The welding equipment does not need to be in contact with the bonded plastic parts.
Soon.
The equipment has a high degree of automation and is convenient for processing complex plastic parts.
There will be no flash.
Welding is firm.
High-precision weldments can be obtained.
Vibration-free technology.
An airtight or vacuum seal structure can be produced.
Minimize thermal damage and thermal deformation.
Resins of different compositions or colors can be bonded together. The advantages of laser welding in plastic parts welding include: accurate and firm welding, airtight and watertight, less resin degradation during welding, less debris generated, and the product surface can be closely connected around the weld. Laser welding has the advantage of no residue, which makes it more suitable for medical products and electronic sensors controlled by the US Food and Drug Administration.
It is easy to control and can weld workpieces with small size or complex shape and structure. Because laser is easily controlled by computer software, the output of fiber laser can reach all the fine parts flexibly, so laser welding can be used to weld areas that are difficult to reach by other welding methods, and products with complex shapes or even three-dimensional geometric shapes can be welded.
Compared with other welding methods, laser welding greatly reduces the vibration stress and thermal stress of products. This means that the aging speed of internal components of products or devices is slow, which can be applied to easily damaged products. Many different materials can be welded. For example, polycarbonate transmitted by near infrared laser and black polybutylene terephthalate reinforced by glass fiber can be connected together, but it is impossible to connect two polymers with such different structures, softening points and reinforcing materials by other welding methods. The most commonly used form of laser welding is called laser transmission welding. The process of this technology is as follows: firstly, two plastic parts to be welded are clamped together, and then the laser beam in the short-wave infrared region is aimed at the parts to be bonded. The laser beam passes through the upper transparent material and then is absorbed by the lower material. The absorption of laser energy makes the temperature of the lower material rise, melting the upper and lower plastics. The material of the vamp can be transparent or colored, but it must ensure enough laser to pass through.
Figure 1: schematic diagram of laser transmission welding.
In the past, because two transparent plastic layers could not absorb enough laser energy, they could not be welded together by transmission technology. Similarly, because the light beam does not have enough penetration ability to heat the welding contact surface, it is impossible to weld the materials of two black layers together by transmission technology. However, recent technological progress has made it possible to weld these two materials together. Laser transmission welding technology mainly uses two types of laser equipment: one is Nd3+:YAG crystal, and the other is semiconductor diode. The wavelength of Nd3+:YAG laser is 1064 nanometer (nm), which is easily absorbed by plastics containing special fillers or pigments. Laser can be easily transmitted to the laser head through optical fiber, especially in welding technology using automatic equipment.
The wavelength range generated by diode laser is between 800- 1000nm, which is the most effective energy region for welding. It is compact in structure and easy to install on automation equipment. The absorption characteristics of diode laser are similar to those of Nd3+:YAG.
Plastic welding sometimes uses carbon dioxide (CO2) laser. It can generate 10600nm light wave, which is more easily absorbed by plastics than Nd3+:YAG and diode laser. However, the penetration performance of carbon dioxide laser is not as good as the other two lasers. Therefore, carbon dioxide laser is mainly used for welding thin film materials.
Laser CO2 Nd3+:YAG diode
Wavelength (micron)10.61.060.8-1.0
Maximum energy (w) 60,000 6,000 6,000
Efficiency 10% 3% 30%
Mirror reflection optical fib, mirror optical fiber and mirror for transmit light beam
Minimum spot size (mm) 0.2-0.7 (diameter) 0. 1-0.5 (diameter) 0.5x0.5
Table 1: comparison of common plastic laser processing technologies in the market
Using Nd:YAG or diode laser transmission welding technology, plastic parts with a thickness greater than 1mm can be welded together at a linear speed greater than 20 m/min. The speed of carbon dioxide laser welding film can be as high as 750 m/min. Almost all thermoplastics and thermoplastic elastomers can use laser welding technology. Commonly used welding materials are PP, PS, PC, ABS, polyamide, PMMA, polyformaldehyde, PET and PBT. However, some other engineering plastics, such as PPS and liquid crystal polymer, are not suitable for laser welding technology because of their low laser transmittance. Therefore, carbon black is often added to the bottom material to make it absorb enough energy to meet the requirements of laser transmission welding.
Figure 2: Polymer for laser welding
Unfilled or glass fiber reinforced polymer materials can be used for laser welding. However, if the glass fiber content is too high, the infrared laser will be scattered and the penetration of the light beam through the polymer will be reduced. Colored plastics can also be used for laser welding, but with the increase of pigment or dye content, the penetration ability of laser beam to plastics will decrease. There are several different welding methods for plastic laser welding.
Contour welding: the laser moves along the contour of the plastic welding layer and melts, so that the plastic layers are gradually bonded together; Or moving the interlayer along a fixed laser beam to achieve the purpose of welding.
Simultaneous weld: laser beam from multiple diodes point to that contour line along the welding layer and melt the plastic, so that the whole contour line is simultaneously melted and combined.
Quasi-synchronous welding: this technology combines the above two welding technologies. The high-speed laser beam (at least 10 m/s) is generated by the reflector and moves along the part to be welded, so that the whole welding part gradually heats up and fuses together.
Mask welding: the laser beam locates, melts and bonds the plastic through the template, only exposing the small and accurate welding part of the plastic layer below. Using this technology, high-precision welding as low as 10 micron can be realized.
Figure 3: Sequential girth welding, synchronous welding and quasi-synchronous welding technology (from left to right)
GLOBO Welding is welding along the product contour, which is a patented technology of Leister company in Switzerland. The laser beam is focused on the welding interface point by point through an air cushion glass ball that can roll freely without friction. The glass ball can not only focus, but also act as a mechanical fixture. When the ball rolls on the surface, it provides continuous pressure on the joint surface. This ensures that there is pressure clamping when the laser heats the material. The glass ball replaces the mechanical fixture, which expands the application range of laser welding in continuous three-dimensional welding. In automobile industry, laser welding plastic technology can be used to manufacture many automobile parts, such as fuel injector, gear rack, engine sensor, cab rack, hydraulic oil tank, filter rack, headlight and taillight. Other automotive applications include the manufacture of intake manifolds and auxiliary water pumps.
Fig. 4: car headlights processed by laser welding technology adopts glass spherical surface, which can not only focus the laser, but also play the role of clamping tool.
In the medical field, laser welding technology can be used to manufacture liquid storage tanks, liquid filtration equipment, hose connectors, ostomy bags, hearing AIDS, implants, microfluidic devices for analysis and so on.
Fig. 5: Microfluidic device manufactured by laser welding technology, which takes advantage of the high precision of this technology.
Laser welding is a vibration-free technology, so it is especially suitable for processing precision electronic components. The equipment manufactured by laser technology includes mouse, mobile phone, connecting equipment, etc. Automotive electronic products manufactured by laser technology include automatic door locks, keyless entry and exit devices and sensors.
Laser can also weld plastic films together, and the films move along the edges of the films to form a sealed structure of the package by bonding. The operation process can be completed quickly. According to the information of TWI, 100W CO2 laser was used to weld 100 m/min polyethylene film.
Fig. 6: Microscopic images of two polyethylene films welded by laser.