If the things you're bonding together are made from thermoplastic film, less than half a millimeter in thickness, you can join them by pressing a hot bar against the two layers. Dielectric welding works by using high-frequency radio waves to produce molecular oscillations to generate heat.
The energy is applied between two metal bars that act as pressure applicators, typically on polyvinyl chloride, to produce easy-opening products. The strength of the weld can be controlled by altering the material blends.
You can do induction welding by running a wire in the item you're producing and using a coil placed close to the product to radiate a high-frequency alternating electromagnetic field, with the heat produced depending on the frequency. With Infrared welding, a tungsten filament line heater or a ceramic plate provides the heat.
Components are melted when you bring them close to the heat source and push the parts together to form a weld. Vibration welding relies on friction: you push the surfaces together and then vibrate them to generate melting heat.
Then align the pieces and let them cool. This technique is particularly useful where the objects that are being put together are too large for ultrasonic welding Laser welding is hot new thing in plastic welding, and its applications are growing.
The laser wavelengths you normally use pass through most plastics in their natural state. First check to make sure the plastic is appropriate for laser welding.
Additives that affect how light is transmitted through it like titanium dioxide and carbon fillers will make a difference. Laser energy can be delivered via a fiber optic to allow flexibility and easy operations.
If neither of the items being joined is made of a light absorbing material, a laser absorber that absorbs infrared laser light needs to be painted or printed on.
A solvent can temporarily dissolve the plastic at room temperature, freeing the polymer chains to move and entangle with dissolved chains in the other component. After a while the solvent evaporates and the plastics resolidify, leaving a mass of entangled polymer chains---a solvent weld.
So a PVC pipe can be joined to a PVC fitting using a solvent cement. The cement typically contains a solvent matched to the type of plastic.
You can rely on solvent welding for piping systems and for assembling toys. However, laws aimed at minimizing the use of organic solvents are likely to limit the future use of solvent welding.