Detailed explanation and trend analysis of laser welding process

Whether in the past or now, most plastic products need to be encapsulated after the assembly is completed. As a specialized industrial technology, plastic connection has a history of more than half a century. Initially, screw and adhesive connections were widely used, but these processes are complicated and time-consuming. With the continuous development of the injection molding process and the wider application of plastics in the automotive industry, economically feasible and reliable connection processes have also been further developed. Plastic welding technology was born under this environment. Faced with the huge pressure of increasing fixed costs, plastic welding technology has been widely used in the automotive industry and many other industries due to its huge potential to reduce costs.

　　Innovative connection process

　　 At present, all kinds of welding processes are already well known. The earliest plastic welding process is hot plate welding, followed by welding methods such as ultrasonic and vibration, and laser welding technology is the latest development. Since its development, joining processes including friction welding (vibration welding and spin welding), ultrasonic welding, microwave welding, high frequency welding, hot plate welding and laser transmission welding are widely used worldwide. At the same time, various processes are also competing for the market.

　　Each different welding method has its own unique advantages and disadvantages. Which process is used mainly depends on the characteristics of the material, the specific application and the different requirements for the welding results. Such as hot plate welding, the plastic component to be welded must touch the hot plate. Obviously, this process is not suitable for those types of plastics that are easily adhered to the heating plate. Vibration welding is only suitable for the mutual welding of planar devices, but the relative movement between the welded components can easily cause damage to the components, and this welding method often produces some plastic particles, so its impact on the environment must be considered in advance .

　　 In contrast, laser welding technology is not limited by the above process factors, and has both quality advantages. With the continuous decline of laser prices and the development needs of some new applications, advanced laser welding technology is being widely recognized. In the past 10 years, in some industrial applications, this new process can be seen everywhere. Now, no other plastic welding process can have such a broad application prospect like laser welding technology.

  Laser welding

　　At the beginning of this technology, not only the equipment occupies a large area, but the maintenance is even more cumbersome. With the emergence of advanced laser sources with smaller volumes and longer lifetimes, laser welding technology has undergone fundamental changes, which can well guarantee the application needs of industrialized mass production.

　　 Compared with other processes, laser welding technology is more cost-competitive, especially in the production of auto parts, its cost efficiency is more prominent. As a non-contact welding process, laser welding only applies vertical pressure at the welding point, which can minimize the mechanical stress on the product to ensure welding quality. In addition, there is no vibration during the welding process, so it will not damage the plastic shell and various internal devices.

　　 Now, laser technology has gradually changed from a laboratory application technology to an important process choice in plastic processing and production. At present, this process can be used for welding various electronic plastic shells, and this iconic application has greatly promoted the development of the laser plastic welding market. The main reason is that plastic shells containing electronic devices cannot Ultrasonic welding is used, and the cost of the screw and bonding method is higher.

Laser transmission transmission welding

　　 At present, laser welding technology generally adopts the welding method of laser transmission and transmission. In this process, the two plastic components to be welded have different laser transmission properties, one of which is transmission to a specific wavelength of laser light, and the other is capable of absorbing laser energy. Currently, lasers in the near-infrared band are mostly used.

　　The product needs to be pressed during the welding process. The easiest way is to use a glass plate. The laser passes through the clamping tool with almost no loss, penetrates the upper layer of plastic, and reaches the surface of the bottom layer of plastic. At this time, the laser is completely absorbed by the underlying plastic and converted into heat energy. At the same time, heat is transferred from the bottom plastic to the lower surface of the upper plastic, so that the two layers of plastic melt at the same time. When the soldered device cools down again, the plastic solidifies to form a strong solder joint.

