Laser Welding

Plastics are laser-welded by passing laser light through a (laser transparent) top part onto a (laser absorbent) bottom part.  The absorbed laser energy softens and melts both parts.  With externally applied clamping pressure, the parts are bonded upon cooling. Typically, diode lasers having a wavelength in the (infra-red) range of 800nm-1000nm are used in this process.

Advantages of laser welding:

• Joint design need only be surface to surface.  There is no need for energy directors or collapse of the weld joint.
• Weld lines can be as narrow as 0.1mm (0.004 in.)
• Good welds can be achieved, even to a hermetic seal.  Tensile strength is that of the unreinforced base resin
• There is no relative motion between the parts as happens with vibration welding.  There are no vibrations that could damage electronic components
• Three-dimensional geometries can be welded
• There is no part marking or bleed through
• The joint has less flash than with other methods

The four main laser welding methods:

• Spot laser welding
• Line laser welding
• Mask laser welding
• Simultaneous through welding  

In spot welding, a circular spot of laser energy traverses a pre-programmed contour path.  The simultaneous line method creates a laser line for welding, while the mask method blocks the laser line in a predefined pattern.  Simultaneous through welding delivers laser energy to the entire surface via a fiber optic head and typically runs a three to five second cycle.

Laser welding conditions

Laser welding conditions are a function of laser power, beam focus, and melt pool temperature.  Transmittance and thickness of the IR-transparent part determine how much energy reaches the absorbent part.  While most unfilled polymers are relatively transparent between in the 800nm to 950nm range, incorporation of fillers, reinforcements or pigments can significantly increase energy absorption and require much more laser energy to make a weld.  Because of scattering, more energy is required to weld through semi-crystalline polymers than through amorphous ones.  

The weldability of various Ticona resins is shown in the table below: