Laser Welding of Thermoplastics: Overview of Lasers, Materials, Processes and Quality

Plastic welding is a process used in the production of many plastic devices to join different parts into one component. The surfaces to be joined are melted and the resulting interfacial molten layer solidifies under pressure. There are many technologies for welding thermoplastics, namely: hot tool welding, where the joining surfaces are heated by direct contact with hot metal plates, ultrasonic and linear vibration welding, where heating is done by vibration and friction, resistance implant welding and dielectric welding, where the heat is generated by an electromagnetic heat source and laser welding, where a laser beam is used to melt the thermoplastic joint area. Laser welding has many advantages over other welding methods. It requires a small amount of heat to be applied to a limited area and shows higher joint strength and higher quality welds. In addition, it can weld complex shapes. Other advantages include the absence of contact between the parts and the welding tool and the absence of mechanical stress on the parts. Laser welding, on the other hand, is more sensitive to polymer materials, processing history, pigmentation, and additives. The technology has unique processing and performance characteristics that enable localized precision welding of films, sheets, or molded parts. Other advantages include the absence of contact between the part and the welding tool and the absence of mechanical stress on the part. Laser welding, on the other hand, is more sensitive to polymer materials, processing history, pigmentation, and additives. The technology has unique processing and performance characteristics that enable localized precision welding of films, sheets, or molded parts. Other advantages include the absence of contact between the part and the welding tool and the absence of mechanical stress on the part. Laser welding, on the other hand, is more sensitive to polymer materials, processing history, pigmentation, and additives. The technology has unique processing and performance characteristics that enable localized precision welding of films, sheets, or molded parts.

Generally speaking, the laser welding process for thermoplastics has four common steps to position the parts to be welded. Depending on the technology to be used, the part transparent to the laser radiation must be facing the laser beam. In addition, the parts must be in close contact; (heating the interface area to be joined. The heating process may vary, depending on the geometry of the parts, the desired end product and the materials used.) The purpose of applying pressure is to promote welding by molecular chain diffusion and the formation of molecular entanglements at the interface. It can occur both during the heating phase and in the subsequent step. Cooling phase. During the cooling process, the interface has the necessary stability so that the parts can be handled without affecting the joint. Usually occurs during the cooling phase, the pressure is kept below the pressure value of step iii.

Laser welding technology is strongly influenced by the interaction of laser radiation with the material. The radiation that impacts the material surface can be reflected, transmitted and/or absorbed. However, to use this technology, the laser radiation must be effectively absorbed at the precise location where the weld must be formed. The absorbed light is locally converted into heat. The heat causes the polymer to melt, allowing chain segments to move, the polymer chains to mix with each other, and form entanglements. After cooling, it solidifies to form a weld. The interaction of the radiation with the material surface depends on the material type and additive content, as well as the laser characteristics (wavelength, power, spot size or shape and beam quality). Laser radiation from far-infrared (IR) laser sources (10600nm) is easily absorbed by the surface and is generally used for direct welding. On the other hand, most polymers exhibit high transmittance (i.e. low absorption) in the 400鈥?600nm range. The laser radiation of laser sources operating in this range is mainly transmitted by the polymer and can therefore only be used for laser transmission and welding. In this case, absorbers must be added to the parts to be welded. Since thermoplastics inherently absorb radiation at these wavelengths, the development of new lasers with longer wavelengths (1500鈥?200nm) has opened up the possibility of welding transparent thermoplastics without the use of additives (absorbing layers or reagents).

