The 5 main features that you should know about the insertion of metal parts into thermoplastic materials

(metal-plastic joints)

Thermoplastic polymers often lack the mechanical strength necessary to assure the structural sturdiness of certain components. This is why many moulded products are strengthened with metal parts such as threaded inserts made of steel or brass.

As an alternative to the use of adhesives, in order to join a plastic component to a metal one, the common procedure is to heat the metal insert, which once positioned in its seat, melts the plastic so that it adheres to the metal surface.

The use of ultrasound for the insertion of metal parts into thermoplastic materials is a very precise and controllable technique. The operating principle behind this application is very simple; the ultrasound vibrations propagate through the metal insert and reach the contact surface between the metal and plastic. In this area, the energy generated by the vibrations is swiftly transformed into heat.

The intensity of the heat is sufficient to melt the plastic, therefore allowing the insert to penetrate it.

Fig. 1 Ultrasound insertion

The vibration time is generally inferior to one second. During this time, the plastic penetrates the knurlings and the undercuts of the insert, thereby encapsulating it. According to the shape and size of the inserts, it is possible to obtain a considerable degree of mechanical resistance against tear and torsional stress.

Figure 1 shows the operating principle of ultrasound insertion.

In order to obtain an insertion with minimum stress and good mechanical strength, the following 5 rules need to be followed:

  1. The metal parts need to be well guided. A centering hole is needed with a diameter larger than 0.1 mm.
  2. Blind holes need to be of about 2 mm deeper than the insert length in order to contain the excess plastic material.
  3. To obtain a high degree of tear and torsional resistance, the wall thickness around the insert holes need to be of at least 2 mm.
  4. It is preferable to touch the part with vibrating sonotrode. The descent speed should be slow. The thrust pressure must be very low.
  5. In order to reduce the tensile stress on the inserts, we recommend that the inserts are allowed to jut out of the welding surface by approx. 0.1 mm.


(Gola = groove) Fig. 2 metal insert dimensions

Figure 2 shows the dimensions of a typical metal insert. It must be remembered that different conditions of use may exist, due to particular geometries. In general the height of the groove may vary from 2 to 5 mm. The insertion hole should slightly exceed the diameter D.

Other important rules to assure correct application:

  • The size of the insert and the hole may vary according to particular requirements, but a sufficient amount of plastic must be removed in order to fill the knurlings and undercuts of the inserts.
  • The amplitude of the vibrations, which is generally low, must be carefully assessed in order to avoid the risk of insert breakage or stress.
  • The thrust pressure and the descent speed must be very low.
  • The ultrasound must be activated BEFORE the sonotrode touches the insert.
  • If a manual tool is being used, the sonotrode must have a guide. A small tip penetrating the insert facilitates the operation. The sonotrodes are made of either titanium or tempered steel.
  • The ultrasound insertion operation is usually noisy. There is also a risk of a loss of metal particles during the operation.
  • Should the insertion of very long metal parts be required, it is preferable to work in an opposite sense, i.e. make the plastic vibrate instead of the metal insert.

Sirius Electric has a great experience in ultrasound welding applications on plastics and metals, offering state-of-the-art equipment and solutions capable of ensuring a number of advantages in terms of the appearance and resistance of the welded parts over time.

If you want to know more about ultrasound welding, you might also be interested in “Ultrasound welding: the pros and cons” “Plastics, not everyone knows that...” “Plastic materials, further information”. In alternative, you can get in contact with Sirius Electric tel. +39 0381 325 610 – email: This email address is being protected from spambots. You need JavaScript enabled to view it.

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Sirius Electric S.r.l.
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