　　 Depending on the scope of application, the welding method is different. According to the different movement modes of the laser beam above the welding spot, laser transmission welding includes several different methods such as synchronous welding, quasi-synchronous welding and contour welding. LPKF's hybrid welding is an improved patented contour welding technology.

　　 quasi-synchronous welding

　　 quasi-synchronous welding method is mainly used for products with small welding area. This process uses a laser beam to continuously sweep the welding contour line until the required welding collapse height is reached. During the welding process, the laser beam is driven by the scanning electron microscope (as shown in the figure), and the beam is precisely controlled in the two dimensions of the X axis and the Y axis. Because the scanning speed is very high, the welding material is melted almost at the same time, so it is called quasi-synchronous welding. The collapse height can be monitored while welding to ensure the high quality of the welding.

Quasi-synchronous (scanning) welding system integrated in the production line

The process is very flexible, and scanning parameters can be programmed on demand. When you need to improve or replace the product, you only need to program, without the need to replace the lens or mask. Quasi-synchronous welding occupies a dominant position in the welding of devices with an area of 400mm×400mm, and the fixing fixture required for welding is relatively simple. However, this welding method cannot weld devices with too large area or focal depth (Z direction, that is, the third dimension) changes beyond the limit, and its main application field is the housing of sensors or electronic devices.

　　 contour welding

　　Different from the widely used quasi-synchronous welding and synchronous welding, contour welding uses an automatic position adjustment system (such as a robot) to guide the laser through the welding line once and introduce it into a movable lens that can automatically adjust the focus. More importantly, this welding technology can ensure the cleanliness of the welding, so there is no need for subsequent cleaning procedures.

　　This kind of process is very flexible, theoretically it can weld any shape device. Contour welding has almost no collapse height, and the weld seam can be precisely controlled to achieve a very small width, which satisfies the demanding requirements of the product in terms of optical performance. Generally, contour welding can be used for larger devices and applications that vary greatly in the third dimension, such as automotive lamps and solar panels. In addition, contour welding is also suitable for welding automotive engine compartment components, such as intake manifolds and fuel tanks, and welding transparent components, such as automotive sunroofs, headlights, taillights and speed dials made of PC (polycarbonate) The transparent panel and so on.

　　 Hybrid welding

The 　　 hybrid welding process can be regarded as an improved technique for ordinary contour welding. This technology was successfully developed by LPKF and Bavarian Laser Centre (BLZ) in 2005.

　　 Hybrid welding process does not rely solely on laser energy for welding, but combines laser with other polychromatic light sources. In the hybrid welding process, the second light source has two functions: the two light sources are focused on the same point, and the spot diameter is much larger than that of the laser. In this way, the temperature difference curve between the welding point and the surrounding environment becomes relatively flat. Compared with pure laser welding, the heat affected area is larger, and the temperature peak appears in the center of the beam (here is the area affected by the laser beam) ).

The 　　 halogen light source proved to be a good choice. Its second light source can also produce light in the near-infrared band, but unlike a single frequency laser, it produces polychromatic light with a wider spectrum. When the hybrid welding system is working, the material is slowly heated by the laser to melt, and then gradually cooled-these are the results of the halogen lamp heating. Since slow cooling can be achieved well, the hybrid welding technology can effectively prevent the generation of internal stress.

　　The continuous spectrum polychromatic light generated by the halogen light source hardly absorbs any laser light compared to the transparent upper layer, and most of the polychromatic light can be absorbed by the upper transparent plastic. Therefore, the transparent component does not need to conduct heat at the bottom layer as in ordinary laser welding, but directly absorbs halogen light energy to reach a similar temperature, thereby greatly increasing the window of laser welding.

This special welding process is suitable for welding automobile taillights, headlights, instrument panels and other parts with higher transparency on the upper layer. These parts usually use PMMA or PC materials as the upper plastic parts. Taking the production of car lights as an example, the use of composite welding technology can completely eliminate the "tempering" process. The system is also suitable for production requirements that require high production efficiency and low output (as shown in the figure). At the same time, the process can also be matched with a universal clamping tool, which significantly reduces production costs.

Typical application of hybrid welding system

　　Laser source

　　Laser welding requires the use of a high-power laser source in continuous output mode, and the wavelength is proven to be particularly suitable in the near-infrared band. Many unmodified engineering plastics are transparent to lasers in this frequency range.

