Laser plastic welding is a method of bonding two or more thermoplastic components together and today’s designers and engineers can benefit from understanding transmission laser welding of plastics – particularly surrounding the concept and design phase of new products.
Although there are many methods for joining thermoplastics, laser plastic welding has a few clear advantages: higher joining quality, minimal resulting flash or particulates, higher quality controls, less stress to the component and the ability weld complex and intricate shapes.
When plastics are joined by laser welding, the laser beam passes through the laser-transmissive upper part to transfer its energy to the lower, laser-absorbent part. Moderate clamping pressure ensures reliable heat conduction between the two joining partners. The weld seam plasticizes, and sets after cooling to form a secure, strong weld. There are four important requirements for the laser welding process to occur.
Laser transparent top layer
The upper joining partner must be sufficiently transparent to the wavelength of the laser to allow the laser beam to pass through the component. Plastic that is transparent to laser light is either pure or contains non-absorbent pigments. Laser-transparent plastics are not necessarily transparent to the human eye. The strength of the energy needed in the laser beam to achieve proper welding at the joining line process is dependent on the transparency of the transmissive plastic in the upper layer. The transparency is dependent on the additives, fillers and the thickness of the material.
Laser absorbing bottom layer
The laser absorbent layer is responsible for turning the remaining laser energy, once passed through the transmissive layer, into heat at the interface of the two layers. A commonly used additive to make plastic absorbent for IR laser light is carbon black (typically 0.2-0.4% by volume) since it is very economical. However, there is also a variety of other additives, including colorings, which are IR absorbent.
Note, it is possible to weld two pieces of clear plastic to one another, either using a special additive or by using special laser wavelengths.
Material compatibility
The two polymers, which are to be joined, must be of the same plastic family with similar resin properties to be joined successfully; otherwise one part may melt or burn and the other will be unaffected. The following materials are known to have been successfully welded: PA 6, PA 66, POM, PBT, PC, ABS, PP and PE in their pure form.
Contact
It is paramount that heat energy, generated on the surface of the lower layer, be transferred to the upper layer so that it may become molten as well. In order for conduction to occur the two layers need to be in contact during the welding process. Contact and pressure are typically accomplished with various methods of clamping devices, see letter G from Figure 1.
Design Considerations
The follow requirements need to be addressed in part design for a successful laser plastic weld.
Melt-collapse (D1 and D2, Figure 1) is the distance the joining partners travel as they move together under clamping pressure. This collapse allows for material fusion and a bond to occur. If joining two flat pieces, a weld rib (E, Figure 1) will be required to allow for melt-collapse to take place. Figure 1, above, represents a joint prior to collapse, where figure 2 shows the same joint after melt-collapse has taken place. Notice the weld flash (H, Figure 2) from the compressed rib.
Beam accessibility
Parts need to be designed to allow direct access for the laser beam to the weld joint, shown as A in figure 1. Accessibility should take into consideration joint width plus part and positional tolerances.
Clamping technology
There must be an adequately large clamping surface on the cover to ensure the optimal application of pressure on the zones adjacent to the weld seam. The workpiece carrier must also be designed to provide good support for the housing in the zone around the root face. Very good and reproducible positioning of the component in the workpiece carrier is crucial for uniform clamping, and the correct tracking of the joint line by the laser. This ensures that the quality of the welding results is always consistent.
Finished design examples include examples cross industries, such as in automotive with Hyundai Equus tail light with a large size, complex shape and aesthetically pleasing welds; electronic sensors with hermetic seal and a stress free process will not damage sensitive electronics. In the medical industry, the micro-atomizer has ultra small and precise welds, no contaminating particulates or flash.
by LPKF