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Laser welding of transparent plastics

by:J&D WATER     2020-02-25
Our traditional plastic laser welding refers to laser penetration welding. The Upper plastic should be able to penetrate the laser and the lower workpiece should be able to absorb the energy of the laser. Therefore, the upper plastic parts are generally natural plastic, and the lower plastic is generally black plastic. The biggest problem of plastic laser penetration welding is the welding of transparent and transparent materials, such as the welding between PC and PC, which is widely used in the medical industry. Since the upper and lower plastics are laser-penetrable components and lack the laser absorption layer, if the plastic laser penetration welding process is to be used, the Weld must be coated with a light absorber such as Clear Weld, by absorbing laser energy by light absorber to melt the weld for welding. However, this process is rarely used by customers in mass production, because on the one hand, laser absorber will generate additional consumables cost and increase production cost; On the other hand, one more spraying process in production will add additional equipment, consumables and manufacturing risks. In recent years, new plastic laser welding technology has solved the welding problem of transparent and transparent plastics. The laser welding method of this transparent plastic comes from the different penetration and absorption characteristics of plastics for lasers in different wave bands. Laser penetration rate of different plastics The author found a chart of penetration rate of different materials for laser of different wavelengths. The laser wavelength used in traditional plastic laser penetration welding is generally 900- 1000nm, it can be seen that in this band, the penetration rate of plastic to laser is very high, reaching 85-95%. Since T (Penetration rate)R (Reflectivity)A (Absorption Rate) = 1, the reflectivity of the material to the laser can be ignored, so in this band, the absorption rate of the plastic to the laser is very low, only 5-Less than 10%. And if we look at the laser Band 2000- At 2100nm, we found that transparent materials such as PMMA and PC have only 5- 20%, the corresponding absorption rate of laser will be as high as over 80%. In this way, if a laser of about 2000nm is used, transparent plastic materials can be welded by laser. However, this will create another problem. The upper and lower transparent plastics can absorb the power of the laser. How can effective welding be achieved? The performance when the laser penetrates the plastic, when the laser hits the plastic, if the plastic surface has high light absorption efficiency, such as the material is black. Or at the focal point of the laser, most of the laser energy will be absorbed by the material surface and then melt the plastic for welding. If the material is far away from the laser focus, the energy will be absorbed by the whole plastic when the laser passes through, and the plastic is hot but not enough to melt the material. Therefore, for the laser welding of transparent plastics, we select the laser of 2000nm band to ensure that transparent plastics such as PC and PMMA can absorb the energy of the laser. We also need to use a laser head with a very short focal length to focus the beam into a light spot. The characteristic of this lens is that the beam diameter of the lens's light outlet is much larger than the focal spot. When the laser passes through the upper workpiece, its energy density is not enough to melt the upper workpiece due to the large beam diameter. The focus of the laser is at the weld, because the focus is enough energy density to melt the plastic to complete the welding.
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