Plastic Welding explained in detail

General description

The ultrasonic welding technology for plastics is considered part of the melt welding processes. The process is based on a conversion of friction and vibration energy into heat. Using a welding tool called a sonotrode, the high-frequency vibrations generated by the ultrasonic system, in combination with pressure, are transferred to the parts to be joined, which leads to relative vibrations in the jointing zone. The micro melt resulting from the friction heat presents a vibration barrier, which causes vibration energy to be absorbed and converted into heat, which then causes the weld contour to melt in fractions of a second and thereby form a molecular connection. As a rule, the plastic parts to be welded have to have a weld contour that corresponds to the type of material and the requirements for the weld seam. When sealing with ultrasonic technology the required heat is exclusively generated within the thermoplastic sealing layer. Contours focus the energy input and result in very short sealing times. During ultrasonic sealing, the heat is generated inside the foil. This means that no heat input from the outside is required. The tools used in the welding process remain cold. The heat energy input into the product is very small. The temperature exchange between the carrier and sealing layer takes place very rapidly. This results in very good hot seam strength. 

Longitudinal method

This is the most common ultrasonic method for welding plastic components. Generally the entire welding system is arranged vertically. The vibrations are applied vertically to the connecting pieces; the welding pressure is produced by a cylinder that pushes the entire system along the welding axis towards the upper connecting piece. 

Torsional method

Here the system is also generally arranged vertically, but this time the process is entirely different. This method is a type of high-frequency friction welding. The vibrations are applied tangentially: the sonotrode moves the upper connecting piece horizontally in relation to the lower connecting piece. The friction produces a melt between the two connecting pieces thanks to the high vibration frequency (20 KHz), amplitude, and pressure.
Because of the tangential motion of the upper connecting piece, the lower connecting piece is placed under virtually no strain by the ultrasound. This method is therefore particularly suitable for applications where additional vibrations in the direct vicinity of the ultrasonic weld are undesirable due to a risk of damage, e.g. sensitive parts, foils, fabrics, thin die-cast components (die-injected springs; fine meshes), electronics. Under certain circumstances this method can be used for joining different plastics together.

Application areas

Ultrasonic plastic welding is suitable for virtually all thermoplastics. When planning welds between semi-crystalline plastic pieces of work, it is important to remember that the weld behaviour of these plastics differs considerably from that of amorphous plastics. The longitudinal and torsional methods produce very different welds.

Ultrasonic plastic welding can be used to a limited extent for joining together two different thermoplastics (amorphous plastics). Ultrasonic welding is not suitable for use with thermosetting plastics.
Today ultrasonic welding is used in every industry that processes plastics.

 Customer benefit 

  • Ideal process for individual applications (longitudinal or torsional process) 
  • Innovation "Torsional welding" 
  • High process speed 
  • High and consistent quality through on-going process control 
  • Reliable connection 
  • Leak-proof connections even with leftover products 





































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TELSONIC Ultrasonics Inc.

TELSONIC Solutions Inc.