Laser plastic welding, often referred to as “through-transmission welding,” is a process of bonding plastic using focused laser radiation.
The process much different to 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 transit the more 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 fills.
It should be noted that the upper layer can be colored and even opaque using special no-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 clamping force is applied to the part. This typically accomplished with clamp tooling and nests, but can also be realized through interference fits.
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