Ultrasonic welding of polyamide—influence of moisture on the process relevant material properties

Abstract

The welding of hygroscopic materials such as polyamide can lead to unstable conditions during the welding process. Due to changing material properties, the ultrasonic welding process is influenced heavily by the moisture level of the welding parts. To achieve stable welding processes and high weldline qualities, it is necessary to understand the influence of moisture on the material properties and the ultrasonic welding process. To perform a scientific examination of the influence of moisture on the ultrasonic welding process, the interactions between the material properties and the welding process are determined in relation to the moisture content. With the aid of welding tests, it can be shown that with constant welding parameters, the attainable weld strength decreases with increasing moisture load. Microscopy analyses show gas bubbles in the weld seam. These are formed during the welding process, because of the evaporation of water in the welding zone. Furthermore, it can be seen that the time required to reach a predetermined welding path depends on the moisture content. At first, the welding time increases with increasing moisture content and is decreasing again with further moisture load. In order to explain the welding behaviour, material tests are carried out. Especially, the material damping influences the welding process largely in case of ultrasonic welding. The material damping in dependency of temperature and frequency can be described by storage and loss modulus. To determine loss and storage modulus at the welding frequency of 20 kHz, dynamic mechanical analyses are carried out at different temperatures and frequencies. The results are extrapolated by making use of time/temperature shift to 20 kHz. The results show that at 20 kHz, the maximum material damping is not exceeded in case of the maximum moisture load but at the same moisture load as in case of the maximum welding times. Further welding tests show that the welding time can be decreased again by choosing a higher welding amplitude, which leads to a higher energy input in the specimens. However, the weld seam strength cannot be increased to the stage of the dry-welded specimens by adapting the welding parameters, because of the formation of gas bubbles in the weld seam strength.