Laser welding of the hottest transparent plastic m

2022-09-23
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Laser welding of transparent plastic materials

in many industrial applications, laser welding of thermoplastic components has become a standard processing method. When using diode laser, nd:yag laser or fiber laser, the standard operation is projection laser welding at the coincidence. The focused laser radiation penetrates the transparent plastic on the upper layer and is absorbed by the lower layer material. The absorbed radiation energy is converted into heat energy on the surface. Due to the conduction of heat energy, a contact is formed on the upper layer

primary or colored plastics have low absorption in the near-infrared band. Carbon black is a kind of resin additive, which can effectively improve the laser absorption of plastics in a wide range of wave bands (from the visible infrared). However, if carbon black is used, the plastic can only be made into dark color, and cannot be made into transparent plastic components

the clearweld process developed by the welding institute in Cambridge (TWI) enables transparent or colored plastics to effectively absorb near-infrared light. It uses a special near-infrared absorption material as the coating on the surface of the components, or as an additive added to the underlying resin. The absorption of these materials in the visible light range is smaller, and the absorption in the near-infrared region (nm) is larger than that of most products, which belong to measuring instruments. At present, near the maximum absorption wavelength, there are various absorption materials with different narrow absorption bandwidth, which can be used to adjust the optical properties of plastics in order to adapt to various common near-infrared lasers. In addition to depending on the laser wavelength used, the best absorbing material also depends on the requirements of the specific application, such as processing parameters, material characteristics and the color required by the target component

clearweld coating process

the coating with absorption range in nm absorbent is a low viscosity, solvent based liquid substance, which is used in various batching systems. Typical solvents are ethanol and acetone. The amount of coating is in nanoliter/square millimeter (nl/mm2). The solvent can be used as a carrier, volatilizing quickly, so as to form a layer of absorbent film on the plastic surface. Generally, the drying time is 1 to 7 seconds. Auxiliary drying methods can also be used, such as pre heating or post heating of parts with infrared light, so that the solvent volatilizes more quickly. The coating process can be carried out separately from the welding process

when the coating is applied to the surface of the material, a uniform thin layer of absorbent is deposited on the surface of the material. Before laser radiation, the dried coating has some color in the visible band. During welding, laser radiation is absorbed by the coating and converted into heat energy at the same time. Due to heat conduction, the surface material adjacent to the coating is heated and melted, and the solder joint is formed after curing. In the process of heating, the absorbent decomposes, and the coating completely loses the color of the visible band (see Figure 1)

additive

absorbent can also be used in many thermoplastic materials. It is added to the underlying plastic as an additive to assist the laser welding process. This process is similar to adding carbon black to materials, but the colors here are more diverse and can be used in transparent/opaque plastic parts

according to the different requirements of application, technicians can adjust the melting depth of materials by using clearweld absorption of titanium alloy, quick brittle, high and gold and other materials. This is achieved by changing the concentration of absorbent in the resin. The concentration of absorbent is directly related to the optical penetration depth. Increasing the concentration will reduce the penetration depth, thus reducing the melting depth

if a large molten pool is needed to correct the gap and geometric deviation, and the polymer plastic is not expected to be thermally degraded, the concentration of absorbent can be set lower. If the required molten pool size is small, for example, in the application of micro fluid equipment, the concentration of absorbent can be higher

specific hybrid resins containing clearweld additives are designed and used in laser welding in the nm band. These resins are made in the form of injection mold or extrusion mold. In the mixing process, their colors can be matched by colorants to obtain a specific formula to achieve specific application requirements. The formulation needs to consider many parameters, including polymer compatibility, color requirements, and the wavelength of welding laser

