Laser welding of transparent plastics

In our traditional sense, plastic laser welding refers to laser penetration welding. The upper layer of plastic must be able to penetrate the laser, and the lower layer of the workpiece must be able to absorb the energy of the laser. Therefore, the upper layer plastic is generally light-transmitting plastic, and the lower layer plastic is generally black light-absorbing plastic. Figure 1 shows the ease of plastic welding.

Figure 1

As can be seen from Figure 1, the easiest is the welding of the upper transparent material and the lower black light-absorbing material, and the most difficult is the welding of transparent plastic and transparent plastic laser and the welding of white plastic and white plastic. Because the principle of plastic laser welding is that the upper layer must be a light-transmitting material, and the lower layer is a light-absorbing material; then the transparent plastic is a light-transmitting material, how to realize the welding of transparent plastic and transparent plastic?

The biggest problem with laser penetration welding of plastics is the welding of transparent and transparent materials, such as PC-to-PC welding, which is widely used in medical device applications. Since the upper and lower plastics are laser-penetrable parts and lack a laser-absorbing layer, if the plastic laser penetration welding process is to be used, a light-absorbing agent such as Clear Weld must be applied to the weld to absorb the laser energy by the light-absorbing agent. to melt the weld for welding. However, this process is rarely used by customers in mass production, because on the one hand, the cost of laser absorbers is high; on the other hand, an additional spraying process in production will increase equipment, consumables and manufacturing risks. However, as early as 2017, Huazhuo successfully developed the first transparent plastic laser welding equipment in China, and did not need to add any absorbent to complete double transparent plastic laser welding. Huazhuo is also the only manufacturer in China that masters transparent laser welding technology. Figure 2 is Huazhuo's patented product, a transparent plastic laser welding machine;

Figure 2

The laser welding method of transparent plastics comes from the different penetration and absorption characteristics of plastics for different wavelengths of lasers. Figure 3 is a graph of the transmittance of different materials for different wavelengths of laser light. The laser wavelength used in traditional plastic laser penetration welding is generally 900-1000nm. It can be seen that in this wavelength band, the penetration rate of plastic to laser is very high, which can reach 85-95%. Since T (transmittance) + R (reflectivity) + A (absorptance) = 1, the reflectivity of the material to the laser is negligible, so in this band, the absorptivity of the plastic to the laser is very low, only 5- less than 10%. And if we observe the laser band of 2000-2100nm, we find that the transmittance of transparent materials such as PMMA and PC in this band is only 5-20%, and the corresponding absorption rate of the laser will be as high as 80% or more. In this way, if a laser of about 2000nm is used, laser welding of transparent plastic materials can be performed. But this will cause another problem. Both the upper and lower transparent plastics can absorb the power of the laser. How can we achieve effective welding?

图3

When the laser hits the plastic, if the plastic surface has a high light absorption efficiency, for example, the material is black. Or at the focal point of the laser, where most of the laser energy is absorbed by the surface of the material and melts the plastic for welding. If the material is far from the laser focus, the energy will be absorbed by the plastic as the laser passes through, and the plastic will be hot but not enough to melt the material. Therefore, for laser welding of transparent plastics, we select a laser in the band around 2000 nm to ensure that transparent plastics such as PC and PMMA can absorb the energy of the laser. We also need to use a very short focal length laser head to focus the beam into a spot. The characteristic of this lens is that the beam diameter at the lens exit is much larger than the focal spot. When the laser passes through the upper workpiece, because the beam diameter is large, its energy density is not enough to melt the upper workpiece. The focus of the laser is at the weld, and the plastic is melted to complete the welding due to the sufficient energy density of the focus.