　　 The laser source used at first is the ND: YAG solid-state laser source with a wavelength of 1 to 064 nm. Its laser characteristics basically meet the needs of plastic welding, but the corresponding maintenance cost is relatively high. With the rapid development of high-power diode lasers, solid-state lasers that are cumbersome to maintain have now been replaced. The efficiency of diode lasers exceeds 50%, but water cooling devices are required to reduce the heat generated at the same time. According to reports, this laser source has a life span of tens of thousands of hours and is basically maintenance-free, which well guarantees the actual needs of industrial production.

　　 Fiber laser, as a representative of the third-generation laser technology, has unparalleled technical advantages of other lasers. If the spot quality of the diode laser source cannot meet some special requirements, a fiber laser is a good choice. LPKF said that by using fiber laser technology, they can achieve a weld width of less than 100 m.

　　 process control

　　Avoiding defective products or rejects is as important as ensuring the reliability of good products. In many production-oriented enterprises, a quality management method called "0-ppm" is adopted, that is, the production of waste products must be completely avoided, and unqualified originals cannot flow into the production process. This requires manufacturers to strictly control the quality of plastic parts to prevent the production of bad originals, or the welding equipment can at least identify these defective products and eliminate them in time. This is because individual defective products may be caused by defects in raw materials or errors in the welding process, and these welding defects often lead to poor sealing or poor optical performance of the bonding area, which affects the quality of welding.

Generally, there are several different process control methods for laser welding. Among them, the quasi-synchronous welding technology adopts the method of detecting the welding joint path, that is, measuring and evaluating the collapse height of the welding device. Usually, the welding process stops when the required correct size is reached.

　　 Welding defects can also be detected by a high-temperature remote sensor. This method measures the heat radiation generated by welding to obtain the evaluation result of welding quality. During the inspection process, if any temperature abnormality occurs, the resulting welding defect can be found immediately.

　　 Development of new materials and additives

　　The development of new materials is also one of the important factors for the success of laser plastic welding technology. In the initial stage, only the black component with added carbon and the undyed corresponding component can be laser welded to meet the requirement that the laser energy is completely absorbed by the underlying plastic. As more and more new materials are developed, there are now a variety of color options suitable for laser welding, which can meet the welding requirements between components of any color. At the same time, the huge market potential of this technology is also being valued by more material developers.

　　The progress of flame retardant additives is also another main reason for the rapid development of laser welding technology. In the past, laser welding technology could not be used due to the need to use phosphorus-containing additives. With the successful development of halogen-free flame retardants, the above problems have been well resolved.

　　Future Trend

　　As a relatively novel joining process, laser welding technology has great potential for innovation. As the price of laser sources drops, it is foreseeable that the application of this technology will become more and more widespread. Now, ordinary consumer products are also joining the ranks of laser welding.

　　The automotive industry has always been the main market for laser welding technology, and the economic crisis has forced the industry to find and establish more cost-effective production and connection processes. At present, many examples have repeatedly proved that laser welding technology is more cost-effective than other methods. If this trend continues, advanced laser welding technology will accelerate its popularization and development.

　　 The medical technology industry has also shown a strong demand for laser welding technology. This application field has strict requirements on the high cleanliness of the process, and laser welding technology can well meet these requirements. Compared with other commonly used connection technologies, laser welding technology does not generate slag and debris, and does not need to use any adhesives. The welding work can be completed in a clean room, for example, welding balls used for cardiac interventional treatment catheters. Capsules or similar applications strongly illustrate this problem.

Manufacturing microfluidic chips requires a high-precision welding process. Generally, the micro runner cannot be realized by other welding methods, or only costly other non-welding processes can be used. The use of lasers to weld the edges of the micro-runners in parallel enables the runners to be formed while welding.

　　 In addition, laser welding technology can also be used to weld larger devices, such as car taillights or similar commonly used large plastic parts. Even the welding of large products such as TV casings and washing machines is no longer a problem.