Laser plastic welding, also often referred to as through-transmission welding, is a process of bonding plastic using focused laser radiation. The process is very much different than traditional metal welding.
The concept involves passing a focused laser beam through an upper, laser transmissive part to the interface of the two parts to be joined. The laser light is turned into heat energy as it is absorbed by the lower joining partner. The heat created at the interface creates a molten weld seam and the two plastics are fused.
The process relies on four main factors: a laser transmissive layer, a laser absorbing layer, plastics material compatibility and good contact between the parts.
Natural thermoplastics transmit infrared laser radiation. Most often sources of either 980nm or 808nm are used as they transmit the most energy at these wavelengths.
The lower layer is responsible for turning the light energy into heat. As the majority of thermoplastics are naturally transmissive to IR radiation, additives must be used to give absorbing qualities to the part. Most often carbon black is used, but many additives will promote absorption including various pigments and fillers. It should be noted that the upper layer can be colored and even opaque using special non-absorbing colorants.
Material compatibility is determined by a few factors including surface energy and polymer chain length, but the most important consideration is to find plastics with similar or overlapping melting temperatures.
Once the light energy is converted to thermal energy at the surface of the lower part, some of that energy must make its way into the upper layer in order for melting to occur in both parts. Good contact between the parts ensures that energy can be transferred via conduction. To ensure good contact, a clamping force is applied to the part. This is typically accomplished with clamp tooling and nests, but can also be realized through interference fits.
It is a misconception that laser welding plastics requires one clear part and one opaque part. It is perfectly possible to weld opaque-to-opaque as well as clear-to-clear.
There are multiple ways to weld two clear polymers with laser. One method makes use of a special, optically clear chemical called Clearweld. Clearweld can be applied as an additive or coating, and is optimized for through-transmission welding of clear polyemrs. It is clear to the human eye, but will absorb laser radiation in the IR spectrum.
Secondly, higher wavelength lasers (around the 2,000nm wavelength) can be utilized, we call this process transparent laser plastic welding or TLPW. This process is still considered laser plastic welding, but it is not the same concept as through-transmission laser welding, described previously. The higher wavelength laser will transmit through both parts, but the majority of the beam energy can be focused precisely at the joint interface where heating takes place simultaneously in both parts.
It is also possible to weld two opaque parts. To get a black part typically plastics are doped with carbon black, however, carbon black is also a great infrared absorber making it impossible to use in the upper layer during transmission welding. Instead of carbon black, a very dark red or green dye can be used to achieve an optically black look while still remaining laser transmissive. Other opaque color combinations are possible as well.
The part to the right is a gear sensor housing, a very common application in the automotive industry.