmission laser welding , the upper component (the one exposed to the laser) must be transparent to allow the laser to pass through, while the lower component must be absorptive.

Generally, better welding results are achieved when the upper transmissive material has a transmission rate above 50% , and the lower absorptive material has a transmission rate below 20% .

3. Influence of Material Type and Thickness on Laser Penetration

Different types of plastics respond differently to laser penetration based on their structure:

Semi-crystalline materials contain crystalline particles that may cause scattering and reflection of the laser beam, reducing its penetration capability. Therefore, the thickness of the transmissive layer made from semi-crystalline materials should generally not exceed 2 mm .

The addition of glass fibers also affects laser penetration — the higher the glass fiber content, the thinner the transmissive layer should be to ensure effective energy delivery.

Amorphous materials , on the other hand, do not have such strict limitations on thickness, as they typically allow better laser transmission regardless of thickness.

4. Impact of Color on Laser Welding Performance

Color plays a significant role in laser welding:

For the absorbing (lower) material, if the natural color does not provide sufficient absorption at the laser wavelength, absorbers can be added to improve performance.

For the transmitting (upper) material, colorants—especially inorganic pigments—can reduce transmittance due to light scattering effects. Therefore, careful selection of coloring agents is necessary for laser welding applications involving colored plastics.

In practice, it's advisable to avoid using colored plastics unless specifically formulated for laser welding.

Transparent/Black combinations are relatively easy to weld.

Black/Black is also manageable.

However, Transparent/Transparent or White/White combinations pose greater challenges.

For Transparent/Transparent joints, suitable laser wavelengths or added absorbers can help achieve melting and bonding.

5. Polymer Compatibility and Melting Temperature Considerations

The success of laser welding also depends on the compatibility of the two polymers being joined and the closeness of their melting temperatures . The better the compatibility and the closer the melting temperatures, the easier it is to achieve a strong, high-quality weld.

Examples include:

PC/ABS with PMMA or PC

PC with PBT

ABS with PBT

When the absorbing polymer absorbs laser energy and begins to melt, it expands thermally. Therefore, it is essential to apply appropriate clamping force between the two layers using fixtures to prevent deformation at the weld interface. Additionally, the clamping force helps transfer heat efficiently between the layers, ensuring both materials melt simultaneously and form a stronger bond.

6. Selection of Laser Parameters

Excessively high laser power or overly long welding times can lead to over-melting and material degradation , negatively affecting weld quality. To achieve optimal results:

Choose an appropriate laser power

Set a proper scanning speed

Control the welding duration

These parameters must be balanced to ensure sufficient melting without damaging the materials.

By understanding and following these material-specific requirements, manufacturers can significantly improve the quality, reliability, and efficiency of laser plastic welding processes. This enables high-strength, precision welding across a wide range of industrial applications, from automotive to medical devices.