Laser welding of plastics and its application

2022-06-15
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Plastic laser welding and its application

with the implementation of the concept of green environmental protection in global industrial production and the consideration of production cost control, plastic, as a renewable non-metallic material with excellent performance, is increasingly widely used in the design and manufacture of parts in various industries. Traditional metal parts are increasingly replaced by plastic parts with the same working performance, At the same time, higher requirements are put forward for the welding connection technology and welding quality between plastic parts. These changes provide an opportunity for the application of laser welding technology in the field of plastic materials

the common method of traditional plastic welding

ultrasonic welding

ultrasonic welding forms joints through high-frequency mechanical vibration. The components to be assembled are clamped between the oscillating welding head and the fixed welding head under pressure, and then at right angles to the contact surface, receiving ultrasonic vibration with a frequency of 20-40khz. Alternating high-frequency stress generates heat at the joint interface to form high-quality welding

the tools used in this process are very expensive, so they are suitable for large production. Applications include valves and screening procedures for welding medical equipment on multi head machines, boxes, automotive parts, vacuum cleaner housings, etc

friction welding

thermoplastic friction welding (also known as "rotary welding") is based on the same principle as metal welding. In this welding process, one substrate is fixed and the other substrate rotates at a controlled angular speed. When the parts are pressed together, the friction heat causes the polymer to melt, and welding is formed after cooling. Friction welding can produce excellent welding quality, simple welding process and strong repeatability. It is only suitable for applications where at least one part is circular and does not need angular alignment

vibration welding

vibration welding is also called linear friction welding. The two thermoplastic parts rub against each other under appropriate pressure, frequency and amplitude until sufficient heat is generated to melt the polymer. After the vibration stops, the parts are aligned with each other, and the molten polymer solidifies to form welding

the main advantage of this welding process is that it can weld large and complex linear parts at high speed. Other strengths include the ability to weld multiple parts at the same time, simple welding tools, and the ability to weld almost all thermoplastic materials. It is mainly used in the automotive and household appliance industries

hot plate welding

for plastic bonding, hot plate welding is the simplest mass production technology. The high-temperature hot plate is clamped between the surfaces to be jointed until it softens. At this time, the hot plate is pulled out, and the two surfaces are bonded under controlled pressure, which is kept together for a specific period of time. The molten surface is then allowed to cool to form a weld. The welding tool or heating component is equipped with a built-in electric heater to prevent plastic from adhering to the welding tool

this welding process is used for many daily necessities, such as vacuum cleaner shell, washing machine and dishwasher parts, brake fluid tank, rear lamp, indicator lamp and other automobile parts. The disadvantage of hot plate welding method is that the welding speed is slow

hot gas welding

hot gas welding uses a heated air stream (usually air) to heat and melt thermoplastic substrates and thermoplastic electrodes. The base material and welding rod are melted to form a weld. In order to ensure effective welding, appropriate temperature and pressure must be applied to the electrode, and appropriate welding speed and torch position shall be ensured. The main applications include the maintenance of injection molded parts such as chemical storage containers, ventilation ducts and automobile bumpers. Nitrogen is used for oxygen sensitive materials, such as polyethylene; Oxygen forms a higher welding strength

the main advantage of this welding method is that it can weld large and complex parts, but the welding speed is slow, and the welding quality completely depends on the welder's skills

implant welding

in implant welding, first clamp the metal insert between the parts to be welded, and then heat it by induction or resistance. When resistance welding is used, wires are required to be placed along the seam to conduct the current into the implant; This method is not required for induction welding. Implant welding has been used to weld complex joints of large parts, including car bumpers, electric vehicles and yacht hull

Plastic laser welding

principle of plastic laser welding

the most commonly used form of laser welding is called laser transmission welding. Firstly, two plastic parts to be welded are clamped together under pressure, and then a beam of laser in the short wave infrared region is directed to the part to be bonded

the laser beam passes through the upper transparent material and is then absorbed by the lower material. The laser energy is absorbed and converted into heat energy, because the two layers of materials are pressed together. The heat energy is transmitted from the absorption layer to the light transmitting layer, so that the two layers of materials melt and combine

at the same time, the internal pressure is generated due to the thermal expansion of the material itself. The combination of internal pressure and external pressure ensures the firm welding of the two parts. Figure 1 and Figure 2 intuitively illustrate the principle and process of plastic laser welding

