Monitoring quality and objects in plastic laser welding applications

In plastic laser welding, the surface of the lower material absorbs the laser energy and then melts, and then transfers the heat to the upper material, melting the welding surface of the upper material, and then forming a complete molten pool. During the welding process, the heat of the molten pool will diffuse to the surrounding. The general relationship between infrared radiation and the temperature of the molten pool (the higher the temperature of the molten pool, the higher the infrared energy density):

Only a small part of the infrared signal radiated by the molten pool is received by the thermometer, as shown in Figure 2. After the thermometer receives the infrared signal emitted by the molten pool, it compares its calibration value and obtains the current calibration temperature value of the molten pool. The calibration of the thermometer is generally performed by a special instrument. When leaving the factory, there is a standard heat source, and the temperature can be set (such as 100掳C, 150掳C, 200掳C, etc.). After the thermometer receives the infrared signal of this temperature, it records the reaction temperature value on the thermometer as the temperature set by the instrument through the calibration calculation of the software.

In addition, it can be seen from Figure 2 that the strongest band of infrared signal is in the range above 2um-. If the infrared signal in this range is used for software processing, signal processing and result analysis will be relatively easy.

The infrared signal of the heat of the molten pool emanates from the molten pool and penetrates the upper plastic material before it can be detected by the thermometer. Take nylon materials, such as PA66 and PA66GF30. The infrared penetration rate diagram is shown in Figure 3:

Through the spectrum, we found that the infrared penetration rate of ordinary plastic materials (except amorphous materials) above the 2um band is very low. As we said before, the strongest infrared signal band is above 2um. Therefore, the infrared signal emitted by the molten pool is difficult to process.

Therefore, before using a thermometer as a monitoring object for plastic laser welding, it is necessary to verify whether the method is feasible in the laboratory. If the monitoring band of the thermometer itself is set improperly or the infrared signal is weak, the thermometer cannot be used as a monitoring object for laser plastic welding applications.