Picture: Cross-section through a component which has thin webs and has been joined by means of circular welding

Circular welding is a widespread modern plastics joining process. It is utilised for pump housings, pipes, frames and Sensors - wherever it is necessary to join individual parts made of plastic in order to form one functional part. Not only sandwich parts but also housings with covers can be welded quickly and precisely in this way. Fischer Kunststoff-Schweißtechnik GmbH, Berkatal, has specialised in the manufacture of precision welding machines for joining plastic parts. The range offered by the company includes not only other friction welding variants but also circular welding.

Exact welds with high quantities

The process is frequently utilised in the automobile industry, e.g. whenever it is necessary to weld together the housing shells of radio-operated car keys. Circular welding can be used in order to weld the halves of the housing with each other in an extremely exact way and in a large quantity. The method is also suitable for components containing sensitive electronics. Willi Fischer, the Managing Director of the company, explains: “While damage may arise here with ultrasonic welding (a competing process), such problems do not usually occur with circular welding. Even the thinnest structures, e.g. filaments infuses, remain free from damage."

It is often problematical to connect those joining members with each 0ther which have thin housing walls and are only provided with longitudinally and transversely arranged ribs and webs on the rear side. In one case, it was a question of welding the component halves of a glove compartment flap for cars with each other. In this respect, the decorative face had to remain fiat. It was not allowed to let the injection process form any sink marks. Such joints can also be obtained by means of welding with a heated tool. In this case, a Teflon coat protects the heated faces from contaminations. Nevertheless, higher quantities may lead to the soiling of the heated tool. Contaminations often arise even in the event of contactless heating using radiators. Moreover the Operator must firstly remove the heated tool after each heating Operation before he can press the joining members on to each other. This leads to along process duration. In this respect, the components must be aligned in their moulds with extreme accuracy so that no distortion occurs. However, the joining pressure is only very low in such processes. Therefore, defective welds arise repeatedly and the distortion of the individual parts is unavoidable. "Our customer, a component supplier in the automobile industry, used to weld the glove compartment flaps using a hot joining process." explains Fischer. Because the heat retention capacity of the narrow webs is much too low, the weld often turned out to be insufficient and the components broke apart once again in the final assembly Fischer continues: "The component supplier has made the selection of a suitable process according to our advice. The geometry of the parts with ribs, webs and end edges running in all directions ultimately led to the decision to utilise circular welding." This constitutes not only the most economically viable process but also a reliable process since the process duration is considerably shorter but the accuracy is higher in this respect. Due to the high joining pressure of approx. 2 N/m², the halves of the part are also pressed into their moulds so strongly that they can adopt their nominal shape without any injection distortion.

Laser welding is another alternative for joining such components. However, problems may arise here as well, e.g. whenever the contact between the joining members is not parallel due to the injection distortion. In contrast, the operator of the circular welding machine always has the possibility of enlarging the joining path by a few tenths of a millimetre.

Welded in all directions

In the case of the circular welding process, the joining members are moved relative to each other by the circulating kinematics and are welded together under pressure. The friction leads to the heating and plastification of the joining faces and thus to the welding operation. The circular oscillations are the same at every joint - irrespective of whether the ribs and the webs run transversely or longitudinally. All the joining faces, no matter how narrow they may be, are welded by the multidirectional welding kinematics. Fischer on this subject: "With this process, we can carry out stable welding in all directions. Even if thin webs also shake on occasion, that does not play any role worth mentioning."

Author: Thomas Dickmann, Dinslaken

Source: See in Joining Plastics 2/10 Site 80/81