process of plastic laser welding

(1) wavelength of laser

diode laser or semiconductor laser is commonly used in the process of plastic welding. The beam is in the near-infrared region, and the beam wavelength is 400-1100 μ m. It can be transmitted through optical fiber, and the laser beam in this range can be absorbed by most plastics. The diode laser welding system is compact, and the laser can reach a higher level of power. The wavelength of laser can be designed according to special requirements. The wavelength of semiconductor laser is generally 808-980 μ m。 The semiconductor laser has the advantages of low investment cost, small volume and high starting rate of effective resin pouring

(2) plastic materials

thermoplastics include amorphous plastics and semi crystalline plastics. Plastics that can be laser welded belong to thermoplastics. In theory, all thermoplastics can be laser welded. The requirements of plastic laser welding technology for the welded plastic are as follows: the materials in the heat action zone are required to have good absorption of laser light wave; For materials that do not belong to the heat action zone, it is required to have good transmittance to light waves, especially when overlaying two thin plastic parts. Generally, the purpose can be achieved by adding absorbent to the plastic in the heat action zone

(3) absorbent

the application of absorbent is a very important process in plastic laser welding process. The essence of plastic laser welding is to melt the plastic to be welded in the heat action zone, and then cool it naturally to realize the bonding of plastic parts. Melting the plastic requires that the plastic parts absorb enough laser energy

generally, the ideal absorbent is carbon black, which can basically absorb all the laser energy of infrared wavelength, so as to greatly improve the heat absorption effect of plastics and make the materials in the heat action zone melt faster and better. Some dyes of other colors can also have the same effect of absorbing light waves

there are three methods to add absorbent: one is to directly infiltrate absorbent into the material to be welded. In this way, the plastic parts infiltrated with absorbent should be placed below, while the plastic parts without absorbent should be placed above to let the laser light wave pass through; Second, infiltrate absorbent into the surface of plastic parts to be welded, so that only part of the plastic infiltrated with absorbent will become a heat action zone and be melted; Third, spray or print absorbent on the contact of two plastic parts to be welded

(4) other parameters

different from metal welding, the laser power required for plastic laser welding is not the greater the better

the higher the welding laser power is, the larger and deeper the heat action zone on the plastic part will be, which will lead to overheating, deformation and even damage of the material. The laser power should be selected according to the required melting depth

the speed of plastic laser welding to improve the comprehensive competitive strength is relatively fast. Generally, the welding speed of 1mm thick weld can reach 20m/min; The highest speed of welding plastic film with high-power CO2 laser can reach 750m/min

Application of plastic laser welding

less resin degradation, less debris, non-contact welding process and no pollution. Compared with other connection methods, laser welding produces less mechanical and thermal stress. Therefore, it is especially suitable for precision electronic components such as mouse, mobile and connector parts, as well as complex components that need to be welded in a cleaner way, such as plastic products containing circuit boards. Automotive, medical, consumer electronics, food and other industries are the physical control systems of laser welding: mainly including the application fields of computer control

automotive industry: laser welding plastic technology can be used to manufacture many automotive parts, such as automatic door lock, keyless access equipment, fuel nozzle, gear change frame, engine sensor, cab frame, hydraulic oil tank, filter frame, headlights and taillights. Other automotive applications include the manufacture of intake manifold and auxiliary water pump

medical field: laser welding technology can be used to manufacture liquid storage tanks, liquid filtration equipment, hose connectors, ostomy bags, hearing aids, grafts, microfluidic devices for analysis, etc

packaging industry: for example, for the packaging of advanced industrial finished products, plastic film welding technology can be used to obtain plastic packaging with high processing speed, reliable joints and beautiful appearance. Laser welding connection of plastic outer packaging materials. The plastic materials are thermoplastic and artificial rubber

packaging of electronic devices: the packaging technology of electronic devices has very strict requirements for the welding environment and process, such as low dust, low heat, no strong vibration, high packaging accuracy, reliable and durable joints, beautiful appearance, etc. A large part of the packaging materials are plastic, so plastic laser welding also has a lot of application space

With the development of laser welding technology, the plastic welding technology will be more and more convenient and applicable, and the plastic welding technology will be more and more widely used

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