Laser welding finds more and more new applications, from the medical industry (assembly of various containers and liquid filters, connection of bags, pipes, etc.) [15], the electronics industry (assembly of mobile phones, keyboards, etc.) to the automotive industry (from multimedia panels to headlight housings), [16]. In summary, thermoplastic laser welding has become an independent component of laser technology with important scientific and industrial potential. The paper describes the welding methods of thermoplastic polymers for connecting sealed housings of medical devices. The purpose of this paper is to review some aspects of thermoplastic laser welding, focusing on laser systems, materials, process parameters and quality monitoring technologies. The paper first introduces the basics of welding (Part I), emphasizes the main aspects and advantages relative to other welding methods, and then focuses on various aspects of laser welding. Section II first discusses the various laser systems for thermoplastic laser welding. However, it is the most commonly used laser welding technology today. Some technical variants of this technology, such as the laser welding process using filler materials, are also discussed. In the next section, the LTW technology without additives will be discussed, which was made possible by the advent of lasers emitting in the 1.5鈥?200 nm range. Section 5 discusses the most commonly used materials in thermoplastic laser welding and several examples of them are reviewed and discussed. The first section introduces laser welding of parts of the same material, followed by laser welding of parts of different materials. Then in section 6, various variants or techniques such as contour welding, simultaneous welding, quasi-simultaneous welding and mask welding are discussed. Section 7 discusses the main control variables or process parameters and their influence, including the influence of polymer composition and pre-weld conditions (thickness) on the process. Finally, section 8 discusses several characterization techniques used to evaluate weld quality.

Laser Welding Thermoplastics

The first demonstrations of laser welding appeared in the 1970s, particularly for welding steel sheets or stainless steel [8], [9]. However, it was not until the 1990s that it became a widely used welding technology in a variety of applications, due to improvements in laser sources and methods. Currently, there are four main types of lasers used for welding thermoplastics. Table 1 describes their wavelength, efficiency, and beam quality. Figure 2 shows their emission wavelengths.

Laser Transmission Welding

The technology is based on the fact that unpigmented polymers transmit NIR wavelength radiation, but by adding absorbers they can absorb the laser radiation, thus achieving localized heating and melting of the polymer. NIR lasers (Nd:YAG and diode lasers) can be used in a different way for laser welding compared to far infrared CO2 laser wavelength technology.

Absorber-free welding technology

The most commonly used process in laser welding depends on the use of standard materials. The upper part - the transparent part - should be transparent for the laser radiation, and the lower part - the absorbing part - should absorb the laser radiation. Usually, the lower part must include additives, pigments or dyes to increase its absorption. The most commonly used absorber is CB, which turns the part black. This combination obviously limits the design possibilities.

Laser Welding Materials

Laser welding is a widely used technology method for producing high-quality welded joints of thermoplastic materials. The most common is similar to thermoplastic welding, where both parts to be welded are the same material. Due to the same melting point and chemical structure, better mixing of polymer chains is provided during heating, which facilitates the welding process. Laser welding of similar thermoplastics often shows better results in weld quality.

Plastic Laser Welding Process

There are many methods to choose from for laser welding, which differ in the relative movement of the laser beam and the welded parts. These methods can be divided into five main processes: contour welding, synchronous welding, quasi-synchronous welding, mask welding and hybrid welding.

In contour welding, a robotic arm is usually used to control the laser movement. The laser radiation beam forms a single pass above the joint compared to the parts. Contour welding is very flexible and particularly suitable. Control Parameters for Plastic Laser Welding

Temperature, time and pressure are the three most critical process parameters for all plastic welding technologies. In laser welding, these parameters are controlled by laser power, welding speed, laser spot size, exposure time, laser working distance, clamping pressure and the type and concentration of laser absorbing additives (if any). The energy density in laser welding is determined by the laser power, the size of the laser spot at the joint and the irradiation time (simultaneous evaluation of material and joint characterization techniques Laser welding offers various possibilities for joining thermoplastics that are sensitive to thermal and mechanical stresses through the control accuracy of the local welding energy. In order to control the welding process, various techniques can be used to evaluate the weld quality. These range from simple observation of the weld joint using a magnifying glass to the use of more complex equipment such as thermal imaging.

Conclusion

2em;">In many applications, polymers are increasingly being used to replace metals or other materials, which requires the development of new technologies for joining thermoplastics. Many technologies have been developed over the years, but laser welding has achieved prominence and is therefore becoming an increasingly interesting area of research and development. Thermoplastic laser welding is a highly specific technology used to join plastic components in applications where high-speed welding is required