What are the process requirements for plastic laser welding?

1. Laser wavelength

Laser welding technology generally uses metal material YAG or CO2 laser as the light source, and plastic welding is no exception. With the rapid development of the semiconductor material industry, semiconductor lasers are gradually being used as light sources.

Among them, the first two are widely used in traditional material processing industries because they are easy to obtain high power; since plastic laser welding has low requirements for light source power but high requirements for controllability and operability, semiconductor lasers are also very useful in plastic welding.

The typical values of the wavelength, maximum power, minimum focus diameter and other parameters of the three light sources of CO2, Nd:YAG and semiconductor lasers are as follows:

1. CO2 laser: The wavelength is longer, 10.6 microns belongs to the far infrared band. Generally speaking, plastic materials have good absorption of this wavelength. The current maximum output power is 50. kW, the conversion efficiency is about 10%, and the minimum focus diameter is about 0.2~0.7mm. The depth of the heat action area is deeper, which is suitable for welding thicker plastic materials. CO laser cannot be transmitted by optical fiber, it can only be transmitted by optical fiber$&* Rigid transmission optical path is composed of lens reflectors, which affects the operability of the laser head.

2. Nd:YAG laser: short wavelength, 1.06 microns is a near-infrared wavelength, not easily absorbed by plastic. Maximum output power is 6kW, conversion efficiency is 3%, and minimum focus diameter is 0.1~0.5mm. Nd:YAG laser has the characteristics of small focusing area and convenient optical fiber transmission. The laser head can be installed on the robot arm to realize CNC and precision automatic welding process; on the other hand, the welding material can better absorb the lower welding material or the middle layer through the upper welding material to achieve welding.

3. Semiconductor laser: wavelength 0.8~1.0 micron, maximum output power 6kW, conversion efficiency 30%, minimum focal diameter 0.5mm. Due to its low output power, it is suitable for occasions with low welding laser power requirements, such as precision welding of small plastic devices. Semiconductor laser has high energy conversion efficiency and is easy to achieve miniaturization and portability of laser.
   

2. Plastic materials

Plastics that can be welded by laser are thermoplastics. In theory, all thermoplastics can be welded by laser.

Plastic laser welding technology has the following requirements for welding plastics: the material in the heat action area needs good laser light wave absorption; the material that does not belong to the heat action area needs good light wave permeability, especially in the superposition welding of two thin plastic parts. Generally speaking, adding absorbent to the plastic in the heat action area can achieve this goal. At present, the single-component plastics that use laser welding include: PMMA--polymethyl methacrylate (organic glass), PC plastic, ABS plastic, LDPE-low-density polyethylene plastic, HDPE-high-density polyethylene plastic, PVC-polyvinyl chloride plastic, Nylon6-nylon 6, Nylon66-nylon 66, PS-PS resin, etc. Plastic parts made of the above various plastics, such as molded plastic products, plastic plates, films, artificial rubber, fibers and even textiles can be used as welding objects. Since laser welding has a small heat effect zone that traditional welding does not have, the above monomer materials can also be welded.

2. Absorbent

The application of absorbent is a very important process in the process of plastic laser welding. As mentioned earlier, the essence of plastic laser welding is to melt the plastic to be welded in the heat action zone and then cool the plastic parts for natural connection. The plastic parts need to absorb enough laser energy to melt the plastic. Plastic itself can absorb laser energy at a high absorption rate, but generally no absorbent is added, the light wave absorption is not very good, the absorption efficiency is very low, and the melting efficiency is not ideal.

Generally speaking, the ideal absorbent is carbon black. Carbon black can basically absorb all laser energy in the infrared wavelength, thereby greatly improving the heat absorption effect of plastics and accelerating the melting of materials in the heat action area. The effect is better. Some dyes of other colors can also have the same light absorption effect.

The British Welding Society has developed a transparent visible light dye. Using this dye as an absorbent can obtain a transparent plastic weld. Carbon black not only absorbs laser light waves in the infrared band, but also absorbs visible light waves, which is why carbon black looks black. Using carbon black as an absorber will deepen the color of the laser weld, which is different from the color of the base material. TWI visible light transparent dyes only absorb electromagnetic waves in the infrared band, not visible light, so the weld still seems transparent.

In many cases, plastic welding requires a beautiful and delicate finished product, so dye absorbers that are transparent to visible light are very popular compared to carbon black.

There are three ways to add absorbent: one is to directly penetrate the absorbent into the material to be welded, place the plastic part permeated with the absorbent at the bottom, and place the plastic part without the absorbent at the top, so that the laser wave can pass through; the second is to penetrate the absorbent into the surface of the plastic part to be welded, so that part of the plastic penetrated by the absorbent becomes the heat action zone and melts; the third is to spray or print the absorbent at the contact point of the two plastic parts to be welded.

Fourth, other parameters

Unlike metal welding, the laser power required for plastic laser welding is not the higher the better. The higher the welding laser power, the larger and deeper the heat effect area on the plastic part, and the material will overheat, deform, and even be damaged. The laser power should be selected according to the required melting depth.

Plastic laser welding speed is relatively fast, generally the welding speed of 1mm thick weld can reach 20m/min; while the use of high-power CO2 laser to weld plastic film, the maximum speed can reach 750m/min.

V. Software

In the laser welding system, the role of computer software is the movement trajectory and speed of the laser head. Digitally control general process parameters such as laser power to improve processing speed and accuracy. The purpose of improving processing quality is no different from software control in traditional laser processing, but due to the special role of absorbents in plastic laser welding, the plastic laser welding control system and processing system have their own characteristics.

The input data provided for the software include

1. Plastic material properties: thickness, color;

2. Welding data: complexity of welding area shape, width, welding speed;

3. Laser characteristics: power:, infrared transmittance, etc.

4. Through calculation and screening, the output results given by the software include: absorber type. During the welding process, the energy loss of the laser light wave in the upper material.

5. The calculation results of the software are very close to the actual welding measurement results. Figure 6 shows the heat affected zone (HAZ) generated after welding. The calculated value of the size is compared with the actual measurement value. The plastic material used is PMMA.

6. It can be seen that the software calculation results are very close to the measurement results. Since plastic laser welding has strong regularity and good predictability, it is very effective and feasible to use software calculation and screening methods to predict the results.