Ultrasound, vibration and rotation: hot-tool-free welding

Common features of the methods

In pipe welding, heat is applied externally using a heating plate or a hot-air nozzle. Ultrasound, vibration and rotation do the opposite: energy is applied mechanically, and heat is generated at the interface between the two workpieces because the material is moved against each other under pressure. Melting therefore only occurs where it is needed.

This results in short cycles, minimal thermal impact on the surroundings and the ability to weld workpieces that cannot be reached with a tool. In return, the methods require stationary machines, precisely matched workpieces and geometries designed specifically for that method. They are therefore primarily used in series production and for injection-moulded parts, not for pipes in the field.

Ultrasonic welding

A sonotrode causes one workpiece to oscillate at typically 20-40 kHz with an amplitude of a few hundredths of a millimeter. The oscillations create frictional heat in a designed joint, often an energy director, which melts first and forms a seam between the workpieces.

The method is suitable for small and medium-sized workpieces with a well-defined joint: packaging, housing parts, fittings and assemblies where no heat marks must appear on the outside. It is also used to weld foil and to insert metal inserts into plastics.

The requirements are a joint designed for the method, uniform material and support so that the workpiece does not move. Materials with high melt viscosity and soft materials such as soft PP and PE require more energy, and fibre-reinforced materials can be difficult.

Vibration welding

In vibration welding, one workpiece is moved back and forth relative to the other with a frequency typically around 100-240 Hz and an amplitude of less than a couple of millimeters. Friction melts the surface, and the movement is stopped while the workpieces are held together under pressure until the seam has solidified.

The method is used for larger workpieces with flat joints, e.g. containers, ducts and automotive parts, where ultrasound cannot cover the entire joint. It requires that the workpieces can be clamped rigidly and that the joint can withstand the relative movement.

Spin welding

Spin welding is the oldest of the three: one workpiece rotates against the other under pressure until the frictional heat melts the surface, after which rotation is braked and the workpieces are held together under pressure while the seam solidifies.

The method requires round, axisymmetric workpieces, e.g. floats, containers and cylindrical parts, and it is fast and reproducible for large production runs. The peripheral speed for PE is typically in the range 100-300 m/min, but the specific value is determined by the workpiece geometry and machine.

The method is not used for pipes and sheets in the field. Butt welding, socket fusion / socket-spigot welding and extrusion welding are the correct choices; see the welding methods overview.

If you are working with PVC-U, other conditions apply: lower heating element temperature, short changeover time and extraction. Read how in butt welding of PVC.

When the method is the right choice

Choose ultrasound when the workpiece is small, the joint can be designed and cycle time matters. Choose vibration when the joint is long and flat, and the workpiece can be clamped. Choose rotation when the workpiece is round and is to be produced in large quantities.

Common to all three is that errors most frequently originate in the design and not in the machine. Insufficient support, incorrect joint geometry or excessive tolerances result in weak joints, no matter how precisely the machine operates. Therefore, always test on workpieces from current production and document with tensile or bend testing in accordance with DVS 2203.