A plastic product may be made of multiple materials or components. To combine the components, mechanical fasteners, adhesives, and welding processes can be used. Among the three joining methods, the welding process has the best effect, and the welding forms are various, and different welding processes can be used according to different materials, sizes, and uses. To
Mechanical fasteners, adhesives and welding processes can all join the two engineering plastics. Mechanical fasteners can quickly connect the two components, but the leak-proof function of the joint is poor, and local stress can also cause the polymer material to separate.
Adhesives can provide good properties and form joints with excellent leak-proof function. However, adhesives are difficult to handle and slow to cure. At the same time, when the adhesive is used for bonding, the preparation of the seam and the cleanliness of the surface are required.
Welding can produce a well-bonded seam with mechanical properties close to that of the parent material. Plastic welding is limited to thermoplastic polymers because thermoplastic materials can be softened by heating. After the thermosetting polymer is hardened, it cannot be softened by heating. Compared with metals, thermoplastic polymers require less heat for welding.
Plastic welding processes can be divided into two categories:
A) Mechanical mobile welding process: ultrasonic welding, friction welding and vibration welding.
B) External heating welding process: hot plate welding, hot gas welding and implant welding.
Ultrasonic welding
Ultrasonic welding uses mechanical high-frequency vibration to form joints. The parts 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, and receive ultrasonic vibration with a frequency of 20-40KHz. Alternating high-frequency stress generates heat at the joint interface to form a high-quality weld. The tools used in this process are very expensive, so it is appropriate to use when the production volume is large.
This welding method is only suitable for small parts whose welding length does not exceed a few centimeters. Applications include welding valves and screening procedures for medical equipment, boxes, auto parts, vacuum cleaner housings, etc. on multi-head machines.
Friction welding
Thermoplastic friction welding (also called "spin welding") has the same principle as metal welding. In this welding process, one substrate is fixed and the other substrate is rotated at a controlled angular velocity. When the parts are pressed together, frictional heat causes the polymer to melt, and after cooling, a weld is formed. The main welding parameters include: rotation speed, friction pressure, forging pressure, welding time and melting length.
Friction welding can produce excellent welding quality, the welding process is simple, and the repeatability is strong. However, due to its simple process, it is only suitable for applications where at least one part is circular and does not require angular alignment.
Vibration welding
Vibration welding is also called linear friction welding. Two thermoplastic parts rub against each other under appropriate pressure, frequency, and amplitude until enough 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 a weld. Vibration welding is similar to spin welding, the difference is that the motion is linear motion instead of rotating motion. This welding method is very fast, the vibration frequency is generally 100 to 240 Hz, and the amplitude is 1 mm to 5 mm.
The main advantage of this welding process is that it can weld large and complex linear parts at high speed. Other advantages include: the ability to weld multiple parts at the same time, the welding tools are simple, and almost all thermoplastic materials including injection molded parts, extruded parts, blow molded parts, thermoformed parts, foamed parts and stamped parts can be welded, mainly used in automobiles and Household appliances industry.
