Plastic welding is a process used in the production of many plastic devices to join different parts into one. The surfaces to be joined melt and the resulting interfacial melted layer solidifies under pressure. There are many techniques for welding thermoplastics, namely: hot tool welding, which heats the joint surface by direct contact with a hot metal plate, ultrasonic and linear vibration welding, which heats by vibration and friction, resistive implant welding and dielectric welding, where heat is provided by electromagnetic heat sources 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 the application of a small amount of heat in a limited area and exhibits greater joint strength and higher quality welds. Additionally, it can weld complex shapes. Other advantages include no contact between the part and the welding tool and no mechanical stress on the part. Laser welding, on the other hand, is more sensitive to polymer materials, processing history, pigmentation and additives. This technology has unique processing and performance characteristics that enable localized precision welding of films, sheets or molded parts. Other advantages include no contact between the part and the welding tool and no mechanical stress on the part. Laser welding, on the other hand, is more sensitive to polymer materials, processing history, pigmentation and additives. This technology has unique processing and performance characteristics that enable localized precision welding of films, sheets or molded parts. Other advantages include no contact between the part and the welding tool and no mechanical stress on the part. Laser welding, on the other hand, is more sensitive to polymer materials, processing history, pigmentation and additives. This 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, parts transparent to laser radiation must face the laser beam. Additionally, the parts must be in close contact; (Heat the interface area to be joined. The heating process may vary depending on the geometry of the part, the desired end product and the materials used.) The purpose of applying pressure is to pass the molecules Chain diffusion and the formation of molecular entanglements at the interface facilitate welding. It can occur both during the heating phase and subsequent steps. cooling stage. During the cooling process, the joints have the necessary stability so the parts can be handled without affecting the joints. This usually occurs during the cooling phase, where the pressure is maintained below the pressure value of step iii.

        
Laser welding technology is strongly affected by the interaction of laser radiation with materials. Radiation impinging on a material surface can be reflected, transmitted, and/or absorbed. However, using this technology, the laser radiation must be efficiently absorbed at the precise location where the weld must be formed. The absorbed light is converted locally into heat. The heat causes the polymer to melt, allowing the chain segments to move and the polymer chains to mix with each other, forming tangles. After cooling, it solidifies to form a weld. The interaction of 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 surfaces and is often used for direct welding. On the other hand, most polymers exhibit high transmittance (i.e. low absorbance) in the range 400–1600 nm. The laser radiation of laser sources operating in this range is mainly transmitted by polymers and can therefore only be used for laser transmission and welding. In this case, the absorber must be added to the part to be welded. Since thermoplastics inherently absorb radiation at these wavelengths, the development of new lasers at longer wavelengths (1500–2200 nm) 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 bags, pipes, etc.) [15], the electronics industry (assembly of mobile phones, keyboards, etc.) to the automotive industry (from multimedia panel to headlight housing), [16]. In summary, laser welding of thermoplastic plastics has become an independent component of laser technology and has important scientific and industrial potential. The paper describes welding methods for joining thermoplastic polymers to sealed enclosures for medical devices. The purpose of this article is to review some aspects of thermoplastic laser welding, focusing mainly on laser systems, materials, process parameters and quality monitoring techniques. This article first introduces the basics of welding (Part 1), highlights the main aspects and advantages related to other welding methods, and then focuses on various aspects of laser welding. Section 2 first discusses various laser systems for thermoplastic laser welding. However, this is the most commonly used laser welding technology today. Some technical variations of this technology, such as laser welding processes using filler materials, are also discussed. In the following section, additive-free LTW technology, made possible by the advent of lasers emitting in the 15,000- to 2200-nm range, will be discussed. Section 5 discusses the most commonly used materials in thermoplastic laser welding and reviews and discusses several examples of them. The first part introduces the laser welding of parts of the same material, and then the laser welding of parts of different materials. Then various variations or techniques such as contour welding, simultaneous welding, quasi-simultaneous welding and mask welding are discussed in Section VI. Section 7 discusses the main control variables or process parameters and their effects, including the effect of polymer composition and pre-welding conditions (thickness) on the process. Finally, Section 8 discusses several characterization techniques for evaluating welding quality.
         
Lasers for welding thermoplastics

The first demonstrations of laser welding appeared in the 1970s, specifically for welding steel plates or stainless steel [8], [9]. However, due to improvements in laser sources and methods, it did not become a widely used welding technology in a variety of applications until the 1990s. Currently, there are four main laser types for welding thermoplastic materials. 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 can transmit NIR wavelength radiation, but by adding absorbers they can absorb laser radiation, allowing localized heating and melting of the polymer. NIR lasers (Nd:YAG and diode lasers) can be used in laser welding in different ways compared to far-infrared CO2 laser wavelength technologies.


Absorber-free welding technology

The most common process in laser welding relies on the use of standard materials. The upper part - the transparent part - should be transparent to the laser radiation, the lower part - the absorbing part - should absorb the laser radiation. Often, the lower part must include additives, pigments, or dyes to increase its absorbency. The most commonly used absorbent is CB, which turns parts black. This combination obviously limits the design possibilities.

Laser welding materials

Laser welding is a widely used technical 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, it provides better mixing of the polymer chains during heating, benefiting the welding process. Laser welding of similar thermoplastics often shows better results in terms of weld quality.

Plastic laser welding process

There are several methods available for laser welding that differ in the relative motion of the laser beam and the welded part. These methods can be divided into five main processes: contour welding, simultaneous welding, quasi-synchronous welding, mask welding and hybrid welding.

In contour welding, robotic arms are often used to control laser movement. Compared to the part, the laser radiation beam forms a single channel over the joint. Contour welding is very flexible and particularly suitable. Control parameters for laser welding of plastics

For all plastic welding technologies, temperature, time and pressure are the three most critical process parameters. In laser welding, these parameters are controlled by laser power, welding speed, laser spot size, irradiation 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 laser spot size at the joint and the irradiation time (simultaneously evaluating the characterization technology of the material and the joint). Laser welding provides a method for joining thermoplastics that are sensitive to thermal and mechanical stress through the control accuracy of local welding energy. Various possibilities are offered. To control the welding process, various techniques can be used to assess the weld quality, ranging from simply observing the welded joint using a magnifying glass to using more sophisticated equipment such as thermography.


In conclusion

Polymers are increasingly used to replace metals or other materials in many applications, requiring the development of new technologies for joining thermoplastics. Over the years, many technologies have been developed, but laser welding has achieved prominence and therefore it is increasingly becoming an area of interest for R&D. Laser welding of thermoplastics is a highly specialized technology used to join plastic parts in high-speed welding applications