In plastic ultrasonic welding, the molecules form new cross links
Ultrasonics activates the plastic molecules – friction heat is generated

Welding of plastics using ultrasonics

Ultrasonic welding of plastic materials creates a molecular bond within the weld seam. Longitudinal vibrations in frequencies of 20 kHz, 30 kHz, and 35 kHz with tool amplitudes between 5 µm and 50 µm are introduced into the plastic materials under the action of force. Ultrasonic vibrations are focused by means of special design of the components or tools. This specific geometry within a component is referred to as energy director. The contact points in the joining area are actively molded by means of energy input to achieve elastic deformation. Friction occurring between the contact surfaces and within the molecule chains generates heat that causes the material to melt.

Ultrasonic welding is particularly suitable for:

  • Joining of injection-molded parts
  • Embedding of membranes
  • Embedding of leather, nonwoven materials, and textiles
  • Staking of different types of materials
  • Production of form-fit joints by swaging
  • Inserting of bushes and magnets

Plastic staking has been successfully used in industrial manufacturing for many years. Staking produces permanent joints with hardly any restrictions on material combinations and enables short process cycles, high efficiency, and great freedom of design and technical process planning.
Often alternative plastic joining methods, such as clipping, snapping or gluing do not work for reasons of material properties, operational safety, or efficiency.

Ultrasonic swaging is a closely related method and only differs from staking in that swaging does not use round or longitudinal shafts for processing but melts and reforms a ridge of plastic (either straight or rounded) around the part to be fastened.



  • Ultrasonic energy director ERG
    Energy director
  • Plastic melt
    Melt encapsulation

The following variants are feasible:

Welding of molded components with joint design / Focusing of energy through the weld geometry (= energy director)
To be applied for all injection-molded parts; this process is referred to as molecular bonding. . Focusing of the ultrasonic energy is performed by so-called energy directors (ERG). Energy directors may be tips or edges that must be included in the part design. The melt is formed directly between the joining parts at the contact points of the energy directors.

  • Mash joint before welding
  • Mash joint after welding

Energy focusing with energy director, see example of mash joint.



Reforming using ultrasonics / Focusing of energy through the sonotrode
This is referred to as staking, spot welding or swaging. The melt develops through immediate contact between the weld surface of the sonotrode and the thermoplastic component. With this method it is also possible to join different types of materials, e.g. plastic-metal or glass-plastic, etc., by a form-fit bond.

  • Energy focusing through sonotrode
  • Energy focusing through sonotrode

Energy focusing with sonotrode, see example of staking




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Variants of joint design
for different requirements:

  • Step joint

    This type of joint design is relatively easy to implement in the injection-molding tool. When amorphous plastics are used, this joint design promotes production of visibly flawless, high-strength and air-tight welds. Additional advantages are that the step joint supports self-centering of components and absorption of increased shear and tensile forces.

  • Step joint
    Step joint
  • Tongue and groove joint

    The greatest strength is usually attained by using a tongue and groove joint. Gap dimensions with very small clearances create a capillary effect which causes the generated melt to penetrate through the entire joint area. This joint design requires relatively thick walls and is a fundamental recommendation, provided that all prerequisites are met.

  • Tongue and groove joint
    Tongue and groove joint
  • Mash joint

    The mash joint has proved to be successful for semi-crystalline plastics combined with thin walls. With large joining distances this joint design typically produces air-tight and high-strength welds.

  • Mash joint
    Mash joint
  • Double V joint

    This joint is recommended for components with a wall thickness of less than 1.5 mm. Precise automatic centering of components and the resulting defined joint positioning contribute to high strength after welding. It should be noted that precise injection-molding and the correct adjustment of the joint are absolutely necessary.

  • Double V joint
    Double V joint
  • Staking

    Using ultrasonics for staking allows thermoplastic molded components to be quickly and cleanly joined with metallic or other non weldable materials.

    There is no need for other additional consumables. The heat resulting from the staking process can be dissipated by means of an air-cooled sonotrode. After the actual staking process, the system provides a pre-selected hold time so that the melt can fully solidify under static pressure. In this way, reset forces are blocked, which in turn ensures accurate and zero-clearance joints.





  • Staking
  • Spot welding

    The molded components that are to be welded lie planar on top of one another without prepared joint points and without energy director. The point of the sonotrode penetrates through the upper plate into the lower plate and so plasticizes the plastic in both components. The resulting melt partly collects in the joint and produces a local, spot weld.





  • Spot welding
    Spot welding
  • Swaging

    It is not always possible to mold components with the necessary staking pins. Swaging is a suitable alternative for these kinds of applications. The contact face of the sonotrode must be machined appropriately for the recasting swaging process. The sequence and cycle times of the process correspond to those of staking. Ultrasonic swaging allows for reforming of large formats and could include the entire circumference of the components.




  • Swaging