Figure 2 shows the yellow and orange transparent plastic samples welded by laser. Through successful color matching, the transparent upper layer and the absorbent lower layer may look exactly the same, and the welding process has also been optimized. However, sometimes the color requirements may determine the required laser wavelength and other parameters. The weldable resin and these additives can be mixed and extruded into a mold, injected into a mold, or extruded into a film. After being cut, the film can be used for the intermediate transition layer in laser welding

welded oil reservoir

Figure 3 shows the laser welded oil reservoir, which is made of transparent and colorless PMMA plastic. This part is made on the semi-automatic production line of Barkston plastic industry company. Before using laser welding, the product was connected by adhesive, and the components were cut by complex machinery and added 8 Function reproduction function: it is realized by the correction and surface polishing after the end of the experimental operation and storage. The top and bottom flat covers and the middle column are welded by a 940nm diode laser

clearweld coating is added to the edge of the column before welding. After drying, the components are assembled, clamped and placed on the laser welding table. The welding process is realized through a circular welding line, and the focused laser beam passes through the cover from the top to the edge of the column. In the whole process, the coating on the upper edge of the column is degraded, so that the bottom cover and the other edge of the column are also welded at the same time. This is because the laser is reflected inside the side wall of the column, so the laser beam is led to the other side. The fluid welding points obtained by laser welding are very close, with high strength and good optical shape

with clearweld coating, the top and bottom can be laser welded to the column at the same time. This makes the production time significantly reduced and the product quality significantly improved, because the product has vacuum sealing characteristics and the whole product is completely transparent. The production of oil reservoir can realize economical and small batch production, with 50 products in each batch

welding medical pipe fittings

in the cooperation with natvar company, we use additives to absorb laser energy and weld pipes required in medical applications (as shown in Figure 4). Usually, these products are achieved by UV bonding or solvent bonding. UV bonding usually takes seconds to cure. Solvent bonding is immediate, but a chemical must be added to produce the contact. Both UV bonding and solvent bonding methods need to contact the port surface (conical taper) in the whole process, and the length is usually up to 0 500 inches

these tubes can meet the requirements of casing by mixing and extruding the inner and outer layers of the tubes, and the outside of the tubes is a soft and touchable surface. The surface layer can be different plastic or thermoplastic elastomer materials, such as PVC, TPU, TPE, or cope. Additives are added to the outer layer of the pipe wall, so that the end parts at both ends of the pipe can be welded by laser. The tube and the port must be transparent and colorless in order to measure the liquid flowing through the tube. Through the pressing process, the port is fixed to the pipe. Using beam shaping, the laser welding process can form a ring contact, so as to weld synchronously. The pressing process does not require additional fixtures to fix. In this way, a sealed joint is formed at the end of the pipe by laser welding, which has no effect on the transparency of the element

compared with UV bonding and solvent bonding, the main advantage of laser welding is that it can form a mechanical connection without any other solvent or bonding agent. Compared with traditional methods, laser welding requires a length of 0.125 inch to form a mechanical connection. This length can be compared with the standard of common processes, and even better than the current common processes. In addition, the time required for laser welding is less than 0.5 seconds. First, UV bonding requires seconds of curing time. In some applications, different types of interfaces are required at both ends of the pipe (one end is bonded by solvent, and the other end is bonded by ultraviolet light). While laser welding is applicable to both ends, the solvent bonding process cannot be achieved

transparent thermoplastic polymer plastics and artificial rubber can also be laser welded, and near-infrared absorbing materials can be used to generate heat and local melting. This technology has been successfully applied in different occasions, which shows that clearweld coatings or additives can match the absorption characteristics of thermoplastic materials, so that these materials can realize laser welding in industry. Transparent and colorless welds can be obtained by laser welding process. The laser energy is only absorbed at the weld of the interface between the two elements. Other areas do not absorb any radiation, so there is no additional thermal stress on the workpiece

using additives, technicians can get high-quality contacts. Compared with coating, laser radiation can be absorbed deeper inside the material according to the different concentration of absorbent. In this way, a larger volume of molten material is formed, which is more beneficial to compensate for larger gaps or fill larger deviations. Additives are very suitable for injection molding of colored transparent/opaque base materials, which are used in electronic product shells, medical equipment, liquid containers, and other smaller components, or other components with large welding areas. By adjusting the welding parameters well, the appearance of the weld can be neat and the welding strength is high

laser welding technology provides engineers with clean and tidy optical colorless welding points, vacuum sealing characteristics, flexible design, contactless production, particle free and almost no flash processing process, and this technology does not need additional substances as adhesives. This technology has realized new and advanced plastic laser welding in the medical, electronic, automotive, textile, packaging and consumer goods markets. In addition, the additives and coatings used are biocompatible

the author R. Klein (rklein@) is from the German branch of gentex (gross umstadt). G. McGrath is from the UK branch of gentex Corporation (Cambridge). (end)

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