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Investigations about the weldability of thermoplastics with the aid of ultrasound

The thesis deals with the fundamentals of ultrasonic welding of thermoplastic polymers. Main points are the energy transmission and the energy conversion.

It is shown theoretically and with the aid of model experiments that the geometry of the parts being joined has a big influence on the weldability.The reason for this phenomenon is, that a stationary wave field forms in the parts to be joined. By that both the energy transmission and the conversion depend on the geometry.

The maximum energy conversion into heat takes place in the sections of maximum alternating stress and strain. Here we have the maximum internal friction. Position, contact surface and geometry of the weld seam have therefore a dominant influence on the seam quality. Additionally the interfacial friction between the parts being joined can be used for heating and melting. The type of energy conversion depends on the joining pressure. The joining pressure is defined as the quotient of the static contact force and the cross-sectional area of the parts being joined. In the area of low pressure the energy is made up of an internal friction component and an interfacial friction component. In the area of high pressure energy conversion takes place by internal friction alone.

A distinction is made in ultrasonic welding between near field welding and far field welding. In near field welding – also called the direct method – the distance between the sonotrode and the weld seam is less than 6 mm. In the far field welding – also called the indirect methode – the sound is conducted to the seam over a large distance through the upper part. The welding capacity of the thermoplastics in respect of the two methods can be estimated with the aid of two dimensionless groups which describe the energy loss and the energy conversion.

As concerns plastic films we have to take into account that the sound permability has an additional influence. This is the quotient of the output energy and the input energy. The thickness of the films being joined is here the dominant value.

Lesen Sie die deutsche Zusammenfassung auf Kunststoffe.de
Author

Helmut Potente
Institut für Kunststoffverarbeitung
Rheinisch-Westfälische Technische Hochschule Aachen

Information

Free keywords: amorphous thermoplastics, dimensionless groups (quantities), energy transmission, energy conversion, far field, interfacial friction, internal friction, model experiments, near field, weldability, welding seam, semi-crystalline thermoplastics, theory
Institute / chair: Fakultät für Maschinenwesen der Rheinisch-Westfälischen Technischen Hochschule
Language: German
Technical consultant for expert services: Prof. Dr.-Ing. G. Menges (Betreuer), Prof. Dr. phil. R. Pohlman
Publication year: 1971
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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