Lasers are effective tools for plastic welding, even for clear plastics. The key to success is material selection: it must be a thermoplastic, meaning that it becomes soft or plastic when heated, hardens on cooling, and that process may be repeated.
Let’s review the many benefits of using lasers for plastic welding. First of all, laser welding is a non-contact, tool-less process with a highly configurable heat source. You can achieve a tailored weld width, and perhaps most importantly, there is minimal clamping force used as compared to other welding options.
Two methods can be used for laser welding plastic: transmission/absorption welding and clear plastic welding. Transmission/absorption is most common, and joins parts in a lap geometry in which the laser passes through the top layer to be absorbed at the surface of the bottom layer. The second layer heats up and conducts heat to the top material eventually causing melting. Under force for the entire process, the two plastics fuse and solidify. The graphic in Figure 1 illustrates the process.
A scan head is used to rapidly move the laser over the joint line multiple times which enables controlled heating, and for certain joint geometries ensures such that the top part “seats” uniformly as the bottom layer melts. The force is applied to the parts via a piece of glass (glass being completely transparent to the laser) or in certain cases by using a mechanical interference fit between the parts.
Figure 2 shows automotive component casings, an example of how the method can be used.
The second method, called clear plastic welding, expands the joint configuration to include butt and fillet weld geometries and materials with little or no color or pigmentation. This method, shown in Figure 3, is made possible by using a laser wavelength between 1.5 and 2 microns. At this wavelength, a number of thermoplastics have absorption peaks that are somewhat independent of colors or pigmentation, so suitable heating can occur for welding. With this method, the top layer absorbs a portion of the beam and transmits a portion. This continues through the thickness of the material until all the laser power has been absorbed, creating a molten section through the thickness of the plastics
Figure 4 shows a close-up of low-density polyethylene (LDPE) welding using this method.
Table 1 is a process comparison of the two methods. Usually the transmission method is used, however when both materials show poor absorption to the 900-1100nm laser wavelength, using the longer 1500-2000nm laser can provide a solution. It’s worth noting that there are many plastics and additive materials that can be used to adapt the absorption properties of plastics without changing the color of base properties.
Table 1 - Process Comparison