There are plastic welding techniques that use an extra plastic strip to help connect the two separate pieces, while other methods melt an edge of each plastic part and push them together. After the plastic has cured (dried), a permanent, watertight seal remains. Only thermoplastics, those that are able to melt with the application of heat, are used for welding. These include polyvinyl chloride, high and low density polyethylene, polystyrene, polypropylene, polycarbonate and acrylic.
Plastic welders usually use hot pressurized gas to melt the plastic together, but they also use ultrasonic vibration, lasers and friction. Many different parts are constructed by plastic welding, including tanks, containers like jugs and fuse-welded pipes. It is a similar process to metal welding, but it does not form as strong of a bond and is easier to accomplish. While the same type of plastics are usually welded together, different kinds can also bond together to form a single product.
There are five main methods of welding plastic parts together. Each technique uses a different approach but has similar results. The most popular and widely used is hot gas welding, where a welding gun releases hot, pressurized air and directs it on the pushed-together edges of the plastic parts.
Their molecular chains cross-link to form a single piece of plastic. While hot gas welding is highly effective, it takes large amounts of energy and is therefore inefficient. A plastic filler rod often aids this process. Ultrasonic welding, like its name suggests, generates heat with high frequency, low amplitude vibration, anywhere from 15 to 40 kHz.
It provides the fastest heat sealing and can go on for long periods of time. Friction/vibration welding takes the two separate pieces of plastic and rubs them together at a higher amplitude and lower frequency than ultrasonic welding. They are clamped together until cured. Hot plate welding is used to join two larger parts together, or those that have complex weld design.
Each plastic piece is attached to a platen of a press. A hot plate in the shape of the weld joint is moved over the two parts, melting the interfaces. The platens then press the pieces together and allow the joint to cool. Finally, laser welding uses an intense beam of light that moves along the joining line. This type of welding is for intricate joints, thin plastic pieces and for products that require the least visible seam possible.